Effects of extreme drought on primary production, species composition and species diversity of a Mediterranean annual plant community

Questions What are the effects of extreme drought on above-ground and below-ground net primary production (ANPP and BNPP, respectively) of an annual plant community in a Mediterranean shrubland? How do annual plant communities in Mediterranean shrublands respond to extreme drought, in terms of changes in their species composition, species richness, and species diversity? Location A Mediterranean shrubland in the Judean Hills, Israel. Method Rainout shelters were used to simulate a 66% reduction in annual rainfall, preceded by a baseline year when no treatments were applied. Above-ground and below-ground (i.e., root) biomass was harvested and species surveys were performed following one growing season. Results Above-ground primary production was significantly reduced by drought, while below-ground production remained mostly stable. Consequently, the shoot:root ratio was reduced by drought, indicating either increased community resource allocation to root production at the expense of shoots or stable root biomass production in the face of drought. Community diversity increased in droughted plots, and significant changes were detected in species composition and dominance patterns. Conclusions Extreme drought had a profound effect at the ecosystem and community level on the studied Mediterranean shrubland. Extreme drought seems to increase species diversity and evenness in the short term, likely due to release from competition. We predict this effect to be countered by low biomass production and individual species abundance if extreme drought persists a sufficiently long time, leading to significant changes in long-term plant dominance patterns and community structure.