COS 301-4 - Functional strategies and neighborhood interactions influence self-pruning in tree crowns

Abstract: The functioning of forests is influenced by the three-dimensional structure that emerges from the interactions between the crowns of individual trees. One of the processes that influences a tree’s shape is self-pruning, or the controlled shedding of branches. Self-pruning is most frequent among the oldest and lowest branches of the crown, which may come to have a net negative carbon balance as canopy growth causes shade to deepen. In this project, we examined what factors determine the point below which self-pruning occurs. We carried out measurements in IDENT-Montréal, an experiment designed to test the influence of biodiversity on tree community function; twelve species were planted densely into plots of varying species composition and diversity. During the experiment’s tenth growing season, we quantified self-pruning strategies by measuring the height and the fraction of light at the crown base of trees in neighborhoods composed of different species. Based on the assumption that shade-tolerant and resource-conservative species have a lower light compensation point, we conjectured that they would self-prune at a lower light threshold. We also sought to determine whether neighborhood-level factors, including functional diversity, contribute to intraspecific variation in height and light at the crown base.



As we predicted, the mean light threshold below which tree species self-prune was correlated negatively with shade tolerance (R2 = 0.351; p = 0.025) and a trait-based index of resource-conservatism (R2 = 0.642; p = 0.001). Unlike the height of the crown base, the light threshold remained fairly insensitive to neighborhood characteristics. However, trees did tend to self-prune at a lower light threshold when surrounded by trees of similar resource-conservatism. Given that resource-conservative species also grew less quickly, trees thus also self-pruned at lower light thresholds when their neighbors were similar in size to themselves. Our ongoing work aims to link this dynamic to the emergence of canopy complementarity, which is an important mechanism through which biodiversity influences ecosystem function.