Light, temperature and understorey cover predominantly affect early life stages of tree seedlings in a multifactorial mesocosm experiment

Increasing light availability by opening up the forest canopy is a key tool for forest managers to stimulate natural regeneration of trees. Tree seedlings are also impacted by a complex set of global change drivers and understorey vegetation. Here, we investigated if (altered) environmental resources and conditions due to global change, and understorey vegetation modulate tree seedling responses, i.e. emergence, survival, growth and biomass allocation, to enhanced light availability. Seeds and seedlings of three species (Quercus robur, Fagus sylvatica and Actr pseudoplacanus) were planted in a multifactorial experiment. Understorey communities were grown in 384 mesocosms on soils from ancient and post-agricultural forest, forested before 1850 and after 1950, respectively. The mesocosms were exposed to two-level full-factorial treatments of light addition, warming and nitrogen enrichment. We measured understorey vegetation cover and height, scored tree seedling emergence and seed predation, measured seedling survival during two growing seasons and measured growth after two growing seasons. We found that emergence and early survival formed critical bottlenecks for seedling establishment. Seed predation formed a barrier to emergence, while cover of understorey vegetation reduced seedling survival. The life stages of the three species were varyingly affected by the different treatments, but predominantly by light addition and warming in a positive way. We found few significant interactions between light and the other treatments, presenting limited evidence of the other drivers modulating seedling responses to increased light availability. With our experiment, we were able to further disentangle the confounding effects of light availability and warming on seedling performance. Our results confirm that managers likely facilitate tree regeneration when opening the canopy by increasing both light availability and temperature, even by slight amounts, at the forest floor. However, managers need to be aware of unintended consequences of their actions as too great an increase in understorey vegetation cover may be detrimental to seedling establishment