Shrub climate sensitivity and the heterogeneity of the greening of the tundra

在过去的几十年里，北极和亚北极地区的绿化主要与直立灌木物种的迅速扩张有关。在整个环极地区观察到，灌木扩张深刻地改变了冻土带生态系统的结构和功能，减少陆地生态系统的整体覆盖，从而对气候变化产生积极的反馈。灌木还影响雪沉积、积累和融雪的模式，并有助于加深活动层和加速永久冻土退化。然而，整个北极和亚北极地区的灌木扩张是高度异质性的，这种异质性的驱动因素仍然知之甚少。虽然直立灌木生长的气候驱动因素已部分确定，灌木树木年代学的最新进展表明，灌木气候敏感性是网站和物种特异性，在欧亚大陆和潮湿地区的敏感性较高的证据。然而，这些发现是基于依赖于在灌木个体的不同位置（根颈或茎）收集的样本的研究，因此，他们没有考虑直立灌木物种的多茎生长形式对其气候敏感性的影响。不同的气候敏感性在一个单一的个人可能会妨碍分析和解释大型树木年代学数据集。** 因此，本研究计划的长期目标旨在解开气候，地形和土壤因素对灌木物种生长的影响，并通过扩展，在过去几十年中观察到的冻土带绿化趋势的异质性。在接下来的五年里，我的团队将测试以下研究假设：1。在整个北美北方观察到的绿化趋势的异质性，从不同的空间尺度上的灌木气候敏感性的差异; 2。灌木的气候敏感性与土壤和地形因子密切相关。直立灌木物种的多茎生长形式影响其对气候的敏感性。这一研究方案将灌木扩展、冻土带绿化和灌木气候敏感性联系起来，将产生不仅对科学界而且对负责北方发展的政府机构都具有重大意义的结果。通过确定绿化趋势异质性是否与灌木气候敏感性差异相关，我们的研究结果将为旨在预测下一个世纪植被变化的模型提供数据，并将有助于灌木年轮学的发展，至少为标准采样协议奠定基础。通过不同的研究项目，该计划将大大有助于高素质人才的培训（2博士，2名硕士和5名本科生）。

The greening of Arctic and Subarctic regions over the last decades has been mainly associated with the rapid expansion of erect shrub species. Observed throughout the circumpolar region, shrub expansion deeply alters the structure and function of tundra ecosystems by reducing overall albedo in terrestrial ecosystems, resulting in a positive feedback to climate change. Shrubs also affect patterns of snow deposition, accumulation and snowmelt and contribute to deepening the active layer and accelerating permafrost degradation. Shrub expansion is however highly heterogeneous throughout the Arctic and the Subarctic regions and the drivers of such heterogeneity are still poorly understood. While climatic drivers of erect shrub growth have been partially identified, recent advances in shrub dendrochronology suggest that shrub climate sensitivity is site- and species-specific, with evidence of higher sensitivity in Eurasia and in wetter regions. However, these findings are based on studies that rely on samples collected at different positions on shrub individuals (root collars or stems) and therefore, they did not consider the impacts of the multi-stem growth form of erect shrub species on their climate sensitivity. Differential climate sensitivity within a single individual might hamper analysis and interpretation of large dendrochronological datasets. ******Therefore, the long-term objectives of this research program aim to disentangle the effects of climatic, topographic and edaphic factors on the growth of shrub species and, by extension, on the heterogeneity of the greening trend observed in the tundra over the last decades. Over the next five years, my team will test the following research hypotheses: 1. The heterogeneity in the greening trend observed throughout northern North America results from the differential climate sensitivity of shrubs at various spatial scales; 2. The climate sensitivity of shrubs is closely associated with both edaphic and topographic variables; 3. The multi-stem growth form of erect shrub species influences their sensitivity to climate.******This research program linking shrub expansion, greening of the tundra and shrub climate sensitivity will generate results of significant interest not only for the scientific community but also for governmental agencies responsible for the development of the North. By determining if the greening trend heterogeneity is associated with differential shrub climate sensitivity, our results will feed the models that aim to predict vegetation changes over the next century and will contribute to the development of shrub dendrochronology by, at the very least, laying the foundations for standard sampling protocols. Through the different research projects, this program will contribute significantly to the training of Highly Qualified Personnel (2 Ph.D., 2 M.Sc. and 5 undergraduate students).