Decoupling of tree height and root depth across the globe and the implications for tree mortality during drought events

Abstract

Tree height (Htree) and root depth (Droot) are the two most critical functional traits for plant ecological strategies. However, the global varying patterns of Htree and Droot and the ecological significance for tree survival under changed climate conditions are less understood. Here, we compiled Htree, Droot, and six root traits related to soil resource absorption for 301 field-grown tree species distributed 776 sites worldwide. The relationships between Htree and Droot across different climate conditions and the implications for tree mortality during drought events were investigated. We found Htree and Droot were often decoupled in angiosperms and gymnosperms among different climate conditions. Greater Htree was prone to occur in warm and moist climates, while greater Droot tended to occur in warm and dry climates with low soil water availability. Phylogenetic analysis revealed that trees have evolved towards smaller Htree but deeper Droot from the deep time. Although dense root structures to some extent contributed to sustaining the resource supply–demand balance of large trees with shallow roots (great Htree/ Droot), the large trees with greater Htree/ Droot were more vulnerable to drought-induced mortality. The findings suggest that the linkage of below- and above-ground reaches has a pivotal role in understanding tree mortality during drought events. The insight will allow more accurate vegetation model prediction for tree sorting processes under climate change scenarios.