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

Structurally heterogeneous forest ecosystems are generally considered to be more productive, resistant and resilient to future changes than simplified ones. The quantity of solar radiation available to trees and the vegetation layer has been suggested to be a central driver among the possible mechanisms proposed to explain a greater productivity of heterogeneous stands. The idea that trees of different species and sizes can occupy complementary positions in space has led to the hypothesis that interception of radiation could increase in mixed and multi-layered forest stands, which should thus lead to an increase in production. In addition, the long-term sustainability of structurally heterogeneous forests with multiple coexisting species can only be ensured if the variability in light availability in the understory is sufficient for all species to be able to survive, grow and reproduce. However, better occupancy of canopy space should lead to reduced variability in understory light conditions. In this study, we investigate whether increasing overstory heterogeneity results in greater light capture by trees and greater variability in understory light. Greater light capture has been suggested as a possible explanation for the overyielding of complex stands and the greater variability in understory light as a mechanism for continuous recruitment of juvenile trees of various species. Based on allometric relationships developed in previous studies, we modeled the three-dimensional structure of forest stands (50 x 50 m) with 3 typical forest structures (single-layered, multi-layered and reverse j-shaped), 10 forest compositions combining beech, oak, fir and pine, 3 levels of basal area, and a fixed mean tree diameter (Figure 1). We predicted understory light conditions using the forest light interception model SAMSARALIGHT. The importance of each factor on understory light availability and variability was analyzed with three-way analyses of covariance. We found no evidence that increasing compositional or structural heterogeneity reduces understory light transmittance or increases tree light capture. Understory light transmittance in mixed stands was always intermediate between that is corresponding pure stands of the species involved in the mixture. Moreover, light transmittance slightly increased with increasing heterogeneity in forest structure. Stands composed of a single homogeneous tree layer transmitted less light than stands with two or more tree layers or stands composed of two or more species. Stand composition had the strongest influence on understory light in our simulations. Stand density was the second most important factor while heterogeneity in tree size had little effect. Compositional heterogeneity increased the variability in understory light conditions in some cases such as in fir-pine mixtures but adding beech to mixtures with oak generally limited variability in understory light, except in stands with very low basal area. Size inequality therefore increased understory light variability little and mainly in pure stands. The advantage of heterogeneous forest stands seems to lie in opportunities for the natural regeneration of various species to benefit from a diversity of light conditions in the understory rather than in opportunities to enhance light capture by the overstory although this effect depends on tree species composition