Local tree-tree interactions are crucial for understanding biodiversity-ecosystem functioning relationships in forests. The complementary use of the canopy space increases canopy packing, thereby promoting community productivity. Several studies have measured theoretical volumetric complementarity using the crown complementarity index (CCI). Despite its effectiveness in quantifying vertical stratification, CCI does not account for tree distance and interacting crown volume. Here, we present the crown overlap index (COI). Based on the concepts of crown overlap and distance weighting, the COI quantifies the interacting crown volume within a distance threshold from 3D laser scanning data. We calculated CCI and COI for 136 tree pairs of the BEF-China experiment and found a negative correlation between the two indices. At high CCI values, COI was consistently low; at lower CCI values, however, COI varied greatly, suggesting that the COI complements the CCI in describing complementarity holistically. Dissimilarity in leaf functional traits and tree structural complexity increased CCI and decreased COI, indicating increased vertical stratification, but limited crown overlap. Similar patterns were observable in a dataset of mature, unmanaged beech forests. We attribute these patterns to differences in crown plasticity, which to some extent, were correlated with leaf trait dissimilarity and a more variable crown organization. Crown shyness, a phenomenon closely related to crown plasticity might also have affected the observed effects. Overall, the COI in conjunction with the CCI provide a more detailed characterisation of physical tree-tree interactions. Thereby, the COI could serve as a promising index for deepening our understanding of forest spatial structure.
Figure 1. Conceptual figure.
Figure 2. Generalised linear mixed effects model (GLMM) partial effect of the crown complementarity index (CCI) on the crown overlap index (COI).
Figure 3. Generalised linear mixed effects model (GLMM) partial effects of A: The leaf trait dissimilarity of tree species pairs (LTD) on the crown complementarity index (CCI); B: The TSP structural complexity dissimilarity (SCD) on the CCI; C: The LTD on the crown overlap index (COI); D: The SCD on the COI.
Literature:
Alexander Täuber*, Andreas Fichtner, Helge Bruelheide, Andréa Davrinche, Sylvia Haider, Tobias Käcks, Xiaojuan Liu, Tama Ray, Goddert von Oheimb. 2026. Tree-tree interactions revisited – A refined approach to quantify spatial complementarity in tree crowns. Ecological Indicators. 191:115648. https://www.sciencedirect.com/science/article/pii/S1470160X26010502?via%3Dihub
