Tree light capture and spatial variability of understory light increase with species mixing and tree size heterogeneity

Mixed and multi-layered forest ecosystems are sometimes more productive than monospecific and single-layered ones. It has been suggested that trees of different species and sizes occupy complementary positions in space which would act as a mechanism to increase canopy light interception and wood production. However, greater canopy light interception reduces the average amount and variability of transmitted radiation offering fewer opportunities for all species to regenerate and to maintain forest heterogeneity in the long-run. We investigated whether increasing overstory heterogeneity indeed results in greater canopy light interception and lower variability in transmittance. We modeled the three-dimensional structure of forest stands with 3 typical forest structures, 10 mixtures of four tree species, and 3 different basal areas. We used the forest light interception model SAMSARALIGHT and performed three-way analyses of covariance to analyze the effects of the three varied components of forest heterogeneity. We found no evidence that increasing heterogeneity increases canopy light interception. In contrast, homogeneous stands intercept more light than heterogeneous stands. Variability in transmittance increased in some cases with compositional heterogeneity and, to a lesser extent, with tree size inequalities. The advantage of heterogeneous forests is in opportunities for natural regeneration rather than in opportunities to enhance canopy light interception.