The role of tree species diversity in mitigating negative impacts of reduced water availability in boreal forests: Patterns and mechanisms

生物多样性丧失和气候变化是影响生态系统生产力、元素循环和其他生态过程的两个重要因素。观测研究表明，在过去几十年中，加拿大北方森林和其他地方的树木死亡率有所增加，同时树木的生长幅度较小或没有增加，甚至减少，这表明变暖导致树木可获得的水分减少对森林功能，特别是碳封存产生了负面影响。然而，关于天然林中可用水改变对生态系统功能的影响的直接证据和机制理解令人惊讶地有限。此外，我们对物种丰富的森林是否、如何以及为什么有助于减轻水供应改变对森林功能的负面影响缺乏了解。我过去的研究项目集中在利用观测方法来理解生物多样性丧失和气候变化对生态系统层面（包括地上和地下）生产力和元素循环的影响。这项拟议的研究将探讨树种多样性在减轻北方森林水分供应减少的负面影响中的作用。这项工作建立在我们最近从观测研究中发现的基础上，并依赖于在天然北方森林中进行的水变化实验。本试验以10年火源纯杰克松、纯颤杨及其均匀混合为试验材料，采用环境水处理、生长期降水（5 ~ 10月）减少25%和增加25%的水处理，每组重复3次，共27个试验单元。具体来说，我将讨论：(1)所有植被层的生长、死亡率、净生物量变化、群落结构、组成和多样性的地上响应；(2)凋落物、粗根和细根、土壤碳、养分、微生物群落和呼吸的响应；(3)物种混合（多样性）是否有助于减轻水减少的不利影响，并增强水增加对目标1-2中评估过程的积极影响；(4)与目标1至3中观察到的反应相关的生理机制。我的研究项目的成功很大程度上依赖于HQP的有效培训。从我以前的实验室成员的发表记录和现任职位来看，我的培训工作是成功的。我计划延续过去15年培养HQP的势头。本研究旨在探讨天然林对脉动和压水变化的响应模式和机制。我们的研究结果将有助于制定应对加拿大北方森林气候变化的战略，并为自然系统的多样性对全球变化缓解的影响提供机制见解。

Biodiversity loss and climate change are two significant stressors on ecosystem productivity, element cycling, and other ecological processes. Observational studies have shown that tree mortality of Canadian boreal forests and elsewhere has increased in the past decades, accompanied by smaller or no increase or even decrease in growth, suggesting that reduced water available to trees induced by warming have negatively affected forest functions, particularly carbon sequestration. However, the direct evidence and mechanistic understanding of the impact of altered water availability on ecosystem function in natural forests is surprisingly limited. Moreover, we lack an understanding of whether, how and why species-rich forests help mitigate the negative impacts of altered water availability on forest functions. My past research program is centered on using observational approaches to understanding the consequences of biodiversity loss and climate change on productivity and element cycling at the ecosystem level including above- and belowground. This proposed research will examine the role of tree species diversity in mitigating negative impacts of reduced water availability in boreal forests. This work builds upon our recent discoveries from observational studies and relies upon a water alteration experiment in natural boreal forests. The experiment consists of three water treatments (ambient, 25% decrease and 25% increase of growing season precipitation (May to October) since 2016) in 10-year-old fire-origin pure jack pine, pure trembling aspen, and their even mixture, each replicated three times, with a total of 27 experimental units. Specifically, I will address: (1) the aboveground responses of growth, mortality, net biomass change, community structure, composition and diversity of all vegetation layers, (2) the responses of litterfall, coarse and fine roots, soil carbon, nutrients, microbial community, and respiration, (3) whether species mixture (diversity) help to mitigate adverse impacts of reduced water and enhance the positive impacts of increased water on processes assessed in objectives 1-2; (4) physiological mechanisms associated with the responses observed in objectives 1 to 3. The success of my research program is highly dependent upon the effective training HQP. As evidenced by the publication record and current positions of my previous lab members, I have been successful in my training efforts. I plan to carry forward with the HQP training momentum I have generated over the last 15 years. This proposed research aims at not only the patterns of but also mechanisms for the responses of natural forests to pulse and press water alteration. Our findings will help to develop strategies to cope with climate change in Canada's boreal forests, and shed mechanistic insights into the diversity effects on global change mitigation for natural systems.