The response of canopy height diversity to natural disturbances in two temperate forest landscapes

Context: Structural diversity strongly influences habitat quality and the functioning of forest ecosystems. An important driver of the variation in forest structures are disturbances. As disturbances are increasing in many forest ecosystems around the globe, it is important to understand how structural diversity responds to (changing) disturbances. Objectives: Our aim was to quantify the relationship between forest disturbances and structural diversity with a focus on diversity in canopy height. Methods: We assessed diversity in canopy height for two strictly protected Central European forest landscapes using lidar data. We used a multi-scale framework to quantify within-patch (α), between-patch (β), and overall (γ) diversity. We then analysed the variation in canopy height diversity over an extensive gradient of disturbance rates. Results: Diversity in canopy height was strongly driven by disturbance rate, with highest overall diversity between 0.5 and 1.5% of the forest area disturbed per year. The unimodal responses of overall diversity to disturbance emerged from contrasting within- and between-patch responses, i.e., a decrease in within-patch diversity and an increase in between-patch diversity with increasing disturbance. This relationship was consistent across study landscapes, spatial scales, and diversity indicators. Conclusion: The recent wave of natural disturbances in Central Europe has likely fostered the structural diversity of forest landscapes. However, a further increase in disturbance could result in the crossing of a tipping point (at ~ 1.5% of forest area disturbed per year), leading to substantial structural homogenization.