Climate as a driver of shrub expansion and tundra greening

全球气候变化已被确定为一个严重威胁，已经对北极陆地系统产生了深远影响。一个戏剧性的例子是苔原灌木的扩张。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主要反馈因素。2014年4月的IPCC影响报告对北极生态系统中灌木增加的检测具有很高的信心，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苔原灌木生长和扩张驱动因素的相对重要性仍然知之甚少。在下一个世纪，苔原生态系统预计将升温2 - 10摄氏度。一个重要的未知数是灌木是否会迅速响应变暖的条件，或环境因素，如水的可用性，养分限制，食草动物，病原体爆发，或火灾是否会开始限制未来植被变化的速度。为弥补这一知识差距，我们建议：1）量化气候作为苔原灌木生长和扩展的驱动力在整个苔原生物群落中的重要性，2）测试苔原绿化的遥感数据与地块规模实地观察之间的对应关系，3）分析灌木生长时间序列之间的相关性，苔原绿化和气候和4）评估苔原绿化和植被变化的模式之间的协议在一个小的规模。我们的主要目标是：1）规模苔原植被变化气候相互作用从地块到生物群落：我们将整合从北极周围地区收集的数据和遥感数据，以测试a）生长季节温度是否是苔原植被变化的主要驱动力，以及B）是否远程-感测到的绿化数据捕捉到一个信号，与空间和时间的变化，在当地规模的灌木扩张。2)为了在景观尺度上将苔原绿化和植被变化的模式联系起来：为了在我们的重点研究地点赫歇尔岛从地块到生物群落进行缩放，我们将使用无人驾驶飞机系统（UAS）收集高空间分辨率（< 10厘米像素）苔原绿化数据，以与植被监测数据，植物性状测量，干扰率和地形特征进行比较。3)为了改善苔原植被变化的预测：我们将整合我们的研究成果，使整个苔原生物群落灌木增加的数据驱动的预测。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，并综合我们的研究结果。这项拟议的研究既及时又紧迫，因为所需的生物群落规模的数据集最近才在足够长的时间内可用，而且技术，方法论和统计技术现在才得到充分的改进，以测试灌木在空间和时间尺度上扩展的机制。我们的研究结果将为灌木生长和苔原绿化的驱动因素提供基本的见解，并将改善对未来苔原植被变化的预测。拟议的研究将产生一个可供参考的数据集，为广大科学界提供苔原植被变化的关键估计。我们将通过在国际公认的期刊上及时发表文章和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、指导和建立联系的机会。

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.