Crown structure and competitive interactions in mixed forests: Insights from an individual-based model

Conifers generally exhibit narrow, deep crowns, whereas broadleaf trees typically form spherical crowns. A widely accepted hypothesis attributes this difference to variations in solar angles: conifers, which prevail in high-latitude regions with lower solar angles, optimize light capture differently than broadleaf trees, which dominate low-latitude areas with higher solar angles. Previous studies have suggested that differences in crown morphology mitigate light competition in mixed forests, facilitating coexistence and enhancing productivity. However, these studies relied on simplified structural models that did not fully account for the physiological constraints of crown morphology or the dynamics of crown competition. In this study, we employed the Spatially Explicit Individual-Based Dynamic Global Vegetation Model to examine the effects of crown morphology on competition dynamics and ecosystem productivity in mixed forests. The model incorporates novel elements: (1) competition for space during canopy expansion, (2) self-pruning due to shading (i.e., lower branch dieback), (3) reductions in crown basal area resulting from self-pruning, and (4) reductions in total leaf area per tree following basal area loss. A 100-year simulation of narrow- and wide-crown saplings with distinct crown morphologies revealed that their relative advantages depended on tree density, planting arrangement, solar angle, and the composition of solar radiation (i.e., direct vs. diffuse light). However, contrary to prior assumptions, negative frequency-dependent selection—expected to promote coexistence—was not observed. Moreover, crown shape diversity did not enhance forest productivity. These findings challenge previous models and suggest that factors beyond crown morphology may drive species coexistence and ecosystem productivity in mixed ecosystems.