Climate as a driver of shrub expansion and tundra greening

全球气候变化已被确定为严重威胁，并已对北极陆地系统产生深远影响。一个引人注目的例子就是苔原灌木的扩张。灌木是北极碳循环的重要组成部分，也是促进高纬度气候变暖的反馈的主要贡献者。2014年4月IPCC的影响报告对北极生态系统中灌木增加的检测结果给出了高可信度，并将灌木扩张归因于气候变暖。然而，目前，气候与其他生物或环境因素作为驱动苔原灌木生长和扩张的相对重要性仍然知之甚少。预计在下个世纪，冻土带生态系统将变暖2 - 10摄氏度。一个重要的未知因素是，灌木是否会迅速对变暖条件做出反应，或者诸如水分供应、营养限制、草食、病原体爆发或火灾等环境因素是否会开始限制未来植被变化的速度。为了解决这一知识差距，我们建议：1)量化气候作为冻土带灌丛生长和扩张的驱动因素的重要性，2)测试冻土带绿化遥感数据与样地尺度野外观测之间的对应关系，3)分析灌丛生长、冻土带绿化和气候时间序列之间的相关性，4)在景观尺度上评估冻土带绿化和植被变化模式之间的一致性。我们的主要目标是：1)从样地到生物群落的冻土带植被变化-气候相互作用的尺度：我们将整合来自北极周围站点的本地收集数据和遥感数据，以测试a)生长季节温度是否是冻土带植被变化的主要驱动因素；b)遥感绿化数据是否捕获了与局地尺度灌木扩张的时空变化相关的信号。2)在景观尺度上联系冻土带绿化与植被变化模式：在重点研究地点Herschel岛，利用无人机系统（UAS）收集高空间分辨率（< 10 cm像素）冻土带绿化数据，与植被监测数据、植物性状测量数据、干扰率和地形特征进行比较。3)提高对冻土带植被变化的预测：整合研究成果，以数据驱动的方式预测整个冻土带生物群系灌木的增加。我们将在2017年举办一个数据综合研讨会，以整合我们的研究目标，评估未来的研究重点并综合我们的发现。由于所需的生物群落尺度数据集直到最近才足够长时间可用，并且技术，方法方法和统计技术现在才足够完善，以测试灌木在时空尺度上的扩张机制，因此提出的研究既及时又紧迫。我们的发现将为灌木生长和冻土带绿化的驱动因素提供基本的见解，并将使未来冻土带植被变化的预测更加完善。拟议的研究将产生一个公开可用的数据集，该数据集将为广泛的科学界提供对冻土带植被变化的关键估计。我们将通过及时在国际公认的期刊上发表文章、在重大会议上发表演讲、知识交流、生态评估、公众参与和媒体传播结果。最后，该项目将为早期职业研究人员提供全面的培训、指导和交流机会。

Global climate change has been identified as a serious threat that has already had profound impacts on terrestrial Arctic systems. A dramatic example of this is the expansion of tundra shrubs. Shrubs are an important part of the Arctic carbon cycle and major contributors to feedbacks facilitating high-latitude climate warming. The April 2014 IPCC Impacts report assigned a high confidence in the detection of an increase in shrubs in Arctic ecosystems and attributes shrub expansion to climate warming. However currently, the relative importance of climate versus other biological or environmental factors as drivers of tundra shrub growth and expansion remains poorly understood. Over the next century tundra ecosystems are predicted to warm by 2 - 10C. A significant unknown is whether shrubs will rapidly respond to warming conditions, or whether environmental factors such as water availability, nutrient limitation, herbivory, pathogen outbreaks, or fire will begin to limit the rate of future vegetation change. To address this knowledge gap, we propose to: 1) quantify the importance of climate as a driver of tundra shrub growth and expansion at sites across the tundra biome, 2) test the correspondence between plot-scale field observations with remotely-sensed data of tundra greening, 3) analyse the correlations between time series of shrub growth, tundra greening and climate and 4) assess the agreement between patterns of tundra greening and vegetation change at the landscape-scale.Our key objectives are:1) To scale tundra vegetation change-climate interactions from plot to biome: We will integrate locally-collected data from sites around the Arctic and remotely-sensed data to test a) whether growing season temperatures are the primary driver of tundra vegetation change and b) whether remotely-sensed greening data capture a signal that correlates with spatial and temporal variation in local-scale shrub expansion. 2) To link patterns of tundra greening and vegetation change at the landscape scale: To scale from plot to biome at our focal research site Herschel Island, we will collect high spatial resolution (< 10 cm pixel) tundra greening data using an unmanned aircraft system (UAS) to compare with vegetation monitoring data, plant trait measurements, disturbance rates and terrain characteristics. 3) To improve predictions of tundra vegetation change: We will integrate our research findings to make data-driven predictions of shrub increase across the tundra biome. We will conduct a data synthesis workshop in 2017 to integrate our research objectives, evaluate the future research priorities and synthesize our findings.This proposed research is both timely and urgent, as the required biome-scale datasets have only recently become available in long-enough duration, and the technologies, methodological approaches and statistical techniques are only now refined sufficiently to test mechanisms of shrub expansion across spatial and temporal scales. Our findings will provide fundamental insights into the drivers of shrub growth and tundra greening and will enable improved predictions of future tundra vegetation change.The proposed research will result in the generation of a publically available dataset that will provide key estimates of tundra vegetation change to the broad scientific community. We will disseminate results through timely publications in internationally recognized journals and presentations at major conferences, knowledge exchange, ecological assessments, public engagement, and media. Lastly, the project will provide comprehensive training, mentorship and networking opportunities for early career researchers.