1Centre for Marine Studies, University of Queensland, St Lucia, QLD, 4072, Australia.
2School of Biological Science, University of Queensland, St Lucia, QLD, 4072, Australia
3Rivers and Wetlands Unit, Department of Environment, Climate Change and Water, NSW, Australia
Lovelock C.E., Skilleter, G. and Saintilan, N. (2009) Tidal Wetlands and Climate Change. In A Marine Climate Change Impacts and Adaptation Report Card for Australia 2009 (Eds. E.S. Poloczanska, A.J. Hobday and A.J. Richardson), NCCARF Publication 05/09, ISBN 978-1-921609-03-9.
Expansion of mangroves into salt marsh habitat in south-east Australia and into freshwater wetlands in northern Australia driven by sea-level rise and soil subsidence associated with reduced rainfall (MEDIUM confidence)
Mangrove areas are likely to expand further landward, driven by sea-level rise and soil subsidence due to reduced rainfall (MEDIUM confidence)
Assess the state of tidal wetlands in poorly sampled areas of Australia (e.g. Kimberly), measure wetland accretion and subsidence rates in tropical Australia and improve research into how tidal wetland fauna will respond to climate change
Remove artificial barriers such as seawalls, ditches and buildings to allow tidal wetlands to adapt naturally and allow landward migration of these habitats
Cath Lovelock is an Associate Professor at the Centre for Marine Studies at the University of Queensland. Her research is focused on the ecology and ecophysiology of coastal plant communities, and how they respond to environmental influences, including climate change in determining plant community productivity and diversity. Current experimental work covers a wide range of coastal plant communities including macroalgae, mangroves and cyanobacterial mat communities, with particular interest in assessment of how nutrient enrichment influences mangrove ecosystems, how mangroves mediate exchanges between the land and sea and how fauna influence the productivity of macroalgae.
Greg Skilleter is an Associate Professor with School of Biological Science, UQ. He also holds the 2008 Australian Research Council (ARC) - OzReader. His research interests are diverse and focus on the effects of physical disturbance and climate change on coastal and estuarine systems, including coral reefs, seagrass, mangroves and rocky intertidal communities. Current projects include Management Strategy Evaluation of Ningaloo, assessment of the implications for biodiversity and the sustainability of estuarine fisheries and Marine Representative Areas Programmes, specifically rocky shores.
Dr Neil Saintilan is Principal Research Scientist and head of the Rivers and Wetlands Unit in the NSW Department of Environment and Climate Change. His research interests focus on the ecology of freshwater and saline wetlands, and their response to climate change.
Climate change is likely to have a strong impact on mangroves, salt marshes and other tidal wetlands. Their position in the intertidal exposes them to a multitude of ocean and atmospheric climate change drivers which leads to high vulnerability to climate change. Tidal wetlands are extremely sensitive to sea level rise. For example, too much flooding and mangroves will “drown”, too little and their productivity will be reduced and they may be replaced with salt marsh or cyanobacterial communities. The strong regulation of productivity and species composition by soil salinity and humidity (influenced by rainfall, river flows and groundwater) in tidal wetlands also makes these ecosystems highly sensitive to changes in rainfall. At the southern edge of their distribution mangroves are limited by low temperature and thus rises in air and sea temperatures is likely to allow their movement even further south where they will enter salt marsh habitats.
Despite their vulnerability to climate change, the adaptive capacity of tidal wetlands to climate change is also high. For example, mangroves have persisted on the coast of Australia through past variations in past sea level. Where there are no landward barriers, mangrove forests and other tidal wetlands can move inland as sea level rises. We also know that mangrove forests can grow rapidly on newly deposited sediments and that recovery from storms and other disturbances can be rapid.
Tidal wetlands in Australia occur on our sheltered coasts and are comprised of mangroves, salt marshes, cyanobacterial mats and salt flats. These different tidal wetland types have strong climatic affinities. Diversity of the plants and animals within these differing wetland types are also strongly influenced by climatic factors.
