Structurally complex forests are more resilient to extreme droughts

The increasingly frequent and severe droughts caused by global warming is threating the forest ecosystem health with pervasive tree mortality. Canopy Structure is one of the important factors that regulating drought-induced tree mortality. However, how tree structural influences the spatial and temporal patterns of tree mortality during droughts remains controversial. Through an analysis of nearly 1.5 million trees during the 2012-2016 drought in California, USA, we found tree mortality first decreased with height for small trees, then increased with tree height in the middle sized trees, and decreased again with tree height for matured big trees. We also found relative tree canopy size compared to neighboring trees demonstrates a consistent negative relationship with tree mortality across species. This new finding may be explained by the fact that trees in a structurally complex forest with tall neighboring trees may have higher crown shadow ratio and less water loss to evapotranspiration during the drought. Therefore, the relatively smaller trees in a structurally complex forest have higher survival rate even during an extreme drought. Our findings suggest that a new forest management strategy that re-establishes heterogeneity in tree species and forest structure could improve forest resiliency to severe and extended droughts.