Effects of aridity on the demography, distribution, and biomass of trees and forests in western Canada

最近的观察表明，日益严重的干旱和水分压力将是森林应对全球气候变化的主要驱动因素。这一预测似乎尤其适用于加拿大西部的北方森林，那里与干旱有关的树木死亡增加以及水分限制的生长和补充率证实，森林动态与水的可获得性密切相关。然而，尽管西部森林水分胁迫的影响有很好的经验证据，但生态学家对日益干旱对树木人口统计(即生长、死亡和更新)的影响如何转化为森林生物群的地理分布和现存量的更广泛变化的了解有限。我计划进行一系列协调一致的实地研究和模拟实验，以量化加拿大西部的森林在下个世纪将如何应对日益加剧的水分压力。这项研究计划将检验以下假设：(1)树木死亡和更新如何随水的可获得性而变化；(2)与水分有关的人口统计学变化如何控制西部北方森林的分布和现存量；以及(3)该林区可能如何应对未来的干旱。首先，我将使用无人机收集柏树丘陵地区非常高分辨率的森林景观的3D图像；这些数据将被用来量化雨量和土壤水分的海拔梯度上冠层树木死亡率的空间和时间变化。其次，我将建立并重新普查一组永久林地，以确定不同郁闭度下光和水对幼树动态的影响。第三，我将使用航空和基于地块的人口统计数据来参数化一个基于个体的森林动态模型，该模型可以模拟树木物种的区域分布和丰度如何在气候梯度和林龄之间变化。通过将实地研究的数据和结果整合到动态森林模型中，这项研究将产生有充分依据的定量预测，预测与气候有关的人口变化如何导致加拿大西部森林覆盖、物种组成和碳储量的未来变化。这项研究的结果最终将导致在全球陆地碳循环模型中更好地描述人口过程，并将能够更有力地预测环境变化下的植被动态。这些进展对于寻求减轻或适应气候变化对森林生态系统服务的影响的加拿大和国际政策制定者将是有价值的。

Recent observations indicate that increasing drought and water stress will be major drivers of forest responses to global climate change. This forecast appears to be particularly true for boreal forests of western Canada, where drought-related increases in tree mortality and moisture-limited growth and recruitment rates confirm that forest dynamics are tightly coupled with water availability. Yet, despite good empirical evidence for the effects of water stress in western forests, ecologists have a limited understanding of how impacts of increasing aridity on tree-level demography (i.e., growth, mortality, and regeneration) could translate to broader changes in the geographic distribution and standing biomass of forest biomes. I plan to undertake a coordinated set of field studies and modelling experiments to quantify how forests in western Canada will respond to increased water stress over the next century. This research program will test hypotheses regarding: (1) how tree mortality and recruitment vary with water availability; (2) how moisture-related demographic variation governs the distribution and standing biomass of western boreal forests; and (3) how this forest region may respond to future aridity. Firstly, I will use an Unmanned Aerial Vehicle to collect very high resolution 3D imagery of forest landscapes in the Cypress Hills region; these data will be used to quantify spatial and temporal variation in canopy tree mortality across elevational gradients in precipitation and soil moisture. Secondly, I will establish and re-census a set of permanent forest plots to determine the effects of light and water on juvenile tree dynamics under different levels of canopy closure. Thirdly, I will use aerial and plot-based demographic data to parameterize an individual-based model of forest dynamics that can simulate how the regional distribution and abundance of tree species vary across climate gradients and with forest age. By integrating data and results from field studies into a dynamic forest model, this research will produce well-grounded, quantitative predictions of how climate-related demographic variation could lead to future changes in forest cover, species composition, and carbon storage in western Canada. Findings from this research will ultimately lead to better representations of demographic processes in models of the global terrestrial carbon cycle, and will enable more robust projections of vegetation dynamics under environmental change. These advances will be valuable to Canadian and international policy makers seeking to either mitigate or adapt to climate change impacts on forest ecosystem services.