Mangroves forests dominate the tropical and sub-tropical coasts. The southern limit of mangroves is set by low temperatures (Stuart et al. 2006), but they extend southward where sufficiently warm ocean currents permit and frost damage is reduced (Saintilan et al. 2009). Mangrove forests have highest diversity in the wet tropics with diversity decreasing on southern coasts and in arid regions (Duke et al. 1998). On mangrove dominated coasts salt marshes and cyanobacterial mats tolerant to desiccation occur landward of mangroves, with their species composition and structure dependent on the level of tidal inundation and freshwater inputs from the terrestrial environment. In arid regions and those with strongly seasonal rainfall succulent salt marsh species and cyanobaterial mats dominate the high intertidal zone, while in high rainfall areas salt marsh species that are less tolerant of high salinity (e.g. Juncus and Phragmites) can be found, often transitioning to freshwater wetlands. In southern latitudes, salt marshes are the dominant habitat of the intertidal zone (Adam 2002). These marshes are dominated by the succulent species Sarcocornia quinqueflora at low tidal elevations and by grasses, rushes and sedges (e.g. Juncus kraussii) at higher elevations. Figure 1.Above: Forest of Ceriops tagal, Hinchinbrook Channel, Queensland. Below: Low Isles, Queensland, mangroves on the reef flat.
Mangroves, salt marshes and other tidal wetlands contribute a wide range of ecosystem services. Although they are ecosystems dominated by low diversity plant communities they are highly productive and provide habitat for a high diversity of animals, algae and microbes. Many animals are residents, spending most, or their entire lifecycle within the wetlands while others access tidal wetland habitats opportunistically, when tides allow, to forage and escape predation. Commercially important fish and crustacean species are strongly linked to the area of mangroves and salt marsh in Australian estuaries. Tidal wetland ecosystems are at the boundary of terrestrial and marine environments and thus they are regions of high biogeochemical activity. Materials from both the marine environment and terrestrial environment are deposited and processed. Some materials from mangroves are contributed to marine waters thereby supporting ocean productivity. Mangroves and salt marshes provide protection for our coastal zone against storms and waves, reducing erosion. Observed Impacts:
There are no published accounts of how increasing global temperature has affected the organisms within mangrove forests and tidal wetlands. But we know that temperature affects fundamental physiological processes and that low temperatures limit mangrove reproduction and distributions (Duke 1990, Duke et al. 1998).
Two key processes that determine plant productivity; photosynthetic carbon gain and respiration, are highly sensitive to temperature. Photosynthesis in mangroves over much of the tropics is limited by high midday leaf temperatures which result in high vapour pressure deficits between leaves and air, resulting in stomatal closure (Clough and Sim 1989, Cheeseman 1994, Cheeseman et al. 1997). In contrast, photosynthesis is limited by low temperature at southern latitudes (Steinke and Naidoo 1991). Low temperatures also strongly limit reproductive success in mangroves (Duke 1990). Respiration (CO2 efflux) from plants and microbial communities in soils approximately double with every 10ºC increase in temperature (Davidson and Janssens 2006, Lovelock 2008) we therefore expect the balance between carbon gain and respiratory losses to be altered with increasing air temperature.
Mangroves decrease in diversity with increasing latitude, and population dynamics of individual species at their southern limit may be influenced by temperature and, for the southernmost species, the occurrence of frost. A recent survey of age-structure of Rhizophora stylosa at the southern extent of its range in northern NSW demonstrated strong recruitment and growth of juveniles over the past few decades, which may have been related to higher temperatures over the period (Wilson 2009)
Saltmarsh floristic diversity increases with increasing latitude in Australia. Mean minimum temperature explains nearly 80% of variability in species diversity between coastal bioregions in Australia, with diversity inversely related to temperature (Saintilan 2009). Saltmarsh physiological responses to temperature are poorly understood, though one study has indicated that germination of some species may be inhibited by higher temperatures (Greenwood and MacFarlane 2006).
CO2 concentrations in the atmosphere have already increased from 350 ppm to 370 ppm in the last 20 years and are predicted to approximately double by 2080, with potentially profound effects on physiological and ecological processes in plant communities and their consumers. CO2 is the substrate for photosynthesis and influences respiration. Because of the sensitivity of these key physiological processes to elevated CO2, primary production and foodwebs in mangrove forests and salt marshes are likely to be sensitive to atmospheric CO2 concentrations.
There are few direct studies of the effects of elevated CO2 on mangroves and salt marshes and only one in Australia (Ball et al. 1997). In many plant species photosynthesis and growth is often enhanced at doubled atmospheric CO2 concentrations; however the level of enhancement is dependent on other interacting environmental factors (Drake et al. 1997, Poorter and Perez-Soba 2001). Growth enhancements are also attributed to declines in respiration under enhanced CO2 concentrations that are in the order of approximately 20% (Drake et al. 1999). In mangroves, elevated CO2 conditions (twice ambient) had little effect on growth rates when growth was limited by salinity, but increased growth by up to 40% when growth was limited by humidity (Ball et al. 1997). Faster growing, less salt tolerant species are more responsive to elevated CO2 conditions, having enhanced growth rates compared to slower growing more salt tolerant species. This may suggest that productivity and expansion of mangroves into fresh and brackish wetlands could occur at an accelerating pace (see sea level section below).
