Management alters drought-induced mortality patterns in European beech (Fagus sylvatica L.) forests

The high tree mortality during the dry and hot years 2018-2019 in Europe affected not only even-aged and monospecific conifer stands but also semi-natural Fagus sylvatica L. (European beech) forests. This has triggered concerns on the future of European beech forests under climate change and raised questions as to whether forest management may increase tree mortality rates through creating more open canopies. We compared long-term mortality rates of beech between strict forest reserves and adjacent managed reference stands for three inventory periods at 11 sites in the federal state Hesse, Germany, near the centre of the European distribution of the species. We hypothesized that mortality would increase with climatic water deficits during the growing season, initial stand density, and decreasing dominance (canopy status) of trees. We also hypothesized that mortality would decrease with site moisture status and the intensity of tree removals. To quantify the influence of management, initial stand density and tree removals were used as predictor variables. In addition, we analyzed the influence of the climatic water balance and competitive status of trees on mortality. Mean annual natural mortality rates ranged between 0.5 % and 2.1 %. Even in the drought years beech mortality rates were surprisingly low. We observed no signs of striking canopy disintegration. The significantly higher mortality (1.6 % - 2.1 %) in unmanaged stands during the drought years 2018 and 2019 was largely confined to suppressed trees. There was no significant increase of mortality in managed stands during the drought years, but a shift of mortality towards larger canopy trees. Our study did not confirm a general influence of management in the form of tree removals on mortality rates. Yet, we showed that during drought years management changed the distribution of mortality within the tree community. To analyze the effect of management on mortality rates more comprehensively, a wider gradient in site moisture conditions including sites drier than in this study and longer post-drought periods should be employed.

2018-2019年欧洲遭遇干旱炎热年份期间，高树木死亡率不仅波及同龄单优针叶林分，还影响到半自然的欧洲山毛榉（European beech，Fagus sylvatica L.）林。这引发了学界对气候变化背景下欧洲山毛榉林未来的担忧，同时也提出了疑问：森林经营是否会通过营造更开阔的林冠层而提高树木死亡率？本研究在该物种欧洲分布区中心附近的德国黑森州11个样地中，针对3个清查周期，对比了严格保护林与邻近经营参照林分的山毛榉长期死亡率。我们提出如下假说：树木死亡率将随生长季气候水分亏缺、初始林分密度以及林木优势度（冠层地位）的降低而升高；同时我们还提出另一假说：死亡率将随立地水分状况以及林木采伐强度的提升而降低。为量化经营活动的影响，本研究将初始林分密度与林木采伐强度作为预测变量。此外，本研究还分析了气候水分平衡与林木竞争地位对死亡率的影响。年平均自然死亡率介于0.5%至2.1%之间。即便在干旱年份，山毛榉死亡率仍处于较低水平，这一结果出人意料。未观测到林冠出现显著崩解的迹象。2018-2019年干旱年份，未经营林分的死亡率（1.6%~2.1%）显著更高，且该类死亡个体主要集中于被压木。经营林分在干旱年份的死亡率未出现显著上升，但死亡个体向冠幅更大的林木偏移。本研究未证实以林木采伐为形式的经营活动对死亡率存在普遍影响。但本研究表明，干旱年份的经营活动会改变林木群落内的死亡率分布格局。若要更全面地分析经营活动对死亡率的影响，需纳入比本研究更广泛的立地水分条件梯度（包括比本研究更干旱的样地）以及更长的干旱后监测周期。