元江生态站1公顷样地内三叶漆枝条枯死

三个树种叶片和枝条P50的差值（P50leaf-stem）比较

水力脆弱性分割(Vulnerability segmentation)，即植物的末端器官比近端器官更容易受到干旱影响而发生气穴化，在遭受干旱胁迫时，植物常常以牺牲树叶和根等末端器官维持整株植物的水力安全。同时，植株在干旱条件下往往表现出明显的气孔调控(Stomatal control)，气孔的关闭能最大限度地减少植物的蒸腾失水，以维持木质部较高的水势。在热带季节性森林和萨王纳生态系统中，季节性干旱对植物的水分利用策略将是一个较强的选择压力，从而出现了常绿、冬季落叶和干季落叶等物候类型的分化，三种物候类型均能适应长期的干旱生境。然而，不同物候类型的植物在应对干旱时是否均表现出脆弱性分割和气孔调控却并不清楚。

中国科学院西双版纳热带植物园元江干热河谷生态站的科研人员张树斌在研究员张教林和曹坤芳的指导和帮助下，选择了干热河谷稀树灌丛漆树科三个不同物候类型的植物，即常绿的清香木(Pistacia weinmanniifolia)，干季落叶的三叶漆(Terminthia paniculata)和冬季落叶的厚皮树(Lannea coromandelica)，比较了三个树种的水力安全策略。结果表明：冬季落叶植物采用了避旱的水力安全策略，具有明显的脆弱性分割和气孔调控，在干季初期落叶，以保全可塑性较低，构建成本较高的枝条和主干的水力安全；常绿植物具有抗气穴化较强的枝条和叶片，具有强烈的气孔调控作用，采用了耐旱的水力安全策略应对季节性干旱；干季落叶植物则缺乏脆弱性分割，气孔调控作用也较小，在干季末期牺牲部分枝条及其附着的叶片，只有在极端干旱的年份才会落光所有叶片。

在未来气候变化的背景下，许多地方可能出现更为频繁和严重的干旱事件。而已有的研究表明，森林树木具有狭窄的水力安全阈值(<1 MPa)，严重干旱事件将导致更大范围的树木死亡。科研人员的研究结果暗示：在大尺度范围内，评价和预测干旱导致的树木死亡时，需要考虑不同物候类型植物的水力安全策略。

该研究以Divergent hydraulic safety strategies in three co-occurring Anacardiaceae tree species in a Chinese savanna 为题发表于《植物科学前沿》（Frontiers in Plant Science）。该研究得到了国家重点研发计划“西南生态安全格局形成机制及演变机理”第5课题“西南生态系统对气候变化适应机制”(2016YFC0502102-05)、国家自然科学基金(31600479, 31570406, 31470470)和中科院“西部之光”项目的资助，野外工作也得到了元江干热河谷生态站的大力支持。（来源：中科院西双版纳热带植物园）

Divergent hydraulic safety strategies in three co-occurring Anacardiaceae tree species in a Chinese savanna

Abstract  Vulnerability segmentation, the condition under which plant leaves are more vulnerable to drought-induced cavitation than stems, may act as a “safety valve” to protect stems from hydraulic failure. Evergreen, winter-deciduous, and drought-deciduous tree species co-occur in tropical savannas, but there have been no direct studies on the role of vulnerability segmentation and stomatal regulation in maintaining hydraulic safety in trees with these three leaf phenologies. To this end, we selected three Anacardiaceae tree species co-occurring in a Chinese savanna, evergreen Pistacia weinmanniifolia, drought-deciduousTerminthia paniculata, and winter-deciduous Lannea coromandelica, to study inter-species differentiation in leaf and stem hydraulic safety. We found that the two deciduous species had significantly higher sapwood-specific hydraulic conductivity and leaf-specific hydraulic conductance than the evergreen species. Moreover, two deciduous species were more vulnerable to stem cavitation than the evergreen species, although both drought-deciduous species and evergreen species had drought-resistance leaves. The evergreen species maintained a wide hydraulic safety margin (HSM) in stems and leaves; which was achieved by embolism resistance of both stems and leaves and isohydric stomatal control. Both deciduous species had limited HSMs in stems and leaves, being isohydric in the winter-deciduous species and anisohydric in drought-deciduous species. The difference in water potential at 50% loss of hydraulic conductivity between the leaves and the terminal stems (P50leaf−stem) was positive in P. weinmanniifolia and L. coromandelica, whereas, T. paniculata exhibited a lack of vulnerability segmentation. In addition, differences in hydraulic architecture were found to be closely related to other structural traits, i.e., leaf mass per area, wood density, and sapwood anatomy. Overall, the winter-deciduous species exhibits a drought-avoidance strategy that maintains the hydraulic safety of the more carbon-costly stems by sacrificing cheaper and more vulnerable leaves, while the evergreen species exhibits a hydraulic strategy of drought tolerance with strong stomatal regulation. In contrast, the drought-deciduous species lacks vulnerability segmentation and sheds leaves at the expense of top shoots during peak drought. This study demonstrates that even sympatric tree species that differ in leaf phenology can exhibit divergent adaptive hydraulic safety strategies.

原文链接：http://journal.frontiersin.org/article/10.3389/fpls.2016.02075/full