Australian Saltmarsh Ecology

AUSTRALIAN SALTMARSH ECOLOGY EDITOR: NEIL SAINTILAN AUSTRALIAN SALTMARSH ECOLOGY AUSTRALIAN SALTMARSH ECOLOGY E...

Author: Neil Saintilan | Paul Adam | Rod Connelly | Pat Dale | Pia Laegdsgaard

351 downloads 1785 Views 7MB Size Report

This content was uploaded by our users and we assume good faith they have the permission to share this book. If you own the copyright to this book and it is wrongfully on our website, we offer a simple DMCA procedure to remove your content from our site. Start by pressing the button below!
Report copyright / DMCA form

DOWNLOAD PDF

AUSTRALIAN

SALTMARSH

ECOLOGY EDITOR: NEIL SAINTILAN

AUSTRALIAN SALTMARSH ECOLOGY

AUSTRALIAN

SALTMARSH

ECOLOGY

Editor: Neil Saintilan

© CSIRO 2009 All rights reserved. Except under the conditions described in the Australian Copyright Act 1968 and subsequent amendments, no part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, duplicating or otherwise, without the prior permission of the copyright owner. Contact CSIRO PUBLISHING for all permission requests. National Library of Australia Cataloguing-in-Publication entry Australian saltmarsh ecology/editor, Neil Saintilan. 9780643093713 (pbk.) Includes index. Bibliography Salt marsh ecology – Australia. Salt marshes – Australia. Coastal zone management – Australia. Saintilan, Neil. 577.690994 Published by CSIRO PUBLISHING 150 Oxford Street (PO Box 1139) Collingwood VIC 3066 Australia Telephone: Local call: Fax: Email: Web site:

+61 3 9662 7666 1300 788 000 (Australia only) +61 3 9662 7555 [email protected] www.publish.csiro.au

Front cover photos by: Todd Minchinton (main), Pauline Ross, Jordan Iles, John Manger. Back cover photos by (clockwise from top left): Neil Saintilan, Neil Saintilan, John Manger, John Manger, Pauline Ross. Set in Adobe Minion 10/12 and Stone Sans Edited by Janet Walker Cover and text design by James Kelly Typeset by Desktop Concepts Pty Ltd, Melbourne Index by Russell Brooks Printed in Australia by Ligare The book has been printed on paper certified by the Programme for the Endorsement of Forest Chain of Custody (PEFC). PEFC is committed to sustainable forest management through third party forest certification of responsibly managed forests. CSIRO PUBLISHING publishes and distributes scientific, technical and health science books, magazines and journals from Australia to a worldwide audience and conducts these activities autonomously from the research activities of the Commonwealth Scientific and Industrial Research Organisation (CSIRO). The views expressed in this publication are those of the author(s) and do not necessarily represent those of, and should not be attributed to, the publisher or CSIRO.

Contents

List of contributors

vii

Preface

ix

Chapter 1

Australian saltmarshes in global context Paul Adam

1

Chapter 2

Distribution of Australian saltmarsh plants Neil Saintilan

23

Chapter 3

Geomorphology and habitat dynamics Neil Saintilan, Kerrylee Rogers and Alice Howe

53

Chapter 4

The ecology of molluscs in Australian saltmarshes Pauline Ross, Todd Minchinton and Winston Ponder

75

Chapter 5

Ecology of burrowing crabs in temperate saltmarsh of south-east Australia Debashish Mazumder

115

Chapter 6

Fish on Australian saltmarshes Rod Connolly

131

Chapter 7

Saltmarsh as habitat for birds and other vertebrates Jennifer Spencer, Vaughan Monamy and Mark Breitfuss

149

Chapter 8

Ecology and management of mosquitoes Pat Dale and Mark Breitfuss

167

Chapter 9

Protection and management of coastal saltmarsh Pia Laegdsgaard, Jeff Kelleway, Robert J Williams and Chris Harty

179

Chapter 10 Mapping, assessment and monitoring of saltmarshes Jeff Kelleway, Robert J Williams and Pia Laegdsgaard Index

211

231

v

List of contributors

Paul Adam

School of Biological Earth and Environmental Science University of New South Wales Mark Breitfuss

Queensland Bulk Water Transport Authority Rod M Connolly

School of Environment and Australian Rivers Institute Griffith University Pat Dale

Griffith School of Environment Centre for Innovative Conservation Strategies Griffith University Chris Harty

Chris Harty Planning and Environmental Management Alice Howe

School of Engineering University of Newcastle Jeff Kelleway

Rivers and Wetlands Unit NSW Department of Environment and Climate Change Pia Laegdsgaard

Coastal Ecology and Management Debashish Mazumder

Australian Nuclear Science and Technology Organisation Todd Minchinton

Institute for Conservation Biology and School of Biological Sciences University of Wollongong

vii

viii

Australian Saltmarsh Ecology

Vaughan Monamy

Faculty of Arts and Sciences Australian Catholic University Winston F Ponder

Australian Museum Kerrylee Rogers

Rivers and Wetlands Unit NSW Department of Environment and Climate Change Pauline M Ross

College of Health and Science University of Western Sydney Neil Saintilan

Rivers and Wetlands Unit NSW Department of Environment and Climate Change Jennifer Spencer

Rivers and Wetlands Unit NSW Department of Environment and Climate Change Robert J Williams

Aquatic Ecosystems Research Unit NSW Department of Primary Industries

Preface

As recently as 1990, Peter Fairweather described Australian saltmarsh as the least studied of all marine habitats, and ignorance of the ecological values of saltmarsh had been reflected in the relative lack of protection afforded to the habitat compared to other ecosystems. By way of contrast, mangroves have been recognised as an important fisheries habitat in Australia for nearly a century, and have a long history of protective legislation and regulations. Several decades of sustained research into mangroves through the 1970s and 1980s provided a comprehensive picture of their structure and composition and aspects of their ecology. Over the same period, little attention was give to saltmarshes, in spite of their occupying as much as 16 000 square kilometres of the Australian coastline and supporting more than three times the number of vascular plant species found in mangrove forests. Throughout the 19th and 20th centuries saltmarshes were replaced by playing fields, residential and commercial land and agriculture. We now know that in the closing decades of the previous century, mangroves began replacing saltmarsh from the seaward edge, a trend likely to continue with elevated sea levels as a result of global climate change. The decline of coastal saltmarsh in the southern half of the continent has now come to the attention of policy makers, and in New South Wales coastal saltmarsh has been declared an Endangered Ecological Community under the NSW Threatened Species Conservation Act. Fortunately, the growing awareness of the vulnerability of coastal saltmarsh has prompted more than a decade of research by a number of university and government scientists. While there is still much to be discovered about Australian saltmarshes, the time is ripe to dispel the myth that we know virtually nothing. This book provides the first synthesis of knowledge of Australian saltmarsh ecology. We hope it will stimulate greater interest in this fascinating habitat. The 10 chapters review geomorphology and biogeography, invertebrate ecology, the use of saltmarsh as a habitat by fish, birds and other mammals, and management issues including the control of mosquitos and the threat of invasive species. The picture which emerges is one of a vulnerable habitat which makes a unique and important contribution to the ecology of the coastal zone. Paul Adam’s opening chapter places Australian saltmarsh in a global context. Saltmarshes occur widely on estuarine and sheltered open coasts, and are immediately recognisable through a combination of habitat, vegetation physiognomy and elements of floristics. Australian saltmarshes exhibit patterns of variation at local, regional and continental scales which are similar to those elsewhere, but nevertheless have unique features. The distinctiveness of Australian saltmarshes is strongest in the south. The flora of southern saltmarshes has similarity with that across Gondwana, but with a number of Australian endemic genera and species. Whether patterns in faunal distribution reflect those in the flora is not known at geographic scales, either in Australia or elsewhere. Chapters 2 and 3 explore the biogeography and geomorphology of Australian saltmarshes. The possible impacts of climate change are introduced in these chapters. Saltmarsh diversity increases toward the colder latitudes, and a warming climate may well pose a threat to many species. In Chapter 3, Neil Saintilan, Kerrylee Rogers and Alice Howe present evidence that sea ix

x


level rise in the southern and eastern regions of the continent may already be having a detrimental impact, promoting the colonisation of the saltmarsh environment by mangrove. Several chapters then consider the faunal ecology of Australian saltmarsh. Pauline Ross, Todd Minchinton and Winston Ponder provide a comprehensive account of the mollusc fauna of Australian saltmarshes in Chapter 4. The authors describe the unique adaptations of gastropods to the challenges of the saltmarsh environment, and their close association with the saltmarsh flora, both for habitat and food. In Chapter 5, Debashish Mazumder outlines the ecology of grapsid crabs, the dominant crustacean and in many ways the keystone of the saltmarsh ecosystem. Crabs in Australian saltmarshes have limited opportunity to spawn, but on the few occasions the tide inundates the upper intertidal saltmarsh a mass spawning ensues. This event provides a link between the trophic ecology of crabs and fish, and is explored by Rod Connolly in Chapter 6. Many species of fish enter the saltmarsh on the spring tide, including several species of direct commercial importance. Chapter 7 considers the importance of saltmarsh as a habitat for a range of terrestrial species, including birds, bats and other mammals. The significance of saltmarsh for migratory shorebirds has only recently been appreciated in the published literature, partly because the saltmarsh is primarily used as a night-time roost, a time when few ecologists are active. The shallow pools of the saltmarsh afford good protection from many predators, as well as a secondary feeding habitat. Among the other night-time visitors to the saltmarsh are several species of insectivorous bat, including some threatened species. There are numerous species of insects which may be attracting bats to the saltmarsh. One such species is the saltmarsh mosquito, Aedes vigilax. The ecology and management of the saltmarsh mosquito forms the subject of Chapter 8 by Pat Dale and Mark Breitfuss. The saltmarsh mosquito is a biting nuisance in many coastal communities and in some locations a vector of the Ross River virus. There are a number of other viruses which cause disease in humans which could be transmitted by mosquitoes, and with global warming the incidence of infection may increase. Perceptions of this risk will need to be addressed to ensure that public opinion continues to support wetland conservation. Several strategies for mosquito control are discussed and their ecological consequences described. The final two chapters provide an overview of management issues and responses. Pia Laegdsgaard, Rob Williams, Jeff Kelleway and Chris Harty describe the effects of overgrazing, use of off-road vehicles, dumping of waste and reclamation. The legislative and policy responses of the various Australian jurisdictions are discussed and the importance of community awareness is stressed. Implementation of conservation measures for saltmarsh is dependent upon us knowing where it is, and the final chapter provides guidelines which should improve the mapping and monitoring of saltmarsh by natural resource managers. While the book goes a long way towards redressing the common misconception that little is known about Australian saltmarsh, a common refrain in many chapters is that there is still much to discover. Several fruitful areas of research are proposed, notably an improved understanding of the ecophysiology of saltmarsh plants, the study of saltmarsh insects and their trophic dependencies, and a better appreciation of the ecology of saltmarshes in the tropical north and the arid west of the continent. Studies of ecosystem processes have been out of fashion for some time, although there are indications of a resurgence of interest. Saltmarshes, particularly in the USA, were amongst the earliest ecosystems subject to process studies, and these early results have entered textbooks as generalisations applicable to all saltmarshes. Given the differences in floristic composition, climate, tidal regimes and sediment fertility it is likely that quantitatively, Australian saltmarshes will differ from those in the USA, and it would be highly desirable if we had local studies – although these will require multidisciplinary teams

