Ecosystem Physiology, Faculty of Environment and Natural Resources, University Freiburg, Freiburg, Germany.
Forest Growth and Dendroecology, Faculty of Environment and Natural Resources, University Freiburg, Freiburg, Germany.
Plant Biol (Stuttg). 2022 Dec;24(7):1186-1197. doi: 10.1111/plb.13455. Epub 2022 Aug 4.
The occurrence of hot drought, i.e. low water availability and simultaneous high air temperature, represents a severe threat to ecosystems. Here, we investigated how the 2018 hot drought in Central Europe caused a tipping point in tree and ecosystem functioning in a Scots pine (Pinus sylvestris L.) forest in southwest Germany. Measurements of stress indicators, such as needle water potential, carbon assimilation and volatile organic compound (VOC) emissions, of dominant P. sylvestris trees were deployed to evaluate tree functioning during hot drought. Ecosystem impact and recovery were assessed as ecosystem carbon exchange, normalized difference vegetation index (NDVI) from satellite data and tree mortality data. During summer 2018, needle water potentials of trees dropped to minimum values of -7.5 ± 0.2 MPa, which implied severe hydraulic impairment of P. sylvestris. Likewise, carbon assimilation and VOC emissions strongly declined after mid-July. Decreasing NDVI values from August 2018 onwards were detected, along with severe defoliation in P. sylvestris, impairing ecosystem carbon flux recovery in 2019, shifting the forest into a year-round carbon source. A total of 47% of all monitored trees (n = 368) died by September 2020. NDVI recovered to pre-2018 levels in 2019, likely caused by emerging broadleaved understorey species. The 2018 hot drought had severe negative impacts on P. sylvestris. The co-occurrence of unfavourable site-specific conditions with recurrent severe droughts resulted in accelerated mortality. Thus, the 2018 hot drought pushed the P. sylvestris stand towards its tipping point, with a subsequent vegetation shift to a broadleaf-dominated forest.
高温干旱的发生，即低水资源可用性和同时的高温，对生态系统构成了严重威胁。在这里，我们研究了 2018 年中欧的高温干旱如何导致德国西南部一片苏格兰松（Pinus sylvestris L.）林的树木和生态系统功能发生 tipping point。我们部署了对优势树种苏格兰松的应激指标（如针叶水势、碳同化和挥发性有机化合物（VOC）排放）的测量，以评估树木在高温干旱期间的功能。通过卫星数据的生态系统碳交换、归一化差异植被指数（NDVI）和树木死亡率数据来评估生态系统的影响和恢复。在 2018 年夏季，树木的针叶水势下降到-7.5 ± 0.2 MPa 的最小值，这意味着苏格兰松的水力损伤严重。同样，碳同化和 VOC 排放也在 7 月中旬后大幅下降。从 2018 年 8 月开始，检测到 NDVI 值下降，同时苏格兰松严重落叶，阻碍了 2019 年生态系统碳通量的恢复，使森林成为全年的碳源。到 2020 年 9 月，共有 47%（n=368）的监测树木死亡。2019 年 NDVI 恢复到 2018 年之前的水平，这可能是由于新兴的阔叶林下物种的出现。2018 年的高温干旱对苏格兰松造成了严重的负面影响。不利的特定地点条件与反复出现的严重干旱同时发生，导致死亡率加速。因此，2018 年的高温干旱使苏格兰松林接近 tipping point，随后植被向以阔叶树为主的森林转变。
