Functional diversity of neighbours mediates sap flow density and radial growth of focal trees, but in different ways between evergreen and deciduous broadleaved species

The functioning of a tree is shaped by the neighbouring species through the interspecific interaction and local environments. The functional trait composition of the neighbourhood could provide mechanistic insights into the effects of neighbours on the resource strategies of focal trees. In this study, we deployed an automated high‐frequency measurement of the sap flow density (SFD) and the radial growth of 48 trees of 12 species at BEF‐China, a large‐scale forest biodiversity manipulation experiment, to investigate the consequences and underlying mechanisms of the functional trait composition of the neighbourhood on the sap flow and radial growth of focal trees. We found a positive relationship between SFD and growth, reflecting the important supportive role of sap flow in tree growth. High functional diversity (FD) of the neighbourhood depressed SFD but promoted growth when considering all species, and thus promoted water‐use efficiency. Acquisitiveness of neighbouring trees positively affected growth, suggesting interspecific facilitation. Furthermore, neighbourhood FD benefits evergreen focal trees by promoting growth. However, in deciduous focal trees, neighbourhood FD reduced SFD but had no significant effects on growth. Our findings suggest that considering the functional trait composition of neighbourhood communities will support effective afforestation and forest management. Read the free Plain Language Summary for this article on the Journal blog.