Preface

and substantial budgets. It is the hope of the authors that this book will inspire the next generation of saltmarsh ecologists to answer some of these questions. Finally, there are a number of people who deserve our thanks, not the least the numerous honours and graduate students whose diligent work has contributed so much to our present understanding of Australian saltmarshes, braving hot days, cold nights, mud and mozzies. Thanks are also due to the team at CSIRO Publishing for their enthusiasm and support, including John Manger, and Briana Elwood. Janet Walker did a superbly efficient job with editing, as did Frank Saintilan. Neil Saintilan and Paul Adam August 2008

xi

CHAPTER 1

Australian saltmarshes in global context Paul Adam

Introduction Coastal saltmarshes are recognised globally as ecosystems of high ecological value which are increasingly under threat (Adam 2002; Valiela 2006). While there is increasing acknowledgement of their importance in Australia, and their ‘Cinderella’ status, demonstrated by Fairweather (1990), has improved over more recent times, they are still relatively unknown compared with the intensively studied marshes of Europe and North America. Coastal saltmarshes can be defined as intertidal communities dominated by flowering plants, principally herbs and low shrubs. They are found on soft substrate shores of estuaries and embayments, and on some open low wave energy coasts. Coastal saltmarsh is also found on the shores of intermittently open saline coastal lagoons. When these lagoons are open to the sea they are tidal (although tidal amplitude may be attenuated in comparison to nearby open shores), but when closed, which is often the majority of times, water level fluctuations are climate driven and lack predictable periodicity. Saltmarsh is distinguished from adjacent communities by both floristic composition and structure. Mangroves are dominated by trees (and amongst the world’s forests are unusual in the virtual absence of an understorey). The boundary between saltmarsh and mangrove is normally sharp, but on temperate coasts there are sites with mosaics of saltmarsh and mangrove where there are groves of Avicennia marina interspersed amongst saltmarsh and at the southern global limit of mangroves in Victoria mangroves are stunted and may be lower than the chenopod shrub Tecticornia arbuscula on adjacent saltmarsh. Seagrass beds are predominantly subtidal and are dominated by various monocots (although none are true grasses). The upper limit of saltmarsh is set by the level of the highest tide, but the nature of the transition to terrestrial vegetation will be determined by topography, and, in urban areas, human activity. Coastal lowlands have been very heavily modified in temperate and subtropical regions, so that natural transitions to terrestrial vegetation are becoming uncommon (Figure 1.1). Swamp forests on coastal flood plains often have an understorey of saltmarshes in the transition zone, which may be inundated with brackish water during storms. Species found in intertidal saltmarsh are also characteristic of seepage zones on seacliffs and rock platforms above the tidal limit, and on some of the most exposed cliffs and headlands, extensive swards (covering hectares) of saltmarsh species are found tens of metres above the sea but subject to high inputs of aerosolic salt. Australia has very extensive areas of saline soil inland – some of these are of natural origin, but salinisation of agricultural and urban land is one of the major environmental problems to 1

2


Figure 1.1 Temperate saltmarsh. Newington, Parramatta River NSW. Marsh dominated by Sarcocornia quinqueflora, and fringing stand of Casuarina glauca.

be addressed as a national priority. Some of the species of inland saline areas also occur in coastal saltmarshes, but the majority of the vegetation comprises species in the same genera as those on the coast, but endemic to the inland. At various geological times parts of the inland would have been shallow seas, and, at others, seed transporting birds could have carried propagules between the coast and inland, so that there will have been ample opportunity for gene and species exchange, as well as periods of isolation of local populations and opportunities for speciation. Towards the head of estuaries, conditions may be brackish or fresh, but still subject to tidal influence. Fringing reed and tall sedge communities in the freshwater tidal zone have been very heavily impacted by urban and agricultural development, and by hydrological change as upstream water abstraction reduces freshwater input. There have been few studies of freshwater tidal marshes in Australia. In tropical Australia the upper intertidal, flooded by the tides only infrequently, develops hypersalinity during the dry season. The vascular vegetation of these hypersaline flats is extremely sparse and contains only a small number of mostly succulent species (Batis argillicola, Cressa cretica, Sesuvium portulacastrum and Tecticornia australasica). Although the vascular plant cover is very low, there is a skin of microalgae and cynobacteria amongst the salt crust and extending some millimeters into the underlying sediment. The ecology of these flats has not been extensively studied, but they may make a considerable contribution to estuarine productivity; around the Gulf of Carpentaria considerable quantities of salt and nutrients are released from hypersaline flats during king tides (Ridd et al. 1988). Similar extensive hypersaline flats occur on arid coastal zones elsewhere, and are known as ‘sabkha’ in the Middle East. There is no consensus as to whether these flats should be regarded as saltmarsh or as a separate ecosystem. This uncertainty renders it difficult to determine the

Australian saltmarshes in global context

extent of saltmarsh in Australia, as different estimates have been made on different bases. However, the area of flats is probably roughly the same as the area of fully vegetated saltmarsh. While extensive hypersaline flats are a feature of tropical coasts, smaller bare patches are found within temperate marshes. On the central NSW coasts such patches were formerly frequent on the Parrramatta River, Cooks River, Botany Bay / Georges River (Hamilton 1919; Clarke and Hannon 1967). In the last few decades many of the patches have become vegetated, and the few that remain have been damaged by vehicle use (extensive new bare patches have been created by off-road vehicular use – Kelleway 2005). Whether the revegetation of bare areas is a response to natural environmental change, or whether it reflects human influence (such as greater discharge of stormwater into marshes) remains to be determined.

The saltmarsh environment The saltmarsh environment is a challenging one for many plants, explaining the relatively small flora and its general similarity around the world. Although the flora is not a single taxonomic lineage, and the adaptations necessary to survive the saltmarsh environment have evolved independently on a number of occasions, relatively few families are represented within it. The factor which distinguishes saltmarsh (and mangroves) from other vascular plant communities is tidal inundation. Tides are highly predictable, but the interactions between tides, weather, groundwater influences and vegetation result in complex patterns of environmental variation (Figure 1.2).

Figure 1.2 The interaction between environmental factors and vegetation in saltmarshes. Redrawn and modified from Clarke and Hannon (1969).

3

4


The tidal regime varies considerably around the coastline. In south-west Western Australia and parts of the Gulf of Carpentaria, tides are diurnal, with a single low and high tide a day on the cycle of 24 hours and 50 minutes. Elsewhere tides are semi-diurnal, with two highs and lows a day, or mixed, when the two highs per day differ considerably in height. Tidal ranges are also extremely variable – in southern Australia mostly low (micro- to mesotidal) but with higher ranges in bays and inlets, while in northern Australia tidal ranges are generally high with a maximum of more than 8 m in north-west Western Australia. The tidal range determines the vertical extent of saltmarsh, but the horizontal extent will depend on the local topography and geomorphology and there are laterally extensive stands at sites with low tidal range and narrow fringes on coasts with high tidal ranges, although for a given surface gradient the higher the tidal range the wider the saltmarsh. As a consequence of tidal inundation the soils in saltmarsh are saline; the lower on the shore the more frequent the inundations and the less variable the soil salinity. However, at higher levels of the shore salinities can vary considerably depending on the balance between rainfall and evaporation. Inundation will also result in anaerobic soil, although the duration of waterlogging will depend on sediment type and local drainage. Tidal flooding has other effects on plants. Tidal currents, which increase with tidal range, may dislodge seedlings, so that recruitment may require sufficient long windows of opportunity between inundation to permit germination and development of sufficiently robust seedlings. Estuarine water may be turbid so that after tidal flooding vegetation may be coated with sediment, possibly reducing photosynthesis. Submergence may also change the effective day length and expose plants to a sudden temperature shock. The physiological consequences of these stresses have not been studied. The interaction between the environmental conditions and species results in a general zonation of species (Adam 1990); the more frequently inundated lower marsh providing habitat for fewer species than the higher levels. Communities are also zoned, but at any given level on the shore there is often a mosaic of communities rather than a continuous band of a single community (Zedler et al. 1995). Local microtopographic change to drainage conditions is often reflected in the vegetation mosaic (despite the absence in many Australian saltmarshes of the well developed creek and pan systems which are a feature of saltmarshes elsewhere – Adam 1990, 2000). The zonation of saltmarshes is often interpreted as the spatial expression at one point in time of succession. Conceptual models have been developed in which species colonise mud or sand flats and promote accretion and stabilisation of sediment. As the elevation of the marsh surface rises, frequency of tidal inundation declines and environmental conditions permit the establishment of other species which displace the primary colonists. Continued expansion of primary colonists seawards results in zonation. This basic model of sedimentation driven succession, with various additional complexities to account for variation in relative sea level, is sustained by empirical evidence, but interpretation of zonation as a reflection of succession in Australia is less certain. Pidgeon (1940), influenced by the Clementsian approach which was then one of the major paradigms of ecology, proposed that the zonation of intertidal communities on the New South Wales coast could be interpreted as resulting from succession, and this view has become part of received wisdom. If true, it would be a very atypical successional sequence as it would imply that the primary colonists were trees (mangroves), subsequently replaced by dwarf shrubs and herbs. Pigeon’s model also postulated that the succession continues above the highest astronomical tide level through Casuarina glauca forest to eucalypt swamp forests. In the absence of a drop in relative sea level it is difficult to see that the proposed succession would be driven by allochthonous sedimentation or that autochthonous sedimentation (peat formation) would be sufficient to elevate the surface out of the tidal frame.


The relationship between saltmarsh and mangrove is complex (see Chapter 3) but it is difficult to accommodate a transition from mangrove to saltmarsh within the standard model of saltmarsh formation. There are few sites where active formation of new saltmarsh is occurring, with the exception of invasion by Spartina anglica. Long-term successional development in Australian Spartina marshes, if it occurs, has yet to be described.

