Self-pruning in tree crowns is influenced by functional strategies and neighborhood interactions

As canopy closure causes forest stands to face increasing light limitation, trees lower branches begin to die back. This process, called self-pruning, defines a crowns base and depth and shapes the structure of entire stands. Self-pruning is often thought to occur after shading causes individual branches to transition from net carbon sources to sinks. Under this explanation, we would expect resource-conservative and shade-tolerant species to initiate self-pruning under deeper shade because their branches need less light to maintain a positive carbon balance. However, the notion that branches are fully autonomous may be complicated by correlative inhibition, in which plants 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 plot-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 characteristics of crown size and position as well as the light level at the crown base (denoted Lbase), which we took as an estimate of the light threshold of self-pruning. As we predicted, more shade-tolerant and resource-conservative species self-pruned at a deeper level of shade (lower Lbase). In addition, most species had higher Lbase when they had more light at the crown top, suggestive of correlative inhibition. With respect to their neighbors traits, though, conservative and acquisitive species showed contrary patterns of plasticity: conservative species had lower Lbase around conservative neighbors, while acquisitive species had lower Lbase around acquisitive neighbors. However, all species declined in crown depth when they grew alongside larger, more acquisitive neighbors. As predicted, plots with a greater interspecific diversity of Lbase had greater basal area and crown volume. Using simulations, we showed that plasticity in crown depth between monocultures and mixtures strengthened the relationship with crown volume, primarily due to competitive release experienced by acquisitive species. By placing shade-induced self-pruning in a comparative context, we clarify how forest function emerges from competition for light between individual trees.