University of Tasmania
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A biogeography of Tasmanian coastal saltmarshes

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posted on 2023-05-28, 09:21 authored by John Aalders
Visually, Tasmanian coastal saltmarshes appear to contrast greatly, both within and between sites, with a strong display of vegetation patterning. Some marshes can be dominated by saline graminoids or lawn-like herbaceous succulents, while others present a complex mosaic of plant species that thrive in the saline environment. A key question in our understanding of coastal wetland ecological features is whether mapped coastal saltmarsh reflects other biophysical patterns in the landscape. To this end, the historical and descriptive aspects of key Tasmanian natural regionalisations and domains are critically examined which focus on the terrestrial/maritime interface. Existing classifications including the Interim Biogeographic Regionalisation of Australia (IBRA), the Interim Marine and Coastal Regionalisation of Australia (IMCRA), weather forecasting districts, geographical position and a state-wide estuarine classification system were tested to their suitability in determining saltmarsh vegetation patterning. From a Tasmanian perspective, the national definition of coastal saltmarshes has some shortcomings. Position in the landscape plays a key role in defining saltmarsh type and Tasmanian examples are examined to test their fit to National (Environment Protection and Biodiversity Act, 1999) and State (TASVEG) interpretations and guidelines. When sea-level rise and climate change ramifications are considered, a carefully measured and workable definition of Tasmanian coastal saltmarshes is derived that largely aligns with, but expands, the current guidelines. Coastal saltmarshes in Tasmania display a range of vegetation configurations from multiple to single species communities. Tasmanian saltmarshes are historically classified to two vegetation classes, useful at a broad scale, but less effective at the fine scale required for ecological studies. A comprehensive state-wide (including off shore islands) vegetation assessment was carried out at 21 sites involving 110 plots. Multivariate analysis methods determined eight vegetation groups, and a diagnostic key to these was developed. Subsequently, the key was field tested on 70 sites (297 plots) and improved. The new vegetation community groupings were aligned to current vegetation classes, community indicator plant species were identified, and a typology produced that reflected each community, yielding a useful tool for future citizen science and management. The distribution of vegetation communities did not align at all with most available natural regionalisations, but the national IMCRA classification provided a mediocre fit. Field and laboratory analyses of bare ground cover, organic layer depth, pH, electrical conductivity (EC), moisture, bulk density, loss on ignition, and composition were carried out on soils from a state-wide collection of 407 coastal saltmarsh plots (91 sites). Multivariate analysis determined eight major soil types, yet, no clear alignment was identified with the vegetation communities. No individual or group of plant species could be exclusively aligned to a particular soil type. It was found that all eight vegetation communities were tolerant of wide ranges in most edaphic factors, but more constrained by climatic variables, such as temperature and rainfall. The distribution of saltmarsh soil types did not conform well with most natural regionalisations, although IMCRA showed modest alignment to soil type. The soils of tidal wetlands are known to be important as carbon stores, however, few reliable estimates are available for Australian coastal saltmarshes and none for Tasmania. An extensive investigation reports in detail on carbon levels for Tasmanian coastal saltmarshes providing an accurate account of the quantum and distribution of this carbon store. Tasmania saltmarsh soils are found to be shallower on average than in other States, limiting the amount of stored carbon when compared to elsewhere in Australia. Tasmanian coastal saltmarshes contain a total carbon stock of 390,000 tonnes, currently valued at $19.8 million (AUD), with an average carbon offset value of $3,380 per hectare. Carbon content was found to vary two-fold across IMCRA bioregions. Limitations in current reporting were identified, and an improved protocol was proposed to account for common errors and uncertainties in carbon calculation. The distribution of halophytes in coastal saltmarshes is believed to be determined by several abiotic factors such as tidal cycles, elevation and salinity. The state-wide vegetation and soil survey of coastal saltmarshes enabled the environmental ranges of key plant species to be classified in terms of EC (as a proxy for salinity), pH, moisture, organic matter, composition, temperature, rainfall and solar exposure. The key abiotic factors that played a role in species incidence were identified allowing decision tools to be produced as an aid in the appropriate selection of plant species suitable for saltmarsh restoration. Rapid sea-level rise and climate change will threaten the integrity of coastal saltmarshes. The resilience of four key halophyte communities and their soils to pulses of increasing salinity (mimicking increasing sea-level) and decreasing salinity (mimicking increasing rainfall) was tested over a 16-month period. Key factors (electrical conductivity as a proxy for salinity, and pH) were measured at intervals in soils, and commencement and conclusion samples of four key plant species. Soils showed mostly unresponsive pH values across all treatments, with generally small changes of EC in increasing/ decreasing salinity plots, yet substantial increases of EC in the control plots. Changes within plant species were not a reflection of soil responses with pH and EC values increasing across all treatments. No plants died during the trial, further suggesting a level of resilience to changes in water conditions. To conclude, the association between the eight vegetation communities, regionalisations and regions was at most unsystematic, although from a field-based view, IMCRA appeared to be somewhat appropriate. Similarly, saltmarsh soil types were not associated to any regionalisation, although again, IMCRA regionalisation was determined as a possible candidate. Carbon stocks differed between vegetation communities, but the differences were not significant. Carbon values for similar vegetation communities differed between regions of all regionalisations including IMCRA, suggesting no association between sequestered carbon and regionalisation. The strongest alignment of saltmarsh vegetation patterning appears to be with two climate variables, temperature and rainfall. This particularly relates to two climatic sectors, that of wet + cold, and dry + warm, with patterning association also evident between wet + warm, and cool + dry.


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