Flora and vegetation of Australian saltmarsh Accounts of saltmarsh flora and vegetation have been published for a number of parts of the Australian coast, including: inter alia by Hamilton (1919); Saenger et al. (1977); Adam (Adam 1981a, b; Adam et al. 1988; Adam and King 1990; Adam 1994); Bridgewater (Bridgewater, Rosser and de Corona 1981; Bridgewater 1982; Bridgewater and Cresswell 1993, 2003; Cresswell and Bridgewater 1998); Craig (1983); Kirkpatrick and Glasby (1981); Thannheiser (2001); Johns (2006); and Kelleway et al. (2007). Saltmarshes occur globally, and most, although exhibiting local characteristics, have immediately recognisable similarities. Australian saltmarshes are no exception; in terms of physiognomy and composition (particularly at generic level) they are similar to saltmarshes elsewhere. Within terrestrial biomes, as a general rule, species richness is highest in the tropics and declines at higher latitudes. This is also the case with mangroves, but saltmarshes show a strikingly different pattern (Adam 1990). Tropical saltmarshes in Australia are extremely depauperate, but species richness increases in temperate latitudes, with the highest number of species being recorded from Tasmanian marshes (Saenger et al. 1977; Bridgewater and Cresswell 2003). Within individual marshes, species richness is generally lowest at low, more frequently tidally inundated elevations and increases higher up the shore, although if freshwater input permits the establishment of tall competitive dominants, such as Phragmites or Typha, in the upper marsh, species richness is again low. The broad geographic scale patterns of variation in species and community distribution within Australian saltmarshes are similar to those elsewhere (Adam 1990). The distinction between tropical and temperature saltmarshes is seen not just in changes in species richness, but also in the distribution of individual species. Some of the tropical species are widespread outside Australia on hot dry shores (Batis, Sesuvium, Cressa) but Tecticornia australasica (Figure 1.3) is an Australian endemic element in the flora. On more temperate shores the flora has a large, widely distributed element (at both generic and species level) as well as a strong Gondwanan element. Adam (1990) has argued that there

Figure 1.3 Distribution of Tecticornia australasica (from literature records, personal observation and records in the Australian Virtual Herbarium).

5

6


Figure 1.4 Distribution of Selliera radicans (from literature records, personal observation and records in the Australian Virtual Herbarium).

is an overall similarity of flora and vegetation in saltmarshes in South Africa, south-western and south-eastern Australia, New Zealand and temperate South America. Links include the upper marsh rush Juncus kraussii, Sarcocornia spp, Triglochin striata, Cotula coronopifolia and Samolus spp. One species common to Australia and New Zealand is Selliera radicans (Figure 1.4), from a family (Goodeniaceae) absent from northern hemisphere saltmarsh floras. (On the central NSW coast there is evidence of a recent decline in S. radicans, only partly explained by habitat loss (Adam et al. 1988; Kelleway et al. 2007). On coasts with a strongly seasonal Mediterranean climate, saltmarsh vegetation is characterised by shrubby chenopods (formerly in the genera Halosarcia and Sclerostegia, but following a recent taxonomic revision by Shepherd and Wilson (2007), Tecticornia spp.). The dwarf subshrub Frankenia is also characteristic of Mediterranean zone saltmarshes. Some of the less common species from brackish upper marsh communities exhibit remarkable transhemisphere disjunctions, which, if they have been correctly identified, may reflect the legacy of past long distance dispersal events, possibly by migratory waders. Examples include Limosella australis, which, amongst other occurrences in Australia, is found in upper saltmarshes on the south coast in NSW, but is an endangered wetland plant in Wales, and Isolepis cernua, widespread, although not abundant, in upper marsh flushes in eastern Australia but which is much rarer in northern Europe. A recent discovery in Australian saltmarshes is the dwarf Eleocharis parvula, one of the most inconspicuous saltmarsh species. Is this a recent introduction, or a cryptic species which had been previously ignored? In the northern hemisphere E. parvula has a circumboreal distribution, but with many disjunctions. Clearly E. parvula has not been deliberately introduced into Australia, and it is difficult to envisage a mechanism for accidental introduction by human agency. The habitat of E. parvula in Australia is similar to that in which it occurs in the northern hemisphere and it is not impossible that it was introduced to Australia by birds, possible a long time ago. We may never be able to determine the origin of E. parvula in Australia, although molecular comparison with northern hemisphere populations may in the future provide insights. Many saltmarsh species have very wide distributions, both at the local scale, within individual marshes, and geographically. This wide amplitude is made possible by the species being made up of many genotypes (Adam 1990). There have been few studies of the genecology of Australian saltmarsh species, but one of the most widespread saltmarsh grasses, Sporobolus virginicus, has been shown to be genetically very variable (Smith-White 1981, 1988). The presence


of genetic variation may facilitate the response of species to climate change, but also has implications for the use of planting material in rehabilitation or recreation projects. The wide distribution of species may suggest that any material could be used (including commercial cultivars) in these projects, without care being taken to match the genotype to the new environment.

Non-vascular flora The vascular plants are of the visibly dominant component of saltmarshes. Other plants may, however, play important roles in the ecosystem. While there have been many studies of algae in Australian mangroves, saltmarsh algae have been rarely studied. However, they are likely to be as important as algae in saltmarshes elsewhere – contributing to primary productivity, stabilising sediment surfaces, being the food source for filter and surface feeders and, in the case of cyanobacteria, which form part of the algae skin on the sediment surface, functioning as nitrogen fixers. Bacteria and fungi play a major, although in the Australian context largely unquantified, role in decomposition and chemical transformation and provide food for filter feeders. Although it has long been known that a number of European vascular halophytes are vascular-arbuscular mycorrhizal (VAM, summarised in Adam 1990), it is only recently that more detailed studies have been undertaken (Davy et al. 2000). There has been no systematic investigation of VAM in Australian halophytes, although Samolus repens does support VAM (pers. obs). Bryophytes and lichens are not generally considered to be components of saltmarshes, although in some northern hemisphere marshes bryophytes form a characteristic element in the vegetation (Adam 1990). In Australia, bryophytes and lichens are largely absent from saltmarshes, although there are occasional occurrences at the highest driftline or as epiphytes on shrubby chenopods.

Additional factors The environmental factors incorporated in Figure 1.2 are universal in space and time – the interplay of the factors with the pool of Australian halophytic species results in a range of distinctive communities, in the same way that the same factors applied to a different range of species elsewhere would produce a different suite of communities. Human activity results in additional factors coming into play. Climate change Increased concentrations of greenhouse gases, including carbon dioxide and methane, are predicted to lead to global warming and other changes in climate conditions. One consequence of global warming will be a rise in sea level, initially as a result of thermal expansion but in the longer term further contributed to by melting of icecaps. The effects of global sea level rise will not necessarily be translated uniformly into changes in relative sea level, as tectonic movement or increased sedimentation could counter the rise in water level at the local scale. However, the Australian coast is tectonically relatively quiescent, and sediment supply, although increased through catchment erosion since European settlement, is still not great. The rise in sea level is thus likely to result in increased inundation of saltmarsh and the retreat of the tidal limit inland. In southern Australia, where many saltmarshes now abut infrastructure and urban or agricultural development, this will result in coastal squeeze, as the seaward edge of saltmarsh is lost to mangrove invasion or inundation beyond flooding tolerance, and the expansion inland is prevented by lack of habitat. However, in northern Australia, where much of the coast

7

8


is still undeveloped, there are few impediments to the establishment of saltmarsh over what are now terrestrial communities. Although warming is likely to be a universal phenomenon, other aspects of the climate – rainfall amount, intensity and temporal distribution, general storminess and frequency and intensity of major storms – are likely to vary at local and regional scales although currently available mathematical models do not permit detailed modelling of probable changes. However, changes in any of these factors are likely to be reflected in changes in saltmarshes – changes in rainfall regimes will alter the patterns of variation in soil salinity, change in storminess, but particularly in the intensity of major storm events, could result in erosion of saltmarsh vegetation. Warming may result in an expansion southwards of the range of northern species, and through competition this might produce a contraction in southern species. Increased temperatures might favour mangroves at the expense of saltmarsh, and might also favour some weeds over native species. While there remains uncertainty over the details of climate change and the biological consequences, even the most extreme climate change deniers would recognise that the atmospheric carbon dioxide concentration has increased and will continue to do so. This in itself will have profound effects on saltmarshes. Simplistically it might be thought that an increase in carbon dioxide will result in greater photosynthesis and, hence, greater ecosystem productivity. However, an increase in carbon dioxide will lead to greater growth of plants with the C3 photosynthetic pathway, altering the current competitive balance between C3 and C4 species. There will also be an increase in water use efficiency so plants will transpire less and the soil moisture regime will be changed. An increased carbon dioxide concentration is also likely to result in a decreased leaf protein content, which will have flow-on effects through the ecosystem. On current information it is not possible to predict whether lower leaf nitrogen will result in greater herbivory as herbivores need to consume more to achieve the same nitrogen input or lesser herbivory as the decline in resource quality deters herbivores. Experimental studies in the glasshouse, and in the field in American saltmarshes, confirm that carbon dioxide concentrations to the levels predicted over the next century will result in shifts in the relative abundance of C3 and C4 species (in favour of C3), changes in water use efficiency, root-shoot ratio and in nitrogen content (Drake et al. 1989), and it is probable that such changes will be experienced on saltmarshes globally. Pollution Climate change could be regarded as a global consequence of pollution, but there is a range of more local pollution events which could have considerable impacts on saltmarshes. Given that estuaries have been sites for industrial development for centuries, there is a considerable legacy of industrial pollution in estuarine saltmarshes. Metals released into estuaries may be incorporated into saltmarsh sediment, where under reducing conditions their biological availability is lessened. Disturbance to sediments (through dredging or reclamation) may result in oxidation of these metals and their release into the environment in much more biologically available and toxic forms. High levels of heavy metals in saltmarsh sediments have been recorded in many estuaries around the world, and in Australia the association of industry, particularly smelting, with estuaries in the southern part of the continent is reflected in elevated levels of metals in waters, sediments and vegetation (see for example Woods et al. 2007). While the Precautionary Principle would indicate that metal accumulation should be regarded as a concern, and measures taken to prevent new discharges and reduce or eliminate existing sources, the ecosystem-level consequences of metal contamination are less clear (Williams et al. 1994; Valiela 2006) possibly because of the lack of studies which have examined ecosystem processes. Most saltmarsh plants, which are physiologically


adapted to a stressful environment, may be constitutively tolerant of metal pollution. In Europe, where a number of saltmarsh species are harvested for human consumption, concern has been expressed about possible adverse consequences for human health of eating saltmarsh plants with high metal concentrations (Beeftink et al. 1982), but the effects of metals in saltmarsh plants on the detrital food chain and on direct herbivory by fauna have been poorly studied. One of the most dramatic forms of pollution to saltmarshes is from oil spills. A number of major spills from shipping accidents have affected saltmarshes, but Australia has fortunately not yet experienced a very large spill, although a number of small spills have affected small areas of marsh. Experience around the world suggests that in most cases, as much or more damage, is done in attempting to clean up oil spills in saltmarshes as that incurred directly from the spill (Baker et al. 1994). In general the recommendation is to prevent, as far as possible, oil reaching saltmarsh but if oiling does occur, to permit it to degrade naturally. In tropical regions breakdown is likely to be fairly rapid, but in temperate and polar latitudes residues may remain in sediment for decades and continue to have biological effects throughout this time (Culbertson et al. 2007). In Australia saltmarshes have been identified as ecologically sensitive communities in oil spill contingency planning; if at all possible booms would be deployed to prevent oil reaching saltmarshes, and if oiling of saltmarshes does occur, dispersants would not be used (for example see Carter 1994). As well as tide-borne incursions of oil, terrestrial chemical and oil spills (from road or rail accidents) could reach saltmarshes, and the emergency services would need to manage any oil and chemicals in stormwater drains and waterways so as to minimise impacts on saltmarshes. Runoff of nutrients, herbicides and pesticides into estuaries has impacts on saltmarshes. Eutrophication of waterways has resulted in algal blooms smothering saltmarshes. For the Peel-Harvey estuary (WA), addressing this eutrophication was a very expensive catchment wide process, involving measures to reduce fertiliser application and runoff, and re-engineering a new opening of the estuary into the ocean (Brearley 2005). Herbicide residues (particularly simazine) have been shown to adversely affect microalgae in European saltmarshes (Mason et al 2003). As these algae help stabilise the marsh sediment this is regarded as an important issue to be addressed. Few data are available from which to determine whether there are similar impacts in Australia. A range of other human impacts has been identified on saltmarshes overseas (Adam 2002), many of which are also likely to be relevant in Australia.

