Climatic disequilibrium of recruit communities across a drought-induced die-off gradient in Mediterranean shrubland

Positive plant-plant interactions (facilitation) may enhance the recruitment and establishment of species less adapted to local macroclimatic conditions. A major cause of this effect is climatic buffering, which implies an increased mismatch between the macroclimatic conditions and the climatic requirements of the existing community - climatic disequilibrium - of plants living under canopies. Here we explore the effect of drought-induced defoliation of Mediterranean shrubland canopy on the recruitment of woody species. We analyzed the differences in the climatic disequilibrium across different categories of canopy defoliation and plant-plant interactions: facilitation, neutral and inhibition. Climatic disequilibrium was estimated as the Euclidean distance in the multivariate environmental space between observed macroclimate and community inferred climate. The inferred climate was calculated by averaging the coordinates of the species' climatic niche centroids, obtained from species distribution, weighted by the species' relative abundances in each community. We found that the recruiting community growing under canopy showed higher climatic disequilibrium than the community growing in the gaps. The facilitated recruiting community growing under dead and living canopy showed the highest disequilibrium, followed by the community growing under mid-affected canopy. The climatic disequilibrium of the recruiting communities experiencing neutral and inhibited interaction was not affected by canopy defoliation. These findings indicate that the climatic disequilibrium of the recruiting community is determined by the facilitation-competition balance. Living canopy provides climatic buffering, but it also implies competition, while dead canopy may provide some structural climatic buffering, without implying competition for resources. These results highlight the relevance of incorporating plant-plant interactions, particularly facilitation, to better forecast plant community responses to extreme climate events and climate change.