Effects of forest management on the dynamics of forest structural complexity and their impacts on forest microclimate

Many current management approaches in temperate European forests aim to promote forest structural complexity. Yet, the dynamics of three-dimensional forest structure under different silvicultural systems and their impacts on forest microclimate remain unexplored. Here, we quantified the temporal dynamics of forest structural complexity in differently managed and unmanaged European beech ( Fagus sylvatica L.) forests (n=100) over nine years by using terrestrial laser scanning (TLS)-derived measures. We then tested whether changes in forest structural complexity are driven by forest management intensity and if the observed changes in three-dimensional forest structure drive changes in forest microclimate, quantified by the diurnal temperature range (DTR). Overall, we observed a net increase in forest structural complexity of 24.76% across all forest management systems and developmental stages. We found a delayed but positive effect of harvested basal area on forest structural complexity change, which we attributed to improved growing conditions for under- and midstory trees following silvicultural interventions. Compared to the managed forests, unmanaged forests showed little changes in forest structural complexity due to the absence of harvesting or natural disturbances. Ultimately, we observed that the dynamics of three-dimensional forest structure trigger changes in DTR, whereby the effect size was small in comparison with the effect of weather conditions. We conclude that silvicultural interventions accelerate the structural dynamics in European beech forests and may promote the development of complex forest structures in the long term. While these management-related changes in three-dimensional forest structure also lead to changes in DTR, their relevance for mediating DTR is relatively small compared to macroclimate changes.