Surviving and thriving Plants which characteristically occur in saltmarshes are referred to as halophytes, and in recent decades considerable attention has been given to the mechanisms permitting survival at high salinity. Plants unable to tolerate high salinity are referred to as glycophytes. There is a spectrum of salt tolerance amongst angiosperms, from those intolerant of very low salinities (for example, avocado) to those capable of completing their lifecycle in salinities well above that of seawater. Although many saltmarsh plants can grow in salinities well above seawater it is unlikely that any species has an obligate requirement for high levels of salts (Barbour 1970), and, in the absence of competition from glycophytes, most halophytes grow well under non-saline conditions. The mechanisms of salt tolerance have attracted a great deal of study and we have a good overall understanding of how they operate, although we do not have a complete detailed account of the physiology of every single species.

9

10


For any plant to grow, its cells must be turgid. In order to maintain turgor the water potential (y) within the tissues must be lower (more negative given that, arbitrarily, the y of pure water is defined as 0) than that of the water in the soil surrounding the roots. The problem for halophytes is how low water potentials can be generated without impairing metabolism or survival. In general, halophytes accumulate large quantities of inorganic ions in their tissues sufficient to generate the necessary low water potentials. However, enzymes of halophytes are no more salt-tolerant than these of glycophytes, so there must be internal compartmentation of salt within the cell. If inorganic ions are accumulated in vacuoles (which comprise the majority of cellular volume in most plant tissues) then they must be osmotically balanced by other solutes in the cytoplasm. These so-called compatible solutes must not impair enzyme function even at high concentration, and preferably be of low molecular weight, uncharged and neutral. A small number of types of compound have been identified as functioning as compatible solutes – proline, quaternary ammonium compounds (particularly glycine betaine) and sugar alcohols. Even though halophytes accumulate ions there must be an ability for plants to control the rate of supply of ions to shoots and to adjust tissue salt content. For all halophytes which have been studied there is exclusion of salt from the xylem sap, such that the salt concentration in the transpiration stream is always much less than that at seawater (rarely exceeding 10% seawater); this selective control of ion uptake is one of the major distinguishing differences between glycophytes and halophytes – glycophytes cannot exclude salt, and accumulate lethal concentrations in their tissues. Even with a high degree of exclusion other mechanisms to regulate ion content are required. Reduction in transpiration rate lowers the rate of supply of ions to the leaves – this explains the apparent paradox of many halophytes appearing xeromorphic while growing in an environment with abundant water. A number of species, but far from all, actively excrete salt from stems and leaves through glandular structures known as salt glands which appear to have evolved independently in a number of different flowering plant lineages. Examples of widespread saltmarsh species with salt glands include Frankenia spp., Limonium spp., Samolus repens and S. junceus, and Sporobolus virginicus (Adam 1990; King et al. 1990). Maintenance of tissue salt concentration can also be achieved through becoming succulent – so leaves of species grown in saline conditions are often markedly more succulent than those of the same species grown under non-saline conditions. Succulence is also a feature of plants employing the CAM photosynthetic pathway, but while succulent coastal plants such as Carpobrotus are CAM, no intertidal saltmarsh CAM plant has been reported. There are considerable metabolic costs associated with salt tolerance – including diversion of photosynthate from growth to the synthesis of compatible solutes, and the energy consumption involved in selective ion uptake and secretion. Low transpiration rates inevitably result in low carbon dioxide uptake and hence low growth. These costs are probably the major explanation for the exclusion in the field of halophytes from non-saline conditions. Although the growth of halophytes is maximal under non- or low saline conditions, the maximum growth rates are lower than those of glycophytes under the same conditions. Under non-saline conditions halophytes are likely to be out-competed by more vigorous glycophytes. The other major physiological problem for many plants growing in saltmarsh is the presence, either permanently or temporarily, of water-logged soils, which lack oxygen and may contain phytotoxins. Few angiosperms can survive such conditions for long periods but mostly they do not experience anaerobic conditions because of the presence of efficient internal aerenchyma which permits the passage of oxygen into the root system, and may also permit


leakage of oxygen into the soil, detoxifying potentially hazardous chemicals, and providing an oxygenated rhizosphere occupied by aerobic microbes and micro and meiofauna. Although there are metabolic costs associated with salt tolerance and surviving flooding, saltmarshes are widely regarded as highly productive, although the data supporting this claim are primarily from North American Spartina marshes. The recognition of the high productivity of Spartina gave rise to interest in the fate of the plant material and led to the ‘outwelling hypothesis’ suggesting that saltmarsh production exported to estuarine and coastal waters sustained the food chain, including commercially and recreationally important fisheries. This in turn led to changes in public perceptions of the value of saltmarshes and, in the USA, introduction of legislation to protect saltmarsh habitat. While aspects of the outwelling hypothesis have been modified (Adam 1990) the high productivity of Spartina on the Atlantic and Gulf Coasts of the USA is firmly established. An assumption that estimates of productivity based on American data can be extrapolated to Australia underpinned early moves to conserve Australian saltmarshes. However, there have been few studies which have measured above-ground standing crops in Australia and even fewer which have estimated productivity. There are reasons to suggest that American results are not directly applicable to Australia: Spartina alterniflora dominated marshes occupy a lower part of the intertidal than most Australian marshes, and have a growth form (a very tall grass) not represented in the lower zones of Australian marshes. In many ways S. alterniflora marshes represent a unique biome, differing not just from Australian saltmarshes, but from saltmarshes in other continents as well. In Australia, the lower intertidal zone is usually occupied by mangroves rather than saltmarsh, except in Tasmania. There has been no study of productivity of introduced Spartina anglica in Tasmania and Victoria. It would be of interest to know whether the species’ productivity is comparable to that overseas, and whether the advent of flowering plants into a zone previously unvegetated has had repercussions throughout the estuarine ecosystem. The studies that report biomass and productivity in Australian saltmarshes are both from temperate coasts (south-west Western Australia, Congdon and McComb 1980; NSW, Clarke and Jacoby 1994), so that again they do not provide a strong basis for extrapolation to the whole coastline of the continent. The significance of algal production in saltmarshes has been little studied anywhere, but algae, which are often abundant on the sediment surface, potentially represent a high quality food for detritivores. The recent studies on tracing the flow of energies between trophic levels in marshes and out into adjacent estuaries (Chapters 5 and 6) will provide us with a more secure basis for understanding the role of saltmarshes in estuaries.

Introduced species Globally the spread of introduced species is regarded as a major threat to biodiversity, second only to habitat loss in its impact (Mooney and Hobbs 2000). While few terrestrial ecosystems are free from weed invasion, saltmarshes would a priori be unlikely candidates to be seriously affected for two reasons. Firstly, the environmental conditions of saltmarsh present a severe physiological challenge which many potential weeds would be unable to overcome, and secondly, many saltmarsh species have propagules which can be dispersed easily, either by flotation or by birds. The very wide distribution of some saltmarsh species, in many cases extending across several continents, reflects the ease of dispersal, and suggests that many species have reached their distributional limits without deliberate or accidental assistance from humans. Long distance transport of viable propagules attached to, or carried within, birds has been demonstrated, and the major migratory pathways of waterfowl and waders are predominantly north-south in orientation. In the mid and high latitudes of the southern hemisphere there is a

11

12


powerful circumpolar atmospheric and oceanic circulation which could potentially carry propagules from west to east between continents (Kloot 1984). It is probable that much of the saltmarsh flora of Australia originated elsewhere, but colonised the continent prior to human occupation. It is important, however, to be able to identify those species which have arrived by human agency, so that management resources are not devoted to conserving them, but rather that attention is given to controlling or preventing further spread. The wide distribution of species may generate debate as to their native status. In some cases the debate can be resolved by closer examination of the species and taxonomic re-evaluation. For example, the common rush of upper saltmarshes in temperate Australia and New Zealand was for many years referred to Juncus maritimus, but systematic investigation by Snogerup (1993) confirmed that it was distinct from the northern hemisphere J. maritimus and that the valid name was J. kraussii. As J. kraussii, it is now clearly seen to be a member of a Gondwanan flora element, rather than having a disjunct bipolar distribution if regarded as part of J. maritimus. Most of the taxonomic issues in the saltmarsh flora have been resolved, but there is still confusion over what species of Spergularia (Caryophyllaceae) occur in Australia and whether or not they are native. Even if the taxonomy and nomenclature are well established there may still be disagreement as to whether species have been introduced by human agency. There has been long running uncertainty in this respect in regard to a group of species characteristic of temperate Gondwanan saltmarshes, including inter alia Cotula coronopifolia, Lobelia alata, Samolus repens and Triglochin striata. Doubts about the native status of members of this group in Australia were first expressed by Mueller (1868) and reinforced by Kloot (1984). However, there is no evidence of deliberate introduction, nor are there indications of spread within the period of historic records; all occupy comparable habitats in the different continents. Cotula coronopifolia may fluctuate in abundance, and can become locally dominant in ephemeral brackish or freshwater accumulations in upper saltmarsh. I would regard this as an opportunistic response to favourable environmental conditions rather than offering support for an hypothesis of recent invasion. Spartina anglica Globally the major exotic invasive species in saltmarshes are grasses in the genus Spartina. Although a number of species have been introduced around the world (for example Ayres et al. 2004 describe the invasion of four exotic Spartina species in San Francisco Bay), in Australia only one species – S. anglica has been introduced (Figure 1.5). Spartina anglica originated in Southampton Water on the south coast of England as a result of spontaneous hybridisation between the native European S. martina and the American S. alterniflora which had been accidentally introduced to Britain. The first collections in 1870 have subsequently been identified as a sterile F1 hybrid and named S. townsendii. In the 1890s there was rapid spread of grass similar to S. townsendii, but which was fertile. Marchant (1963, 1968) showed that the fertile form was an amphidiploid derived from the original sterile hybrid by chromosome doubling. This fertile form was later named S. anglica (Hubbard 1968). S. anglica spread rapidly, and was able to colonise lower levels on the shore than existing saltmarshes and promoted sediment accretion and stabilisation. These properties were viewed as highly desirable for coastal engineering and as fodder, and seeds and transplant material were exported from Britain to countries around the world (Ranwell 1967). The history of the introduction of Spartina to Australia was described by Boston (1981). The first known introduction was a planting in Corner Inlet, Victoria, probably in the 1920s, but this failed to establish. From 1927 and through the 1930s numerous introductions, prima-


