Global Synthesis Reveals Context-Dependent Plant Diversity Responses to CO2, Climate Change, and Nitrogen.

Global understanding of how plant diversity responds to multiple, co-occurring global change drivers (e.g., elevated CO2, climate change, and nitrogen addition) remains fragmented, largely due to the highly context-dependent nature of responses and disparate experimental evidence. To address this, we conducted a global meta-analysis synthesising 6832 control-treatment comparisons from 465 studies. We found that for individual drivers, elevated CO2 had no effect on species richness, whereas increased precipitation enhanced it. In contrast, warming, decreased precipitation, and nitrogen addition generally reduced richness, with nitrogen addition consistently decreasing Shannon diversity and Pielou evenness. When drivers combined, warming-driven losses were offset by elevated CO2 and increased precipitation and those of nitrogen addition were counteracted by both increased and decreased precipitation. Interaction analyses further revealed that additive effects between drivers predominated, though significant antagonisms emerged for specific pairs. Importantly, these responses were strongly context-dependent, being mediated by a combination of site-specific conditions (i.e., local climate, soil, and background diversity) and experimental methodologies (i.e., treatment duration, intensity, and plot and sampling area). Our synthesis underscores that predicting future biodiversity trajectories necessitates that models evolve beyond main effects to explicitly represent both multi-driver interactions and the critical, often-dominant, moderating role of local contexts and experimental conditions.