COS 76-5 - Tidal marsh grass exhibits trait plasticity mediated by genetic identity along colonization front into coastal forest

Abstract: The role of genetic vs. plastic responses of plants to rapid environmental change can be better understood through work in coastal systems affected by sea-level rise, where salt-water intrusion and coastal storm events are driving conversion of maritime forest to salt marsh habitat. Native high marsh plants can establish in maritime forest understory prior to forest dieback and the success of this early establishment may impact the long-term habitat trajectory. We established an in-situ mesocosm experiment to characterize the relative roles of genetic identity vs. phenotypic plasticity in responses of the salt marsh grass Spartina patens to canopy shading and soil salinity. Individuals from 10 clonal lines of S. patens from migrating populations were grown in open marsh or neighboring maritime forest and in one of three salinity treatments. We found strong shifts towards aboveground resource allocation under light limitation by canopy shading, with increased specific leaf area and plant height. Total biomass and sexual and clonal reproduction were negatively affected by shading. Interestingly, the negative fitness consequences of the combined stressors of shading and high salinity were less than would be expected under an additive model. This may be due to generally reduced osmotic stress under canopy cover (due to lower irradiance, temperature, greater humidity, etc.). These results suggest that low light availability is the main driver of S. patens traits and fitness in transitioning upland habitats, and that if S. patens populations are able to persist, they will be able to respond plastically as the forest canopy opens.