Responses of Spartina and Phragmites to climate change: Comparisons of photosynthetic metabolism and salt tolerance in Phragmites australis and Spartina species and their significance for salt marsh responses to climate change.

Research output: Book/anthology/dissertation/reportPh.D. thesis

Ecosystem engineers, Phragmites and Spartina, are dominant species in coastal wetlands around the world. Understanding how they respond to climate change is vital in order to predict changes in the ecosystem services of coastal wetlands. While both Phragmites and Spartina are keystone species in the wetlands where they occur and exhibit invasive behavior outside their native range, they differ in multiple ways; C3 vs C4 photosynthetic pathway, tall vs short stature, low vs high salinity tolerance. Their response to different direct and indirect aspects of climate change is therefore also expected to differ.
A higher temperature will increase enzyme activity, including photorespiration by Rubisco, something that C4 species avoid by concentrating CO2 in bundle sheath cells. For the same reason increased CO2 will benefit C3 species by lowering photorespiration, while it will have little effect on C4, as it already operates near saturation at present CO2 concentrations. These predictions are further muddled by the fact that climate change is more likely to benefit invasive species over native ones. Increased sea level rise may be a danger to their habitat, unless the natural, ongoing vertical elevation of the marsh surface can keep pace. Here their role as ecosystem engineers becomes important. Increased sea level, as well as changes in frequency of storm events and precipitation patterns, will also affect salinity in coastal areas. How they respond to salinity, and inter-specific competition for light, is therefore important for improving predictions about how coastal wetlands in western Europe will respond to the hydrological changes they will inevitably experience.
This PhD thesis aimed to increase our understanding of these complex interactions through a series of experiments.The results from paper IV indicate that the photosynthetic strategy alone is not enough to predict the outcome of increased temperature if the plant has high plasticity, both genotypic and phenotypic, as is the case in the widespread cosmopolite Phragmites australis. While paper V hints at increased evaporative cooling of the leaf as a mechanism for P. australis to acclimate to higher temperature. At Skallingen, the coastal wetland examined in the Danish Wadden Sea in paper III, the rate of elevation change appears to keep pace with the current rate of sea level rise. Here the high marsh, dominated by native Phragmites australis, and the low marsh, dominated by invasive Spartina anglica, showed similar rates of elevation change, but it was driven by different processes. The low marsh zone generally had higher rates of vertical accumulation, with some shallow subsidence, while the high marsh exhibited belowground expansion to reach the same rates of elevation change. Singular events had a large influence on the high marsh, and a clear seasonal variation was apparent in both zones. However, the future of Skallingen is uncertain under all but the most optimistic climate change projection by IPCC and will most likely depend on how the area responds to increased sea levels. Will rate of elevation change accelerate alongside the rate of sea level rise? Coastal wetlands exhibit clear plant zonation patterns across an elevation gradient, where the physical tolerance of a species limits its distribution toward the low end of the gradient, and its competitive ability towards the other. We found that increased salinity decreased the competitive superiority of P. australis. Though this was not apparent in decreased photosynthetic capacity in the remaining leaves (Paper I). The energy expenditure associated with maintaining low Na concentration in the tissue, when external Na increased could account for the lower biomass in P. australis at high salinity (Paper II). Increased salinity intrusion in coastal wetlands might therefore increase the presence of the invasive S. anglica, which could alter the ecosystem and have implications for salt marsh conservation in Europe.
Original languageEnglish
Number of pages234
Publication statusPublished - 31 Mar 2020

    Research areas

  • Phragmites australis, climate change, light- and CO2 response, coastal wetlands, Spartina anglica, Spartina alterniflora

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