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

加拿大北方森林的大部分生物群落都生长在永久冻土层上。在加拿大西北部，由于最近的气候变暖（过去50年约3°C）和相关的永久冻土融化，这些森林正在迅速变化，由于这些系统中储存了大量的碳，这可能会对全球产生影响。尽管如此，我们对这些森林的生态知之甚少，部分原因是它们的商业价值有限，而且相对难以进入。预计大气二氧化碳浓度的增加和相关的变暖将通过以下方式提高生产力和北方森林的向北扩展：(a)延长生长季节；(b)土壤温度升高，改变了季节性冻土和永久冻土的冻融动态，增强了土壤微生物活动、分解和养分矿化；(c)由于大气二氧化碳浓度升高导致的生理速率增强。最近的研究表明，与预测相反，高纬度的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这些变化发生的原因，以提高我们预测未来条件的能力。

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.