Divergence of ecosystem functioning and stability under climatic extremes in a 24-year long-term grassland experiment

Ecosystem functioning is impacted by the rising intensity and frequency of climatic extremes. Given the substantial evidence of the impacts of climatic extremes on ecosystem productivity, plant ecologists have been fascinated by the role of species richness in sustaining ecosystem functioning and stability under climatic extremes. Using the above-ground net primary productivity (ANPP) and climate data of a long-running (1997-2020) biodiversity experiment in Bayreuth, Germany, we examined the (i) effects of climatic conditions on species richness and ANPP, and (ii) role of species richness on resistance and resilience of ecosystem under different climatic conditions. Bayreuth Biodiversity Experiment was established in 1996, which comprises 64 plots (each plot is 2m×2m in size). Biomass was harvested twice a year (June and September) at 5 cm above the ground within the centre of each plot. We employed the Standardized Precipitation Evapotranspiration Index (SPEI) to classify the growing season (3-month SPEI) and annual (12-month SPEI) climatic conditions (ranging from extreme wet to extreme dry conditions) into a 5-class and 7-class climatic conditions classifications. A number of pairwise tests (ANOVA and post-hoc) were used to assess the differences in species richness and ANPP among various climatic conditions. We utilized generalized linear models to assess the relationships between species richness and ANPP, and linear mixed-effects models to examine the relationships between species richness and resistance and resilience under different directions (e.g., dry or wet) and intensities (e.g., extreme, moderate and mild) of climatic conditions. Results show that ANPP varied greatly with respect to climatic intensity and direction, peaking in extreme wet conditions and declining in extreme dry ones. Species richness and ANPP formed a concave-up (unimodal) pattern for the dry conditions and a negative linear (positive linear) pattern for the wet conditions in June (September) harvests. Species richness increased ecosystem resistance regardless of intensity, direction and classification of climatic conditions, while decreased ecosystem resilience towards dry climatic conditions. Ecosystem resilience remained steady towards wet climatic conditions. Our study stresses the importance of maintaining a community with higher species richness to stabilize ecosystem functioning and enhance resistance to various climatic conditions.