Department of Forest Botany, Dendrology and Geobiocoenology, Mendel University in Brno, Zemědělská 3, 61300 Brno, Czech Republic.
Siberian Federal University, 79 Svobodny pr., 660041 Krasnoyarsk, Russia.
Tree Physiol. 2020 Aug 29;40(9):1147-1164. doi: 10.1093/treephys/tpaa066.
Spring drought is becoming a frequently occurring stress factor in temperate forests. However, the understanding of tree resistance and resilience to the spring drought remains insufficient. In this study, European beech (Fagus sylvatica L.) seedlings at the early stage of leaf development were moderately and severely drought stressed for 1 month and then subjected to a 2-week recovery period after rewatering. The study aimed to disentangle the complex relationships between leaf gas exchange, vascular anatomy, tree morphology and patterns of biomass allocation. Stomatal conductance decreased by 80 and 85% upon moderate and severe drought stress, respectively, which brought about a decline in net photosynthesis. However, drought did not affect the indices of slow chlorophyll fluorescence, indicating no permanent damage to the light part of the photosynthetic apparatus. Stem hydraulic conductivity decreased by more than 92% at both drought levels. Consequently, the cambial activity of stressed seedlings declined, which led to lower stem biomass, reduced tree ring width and a lower number of vessels in the current tree ring, these latter also with smaller dimensions. In contrast, the petiole structure was not affected, but at the cost of reduced leaf biomass. Root biomass was reduced only by severe drought. After rewatering, the recovery of gas exchange and regrowth of the current tree ring were observed, all delayed by several days and by lower magnitudes in severely stressed seedlings. The reduced stem hydraulic conductivity inhibited the recovery of gas exchange, but xylem function started to recover by regrowth and refilling of embolized vessels. Despite the damage to conductive xylem, no mortality occurred. These results suggest the low resistance but high resilience of European beech to spring drought. Nevertheless, beech resilience could be weakened if the period between drought events is short, as the recovery of severely stressed seedlings took longer than 14 days.
春旱正成为温带森林中频繁发生的胁迫因素。然而，树木对春旱的抗性和恢复力的理解仍然不足。本研究以处于早期叶片发育阶段的欧洲山毛榉（Fagus sylvatica L.）幼苗为对象，适度和重度干旱胁迫 1 个月，然后在重新浇水后进行为期 2 周的恢复期。本研究旨在厘清叶片气体交换、脉管解剖结构、树木形态和生物量分配模式之间的复杂关系。适度和重度干旱胁迫下，气孔导度分别下降了 80%和 85%，导致净光合作用下降。然而，干旱并未影响慢速叶绿素荧光指数，表明光合作用器的光部分未受到永久性损伤。在两个干旱水平下，茎水力传导率下降了 92%以上。因此，胁迫幼苗的形成层活动下降，导致茎生物量降低、树木年轮变窄、当前年轮中的导管数量减少，这些导管的尺寸也较小。相比之下，叶柄结构不受影响，但叶片生物量减少。仅重度干旱会降低根生物量。重新浇水后，观察到气体交换的恢复和当前年轮的再生，但在重度胁迫的幼苗中，恢复时间延迟了几天，幅度也较小。降低的茎水力传导率抑制了气体交换的恢复，但木质部功能通过再生长和栓塞导管的再填充开始恢复。尽管木质部受损，但没有发生死亡。这些结果表明，欧洲山毛榉对春旱的抗性低但恢复力强。然而，如果干旱事件之间的间隔很短，山毛榉的恢复力可能会减弱，因为重度胁迫幼苗的恢复需要超过 14 天的时间。