Elevated CO2 also decreases the temperature at which plants freeze making them more susceptible to frost, which could counteract increases in productivity due to enhanced temperatures (Woldendorp et al. 2008). In salt marshes in the USA, elevated CO2 resulted in increased salt marsh surface elevation through positive effects on root growth which was proposed to counterbalance the negative effects of sea level rise on these communities (Cherry et al. 2009, Langley et al. 2009)
Another common plant adaptation to elevated CO2 concentrations is decreased nitrogen invested in leaves and a concomitant increase in the carbon:nitrogen ratio of plant tissues. Changes in the ratio of carbon and nutrients in plant tissue has been observed to have flow on effects on consumers (Stiling et al. 1999) and on decomposition processes (Bosire et al. 2005). Elevated CO2 concentrations are therefore likely to be affecting food webs, carbon and nutrient cycling and the quality of material exported from mangroves to nearshore waters.
Mangroves, salt marsh and salt flats are within the intertidal zone of low energy coasts and are thus highly sensitive to rising sea level. During the past 10,000 years mangroves in some situations were unable to withstand rates of sea level rise that exceeded 1.4 mm yr-1 (Bermuda: Ellison 1993), but the sea level rise threshold for mangrove loss and for changes to intertidal wetland communities will vary depending on a range of interacting factors, including geomorphological setting, tidal range, accretion (e.g. from sediment inputs), subsidence (Ellison 1993, Cahoon et al. 1999), tree growth rates and species composition (Cahoon et al. 1999, Cahoon et al. 2003, Krauss et al. 2003). Current rates of sea level rise in Australia range from 1.3 to 8.0 mm y-1 but there is no evidence to suggest that seaward fringing mangroves are declining. However, increases in mangrove cover through recruitment into landward salt marshes have been documented in southern Australia (Saintilan and Williams 1999, Rogers et al. 2005b) and encroachment of mangroves into fresh water wetlands has been observed in northern Australia (Applegate 1999, Mulrennan and Woodroffe 1998, Winn et al. 2006).
Mangroves have an important role in protecting coasts from storm and tsunami damage (Smith et al. 1994, Massell et al. 1999, Mazda et al. 2002, Dahdouh-Guebas et al. 2005, Danielsen et al. 2005). Storms can have a large impact on mangroves, with catastrophic destruction being observed in the Caribbean and Bangladesh (Smith et al. 1994, Mastaller 1996, Cahoon et al. 2003), often with very slow recovery (Sherman et al. 2001, Piou et al. 2006), or none at all (Cahoon et al. 2003). Intense storms can strongly influence surface elevation of wetlands through erosion, deposition and subsurface processes which can subsequently influence rates of recovery (Cahoon 2006). Quantitative data from Australia on impacts of cyclones on mangroves and their recovery are rare (Bardsley 1985, Woodroffe and Grime 1999).
Data from the Caribbean indicate that mangroves can recover from severe storm damage providing patches of reproductive trees remain and the hydrology and sediments are not altered to an extent where re-establishment is prevented (Smith et al. 1994, Ellison 1998, Sherman et al. 2001). Tree species differ in their responses to cyclones, with species from the Rhizophoraceae being particularly vulnerable as they are unable to resprout (Baldwin et al. 2001). The effects of cyclones on fauna associated with mangroves in Australia are not known, but loss of mangroves from human disturbances in Kenya and Malaysia resulted in declines in diversity and abundance of fauna (reviewed in Manson et al. 2005).
Changes in rainfall will have major effects on tidal wetlands. The predicted changes in rainfall with climate change are complex, with increases in rainfall predicted in some regions and decreases in others. There is also predicted to be increases in the intensity of rainfall events which are likely to influence erosion and other processes in catchments which will have flow on effects on tidal wetlands due to altered delivery of sediments to estuaries.
Rainfall influences species composition, diversity and productivity of intertidal wetlands. Freshwater inputs to intertidal wetlands reduces salinity, increases the water content of soils and delivers sediments and nutrients creating conditions that are favourable for plant physiological function (Smith and Duke 1987, Ball 1998). Rainfall also influences groundwater inputs, which can lead to the maintenance of soil surface elevation through subsurface swelling of soils (Whelan et al. 2005, Rogers et al. 2005). Connectivity of ecosystems through fresh flows, e.g. with flushing accumulated material from salt flats to mangroves and near shore waters, is also strongly influenced by rainfall (Ridd et al. 1988).