Figure 1.5

Spartina anglica, Tamar Estuary, Tasmania.

rily as seed, were made and plantings occurred in all states, including in the tropics (Boston 1981). Most of the introductions failed; Boston (1981) suggests that lack of care in the treatment of seed meant that much of the material that reached Australia was not viable. S. anglica was successfully introduced into the south-east of the continent, with currently extensive stands in Tasmania and Victoria. It is also widespread in New Zealand. From the perspective of those who promoted the introduction of S. anglica its establishment would be regarded as justifying their enthusiasm, while the failure to spread around more of the coast would be a matter of regret. However, there has been a substantial change in attitudes the focus today is on control. Spread of exotic Spartina is viewed with concern because of the threat to biodiversity posed by expansion into previously unvegetated mudflats, gene flow to native Spartina spp., and low species richness of the new communities dominated by the exotic invader. The change in attitude is well illustrated in Tasmania. S. anglica was planted in the Tamar estuary for the Marine Board of Launceston with the hope that if the mudflats were stabilised by vegetation, ‘it would force the stream flow into the central part of the river, creating a scouring effect and keeping the main channel relatively free of mud’ (unpublished letter from GJ Martin, quoted by Pringle 1993). In terms of vegetating mudflats in the Tamar S. anglica was extremely successfully, spreading explosively in the 1960s (Pringle 1993), and also colonising other intertidal areas in northern and eastern Tasmania. (RGAG 2002). By 1997 S. anglica dominated about 600 hectares of intertidal habitat in Tasmania (RGAG 2002). There was growing concern about adverse impacts on the integrity of native saltmarsh, loss of habitat for migratory waders (several of the sites colonised by Spartina are wetlands of international importance listed under the Ramsar convention), potential for changes to estuarine hydrodynamics and nutrient cycling which could threaten aquaculture (a growing and important component of the Tasmanian economy), and loss of aesthetic values and shoreline access, impacting on coastal tourism.

13

14


The control of Spartina is made more difficult by the nature of its habitat. The soft unconsolidated sediment makes access by machinery difficult, and even pedestrian access can be both difficult and hazardous. Experience in Australia and overseas suggests that physical removal by pulling and digging is only likely to be successful in the earliest stages of colonisation. Once Spartina is well established, physical removal is likely to cause considerable environmental disturbance and promote further spread from the establishment of rhizomes/roots released during the removal process. Herbicides have been used to control Spartina in a number of countries but there have been relatively few studies of their effects on estuarine fauna and non-target flora. There are no herbicides for which application to Spartina is an approved on-label use, and use of herbicides in the estuarine environment will require the issuing of permits by the relevant authority on a case-by-case basis. In Tasmania currently the most cost effective and environmentally least damaging herbicide for Spartina control is Fusilade® (the active constituent being fluazifop-P as butyl ester), a selective post-emergence grass killer which does not affect native saltmarsh species or seagrasses, is rapidly degraded and has very low toxicity to estuarine fauna (RGAG 2002). Smothering with black plastic has been successful in eradicating small patches of Spartina in Tasmania (RGAG 2002) but the technique is not practical for treating large-scale infestations. The long-term hope for the control of environmental weeds is the development of biological control. Spartina in Australia is potentially a good candidate for biological control in that there are no closely related native taxa. Research in the USA has identified the plant-hopper Prokelisia marginata as a potential control agenda for introduced Spartina (Daelher and Strong 1997; Wu et al. 1999). The native habitat of P. marginata is on S. alterniflora on the East and Gulf Coasts of America. Before any biocontrol agent could be released in Australia there would be a requirement for a comprehensive risk assessment. In Victoria the ability of S. anglia to tolerate more tidal submergence than native species has permitted its establishment seaward of stands of the mangrove Avicennia marina. The long-term consequences of this are unknown, but Spartina is very effective in promoting sedimentation and so its presence could lead to reduction in sediment supply to the mangrove zone. Spartina spp. utilise the C4 photosynthetic pathway. As carbon dioxide levels in the atmosphere increase, plants with the C4 mechanisms will be relatively disadvantaged compared to C3 species. However, the greater submergence tolerance of S. anglica is likely to permit its continuing invasion of mudflats, although as sedimentation raises the marsh surface, replacement by C3 native species may be enhanced. While there is still the potential for S. anglica to spread to new sites, an increase in geographical distribution is unlikely, not least because at lower latitudes the lower intertidal habitat is pre-empted by more vigorous mangroves than those in Victoria. Juncus acutus Juncus acutus, sharp rush, is a northern hemisphere species, native to Europe, Asia and North America where it occurs in upper saltmarshes and brackish inter-dune wetlands. In parts of its native range, for example in the United Kingdom, it is a relatively rare species of conservation interest. It has been accidently introduced into Australia where it is now a major weed of pasture and coastal saltmarsh (Figure 1.6). In saltmarsh it can form dense monospecific stands, crowding out the native J. kraussii. A number of trials of potential control methods have been conducted in saltmarsh in Homebush Bay (Paul and Young 2006; Paul et al. 2007; see Figure 1.7). Combinations of cutting and herbicide were effective in eradicating J. acutus and regeneration of natives occurred, although re-establishment of J. acutus seedlings and other weeds needed to be prevented.


Figure 1.6 Distribution of Juncus acutus (from literature records, personal observation and records in the Australian Virtual Herbarium).

Although eradication has been demonstrated in small plots, the practicality and cost-effectiveness of control over whole marshes remains to be demonstrated. Is Phragmites australis a weed in Australian saltmarshes? Phragmites australis, the common reed, is one of the most widely distributed vascular plants in the world, and is certainly native in Australia. It is primarily a freshwater species, but occurs intertidally in the upper reaches of estuaries and, where freshwater discharge occurs, at the higher elevations in saltmarshes. If there is increased freshwater input, for example associated with stormwater drainage, then Phragmites, and other species such as Bolboschoenus spp. Schoenoplectus spp. and Typha spp. (Zedler et al. 1990) can spread into saltmarsh, forming tall dense stands shading out lower vegetation. In the United States there has been a dramatic spread of P. australis in saltmarshes in recent decades. In part this has been facilitated by clearing and development in the hinterland of marshes, with concomitant increase in nutrient and freshwater inputs. However, the invasive P. australis has been shown to be an exotic European genotype, rather than a native American form (Saltonstall 2002). It has been suggested that P. australis may be spreading in Australian saltmarshes, but there are very few monitoring data to confirm this speculation, nor is it known whether there are non-indigenous ecotypes in Australia. The other introduced species – sleepers or no problem? The examples discussed above are only a few of the many introduced species to be found on Australian saltmarshes. Some of the others are clearly becoming major threats such as groundsel bush Baccharis halimifolia which from its initial introduction in Queensland is now spreading south on the NSW coast. There are active control measures applied because B. halimifolia is implicated as a serious allergy-causing plant for humans. Most of the other species are relative small plants, often annual. They may be locally abundant, but while detracting from the ‘naturalness’ of vegetation are generally viewed as unlikely threats to ecosystem integrity. The majority of these species are found in the upper marsh, often associated with areas disturbed by foot or vehicle passage. Not only are the species not seen as a threat, there are no obvious techniques available for control even if this were thought desirable. Many of the worst environmental weeds in Australia were for long periods (decades) ‘sleepers’ – present in vegetation but not becoming dominant or appearing aggressive invaders

15

16


Figure 1.7 Control of Juncus acutus at Homebush Bay (a) infestation and (b) mounds left following removal.

until they suddenly underwent a population explosion and became recognised as serious threats. Unfortunately it has not been possible to predict which amongst the very large number of introduced species will become the major problems of the future. It is possible that amongst species already present in saltmarshes are some ‘sleepers’, which means that even if control programs are not practical at present we should nevertheless be monitoring changes in the composition and abundance of the weed flora. One species which may be showing signs of making the transition from being benign to being a problem is Aster subulatus, a North American species which has been present in NSW saltmarshes for many decades. Although almost ubiquitous it is rarely abundant, but Keith et al. (2007) recently published a photograph of a dense stand of A. subulatus on a saltmarsh on the south coast of NSW; perhaps a sign of what is to come.


We also need to reconsider the perception that introduced species, which, while numerically abundant contribute relatively little to stand biomass, are ecologically benign. The largest numbers of ‘minor’ introduced species are found in saltmarsh in the Mediterranean climate zone, and many of these species are also found as weeds in Californian saltmarshes. Recent investigation in California show that there may be adverse interactions between native species and exotics – for example the introduced annual grass Polypogon monspeliensis (also present in Australia) out-competes the native Salicornia virginica (correctly Sarcocornia virginica, Callaway and Zedler 1997). Control of P. monspeliensis invasions requires addressing the environmental changes which facilitate invasion, rather than only the symptom. Callaway and Zedler (1998) showed that alteration of hydrologic regimes, affecting both soil moisture status and salinity, have created conditions favourable for exotic plants, so that controlling existing invasive populations, and reducing the potential for future invasions, will require landscapescale restoration of hydrology. The practicality of this approach in the Australian context remains to be determined, but in order to justify the need, ecological studies to examine interactions between native and introduced species are required.

