Maintaining the coexistence of tree species of different shade tolerances with uneven-aged silviculture

Maintaining the coexistence of multiple tree species is one of the greatest issues of uneven-aged silviculture as it is of great interest, for example, to maintain biodiversity or increase resilience to global changes. In many places, while forest managers have successfully maintained complex stand structure with uneven-aged silviculture, they have often faced difficulties in maintaining some desired species mixtures. Naturally, the composition of uneven-aged forests evolves so that a few species successfully regenerate and suppress the others. Since a continuous canopy cover is usually maintained by uneven-aged silviculture, when nutrients and moisture are not limiting, the availability of light in the understory is the driving factor of natural regeneration dynamics. The amount of transmitted radiation determines, at least partly, the composition of the regeneration layer because of interspecific differences in growth and survival in shade. In theory, shade-tolerant species generally dominate the regeneration under closed canopies whereas less shade-tolerant species dominate the regeneration under partially open canopies. We hypothesize that, along a gradient of light availability, shade-tolerant species suppress less shade-tolerant species in understory with low availability of light whereas the opposite situation occurs in understory with high availability of light. In addition, we expect that forest managers can effectively control understory light with appropriate modifications of forest structure and density. We examined mixtures of sessile oak (Quercus petraea (Matt.) Liebl.) and European beech (Fagus sylvatica L.) in the Belgian Ardennes. We selected 23 sites with favorable water and nutrient supply. Regeneration areas (100-6500m²) were fenced off to protect them from browsing by deer. Saplings were sampled in 241 subplots and their height growth was annually monitored during two years. Understory light transmittance was measured in each subplot using hemispherical photography. The height growth of beech and oak saplings was modeled with mixed non-linear models. In order to simulate various silvicultural treatments, we used a model of light interception by heterogeneous canopies (SAMSARALIGHT). We simulated selective thinnings of 5 different types: harvesting preferentially small trees, large trees, or trees of shade-tolerant species or creating circular gaps. Understory light was found to be a key parameter in the dynamics of uneven-aged stands, as it affects regeneration growth and composition. There were interspecific differences in growth response to light availability. The optimum height growth of beech and oak regenerations were reached at 10% and 20% of transmittance, respectively. Our simulations highlighted that various silvicultural treatments can effectively be used to control understory light as long as harvest intensity is adapted to the chosen strategy (Figure 1). The results of this study also underscore the problem that even under good light conditions (transmittance > 20 %), regeneration of less shade-tolerant species might not overcome the regeneration of shade-tolerant species. Contrary to our hypothesis, we found no rank reversal of the height growth of the two study species along a wide light gradient. Consequently, maintaining less shade-tolerant species in stands with shade-tolerant species might require silvicultural interventions jointly in the overstory and regeneration layers