In tropical and subtropical environments in Australia, the proportion of the intertidal zone colonised by mangrove is correlated with rainfall (Bucher and Saenger 1991). Rainfall will modulate the size of the seasonally hypersaline zone from which mangroves are excluded and thus alter the abundance of habitat types.
Sediment delivery to the estuary co-varies with rainfall (Davies and Eyre 2005) and increases with human development of the catchment (Furnas 2003). Sedimentation increases surface elevation of wetland soils relative to sea level as well as increasing habitat for mangrove colonization (e.g. Trinity Inlet, Cairns; Duke 1997, Duke and Wolanski 2001). In addition to increasing soil surface elevation, delivery of sediments has a direct positive effect on wetland plant growth (Pezeshki et al. 1992, Hemminga et al. 1998, Ellis et al. 2004, Lovelock et al. 2007), although it can lead to reduced diversity of fauna (Ellis et al. 2004) and tree mortality if sedimentation is excessive (Ellison 1998). Increases in the frequency of intense rainfall events combined with land use change in catchments will increase sedimentation which will increase the availability of suitable mangrove habitat and enhance mangrove growth (Lovelock et al. 2007), however excessive sedimentation events could result in forest losses (Ellison 1998).
Potential Impacts by the 2030s and 2100s:
Plant, soil and fauna will be affected by increases in water and air temperatures. The predicted 2ºC increase in temperature may increase growth of mangroves at their southern limits through enhanced photosynthesis over longer growing seasons and because of increased reproductive potentials. This may have large effects on southern coastal ecosystems as mangroves have different faunal associations to salt marshes. Saltmarsh diversity may also decrease, both in competition with mangroves but also due to the potentially negative effects of higher temperatures on germination.
Increasing temperature may increase plant and soil respiration by approximately 20%, resulting in reduced net carbon gain, increased methane emissions and decreases in soil carbon storage (Davidson and Janssens 2006). As mangroves and salt marshes have large carbon and nutrient stores in soils and plant biomass (Robertson et al. 1992, Twilley et al. 1992, Chmura et al. 2003) increases in temperatures and associated increases in respiration may have negative effects on carbon balance that may not be matched by increases in production, which in some cases, particularly in northern regions may be reduced (e.g. see Clark 2004 for terrestrial forest ecosystems).
The available data suggests that under future elevated CO2 primary production is likely to be enhanced, although not uniformly over the range of tidal wetlands. Increases in CO2 concentrations may partially reduce the negative effects of reduced humidity and rainfall expected where temperatures increase in northern regions. Increasing levels of CO2 may also change patterns of species dominance in wetlands, accelerating mangrove encroachment into adjacent brackish and freshwater wetlands, and alter the competitive interactions between saltmarsh species which are comprised of species that use both C4 and C3 photosynthetic pathways. The dominant saltmarsh grass, Sporobolus virginicus, is C4. C4 species are predicted to be achieve only moderate enhancements in photosynthesis under elevated levels of CO2 compared to C3 species.. Elevated CO2 may exacerbate the negative effects of freezing temperature on mangroves which may limit their expansion into southern regions.
The effect of sea level rise will be moderated by coastal geomorphology. Many of the mangrove areas of northern Australia are associated with broad, flat coastal plains that often have large areas of intertidal salt flat and high intertidal salt marsh (e.g. Fitzroy River, Duke 1997). Around 5500-7500 years ago salt flats and salt marsh areas were covered in mangrove forests in what Woodroffe (1990, 1992) has called the ‘big swamp’ phase of estuary development. In northern Australia, sediment deposition raised the level of soils and sea level has dropped (by approximately 1 m in the last 6000 years) resulting in mangrove forests that currently occupy the edges of coastal plains with the development of salt marsh and salt flats behind them (landward), and in areas of high rainfall, the development of extensive fresh water marshes. Landward migration of mangroves into salt marshes, fresh water wetlands or agricultural lands (where there are no significant human barriers to prevent this) is highly probable. Significant changes in faunal diversity and ecosystem function s likely (Eliot et al. 1999, IPCC 2002a,b, Millenium Ecosystem Assessment 2005, Nicholls 2004). Impacts of sea level rise will also depend on tidal range. We expect areas with smaller tidal ranges to be more vulnerable to sea level rise compared with those with larger tidal ranges (Semeniuk 1984, Woodroffe 1986).
