Resilience of montado understorey to experimental precipitation variability fails under severe natural drought

Future climate scenarios for the Iberian Peninsula predict increasingly variable precipitation regimes, challenging key ecosystem processes in the Mediterranean biome. This study presents a large-scale water manipulation experiment, exposing the understorey vegetation in a Mediterranean oak woodland to simulations of precipitation variability. We hypothesized that, with no net changes in rainfall quantity, large infrequent precipitation events extend the period of soil moisture deficit, thereby decreasing productivity and soil nitrogen availability, accompanied by changes in plant community structure. However, the herbaceous understorey was highly resilient to increased precipitation variability. Extending the dry period between precipitation events from three to six weeks had no effect on productivity and community structure, this lack of responsiveness being attributed to phenotypic and physiological adaptations of the vegetation. However, vegetation senesced earlier, the shorter life cycle potentially influencing reproductive success. Additionally, experimental water manipulation did not affect soil nitrogen dynamics, with nitrogen being limiting in both treatments. In comparison, non-manipulated control plots experienced a severe natural wintertime drought, significantly reducing productivity and affecting species composition, but showed less indication of nitrogen limitation. Thus, although the understorey vegetation was resilient to changes in precipitation variability, long drought periods exerted highly negative effects, with the vegetation not being able to buffer the drought effects through adaptive strategies when the length of the dry period exceeded ∼10 weeks. Our results highlight the necessity for further studies investigating how climate change will influence the co-limitation of water and nitrogen availability, which in turn might affect plant productivity in Mediterranean ecosystems.