Forest composition and diversity buffer microclimates and enhance productivity

Abstract Tree diversity may play a role in buffering forest ecosystems against climate extremes. We asked how diverse tree communities could (1) influence local atmospheric water demand, and (2) how changes in microclimate could be linked to the productivity of trees within forest communities. To assess forest canopy characteristics and calculate above‐ground productivity, we measured plant fractional cover using UAV‐LiDAR and conducted growth inventories in 148 forest plots of a young temperate tree diversity experiment. We monitored the microclimate and tracked soil moisture in a subset of forest plots to understand the relative influence of above‐ground and below‐ground water status on tree growth. Gas exchange, spectral reflectance and leaf mass per area were measured on four tree species distributed throughout the experiment to assess the influence of microclimate on tree physiology. We found that fast‐growing, early successional tree species increased forest cover, which was linked to buffered daily amplitudes of vapour pressure deficits (VPD) within forest plots and increases in the annual wood production of slower‐growing, shade‐tolerant tree species. In assemblages with more buffered microclimates, several slower‐growing tree species showed increased leaf water content and reduced physiological stress. Plots with reduced amplitude in VPD exhibited greater complementarity and productivity relative to those with higher variability. Synthesis . Our findings show that tree diversity and the proportion of fast‐growing, early successional tree species can increase canopy cover and buffer the microclimates of young forests, benefiting the physiology and productivity of slower‐growing, shade tolerant tree species. We reason that a self‐reinforcing dynamic of tree growth and microclimate buffering is likely at play. In the context of a rapidly changing climate, our findings suggest that diverse forest communities can shape local microenvironments to support sustainable ecosystem functioning.