Tasmanian coastal saltmarsh community transitions associated with climate change and relative sea level rise 1975-2009

Coastal saltmarshes are reputed to be one of the most vulnerable communities to global warming, with widespread evidence of retreat and movement of lower marsh vegetation into areas previously occupied by upper marsh vegetation in response to rising sea levels, and potential changes in community composition from changes in rainfall, temperature and wind. We undertook an investigation of decades scale change in the distributions of saltmarsh communities defined by dominant species, using historic vegetation maps, remote sensing imagery and extensive field data collection. Our study area in south-eastern Tasmania has suffered a marked increase in temperatures and wind speeds and a marked decrease in rainfall since 1975, with sea level rising at a rate of 0.8 mm per annum. We therefore tested the hypothesis that these changes would result in a shift in saltmarsh community composition towards more salt- and inundation-tolerant communities and salt scalds. Eighteen percent of the 1975 marsh was lost to direct human modification and a net 4% was lost to coastal retreat. One large marsh was cut off from the sea then burned, then reconnected with the sea. The vegetation change between 1975 and 2009 in other parts of the saltmarshes occurred in 21% of their 1975 area. Most of the community transitions were consistent with increasing aridity. Thus, our results indicate that global warming has already caused marked changes in community composition in saltmarsh in Tasmania.