Climate change threatens endangered plant species by stronger and interacting water-related stresses

Atmospheric CO2-concentration, temperature and rainfall variability are all expected to increase in the near future. The resulting increased dynamics of soil moisture contents, together with increased plant physiological demands for both oxygen and water, will lead to an increased occurrence of wet and dry extremes of plant stresses, i.e. of oxygen and drought stress, respectively, alone and in interaction. The use of indirect environmental variables of previous studies and their focus on one stress at a time has hampered understanding the causal impact of climate change on plant species composition through changes in abiotic site conditions. Here, we use process-based simulations of oxygen and drought stress and show that both stresses will increase (on average with ca. 20% at sites where both stresses occur) in a warmer and more variable future (2050) climate (applying a national downscaled version of IPCC scenarios). These stresses will increasingly coincide, i.e. both stresses will occur more often (but not at the same time) within the same vegetation plot. We further show that particularly this increased coincidence of water-related stresses will negatively affect the future occurrence of currently endangered plant species (a reduction of 16%), while such a decrease is not apparent for common species. Individual stresses did not affect the occurrence of endangered plant species. Consequently, the species that are already threatened under the current climate, will suffer most from climate change