Whether mangroves and saltmarshes will persist in their current distribution and abundance will depend on the rate of sea-level rise. Preliminary results from the SE Australian surface elevation table (SET) network indicate that mangroves have been able to increase elevation primarily as a result of sediment accretion, at a rate comparable with the rate of sea-level rise over the past decade, of 1-2 mm per year. Saltmarshes have not been able to maintain this rate of vertical accretion and have been colonised by mangroves. The Fourth Assessment Report of the IPCC has projected acceleration in the rate of sea-level rise to 2100, with rates of rise exceeding 10 mm yr-1 towards the end of this period. It is unlikely that mangroves will persist in their current position in the landscape given these rates of sea-level rise, although thresholds of sea level rise precipitating ecosystem collapse have not been determined and are likely to vary between geomorphic and geographic settings. The nascent SET network in Australia and globally should provide knowledge of processes determining the resilience of tidal wetlands and thresholds over the coming decades.
Increases in the frequency and intensity of cyclones and intense storms will increase the destructive effects on mangroves and salt marshes. There are gaps in our knowledge of the effects of both the predicted changes in the frequency and intensity of cyclones and intense storms as there are few data documenting the extent of damage and recovery of mangroves and salt marshes in Australia.
Where rainfall is reduced productivity and diversity of mangroves will decline and increases in the area of salt marsh and salt flats are likely (Smith and Duke 1987). Reduced rainfall will lead to reductions in sedimentation and to reduced plant growth. Both of these factors will result in reduced tidal wetland surface elevation increments and thus increase the susceptibility to sea level rise. Sedimentation in mangroves has been observed to vary between -11 mm y-1 (erosion) to 10 mm y-1 (Furukawa and Wolanski 1996, Bird and Barson 1977, Spenceley 1977, 1982, Bryce et al. 2003). At the higher end, sedimentation is higher than projected sea level rise, but there is not sufficient data to determine what levels of sedimentation occur over most of Australia and whether these exceed tidal wetland surface subsidence. In a study of sedimentation in southern Australia, sedimentation was higher in mangroves compared to salt marsh (approx 5 mm y-1 in mangroves and 2.5 mm y-1 in salt marsh, Rogers 2005). Sedimentation increased linearly with tidal range (Rogers 2005). Extrapolation over tidal ranges in Australia, and assuming a similar sediment supply suggests sedimentation could vary from 1.6 to 2.8 mm y-1, which is at the low end of the published range and is slightly lower than current rates of sea level rise. Thus areas with low tidal ranges, low rainfall and limited sediment supply are more likely to experience a landward retreat of tidal wetlands with sea level rise compared to those areas with high tidal range, high rainfall and an ample sediment supply, which are conditions under which tidal wetland expansion is likely to occur.
Climate change factors interact with each other, and with direct human influences on the coast, to affect outcomes for tidal wetlands. For example, nutrient enrichment and elevated CO2 have positive influences on wetland plant root growth (McKee et al. 2007, Cherry et al. 2009) which may decrease vulnerability to sea level rise (Langley et al. 2009). However, nutrient enrichment also results in increased mortality when mangroves experience drought (Lovelock et al. 2009), thereby increasing vulnerability to climate change factors. Similarly, elevated CO2 increases vulnerability to low temperatures (Woldendorp et al. 2008) potentially limiting the effects of increased temperatures. Increases in the area of mangroves may be particularly likely if high sediment deposition rates due to land-use change in the catchments are sustained or increased with increasing rainfall resulting in the creation of new habitat for mangrove colonisation.
Losses in mangrove and salt marsh area may occur if high temperatures and aridity depress productivity and if sediment delivery is reduced. Pollution and storm damage could accentuate these losses (Duke et al. 2005). Under scenarios of negative human influence (e.g. pollution and impoundment by building of barriers) reductions of fringing mangroves may be substantial, and forests establishing landward may have reduced productivity.
A significant threat to the resilience of intertidal wetlands with climate change is the presence of barriers (e.g. Figure 2) that will prevent the landward migration of intertidal wetland communities. Barriers to landward migration of intertidal communities can be imposed by natural features e.g. steep slopes, but urban, agricultural and other human developments that build berms, bunds, seawalls and roads on coastal plains impose significant threats to resilience of mangroves, salt marsh and salt flats with sea level rise. Barriers also reduce connectivity between ecosystems and overall productivity. Landward barriers to wetland migration will have particularly negative consequences for salt marsh and salt flat communities that are compressed between human imposed landward barriers and encroaching mangroves (Saintilan and Williams 1999, Adam 2002, Saintilan et al. 2009). Figure 2. Mangrove up against sea wall, South East Queensland.
