Climate sensitivity of regional carbon stocks in Canada's western boreal forest

加拿大西部的北方针叶林储存了大量的碳。从马尼托巴省一直延伸到育空地区，这片森林区域被管理的部分包含了相当于大约130年国家温室气体排放量，代表着一个全球重要的碳库。然而，近几十年来干旱加剧导致整个区域的森林碳损失，因为水分压力降低了生产力(减缓了树木的生长)并增加了死亡率。这些变化构成了重大威胁，因为变暖的气温和更大的干旱共同加剧了火灾风险，使树木容易受到虫害的攻击，减少了可销售木材的供应，并在碳储存释放到大气中时引发了破坏性的气候变化反馈。在接下来的五年里，我们将使用一套协调的实地研究和模拟实验来：(1)评估水分可获得性对个体树木的生长和死亡的影响如何在物种之间、在当地竞争中以及随立地条件而变化；(2)评估林分内生产力和生物量周转的气候变化如何影响生物量随时间积累的速率；(3)在未来气候和干扰情景下，整个区域林木的当前物种组成、树干大小分布和碳密度预计将如何变化。我和我的学生将在加拿大西部三个主要的北方生态区建立一个由400个地点组成的网络。在每个地点，我们将使用无人驾驶飞行器进行航空调查，并利用由此产生的图像的年际变化来衡量树木生长和死亡的年率。我们将使用这些数据来开发统计模型，以了解气候因素如何以不同的方式影响单株树木和较大林分的生产力和死亡率。然后，我们将把这些关系合并到一个基于个体的森林模拟器中，以预测树木生长和死亡的差异如何转化为森林结构和组成随时间的变化。最后，我们将使用这个模拟器进行基于模型的实验，以评估水分压力的增加如何影响21世纪的区域碳平衡、森林结构和物种组成。这项研究计划将提供对加拿大一个主要林区碳动态的更清晰的理解，目前关于这一地区的数据很少。尽管传统观点认为这些森林是一个净碳汇，但基于大量新数据的模型预测可能会揭示它们成为碳源的潜力，如果未来几十年湿度压力如预期的那样增加的话。更好地了解西部北方森林对气候变化的敏感性，将解决有关其对加拿大国家温室气体排放和全球碳-气候系统的影响的重大不确定性。

Canada's western boreal forest stores vast amounts of carbon. Stretching from Manitoba to the Yukon, the managed portion of this forest region contains the equivalent of about 130 years' worth of national greenhouse gas emissions and represents a globally significant carbon pool. However, increases in aridity in recent decades have caused region-wide losses of forest carbon as moisture stress has both reduced productivity (slowed tree growth) and increased mortality. These changes pose a significant threat as warming temperatures and greater aridity together exacerbate the risk of fire, predispose trees to attack by insect pests, reduce supplies of merchantable timber, and trigger damaging climate change feedbacks when carbon stocks are released to the atmosphere. Over the next five years, we will use a coordinated set of field studies and modelling experiments to: (1) evaluate how effects of water availability on the growth and mortality of individual trees vary among species, with local competition, and with site conditions; (2) assess how climatic variation in productivity and biomass turnover within forest stands influence rates of biomass accumulation over time; (3) project how the current species composition, stem size distribution, and carbon density of forest stands across the region are expected to change under future climate and disturbance scenarios. My students and I will establish a network of 400 sites across three major boreal ecozones in western Canada. At each of these sites we will conduct aerial surveys with an unmanned aerial vehicle, and use year-to-year changes in the resulting imagery to measure annual rates of tree growth and mortality. We will use these data to develop statistical models for how climatic factors influence the productivity and mortality of individual trees and larger stands in different ways. We will then incorporate these relationships into an individual-based forest simulator to project how differences in tree growth and mortality translate into changes in forest structure and composition over time. Finally, we will run model-based experiments with this simulator to evaluate how increases in moisture stress affect regional carbon balance, forest structure, and species composition over the 21st century. This research program will provide a clearer understanding of carbon dynamics across a major Canadian forest region for which there is currently little data. Although the conventional thinking is that these forests are a net carbon sink, model projections based on extensive new data could reveal their potential to become a carbon source if moisture stress increases as expected over the coming decades. A better understanding of the sensitivity of the western boreal forest to climate change would resolve important uncertainties concerning its influence on Canada's national greenhouse gas emissions and the global carbon-climate system.