气候变暖影响下城市树木衰退死亡的生理机制研究

Extreme weather events are occurring with increasing frequencies and extents under the influence of climate warming, which have been shown to cause accelerating rates of tree decline and mortality at the global scale. Climate warming is most directly reflected by the extreme high temperature weather events that frequently occur in recent years. Extreme high temperatures are particularly obvious in cities on top of the Heat Island Effect and thus show stronger destructive influences. Heat waves have caused severe mortality of urban trees but the underlying mechanisms are still poorly understood. Here, we plan to investigate the physiological mechanisms of urban tree mortality through interspecific comparisons of key functional traits, tree physiological responses along a gradient of urban-related environmental stresses, and dynamic measurements of ecophysiological processes during summer heat waves. Firstly, by the implementation of this project we would be able to identify key functional traits determining the stress tolerance of urban trees as well as identifying stress tolerant tree species in Northeast China. Secondly, this investigation will contribute to the demonstration of coordinations between water and carbon physiology by studying the patterns of responses along the urban environmental stress gradient in terms of plant water relations, photosynthetic gas exchange and carbon economy. We will also investigate the role hydraulic failure plays in causing intensified tree mortality by dynamically monitoring the water transport related physiological processes in tree xylem. This project would provide fundamental knowledge that contributes to establishing and maintaining urban forests for human well-being in the framework of the national ecological civilization construction.

极端高温热浪天气频发是气候变暖的重要表现，其干热胁迫效应是全球范围内森林大量衰退和死亡的重要诱因，城市热岛效应使得热浪天气具有更大的破坏力，近年来也频繁造成城市树木严重的衰退和死亡。因此，亟需开展相关的抗逆生理机制研究，以便为相应的防治措施提供基础科学依据。本项目拟以沈阳市的主要绿化树种为对象，通过同质园下种间功能性状对比、沿城市胁迫梯度树木的生理响应分析、热浪发生时生理生态过程的原位连续监测，深入研究热浪产生伤害的胁迫生理过程和树木的抗逆机制。将明确东北气候条件下决定城市树木抗逆性的主要功能性状，探明主要城市树种的抗逆性差异；理清沿城市环境压力梯度的变化，树木在水分关系、光合气体交换和碳经济等方面的响应规律，揭示城市树木胁迫响应中的水－碳耦合机理；明确严重热浪天气下水力失败发生的动态过程，并揭示其加剧城市树木衰退死亡的生理机制。该研究的预期成果将服务于我国新时代的城市林业和生态文明建设。

极端天气频发是气候变暖的重要表现，而城市生态系统是气候变暖响应的敏感区和脆弱区，加之城市热岛效应，极端高温热浪天气对城市园林生态系统具有更大的破坏力。此外，在较高纬度地区，普遍存在冻融交替以及冬季干化事件，会导致树木导水功能严重受损，影响植物的正常生理活动。因此亟需开展相关的抗逆生理机制研究，以便为相应的树木受害防治措施提供基础科学依据。. 针对课题中有关城市树木热胁迫适应和响应的内容，开展了沈阳市主要绿化树种热胁迫生理相关测定，明确了干热胁迫条件下决定城市树木抗逆性的主要功能性状，探明了主要城市树种的抗逆性差异。针对沈阳市典型的园林绿化树种银杏衰退死亡现象，揭示了其对城市胁迫响应的水－碳耦合机理。同时，针对气候变暖背景下极端寒潮频发的现象，围绕沈阳市主要城市园林树种早春物候和抗寒性机制开展了研究，探讨了树木水力安全与早春物候之间的关系及其生理机制。. 主要获得以下三方面的结果：1）叶片高温半致死温度、机械强度与叶片解剖结构之间存在密切关联，表明叶片的解剖性状能够影响其耐热性和抗虫性；叶片高温半致死温度与叶片失水曲线、叶片机械强度之间的关系，表明叶片的耐热性、保水性、抗虫性三者之间存在一致性。2）沿着城市环境梯度的变化，银杏的水力结构特征、木质部解剖特征和非结构性碳水化合物存在显著变化，在城市中心较大环境压力下树木的水－碳耦合生理的变化是其衰退死亡的重要机制。3）温带环孔材和散孔材树种的早春物候差异和水力安全维持具有密切关联，早春寒潮的存在使环孔材树种进化出较晚的物候以规避木质部栓塞风险，证实了水力结构介导了环孔材和散孔材树种在春季物候和生长策略上的显著差异，这些差异也可能导致两个功能类群树种对全球气候变暖存在响应差异。此外，使用高精度树木生长仪，在同质园中连续监测了6个树种整个生长季的径向生长动态，分析了热浪天气对其生长的影响，有望在近期发表高质量成果。

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