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。在环极地地区观察到，灌木的扩展通过减少陆地生态系统的总体反照率，深刻改变了苔原生态系统的结构和功能，从而对气候变化产生了积极的反馈。灌木还影响积雪、积雪和融雪的模式，有助于加深活动层和加速永久冻土退化。然而，在整个北极和亚北极地区，灌木的扩展是高度异质性的，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经部分确定，但灌木树干年代学的最新进展表明，灌木对气候的敏感性是因地而异的，有证据表明，在欧亚大陆和较潮湿的地区，灌木对气候的敏感性更高。然而，这些发现是基于对灌木个体(根颈或茎)不同位置采集的样本的研究，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。单个个体内不同的气候敏感度可能会阻碍对大型树种年代学数据集的分析和解释。*因此，这项研究计划的长期目标是弄清气候、地形和土壤因素对灌木物种生长的影响，进而对过去几十年在冻土带观察到的绿化趋势的异质性的影响。在接下来的五年里，我的团队将检验以下研究假设：1.整个北美北部观察到的绿化趋势的异质性是由于灌木在不同空间尺度上对气候敏感性的差异；2.灌木对气候的敏感性与土壤和地形变量密切相关；3.直立灌木物种的多茎生长形式影响它们对气候的敏感性。*这个将灌木扩展、苔原绿化和灌木气候敏感性联系在一起的研究项目将产生不仅科学界而且负责北方发展的政府机构的重大兴趣的结果。通过确定绿化趋势的异质性是否与不同的灌木气候敏感性有关，我们的结果将为旨在预测下个世纪植被变化的模型提供支持，并将至少通过为标准采样方案奠定基础，为灌木树干年代学的发展做出贡献。通过不同的研究项目，该计划将大大促进高素质人才的培养(2个博士，2个硕士)。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).