• Tidal wetlands provide valuable ecosystem services, including supporting high biodiversity and productivity of the coastal zone.
• Effects of increasing temperature have not been documented but may include changes in reproductive phenology of mangrove trees and increased productivity of forests in southern latitudes.
• Elevated CO2 is likely to result in increased productivity of trees, particularly where growth is not limited by high salinity or low humidity. But decreased quality of food for consumers and increased susceptibility of trees to freezing damage are also likely.
• Reductions in rainfall will decrease diversity and productivity of forests while increases in rainfall will increase diversity and productivity. Pollution with nitrogen fertilizers enhances negative effects of lower rainfall.
• Overall, total area of mangrove forest in Australia is likely to increase with sea level rise as low lying coastal areas become inundated. Salt marsh area is likely to decline as mangroves invade and landward migration is prevented by human and natural barriers.
• Changes in rainfall, catchment development, elevated CO2, sea level rise and nutrient availability interact to modify outcomes as all factors affect process that maintain wetland surface elevation and thus vulnerability to sea level rise
Confidence Assessments >
Evidence: There is weak evidence for tidal wetland response to warming temperatures. There is no direct evidence that mangroves are moving south with climate change though there is evidence that the red mangrove Rhizophora stylosa is expanding within its range at its southern limits. There is strong evidence that mangrove forest productivity and reproduction is limited by low temperature in temperate regions. Mangroves are highly frost sensitive. There is strong global evidence of changes in the phenology of plant and animal species with increased temperature. There is a strong inverse relationship between saltmarsh diversity and temperature. There is a medium level of agreement that warming temperatures are affecting tidal wetlands. There is little direct evidence that climate change is resulting in increases in productivity and abundance of mangroves at southern latitudes. Factors such as enhanced sedimentation may explain mangrove expansion where it has occurred (e.g. Lovelock et al. 2007).
Confidence: We have MEDIUM confidence that warming temperatures are impacting tidal wetlands. Knowledge of thermal limits to mangrove and salt marsh flora and fauna is balanced against a lack of direct evidence of change.
Evidence: We have evidence from experimental studies that elevated atmospheric CO2 levels increases plant productivity under some conditions. One USA salt marsh study documents increased surface elevation with elevated CO2 suggesting high CO2 will mitigate effects of rising sea level. There is weak evidence of impacts on Australian tidal wetlands. There are no field studies in mangrove forests and none in Australian salt marshes. There is a low level of agreement that elevated atmospheric CO2 levels will benefit tidal wetlands. Enhancements in growth with elevated CO2 can be reduced with high salinity, low humidity and low nutrient availability. Positive effects of root growth on tidal wetland surface elevation may only occur in ecosystems with highly organic soils and not those with mineral soils.
Confidence: We have LOW confidence that elevated atmospheric CO2 levels are impacting tidal wetlands due to insufficient evidence.
Evidence: There is strong evidence globally that sea level rise will affect wetlands. Loss of wetlands in the USA is well documented. Models indicate tidal wetland loss should be expected where surface elevation of wetlands does not keep pace with sea level rise (Cahoon et al. 2007). There is a high level of agreement that sea level rise is an important factor affecting wetlands in Australia and globally.
There is evidence of impacts in Australia. Published accounts of wetland loss and change due to sea level rise are not common in Australia (with exceptions, e.g. Winn et al. 2006; Rogers et al. 2006), although invasion of marsh communities by mangroves is documented in Australia. Monitoring in Australia is geographically limited with some regions lacking information. There is a limited information of short term adaptive capacity to sea-level rise of tidal wetlands in Australia, although changes in mangroves’ distribution during sea level change in the Holocene are known. There is a moderate level of agreement that sea-level rise is the major factor causing mangrove invasion of marshes in Australia. Other hypotheses, e.g. subsidence due to groundwater loss, are also proposed.
We have high confidence that sea-level rise is affecting Australian tidal wetlands. Although there is ample international evidence of wetland loss and change, spatial variation in sea level rise in Australia, large and variable tidal ranges and data gaps for tidal wetlands for large portions of the Australian coastline reduce our confidence.
Evidence: There is evidence that alteration of rainfall regimes is affecting Australia tidal wetlands. Variation in tidal wetlands over climatic gradients within Australia provides strong evidence that patterns in rainfall affect tidal wetland diversity and productivity. Changes from salt marsh to mangroves were linked to low rainfall (Rogers et al. 2005). Drought resulted in mangrove mortality (Lovelock et al. 2009). There is a high level of consensus on the importance of increasing and decreasing rainfall on tidal wetlands despite few experimental studies.
