Self‐pruning in tree crowns is influenced by functional strategies and neighbourhood interactions

As canopy closure imposes light limitation in forests, the dieback of trees' lower branches, known as self‐pruning, defines their live crown base and shapes the structure and function of entire stands. Self‐pruning is often thought to occur after shading causes individual branches to transition from carbon sources to sinks. Under this assumption, we would expect resource‐conservative and shade‐tolerant species to initiate self‐pruning under deeper shade because their branches require less light to maintain a positive carbon balance. However, this pattern may be complicated by ‘correlative inhibition,’ which may cause plants to preferentially allocate resources towards sunlit branches. Consistent with this idea, we predicted that within species, trees with sunlit tops would initiate self‐pruning at a higher light threshold. Lastly, we predicted that community‐level diversity in self‐pruning strategies would correlate with productivity and total crown volume. We tested these predictions in an experiment where 12 temperate tree species were planted in plots of varying diversity and composition. We measured crown dimensions and position as well as the fraction of light reaching the crown base (denoted L base ), which we took as an estimate of the light threshold of self‐pruning. As predicted, shade‐tolerant and resource‐conservative species self‐pruned at a deeper level of shade (lower L base ). Within species, trees generally had higher L base when they had more light at the crown top, suggestive of correlative inhibition. With respect to their neighbours' traits, though, conservative and acquisitive species showed contrary patterns of plasticity: conservative species had lower L base around conservative neighbours, and acquisitive species around acquisitive neighbours. As predicted, plots with a greater diversity of L base had greater basal area and crown volume. Using simulations, we showed that adjustment of crown depth from monocultures to mixtures strengthened the relationship between diversity of L base and crown volume, primarily due to competitive release that benefited acquisitive species. We provide evidence that self‐pruning strategies are intimately connected to resource acquisition strategies and propose that L base may serve as a functional trait to quantify them. Our results reinforce the role of tree architectural diversity in the functioning of light‐limited forests. Read the free Plain Language Summary for this article on the Journal blog.