Xu Gui-Qing, Chen Tu-Qiang, Liu Shen-Si, Ma Jie, Li Yan
State Key Laboratory of Desert and Oasis Ecology, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China; Fukang Station of Desert Ecology, Chinese Academy of Sciences, Fukang 831505, China; University of Chinese Academy of Sciences, Beijing 100049, China.
State Key Laboratory of Desert and Oasis Ecology, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China; Fukang Station of Desert Ecology, Chinese Academy of Sciences, Fukang 831505, China; University of Chinese Academy of Sciences, Beijing 100049, China.
Sci Total Environ. 2023 Jan 1;854:158802. doi: 10.1016/j.scitotenv.2022.158802. Epub 2022 Sep 14.
In recent year, widespread declines of Populus bolleana Lauche trees (P. bolleana, which dieback from the top down) and Haloxylon ammodendron shrubs (H. ammodendron, which dieback starting from their outer canopy) have occurred. To investigate how both intra-canopy hydraulic changes and plasticity in hydraulic properties create differences in vulnerability between these two species, we conducted a drought simulation field experiment. We analyzed branch hydraulic vulnerability, leaf water potential (Ψ), photosynthesis (A), stomatal conductance (gs), non-structural carbohydrate (NSCs) contents and morphological traits of the plants as the plants underwent a partial canopy dieback. Our results showed that: (1) the hydraulic architecture was very different between the two life forms; (2) H. ammodendron exhibited a drought tolerance response with weak stomatal control, and thus a sharp decline in Ψ while P. bolleana showed a drought avoidance response with tighter stomatal control that maintained a relatively stable Ψ; (3) the Ψ of H. ammodendron showed relative consistent symptoms of drought stress with increasing plant stature, but the Ψ of P. bolleana showed greater drought stress in higher portions of the crown; (4) prolonged drought caused P. bolleana to consume and H. ammodendron to accumulate NSCs in the branches of their upper canopy. Thus, the prolonged drought caused the shoots of the upper canopy of P. bolleana to experience greater vulnerability leading to dieback of the upper branches first, while all the twigs of the outer canopy of H. ammodendron experienced nearly identical degrees of vulnerability, and thus dieback occurred uniformly. Our results indicate that intra-canopy hydraulic change and their plasticity under drought was the main cause of the observed canopy dieback patterns in both species. However, more work is needed to further establish that hydraulic limitation as a function of plant stature was the sole mechanism for causing the divergent canopy dieback patterns.
近年来，胡杨（Populus bolleana Lauche，自上而下枯死）和梭梭（Haloxylon ammodendron，从树冠外围开始枯死）出现了广泛衰退。为了研究冠层内水力变化以及水力特性的可塑性如何导致这两个物种在脆弱性上产生差异，我们进行了一项干旱模拟田间试验。在植物经历部分冠层枯死的过程中，我们分析了植物的枝条水力脆弱性、叶水势（Ψ）、光合作用（A）、气孔导度（gs）、非结构性碳水化合物（NSCs）含量以及形态特征。我们的结果表明：（1）这两种生活型的水力结构差异很大；（2）梭梭表现出耐旱反应，气孔控制较弱，因此叶水势急剧下降，而胡杨表现出避旱反应，气孔控制更严格，从而保持相对稳定的叶水势；（3）随着植株高度增加，梭梭的叶水势表现出相对一致的干旱胁迫症状，但胡杨的叶水势在树冠较高部分表现出更大的干旱胁迫；（4）长期干旱导致胡杨消耗而梭梭在其上部冠层枝条中积累非结构性碳水化合物。因此，长期干旱导致胡杨上部冠层的嫩枝更容易受到影响，导致上部枝条首先枯死，而梭梭树冠外围的所有嫩枝受到的脆弱性程度几乎相同，因此枯死均匀发生。我们的结果表明，干旱条件下冠层内水力变化及其可塑性是观察到的这两个物种冠层枯死模式的主要原因。然而，需要更多的工作来进一步确定作为植株高度函数的水力限制是导致不同冠层枯死模式的唯一机制。