Discussion Australian saltmarshes resemble those elsewhere in the world in terms of general appearance and physiognomy of the dominant plants. There are fewer data on which to establish whether, faunistically, Australian saltmarshes fit a general global model. Nevertheless, there are some distinctive Australian elements in the flora – Tecticornia spp., Wilsonia spp. – and a number of wider Gondwana links which differentiate Australian saltmarshes from those in the northern hemisphere. Within Australia there is regional differentiation of saltmarsh flora and vegetation which is correlated with climate, and a particularly important feature is the extent of tropical saltmarshes. These have been little studied, but given that much of the tropical coast of Australia is still undeveloped we have unique opportunities and responsibilities to conserve large areas of tropical saltmarshes in association with mangroves and hypersaline flats. Globally saltmarshes are facing a range of threats (Adam 2002) and Australian saltmarshes are not immune from these threats, at both the local, site-specific level and more widely, changes in relative sea level will affect all saltmarshes over time. There is an active need for conservation management now, even if the impacts of some threats will not be apparent for decades hence. The listing of saltmarsh in NSW as an endangered ecological community provides an incentive, at least in that state, for the development of conservation plans. In other states, but most notably in Queensland through provisions of fisheries legislation, saltmarsh is also accorded high conservation status. Many of the saltmarshes in Australia are publicly owned, but despite this there is often no active recognition of this by a relevant management authority. Among the public lands, saltmarsh is frequently an orphan, so a major challenge is to develop a culture in which public agencies are more pro-active in saltmarsh management. While recognition of the importance of saltmarsh can be used reactively to modify or reject proposals which would destroy saltmarsh, our ability to develop proactive management is limited by lack of information. There is still much to be done to document the occurrence and composition of saltmarsh, and we have scarcely begun studies on ecological processes and functions in Australian saltmarshes. The detailed study of the Towra Point saltmarsh by Clarke and Hannon (1967, 1969, 1970, 1971) was, at its time, one of the most detailed in the world, but since then there has been little follow up in Australia. Many aspects of the biology of saltmarsh plants, such as population dynamics, pollination, regeneration biology and responses to

17

18


particular threats have scarcely been investigated. Without information, there will be a very uncertain foundation on which to plan for the future

References Adam P (1981a). Australian saltmarshes. Wetlands (Australia) 1, 8–10. Adam P (1981b). Saltmarsh plants of NSW. Wetlands (Australia) 1, 11–19. Adam P (1990). Saltmarsh Ecology. Cambridge University Press: Cambridge. Adam P (1994). Saltmarsh and mangrove. In Australian Vegetation, 2nd edn. (Ed. RH Groves) pp. 395–435. Cambridge University Press: Cambridge. Adam P (2000). Morecambe Bay saltmarshes: 25 years of change. In British Saltmarshes. (Eds BR Sherwood, BG Gardiner and T Harris) pp. 81–107. Linnean Society of London, Forrest Text: Cardigan, UK. Adam P (2002). Saltmarshes in a time of change. Environmental Conservation 29, 39–61. Adam P and King RJ (1990). Ecology of unconsolidated shores. In Biology of Marine Plants. (Eds MN Clayton and RJ King) pp. 296–309. Longman Cheshire: Melbourne. Adam P, Wilson NC and Huntley B (1988). The phytosociology of coastal saltmarsh vegetation in New South Wales. Wetlands (Australia) 7, 35–57. Ayres DR, Smith DL, Zaremba K, Klohr S and Strong DR (2004). Spread of exotic cordgrasses and hybrids (Spartina spp.) in the tidal marshes of San Francisco Bay, California, USA. Biological Invasions 6, 221–231. Baker JM, Adam P and Gilfi llan E (1994). Biological Impacts of Oil Pollution: Saltmarshes. International Petroleum Industry Environmental Conservation Association: London. Barbour MG (1970). Is any angiosperm an obligate halophyte? American Midland Naturalist 84, 105–120. Beeftink WG, Nieuwenhuize J, Stoeppler M and Mohl C (1982). Heavy-metal accumulation in salt marshes from the western and eastern Scheldt. Science of the Total Environment 25, 199–223. Boston KG (1981). The introduction of Spartina townsendii (s.l.) to Australia. Melbourne State College: Occasional Papers No. 6, 1–57. Brearley A (2005). Ernest Hodgkin’s Swanland: Estuaries and Coastal Lagoons of Southwestern Australia. University of Western Australia Press: Crawley. Bridgewater PB (1982). Phytosociology of coastal salt-marshes in the mediterranean climatic region of Australia. Phytocoenologia 10, 257–296. Bridgewater PB and Cresswell ID (1993). Phytosociology and phytogeography of Western Australian salt marshes. Fragmenta Floristica et Geobotanica Supplementum 2, 609–629. Bridgewater PB and Cresswell ID (2003). Identifying biogeographic patterns in Australian saltmarsh and mangal systems: a phytogeographic analysis. Phytocoenologia 33, 231–250. Bridgewater PB, Rosser C and de Corona A (1981). The Saltmarsh Plants of Southern Australia. Monash University Botany Department: Melbourne. Callaway JC and Zedler JB (1997). Interactions between a salt marsh native perennial (Salicornia virginica) and an exotic annual (Polypogon monspeliensis) under varied salinity and hydroperiod. Wetlands Ecology and Management 5, 179–194. Carter S (1994). Coastal Resource Atlas for Oil Spills from Barrenjoey Head to Bellambi Point. Environment Protection Authority of New South Wales: Chatswood. Clarke LD and Hannon NJ (1967). The mangrove swamp and salt marsh communities of the Sydney district: I. Vegetation, soils and climate. Journal of Ecology 55, 753–771. Clarke LD and Hannon NJ (1969). The mangrove swamp and salt marsh communities of the Sydney district: II. The holocoenotic complex with particular reference to physiography. Journal of Ecology 57, 213–234.


Clarke LD and Hannon NJ (1970). The mangrove swamp and salt marsh communities of the Sydney district: III. Plant growth in relation to salinity and waterlogging. Journal of Ecology 58, 351–369. Clarke LD and Hannon NJ (1971). The mangrove swamp and salt marsh communities of the Sydney district: IV. The significance of species interaction. Journal of Ecology 59, 535–553. Clarke PJ and Jacoby CA (1994). Biomass and above ground productivity of salt-marsh plants in south-eastern Australia. Australian Journal of Marine and Freshwater Research 45, 1521–1528. Congdon RA and McComb AJ (1980). Productivity and nutrient content of Juncus kraussii in an estuarine marsh in south-western Australia. Australian Journal of Ecology 5, 221–234. Craig GF (1983). Pilbara Coastal Flora. Western Australian Department of Agriculture: Perth. Cresswell ID and Bridgewater PB (1998). Major plant communities of coastal saltmarsh vegetation in Western Australia. In Wetlands for the Future. (Eds AJ McComb and JA Davis) pp. 297–326. Gleneagles Publishing: South Australia. Culbertson JB, Valiela I, Peacock EE, Reddy CM, Carter A and VanderKruik R (2007). Longterm biological effects of petroleum residues on fiddler crabs in salt marshes. Marine Pollution Bulletin 54, 955–962. Daehler CC and Strong DR (1997). Reduced herbivore resistance in introduced smooth cordgrass (Spartina alterniflora) after a century of herbivore-free growth. Oecologia 110, 99–108. Davy AJ, Costa CSB, Yallop AR, Proudfoot AM and Mohamed MF (2000). Biotic interactions in plant communities of saltmarshes. In British Saltmarshes. (Eds BR Sherwood, BG Gardiner and T Harris) pp. 109–127. Linnean Society of London, Forrest Text: Cardigan, UK. Drake BG, Leadley PW, Arp WJ, Nassiry D and Curtis PS (1989). An open top chamber for field studies of elevated atmospheric CO2 concentration on saltmarsh vegetation. Functional Ecology 3, 363–371. Fairweather PG (1990). Ecological changes due to our use of the coast: research needs versus effort. Proceedings of the Ecological Society of Australia 16, 71–77. Hamilton AA (1919) An ecological study of the salt marsh vegetation of the Port Jackson district. Proceedings of the Linnean Society of N.S.W 44, 463–513. Hubbard CE (1968). Grasses. Penguin: Harmondsworth. Johns L (2006). Field Guide to Common Saltmarsh Plants of Queensland. DPI&F Publication: Brisbane. Keith DA, Simpson C, Tozer MG and Rodoreda S (2007). Contemporary and historical descriptions of the vegetation of Brundee and Saltwater Swamps on the lower Shoalhaven River floodplain, southeastern Australia. Proceedings of the Linnean Society of N.S.W. 128, 123–154. Kelleway J (2005). Ecological impacts of recreational vehicle use on saltmarshes of the Georges River, Sydney. Wetlands (Australia) 22, 52–66. Kelleway J, Williams RJ and Allen CB (2007). An Assessment of the Saltmarsh of the Parramatta River and Sydney Harbour. NSW Department of Primary Industries: Cronulla. King RJ, Adam P and Kuo J (1990). Seagrasses, mangroves and saltmarsh plants. In Biology of Marine Plants. (Eds MN Clayton and RJ King) pp. 213–240. Longman Cheshire: Melbourne. Kirkpatrick JB and Glasby J (1981). Salt Marshes in Tasmania: Distribution, Community Composition and Conservation. Occasional Paper No. 8. Department of Geography, University of Tasmania: Hobart. Kloot PM (1984). The introduced elements of the flora of Southern Australia. Journal of Biogeography 11, 63–78. Marchant CJ (1963). Corrected chromosome numbers for Spartina × townsendii and its parent species. Nature 199, 929.

19

20


Marchant CJ (1968). Evolution in Spartina(Gramineae). II. Chromosomes, basic relationships and the problem of S. x townsendii agg. Journal of Linnean Society (Botany) 60, 381–409. Mason CF, Underwood GJC, Baker NR, Davey PA, Davidson I, Hanlon A, Long SP, Oxborough K, Paterson DM and Watson A (2003). The role of herbicides in the erosion of salt marshes in eastern England. Environmental Pollution 122, 41–49. Mooney HA and Hobbs RJ (2000). Global change and invasive species:Where do we go from here. In Invasive Species in a Changing World. (Eds HA Mooney and RJ Hobbs) pp. 425– 434. Island Press: Washington, DC. Mueller F (1868). Fragmenta Phytographiae Australiae 6. Auctoritate Guberni Coliniae Victoriae, ex officina Joannis Ferres: Melbourne. Paul S and Young R (2006). Experimental control of exotic spiny rush, Juncus acutus from Sydney Olympic Park: I. Juncus mortality and re-growth. Wetlands (Australia) 23, 1–13. Paul S, Young R and MacKay A (2007). Experimental control of exotic Spiny Rush, Juncus acutus from Sydney Olympic Park: II. Effects of treatments on other vegetation. Wetlands (Australia) 24, 1–13. Pidgeon I (1940). The ecology of the central coastal area of New South Wales. III. Types of primary succession. Proceedings of the Linnean Society of N.S.W 65, 221–249. Pringle AW (1993). Spartina anglica colonisation and physical effects in the Tamar estuary, Tasmania 1971–91. Papers and Proceedings of the Royal Society of Tasmania 127, 1–10. Ranwell DS (1967). World resources of Spartina townsendii (sensu lato) and economic use of Spartina marshland. Journal of Applied Ecology 4, 239–256. RGAG (2002). Strategy for the Management of Rice Grass (Spartina anglica) in Tasmania, Australia. Department of Primary Industries, Water and Environment: Tasmania. Ridd P, Sandstrom MW and Wolanski E (1988). Outwelling from tropical tidal salt flats. Estuarine Coastal and Shelf Science 26, 243–253. Saenger P, Specht MM, Specht RL and Chapman VJ (1977). Mangal and coastal saltmarsh communities in Australasia. In Ecosystems of the Worlds: Wet Coastal Ecosystems. (Ed. VJ Chapman) pp. 293–345. Elsevier: Amsterdam. Saltonstall K (2002). Cryptic invasion by a non-native genotype of the common reed. Phragmites australis in North America. Proceedings of the National Academy of Sciences 99, 2445–2449. Shepherd KA and Wilson PC (2007). Incorporation of the Australian genera Halosarcia, Pachycornia, Sclerostegia and Tegicornia into Tecticornia (Salicornioideae, Chenopodiaceae) Australian Systematic Botany 20, 319–331. Smith-White AR (1981). Physiological differentiation in a salt-marsh grass. Wetlands (Australia) 1, 20–21. Smith-White AR (1988). Sporobolus virginicus (L.) Kunth in coastal Australia: the reproductive behaviour and the distribution of morphological types and chromosome races. Australian Journal of Botany 36, 23–39. Snogerup S (1993). A revision of Juncus Subgen. Juncus (Juncaceae). Willdenowia 23, 23–73. Thannheiser D (2001). Studien zur Küstenvegetation Victorias (Südaustralien). Bamberger Geographische Schriften 20, 271–285. Valiela I (2006). Global Coastal Change. Blackwell Publishing: Carlton. Williams TP, Bubb JM and Lester JN (1994). Metal accumulation within salt marsh environments: A review. Marine Pollution Bulletin 28, 277–290. Woods JLD, Brown TH, Gangaiya P and Morrison RJ (2007). Water quality in Tom Thumb Lagoon, a highly disturbed urban estuary in Port Kembla, New South Wales, Australia. Wetlands (Australia) 24, 44–66.


