Karlsruhe Institute of Technology, Institute of Meteorology and Climate Research-Atmospheric Environmental Research (KIT/IMK-IFU), Kreuzeckbahnstraße 19, 82467 Garmisch-Partenkirchen, Germany.
Tree Physiol. 2022 Aug 6;42(8):1532-1548. doi: 10.1093/treephys/tpab141.
Forests are increasingly affected by heatwaves, often co-occurring with drought, with consequences for water and carbon (C) cycling. However, our ability to project tree resilience to more intense hot droughts remains limited. Here, we used single tree chambers (n = 18) to investigate transpiration (E), net assimilation (Anet), root respiration (Rroot) and stem diameter change in Scots pine seedlings in a control treatment and during gradually intensifying heat or drought-heat stress (max. 42 °C), including recovery. Alongside this, we assessed indicators of stress impacts and recovery capacities. In the heat treatment, excessive leaf heating was mitigated via increased E, while under drought-heat, E ceased and leaf temperatures reached 46 °C. However, leaf electrolyte leakage was negligible, while light-adapted quantum yield of photosystem II (F'v/F'm) declined alongside Anet moderately in heat, but strongly in drought-heat seedlings, in which respiration exceeded C uptake. Drought-heat largely affected the hydraulic system as apparent in stem diameter shrinkage, declining relative needle water content (RWCNeedle) and water potential (ΨNeedle) reaching -2.7 MPa, alongside a 90% decline of leaf hydraulic conductance (KLeaf). Heat alone resulted in low functional impairment and all measured parameters recovered quickly. Contrary, following drought-heat, the recovery of KLeaf was incomplete and stem hydraulic conductivity (KS) was 25% lower than the control. However, F'v/F'm recovered and the tree net C balance reached control values 2 days post-stress, with stem increment rates accelerating during the second recovery week. This indicates a new equilibrium of C uptake and release in drought-heat seedlings independent of hydraulic impairment, which may slowly contribute to the repair of damaged tissues. In summary, Scots pine recovered rapidly following moderate heat stress, while combined with drought, hydraulic and thermal stress intensified, resulting in functional damage and slow recovery of hydraulic conductance. This incomplete hydraulic recovery could critically limit evaporative cooling capacities and C uptake under repeated heatwaves.
森林越来越容易受到热浪的影响，热浪通常与干旱同时发生，这对水和碳（C）循环有影响。然而，我们预测树木对更强烈的热干旱的适应能力仍然有限。在这里，我们使用单个树木室（n=18）来研究控制处理和逐渐加剧的热或干旱-热胁迫（最高 42°C）期间，辐射松幼苗的蒸腾作用（E）、净同化（Anet）、根呼吸（Rroot）和茎直径变化，包括恢复。与此同时，我们评估了压力影响和恢复能力的指标。在热处理中，通过增加 E 来缓解叶片过热，而在干旱-热胁迫下，E 停止，叶片温度达到 46°C。然而，叶片电解质渗漏可以忽略不计，而光适应的光合作用系统 II 量子产量（F'v/F'm）在热胁迫下适度下降，但在干旱-热胁迫下强烈下降，其中呼吸作用超过了 C 的吸收。干旱-热胁迫主要影响了水力系统，表现为茎直径收缩、相对针叶水含量（RWCNeedle）和水势（ΨNeedle）下降到-2.7 MPa，同时叶片水力导度（KLeaf）下降了 90%。单独的热胁迫导致的功能损伤较小，所有测量的参数都能迅速恢复。相反，在干旱-热胁迫后，KLeaf 的恢复不完全，茎水力导率（KS）比对照低 25%。然而，F'v/F'm 恢复，树木净 C 平衡在应激后 2 天达到对照值，茎增量率在第二个恢复周内加速。这表明在干旱-热胁迫下，树木吸收和释放 C 的新平衡与水力损伤无关，这可能会缓慢地促进受损组织的修复。总之，辐射松在中度热胁迫后迅速恢复，而与干旱相结合，水力和热胁迫加剧，导致功能损伤和水力导率的缓慢恢复。这种不完全的水力恢复可能会严重限制在反复热浪下的蒸发冷却能力和 C 的吸收。
