How do trees fail in intraspecific competition? A test for the roles of non-structural carbohydrates and stoichiometries in Pinus massoniana.

Competition is ubiquitous and an important driver of tree mortality. Non-structural carbohydrates (NSCs, including soluble sugars and starch) and C-N-P stoichiometries are affected by the competitive status of trees and, in turn, physiologically determine tree growth and survival in competition. However, the physiological mechanisms behind tree mortality caused by intraspecific competition remain unclear. Here, we ask how the performance (growth vigour) of trees in intraspecific competition relates to NSC and C-N-P stoichiometry traits. Through the field surveys at neighbourhood levels, we demonstrated that competition is responsible for tree mortality in an even-aged Pinus massoniana forest. The whole NSCs and C-N-P stoichiometries of trees in different growth vigour classes (i.e., flourishing, moderate, and dying) were then analysed to elucidate how trees fail in competition. We found that (1) the concentrations of NSCs and their components in stems, coarse roots and fine roots were constant across tree growth vigour classes, but were significantly lower in the leaves, twigs and branches of moderate and dying trees than those of flourishing trees, and (2) the C, N and P concentration and their respective ratios were constant in all the tissues across tree growth vigour classes, but the nitrogen stoichiometric homeostasis index (H_N) of flourishing trees was significantly higher than that of moderate and dying trees. The results demonstrated that both carbohydrate deficiency and low stoichiometric homeostasis are potential physiological drivers underlying tree mortality caused by intraspecific competition. This study also emphasizes the importance of considering stoichiometric homeostasis in research on tree competition and forest dynamics.