Confidence: There is high confidence that changes in rainfall influences tidal wetlands. Despite few experimental studies there is a high level of consensus that increasing and decreasing rainfall will strongly influence tidal wetlands because of fundamental importance of water availability to plant and animal communities.
Potential Impacts by the 2030s and 2100s:
Evidence: There is evidence that warming temperatures will influence tidal wetland distributions. There is fossil evidence of tropical mangrove forest communities at lower latitudes in the past in Australia (Hashimoto et al. 2006). There is strong evidence that productivity and reproduction will be affected by warming temperatures. Mangrove forest productivity and reproduction is limited by low temperature in temperate regions. Mangroves are highly frost sensitive. There is strong global evidence of changes in the phenology of plant and animal species globally with increased temperature. There is a medium level of consensus that warming temperatures will affect tidal wetlands. There is a lack of evidence of change but there is high confidence in physiological models. Dispersal barriers and faunal dependencies are not known.
Confidence: There is medium level of confidence that warming temperatures will affect tidal wetlands over the coming century. Knowledge of thermal limits to tidal wetland flora and fauna is balanced against a lack of direct observations.
Evidence: There is evidence from experimental studies that CO2 increases plant productivity under some conditions. One USA salt marsh study documents increase in surface elevation with elevated CO2. There is weak evidence that Australian tidal wetlands will be affected. There are no field studies in mangroves and none in Australian salt marshes. There is a low level of agreement that increasing elevation of atmospheric CO2 levels will benefit tidal wetlands. Enhancements in growth with elevated CO2 can be reduced with high salinity, low humidity and low nutrient availability. Enhancement in root growth and the positive effects expected for tidal wetland surface elevation gain may only occur in ecosystems with highly organic soils and low nutrient availability and not those with mineral soils or higher nutrient availability (where decomposition of organic matter may be enhanced).
Confidence: There is a low confidence that increasing elevation of atmospheric CO2 levels will benefit tidal wetlands over the coming century. There is insufficient evidence to be confident of predictions.
Evidence: There is strong evidence that accelerating sea-level rise will impact tidal wetlands. Models predict loss of wetlands as sea level rise accelerates. Loss of salt marsh is expected as landward barriers restrict wetland upland migration. Changes in tidal wetland fauna are expected. Medium confidence arises due to uncertainty in sea level rise predictions and lack of information on adaptive capacity. There is a HIGH level of agreement that sea level rise is an important factor that will affect tidal wetlands in Australia and globally.
Confidence: There is medium to high confidence that sea-level rise over the coming century will impact tidal wetlands. Spatial variation in sea-level rise in Australia, large and variable tidal ranges and data gaps of wetlands for large portions of the Australian coastline reduce our confidence in predicted outcomes.
Evidence: There is evidence that changing rainfall over the coming decades will affect tidal wetlands. Variation in tidal wetlands over climatic gradients within Australia provides evidence that patterns in rainfall will have a strong impact on tidal wetland diversity and productivity. Subsidence within salt marsh and increased vulnerability to mangrove encroachment has been linked to low rainfall (Rogers et al. 2005). Drought resulted in mangrove mortality (Lovelock et al. 2009). There is a MEDIUM consensus, despite few experimental studies, on the strong effects of increasing and decreasing rainfall on tidal wetland communities.
Confidence: There is MEDIUM confidence that alteration of rainfall patterns and intensity will affect tidal wetlands this century. Despite few experimental studies there is consensus on effects of increasing and decreasing rainfall because of fundamental importance of water availability to plant and animal communities.
Adaptation Responses >
Mangrove forests and other tidal wetlands may adapt to rising sea level and remain stable if the rate of vertical accretion of the soil surface of the wetland equals or exceeds the rate of sea level rise (Cahoon et al. 1999, Morris et al. 2002). This simple idea underpins many of the current models used to assess wetland stability with rising sea level (e.g. Nicholls et al. 1999, Simas et al. 2001, Nicholls 2004). Wetland soil surface elevation and its response to sea level are influenced by a suite of interacting processes and feedback mechanisms (Figure 3) that occur on both the surface and subsurface of soils. Elevation of the wetland soil surface is directly influenced by soil volume, which is related to several interrelated processes. Tidal floodwaters deliver sediments to wetlands, where aerial roots and pneumatophores of mangroves enhance sediment deposition, adding to soil volume (Krauss et al. 2003). In addition, soil volume is related in part to soil organic matter accumulation, which is the net result of root growth (positive soil volume) and root decomposition (negative soil volume) (Cahoon and Lynch 1997, Cahoon et al. 2003, 2006).
