Plant responses to simultaneous stress of waterlogging and shade: amplified or hierarchical effects?

Community ecologists often assume a hierarchy of environmental sieves to predict the impact of multiple stresses on species distribution. We tested whether this assumption corresponds to physiological responses using impact of water level and shade in wetland vegetation as a model. Seedlings of four wetland species were grown under full light and simulated canopy shade, both in drained and waterlogged soils. When subject to both stresses simultaneously, waterlogging and shade independently affected growth of the two waterlogging tolerant species. For the intolerant species, however, waterlogging had the largest impact and the additional effect of shade was smaller than the effect of shade in drained soil. Soil flooding decreased specific leaf area but only if plants were in full light. Waterlogging did also not constrain a higher investment in stems of shaded plants. These results demonstrate that light limitation in flooded habitats only plays a role if species can tolerate waterlogging and therefore correspond with the notion that water level determines the potential species pool and that standing crop consequently determines which species can actually persist.