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

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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.