The effect of forest structure on drought stress in beech forests (Fagus sylvatica L.)

Abstract

The unprecedented drought between 2018 and 2020 had a significant impact on European beech (Fagus sylvatica L.) forests in Central Europe. The role of different forest structures in mitigating drought stress remains controversial. This contentious debate prompted our study, in which we aimed to quantify the effect of forest structure on drought stress in beech forests in two ecoregions of northern Bavaria, Germany. Using a mobile laser scanner, we surveyed 240 plots in drought-stressed forests. We analyzed the responses of beech trees to the drought period through radial growth, wood-derived δ13C signal, and crown defoliation. Results revealed significant responses of beech forests in both regions to the drought event, including increased crown defoliation, reduced tree growth, and altered δ13C signatures compared to pre-drought conditions. Our results show a relationship between crown closure and crown defoliation in beech, suggesting an increased vulnerability of beech to drought in more open canopies. However, the potential for silvicultural intervention to mitigate drought stress, as measured by BAI and δ13C signal, appears limited. Neighboring trees and forest structure had little influence on average drought resistance. The δ13C signal showed minimal responsiveness to variations in canopy openness, as well as to distinctions between single and multi-layered forests. However, increased structural complexity within stands tended to increase resistance due to the compensatory effects of understory trees. Future forest management strategies could focus on promoting structural diversity, selecting resilient individuals but also actively enrich the forests with more drought-adapted species to increase the adaptive capacity of beech-dominated forests in the face of changing climate conditions.