Groundwater drainage and storage result in shrinking and swelling of soils (negative and positive soil volume) (Whelan et al. 2005; Rogers and Saintilan 2008), and soil compaction (reduced soil volume) also influence soil elevation. As soil elevation increases the frequency, depth, and duration of tidal flooding decreases (negative feedback) (Cahoon and Reed 1995). When sea level rises, hydroperiod increases. So as long as soil elevation gain matches sea level rise, the wetland will maintain the same relative elevation within the tidal frame, migrating upslope if need be. This model (Figure 3) and the instrumentation devised to test the model, the rod surface elevation table (RSET, Cahoon 2002; Figure 4) have been used to understand the vulnerability of wetlands to sea level rise and capacity for adaptation by describing the trajectory of the elevation of coastal wetlands in responses to a range of environmental conditions. In Australia RSETs (or earlier version SETs) have been deployed in southern salt marshes and mangroves (Rogers et al. 2006) and in Moreton Bay, South East Queensland (Lovelock CE unpublished). The RSET and SET network will provide greater knowledge of possible thresholds of sea-level rise that prompt exceed the adaptive capacity of coastal mangrove and saltmarsh ecosystems to keep pace with sea level rise, and increase our knowledge of the processes that contribute to variation in adaptation among tidal wetlands in different geomorphic and biogeographic settings. Adaptation can also be enhanced by management of tidal wetlands and adjacent ecosystems. Figure 3. Diagram showing direct and indirect biotic controls on vertical accretion and elevation change. From Cahoon et al. 2006. Figure 4. Left: Measuring wetland soil surface elevation, North Stradbroke Island, Queensland. Right: Installing surface elevation tables for measuring changes in soil elevation in salt marsh, South East Queensland.
Management options for adaptation
• Removal of barriers to landward migration of tidal wetlands is an adaptation strategy that could be used to maintain wetland areas.
• Detailed elevation mapping of coastal wetland and hinterland environments, using remote sensing platforms such as LiDAR. This will determine areas where landward migration of critical coastal wetland communities can be accommodated.
• Improved models of coastal wetland migration based on quality elevation models and dynamic elevation responses derived from the SET data and landscape scale process models
• Limit construction of future barriers, thereby managing the landward retreat (Gilman et al. 2008)
• Remove and limit additional drivers that have negative effects, e.g. pollution, nutrient enrichment, delivery of upstream sediments, clearing and cutting in catchments.
• Manage catchments for processes that modify surface elevation (e.g. groundwater extraction, erosion, fertilization and river flows).
• Restore degraded tidal wetlands.
• Protect and restore tidal wetlands that have a high capacity for adaptation, such as those with suitable topography, and where human barriers are absent or can be removed.
Knowledge Gaps >
Much of what we know about how tidal wetlands are being and will be influenced by climate change comes from paleontological research over past climatic fluctuations and from comparisons over sites at different latitudes and climatic conditions, not from direct experimental manipulations or long term observations, although there are exceptions (see Further information section). We have few direct evaluations of adaptations to climate change drivers and little data on which to predict rates of adaptation or limits to adaptation.
• There are few locations with adequate data to describe coastal topography at a sufficiently fine scale to model inundation with sea level rise.
• There are data on wetland surface elevation dynamics (rates of accretion and subsidence of wetland soils) from temperate and subtropical estuarine systems, but none from the tropics.
• There are parts of Australia where little is known of the current state of tidal wetlands, e.g. the Kimberly and other parts of northern Australia.
• Although large tidal ranges are proposed to increase adaption of wetlands to sea level rise there are no direct tests of this hypothesis.
• Most of what we know about how tidal wetlands respond and adapt to climate change is focussed on the plant communities. Very little is known of how animal and microbial communities of tidal wetlands will adapt to climate change. Microbial communities are vital for many of the ecosystem services provided by mangroves. Faunal communities are important to human livelihoods and cultural values of the coastal. These are significant knowledge gaps that require research. Although changes are anticipated in vegetation structure and coverage of intertidal wetlands with sea level rise, understanding of the functional consequences of these changes, or how ecosystem services will change, remains qualitative. The impacts on climate change on tidal wetland faunal abundance, sediment trapping, nutrient and carbon fluxes are currently not known with any certainty.
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