Functionally dissimilar neighbors improve tree water use efficiency through increases in leaf phosphorus concentration

Water use efficiency (WUE) is central to the global cycles of water and
carbon. However, whether increasing tree diversity in plantation can
increase WUE remain poorly understood. Here, we conduct a forest
biodiversity experiment with 32 tree species spread in 14 ha in
subtropical China to assess the effects of neighboring tree diversity on
foliar WUE of Cunninghamia lanceolata, a widespread tree plantation
species in China. We measure foliar δ13C as the proxies of changes in
intrinsic WUE. Folia P concentrations of focal trees increase with trait
dissimilarities between focal trees and neighbors, and the increased
foliar P concentrations improve foliar WUE of focal trees. This
neighborhood complementarity effect on WUE is stronger under more shaded
neighborhood. However, neighborhood biodiversity did not significantly
affect foliar δ18O, a surrogate for stomatal
conductance. These findings suggest that tree biodiversity increases WUE
through the complementary usages of soil P between neighboring tree
species.