Wu M-Y, Hacker S, Ayres D and Strong DR (1999). Potential of Prokelisia spp. as biological control agents of English Cordgrass, Spartina anglica. Biological Control 16, 267–273. Zedler JB, Nelson P and Adam P (1995). Plant community organization in New South Wales saltmarshes: Species mosaics and potential causes. Wetlands (Australia) 14, 1–18. Zedler JB, Paling E and McComb A (1990). Differential responses to salinity help explain the replacement of native Juncus kraussii by Typha orientalis in Western Australian salt marshes. Australian Journal of Ecology 15, 57–72.

21

CHAPTER 2

Distribution of Australian saltmarsh plants Neil Saintilan

Characteristics of saltmarshes and the saltmarsh environment Coastal (intertidal) saltmarsh has been defined by Adam (1996) as an intertidal plant community dominated by herbs and low shrubs. The plants that comprise coastal saltmarsh are not exclusively intertidal, a characteristic that sets saltmarsh plants apart from mangrove. Most saltmarsh species take advantage of inland saline environments, both aquatic and arid, and their intertidal occurrence is sometimes best understood as a continuation of a network of saline aquatic environments. This is particularly true of the South Australian saltmarsh flora. The characteristics which suit plants to this harsh environment are a tolerance of extreme ranges of salinity and soil water content. The periodic inundation of the intertidal environment by seawater salinises the soil, and salts will subsequently concentrate by processes of evaporation and transpiration. While this phenomenon occurs the world over, in Australia the range of salinity encountered is enhanced by the extremes of climatic variability and, in the tropics, seasonality (Adam 1996). Plants occupying the saltmarsh must be able to withstand periodic soil salinity and inundation. There is a range of strategies amongst the 100 or more species found in Australian saltmarshes. The adaptations to saline conditions are often at the expense of growth rate, and it is this that explains the narrow penetration of saltmarsh plants into upslope freshwater terrestrial environments (Adam 1990). Two-thirds of the 103 Australian saltmarsh plant species listed in Appendix 2.1 belong to five families; Chenopodiaceae; Poaceae; Cyperaceae, Aizoaceae and Asteraceae. While there is a high degree of endemism at a species level amongst Australian saltmarshes, at a generic and family level there are numerous commonalities with the saltmarshes of other continents (Adam 1990; Chapter 1, this volume). In this sense the saltmarshes of Australia are not as distinctly Australian as the terrestrial vegetation communities, though there are unique features of the Australian saltmarsh ecologically.

Description of common saltmarsh plants For those beginning their interest in coastal saltmarsh, there are some excellent keys and field guides available, notably: ●

Field Guide to Common Saltmarsh Plants of Queensland (2006) by Louise Johns, and available through the Queensland Department of Primary Industries. 23

24


●

●

Saltmarsh Plants of New South Wales, by Paul Adam and illustrated by John Barclay. Wetlands (Australia) 1, 11–19. The Saltmarsh Plants of Southern Australia (1981) by P. Bridgewater, C. Rosser and A. de Corona, Monash University, Melbourne.

While numerous plant species can be found in the Australian saltmarsh, particularly at the landward fringe, only a few species dominate. These species are likely to be the most significant ecologically, and their characteristics are described briefly in the following section. Plate 2.1 on pages 45–46 displays colour photographs of these species. Sporobolus virginicus The salt couch Sporobolus virginicus is the most widely distributed saltmarsh plant in Australia, occurring in 33 of the 36 coastal bioregions. Its seeds are primarily airborne, though can be dispersed by water (Naidoo and Naidoo 1992). On the east Australian coast, the species increases in prevalence northward, and is the dominant saltmarsh plant in south-east Queensland. It is known to be tolerant of waterlogged acidic soils (Naidoo and Naidoo 1992) and grows particularly well in sandy locations (Johns 2006). Sarcocornia quinqueflora The family Chenopodiaceae contributes more species to the Australian saltmarsh than any other, and the ‘samphire’ Sarcocornia quinqueflora (beaded glasswort) is the most widely distributed member of the Chenopodiaceae. The plant forms a creeping mat, and spreads primarily through vegetative propagation, though the seeds can be tidally dispersed. The colour of the plant ranges from green to red and purple and may change with environmental conditions. This low-growing plant is found in the wetter parts of the saltmarsh zone, often referred to as representative of the ‘wet’ or lower saltmarsh zone. The species is the dominant saltmarsh plant in southern and central New South Wales, and is found throughout the Australian coastline with the exception of the Northern Territory and the northern half of Western Australia. Juncus kraussii The rush Juncus kraussii grows in fresher conditions than Sporobolus virginicus and Sarcocornia quinqueflora, and may form a landward fringe to the saltmarsh, or a dominant species on upstream saltmarshes within estuaries. The plant forms thick stands generally less than a metre high, though taller stands of up to 2 m have been observed (e.g. Belongil Creek, NSW, Nick Wilson pers. comm.). The species can withstand several months continuous inundation around the margins of lagoons (Adam 1981). It is a plant of the southern Australian saltmarshes and has not been found growing north of 20°S latitude. Samolus repens The creeping brookweed is a low-growing herb that rarely forms a dominant stand, though is commonly found throughout its geographic range, which extends through the southern half of the continent from 23°S latitude. S. repens produces small attractive flowers between September and March which may be white or pink. A more upright form of the genus, Samolus junceus, is found in Western Australia. Suaeda australis Another chenopod, Suaeda australis is a small, woody upright perennial herb, a taller plant than Sarcocornia, with succulent leaves approximately 5 cm long. Commonly known as seablite, S. australis is common on the Australian east coast extending from Cape York Peninsula south

Distribution of Australian saltmarsh plants

to Tasmania, throughout coastal South Australia and the south-western corner of Western Australia. Though some records of the species have been reported from the Northern Territory, it is not common there. S. australis prefers somewhat drier, better-drained conditions than S. quinqueflora, though it relies on water for seed dispersal (Clarke and Hannon 1970). Tecticornia pergranulata Following a taxonomic review by Shepherd and Wilson (2007) the genera Halosarcia and Sclerostegia were incorporated into the genus Tecticornia. The Chenopod genus Tecticornia contributes over a dozen species to the Australian saltmarsh. The succulent stems are similar in appearance to S. quinqueflora, though Tecticornia grows predominantly as a shrub, up to a metre high. T. pergranulata is as common in terrestrial saline environments as on the coast. The species is found in all Australian mainland jurisdictions, though is rare in NSW with a single population in Homebush Bay the only recorded occurrence for the state. Triglochin striata Triglochin striata, the three-ribbed or streaked arrowgrass, is common in less well drained depressions on the saltmarsh plain. The leaves are erect, often in groups of three, and may be up to 30 cm long, though more commonly less than 10 cm. The plant is widely distributed, both in Australia and in other southern continents. Gahnia filum Gahnia filum grows as a clumped sedge, slightly taller than Juncus krausii (approximately 1.5 m) and with a long trailing edge to their slender shoots. Gahnia filum is found almost exclusively in the south-east corner of the continent, including South Australia, Victoria and Tasmania, with some occurrences on the NSW south coast north to the Georges River, where it appears to be spreading (Adam pers. comm.). In South Australia, the plant is the primary habitat of the Yellowish Sedge-skipper Butterfly (Hesperilla flavescens flavia) (Coleman and Coleman 2000).

Saltmarsh structural forms and zonation In their detailed consideration of the distribution and ecology of Tasmanian saltmarshes, Kirkpatrick and Glasby (1981) define a series of saltmarsh structural forms which could validly be applied more widely: 1. Communities dominated by succulent shrubs (e.g. the genera Tecticornia). 2. Communities dominated by grasses (e.g. Sporobolus virginicus, Stipa stipoides, Zoysia macrantha). 3. Communities dominated by sedges and grasses (e.g. Juncus krausii, Gahnia filum). 4. Communities dominated by herbs (low-growing creeping plants such as Wilsonia backhousei, Samolus repens, Schoenus nitens). The distribution of these forms varies across the intertidal zone. Within New South Wales, the lower intertidal is dominated by herbs and grasses which gives way to sedges and rushes in the landward sections of the intertidal zone. Within Victorian saltmarshes, the lower saltmarsh zone is dominated by succulent shrubs of the genera Tecticornia and Sarcocornia. The herbs and grasses are more commonly found in a landward, upper-intertidal zones which are also the most species diverse (see, for example, Schindl 2002). Coleman (2005) described plants characteristic of four elevation zones within the saltmarshes of South Australia. A low marsh community is characterised by Suaeda australis and Sarcocornia quinqueflora. Species characteristic of the mid-marsh are Frankenia pauciflora and species of the

25

26


genus Tecticornia. The high marsh is characterised by a diverse array of species including Mimulus repens (in brackish areas), Puccinellia stricta, Wilsonia humilis, Apium annuum, Samolus repens, Disphyma crassifolium, Spergularia spp., Atriplex semibaccata and Trigoichin striata. A landward community of saltmarsh plants, above the level of normal spring high tide inundation, includes Nitraria billardierei, Distichlis distichophylla and Dianella brevicaulis. Most studies have indicated that a combination of moisture content and salinity explain the distribution of vegetation communities within the saltmarsh. Soil moisture content decreases between the mangrove and terrestrial environments. Soil chlorinity varies less predictably, and will respond to micro-scale hydraulic controls (such as evaporative depressions), as well as plant activity (accumulating salts within the root zone). Temporal variability in salinity may also be high, and related to rainfall, groundwater discharge, and the periodicity of the tides. On the New South Wales coast, for example, spring tides reach their maximum inundation in summer (daylight hours) and winter (night), which are also the times of highest variability in tide height. The periods which inundate the upper-intertidal are also those which least frequently inundate the lower saltmarsh. In summer, the additional evaporative losses resulting from higher temperatures make this the period of maximum soil salinity in the saltmarsh (Clarke and Hannon 1969).

