长期氮添加对兴安落叶松和水曲柳水力结构的影响

Nitrogen deposition is one of the important environmental issues in China. Nitrogen deposition can greatly affect tree hydraulic architecture, which would influence plant resistance to drought or low temperature stresses, and further affect the survival and growth of trees. However, there is a surprising lack of integrative study on the responses of tree hydraulic architecture and water relations to nitrogen deposition, and the underlying mechanism requires further investigations. The Larix gmelinii and Fraxinus mandshurica are important afforestation and timber tree species in northeast China, which playing an important role in forest ecosystem function and timber production. These two species not only represent different plant functional groups (conifers vs. broadleaf), but also have contrasting hydraulic strategies (isohydric vs. anisohydric). Therefore, we proposed to measure the hydraulics, economics, and anatomical traits of L. gmelinii and F. mandshurica in the 17-year nitrogen addition experiment. The aims of this project are to (1) examine the impact of long-term nitrogen addition on tree hydraulic architecture, water relations, and growth; (2) compare the responses of L. gmelinii and F. mandshurica to nitrogen addition, and reveal their differences in adaptive strategies. This project is expected to deepen the mechanistic understanding of the productivity formation and maintenance of plantation under future nitrogen deposition scenarios, and provide scientific basis for better assessing the adaptability of tree species with contrasting plant functional types and hydraulic strategies to environmental changes.

氮沉降是当今中国重要的环境问题。氮沉降对树木水力结构的改变会影响干旱或低温胁迫环境下植物的抗逆性，进而影响树木的生存和生长。然而，目前关于氮添加对树木水力结构影响的研究非常有限，其潜在的机制尚不明确。兴安落叶松(Larix gmelinii)和水曲柳(Fraxinus mandshurica)是我国东北地区的重要造林树种和珍贵用材树种，在森林生态系统功能和木材生产中扮演着重要角色。二者不仅代表不同植物功能型(针叶vs.阔叶)，同时还具有不同的水力对策(等水调节vs.非等水调节)。为此，本项目拟利用在兴安落叶松和水曲柳人工林开展的17年的氮添加试验，通过研究两种树种的水力、经济和解剖生理生态性状对氮添加的响应，揭示长期氮添加对树木生长和人工林生产力影响的水分生理机制。其次，通过比较兴安落叶松和水曲柳对氮添加的响应差异，为评估不同功能型、不同水力对策树种对环境变化的适应能力提供科学依据。

氮沉降是当今中国重要的环境问题。氮沉降对树木水力结构的改变会影响干旱或低温胁迫环境下植物的抗逆性，进而影响树木的生存和生长。本项目利用长达17年的氮添加试验，测定了兴安落叶松（裸子植物/等水调节）和水曲柳（被子植物/非等水调节）两种树种的水力、经济和结构性状对氮添加的响应，比较了两种水力对策树种对氮添加响应的差异。研究发现氮添加显著增加了水曲柳的叶导水率和枝比叶导水率，虽然氮添加也显著增加了兴安落叶松的叶导水率，但却降低了其枝比边材导水率。由于叶水力阻力在整株植物水力阻力上所占比例较大，而枝水力阻力占比较小。因此，氮添加诱导的兴安落叶松叶导水率的增加使得气孔保持开放，维持了叶碳同化和整树生长。此外，氮添加增加了水曲柳的叶失膨点水势、叶饱和含水时的渗透势和叶水容，但对兴安落叶松的这些性状没有影响，表明非等水调节的水曲柳对氮添加的响应比等水调节的兴安落叶松响应更加敏感，即氮添加降低了水曲柳的叶共质体干旱容忍能力，但没有改变兴安落叶松的叶共质体干旱容忍能力。此外，氮添加降低了兴安落叶松和水曲柳的叶栓塞阻力，显著降低了两个树种的叶干旱容忍性，但对两个树种的枝栓塞阻力均没有产生显著影响。这会导致两个树种的脆弱性分割机制增强，即在面临干旱胁迫时采取牺牲碳投资小的叶片为代价，保护碳投资较大的枝和主干的水分状况，这种对策有利于适应不断加剧的全球氮沉降和气候变化。本研究为评估不同功能型、不同水力对策树种对环境变化的适应能力提供了科学依据。