Boreal forests on permafrost: functional trait and whole-plant responses to climate warming and permafrost thaw

加拿大北部森林的大部分生物群都在永久冻土(常年冻土)上。在加拿大西北部，由于最近气候变暖(近50年来约3摄氏度)和与之相关的永久冻土融化，这些森林正在迅速变化，这可能会对全球产生影响，因为这些系统中储存了大量的碳。尽管如此，我们对这些森林的生态了解相对较少，部分原因是它们的商业价值有限，而且相对难以接近。预计大气二氧化碳浓度的增加和相关的变暖将通过：(A)更长的生长季节；(B)土壤温度的增加，改变季节性冻结和永久冻土的冻融动态，并加强土壤微生物活动、分解和养分矿化，从而提高生产力和北方森林的范围；以及(C)由于大气二氧化碳浓度的增加而提高生理速率。最近的研究表明，与预测相反，高纬度北方森林对不断变化的环境条件表现出实质性的负面反应，通过拟议的研究，我们试图更好地理解为什么会发生这些变化，以提高我们预测未来条件的能力。 为了实现这一目标，我们将使用树轮研究来量化加拿大西北地区(NWT)纬度和永久冻土梯度上优势树种的生长-气候关系。将进行采样，以捕捉土壤资源条件的当地变异性，以确定土壤资源可用性在观察到的生长-气候关系中的作用(即，树木生长是否因资源限制而对变暖的温度做出负面反应？)。树木年轮不仅包含有关生长速度的信息，还包含通过树木年轮解剖和相关的稳定同位素组成测量的木材形成时的环境条件和个体的生理潜力。因此，对于对近期气候变化表现出不同反应的树木，我们将进行详细的树轮解剖和稳定同位素分析，以辨别生长反应的不同解剖和生理驱动因素。最后，我们希望将这些长时间记录与实际流量和生产率联系起来。为了实现这一点，我们将量化地点内和纬度梯度上的相关生理特征，以确定结构和功能之间的关系。我提议的研究将阐明驱动高纬度北方森林对气候变化反应的机制，以及这些反应如何随纬度和永久冻土条件、当地土壤条件和物种的变化而变化。我们的数据将为驱动高纬度树木生长反应的生理机制提供新的见解。这一认识对于评估在不断变化的环境条件下生长的树木的可塑性范围和恢复能力，以及最终预测高纬度北方针叶林在未来气候情景下的群落和物种水平的反应至关重要。

Much of Canada’s boreal forest biome occurs on permafrost (perennially cryotic ground). In Northwestern Canada, these forests are changing rapidly due to recent climate warming (~3°C in the last 50 years) and associated permafrost thaw, which could have global implications due to the large amounts of carbon stored in these systems. Despite this, we know relatively little about the ecology of these forests due in part to their limited commercial value and relative inaccessibility. Increased atmospheric CO2 concentrations and associated warming are predicted to increase productivity and the northward extent of boreal forests via: (a) longer growing seasons; (b) increased soil temperatures, which alter freeze-thaw dynamics in seasonally frozen as well as permafrost soils and enhance soil microbial activity, decomposition, and nutrient mineralization; and (c) enhanced physiological rates attributable to elevated atmospheric CO2 concentrations. Recent studies have shown that contrary to predictions, high latitude boreal forests are showing substantial negative responses to changing environmental conditions and through the proposed research we seek to better understand why these changes are happening to improve our capacity to predict future conditions. To achieve this goal, we will quantify growth-climate relationships of dominant tree species across a latitudinal and permafrost gradient in the Northwest Territories (NWT), Canada using tree ring studies. Sampling will be conducted with a view to capturing local variability in soil resource conditions to determine the role of soil resource availability in observed growth-climate relationships (i.e., are negative tree growth responses to warming temperatures due to resource limitation?). Tree rings contain information not only on the rates of growth, but also the environmental conditions at the time the wood was formed and physiological potential of individuals as measured through tree ring anatomy and associated stable isotope composition of the rings. For trees showing different responses to recent climate change, we will therefore conduct detailed tree ring anatomical and stable isotope analyses to discern the differential anatomical and physiological drivers of growth responses. Finally, we wish to link these long temporal records with actual flux rate and productivity. To achieve this, we will quantify associated physiological traits within sites and across the latitudinal gradient to determine relationships between structure and function. My proposed research will elucidate the mechanisms driving high latitude boreal forest responses to climate changes, and how these responses vary with latitude and permafrost conditions, local soil conditions, and species. Our data will provide novel insights to the physiological mechanisms driving growth responses in high latitude trees. This understanding is crucial for evaluating the range of plasticity and the capacity for resilience of trees growing under changing environmental conditions and ultimately to predict community and species level responses of high latitude boreal forests under future climatic scenarios.