Continental distribution of saltmarsh plants Coastal specialists versus generalists The majority (over 90%) of saltmarsh species are generalists, distributed across a range of saline aquatic habitats both coastal and inland. On the basis of records held within the Australian Virtual Herbarium, there are a number of species which are coastal specialists, though no distinction is made between coastal saltmarsh and coastal dune and headland environments. These species are: Batis argillicola; Baumea teretifolia; Carpobrotus glaucescens; Limonium solanderi; Limonium australe; Suaeda arbusculoides; Sesuvium portulacastrum and Austratipa stipoides (selected examples in Figure 2.1). Several species (over 20) are found predominantly, though not exclusively, in coastal environments. Common species in this category are: Sarcocornia quinqueflora; Sporobolus virginicus; Zoysia macrantha; Triglochin striata; Suaeda australis; Juncus kraussi;, Isolepis nodosa; Selliera radicans; Gahnia filum and Fimbristylis ferruginea. A third category of saltmarsh plants are those where there is no obvious coastal preference to their distribution. Plants loosely described as shrubs appear to fall into this category, including all of the species of Tecticornia and Maireana found in the saltmarsh. Latitudinal patterns of diversity The inverse relationship between saltmarsh species diversity and latitude, mentioned by several authors (Saenger et al. 1977; Specht 1981; Adam et al. 1988) is confirmed in an analysis of the data of Appendix 2.1 (Figure 2.4). The relationship is particularly strong (r2 = 0.64), and contrasts to the trend for mangroves of decreasing diversity with increasing latitude. Two outliers in Figure 2.4 exhibit low diversity in spite of high latitude, and in both cases this is due to rockdominated coastlines (the Gawler and Tasmanian West bioregions). There is little sediment yield from the few rivers that reach the coast in these bioregions. The bioregion is a useful scale of analysis for conservation planning, and Table 2.1 represents the proportion of the total saltmarsh flora present in each of the 36 coastal bioregions. Centres of biodiversity emerge, with the South Australian bioregions particularly diverse. Three-quarters of the 103 listed saltmarsh species can be found within 200 km of Adelaide.


(a)

(b)

(c)

(d)

Figure 2.1 Distribution of confirmed records of two coastal specialists (Limonium solanderi (a) and Stipa stipoides (b)) and two generalists (Dissocarpus biflorus (c) and Atriplex semibaccata (d)). In the latter cases the disjunct coastal extent is explained by connection through interior saline lakes.

Fischer (1960) described the correlation between floral and faunal diversity and decreasing latitude as one of the most imposing biogeographic features on the planet. This is a trend to which Australian mangroves conform (Saenger et al. 1977; Wells 1983) but clearly not saltmarsh. Variations in intertidal extent between bioregions provide no explanation; indeed, there is an inverse relationship between intertidal extent and saltmarsh diversity in Australia (Figure 2.5). Northern Australia supports a low diversity of saltmarsh in spite of the large intertidal area available to saltmarsh colonisation (Table 2.2 below). To some degree this may be explained by climatic constrains, with hypersaline conditions developing in upper-intertidal elevations during the dry season in many parts of northern Australia. Wide intertidal areas will be devoid of saltmarsh in these situations. Notwithstanding, the four southern States of Tasmania, Victoria, South Australia and New South Wales contain less than 2.5% of the total saltmarsh/saltpan area yet together support 90% of the saltmarsh flora of Australia. The inability of many saltmarsh species to colonise the intertidal flats of tropical Australia is most probably related to an intolerance of higher temperatures, or a combination of higher temperatures and seasonally higher salinities, which appears to inhibit the germination of

27


Box 2.1

Sources of data and methods of analysis

All confirmed records of species occurrences in Australia are now represented in the Australian Virtual Herbarium, an interactive internet-based tool which can be accessed through the websites of participating herbaria. Bridgewater and Cresswell (2003) used a prototype version of this tool, along with published records and personal observations, to define the distribution of saltmarsh species groups throughout the continent. The Interim Bioregionalisation of Australia (IBRA) was used to provide the fundamental spatial unit for this analysis. Their work presented the first systematic presentation of saltmarsh species assemblage distribution in Australia, and is described later in this chapter. Appendix 2.1 of this chapter is a compilation of the known distribution of Australian saltmarsh plants, utilising the completed records for 103 saltmarsh species now found within the Australian Virtual Herbarium. This list was compiled primarily on the basis of species lists in Adam (1981), Adam et al. (1988), Kirkpatrick and Glasby (1981), Johns (2006), Bridgewater and Cresswell (2003). Some adjustments were made where published accounts expanded the range of some species including, for example, the detailed biogeographic work of Kirkpatrick and Glasby (1981) for Tasmanian saltmarshes. Multivariate statistical analyses, such as cluster analysis, can be used to provide an a priori grouping of bioregions in terms of saltmarsh species occurrence (Saintilan 2009). Such an analysis is presented in Figure 2.2. This cluster dendogram represents the similarity or dissimilarity of bioregions in presence and absence of saltmarsh species (data from Appendix 2.1). The Bray-Curtis 20

40

60

80

100

3 4 5 1 2 35 34 32 33 31 36 28 29 30 14 6 7 8 9 17 21 18 19 10 11 12 13 15 16 25 27 24 26 20 22 23

Bray-Curtis Similarity index

28

Northern Humid East

North

Southern Arid West

Humid East

South

Arid West

Figure 2.2 Clustering of IBRA bioregions on the basis of saltmarsh floristic composition. At the highest level of dissimilarity, the continent can be divided north–south along 23° latitude. These groups may be further divided by coastal orientation.


similarity index ranges from zero (no species in common) to 100 (all species in common). The broader divisions on the tree are therefore between groups of sites with a lower proportion of species in common (for example 25% is the first major division). Higher-order branchlets represent similar bioregions in the overall ‘assemblage’ of saltmarsh plants present (up to 90% of species in common in some cases). The abundance of saltmarsh plants (species dominance) in each bioregion was not considered in this analysis. When applied to the Virtual Herbarium data, this divides the continent into a northern and southern region, with a secondary division along coastal orientation (Figure 2.3). Bridgewater and Cresswell (2003) present an alternative analysis, discussed later in this chapter. Coastal Bioregions

34

35

33 31

NW

2

N

32

36

1

3

30

NE 4

29

5

28

6

27 26

23

SW 25

24

20

22

19

S 0

250

500

1000

7 8

21

SE

18 17

9 16

10 15

11

Kilometers

14

12 13

1. Gulf Plains 2. Cape York Peninsula 3. Wet Tropics 4. Brigalow Belt North 5. Central Mackay Coast 6. South East Queensland 7. NSW North Coast 8. Sydney Basin 9. South East Corner 10. South East Coastal Plain 11. Flinders 12. Tasmanian South East 13. Tasmanian Southern Ranges 14. Tasmanian West 15. Tasmanian Northern Slopes 16. King 17. Naracoorte Coastal Plain 18. Murray Darling Depression 19. Kanmantoo 20. Gawler 21. Eyre Yorke Block 22. Nullabor 23. Hampton 24. Esperence 25. Warren 26. Swan Coastal Plain 27. Geraldton Sandplains 28. Carnarvon 29. Pilbara 30. Damperland 31. Northern Kimberley 32. Victoria Bonaparte 33. Darwin Coastal 34. Tiwi Cobourg 35. Arnhem Coast 36. Gulf Coastal

Figure 2.3 Coastal bioregions included in the analysis, with the saltmarsh biogeographic provinces identified by cluster analysis.

some species (Greenwood and MacFarlane 2006). One potential impact of global warming may therefore be a decline in diversity of Australian saltmarsh flora within many bioregions of southern Australia. Clustering of bioregions The northern and southern divisions The primary division is between a northern and southern saltmarsh flora, separated at 23°S, defining a northern division extending from the Central Mackay Coast to the Carnarvon bioregion on the West Coast (see Figure 2.3). The two halves of the content have less than 25% of species in common. Species occurring primarily in the northern bioregions include: Tecticornia australasica; Fimbristylis polytrichoides; Portulaca bicolor; Fimbristylis ferruginea; Batis agrillicola; Xerochloa

29


90 80

Saltmarsh diversity

30

70 60 50 40 30 20 10 0 0

5

10

15

20

25

30

35

40

45

50

Latitude Figure 2.4 Relationship between saltmarsh diversity and latitude for the 36 Australian coastal bioregions. (Saltmarsh plants listed in Appendix 2.1) (r2 = 0.64, P

Australian Saltmarsh Ecology

Australian Palms: Biogeography, Ecology and Systematics

The Saltmarsh Murders

Essential Australian Law (Australian Essentials)

The Australian

Australian Storys

Australian Bustard

Australian Storys

Australian Storys

Australian Storys

Australian Essential Dispute Resolution (Australian Essentials)

Australian Education Union (Australian Education Review)

The Australian

Essential Australian Management Law (Australian Essentials Series)

Ecology in Agriculture (Physiological Ecology)

Tropical Stream Ecology (Aquatic Ecology)

Keywords in Australian Politics

Principles of Australian Evidence

Making Australian Foreign Policy

Cooking the Australian Way

One Hot Australian Night

Australian Intellectual Property Law

Australian Kin Classification

Australian Soil Classification

Legends of Australian Fantasy

Wealthy Australian, Secret Son

Australian Cinema After Mabo

Haematology of Australian Mammals

Beyond Nostalgia: Reshaping Australian Education (Australian Education Review No. 44)

The Australian Crisis

Essential Australian Company Law

Australian Saltmarsh Ecology

Australian Palms: Biogeography, Ecology and Systematics

The Saltmarsh Murders

Essential Australian Law (Australian Essentials)

The Australian

Australian Storys

Australian Bustard

Australian Storys

Australian Storys

Australian Storys

Australian Essential Dispute Resolution (Australian Essentials)

Australian Education Union (Australian Education Review)

The Australian

Essential Australian Management Law (Australian Essentials Series)

Ecology in Agriculture (Physiological Ecology)

Tropical Stream Ecology (Aquatic Ecology)

Keywords in Australian Politics

Principles of Australian Evidence

Making Australian Foreign Policy

Cooking the Australian Way

One Hot Australian Night

Australian Intellectual Property Law

Australian Kin Classification

Australian Soil Classification

Legends of Australian Fantasy

Wealthy Australian, Secret Son

Australian Cinema After Mabo

Haematology of Australian Mammals

Beyond Nostalgia: Reshaping Australian Education (Australian Education Review No. 44)

The Australian Crisis

Essential Australian Company Law

Recommend Documents