Collaborative Research/RUI: Past, Present and Future Productivity of Arctic Woody Vegetation in a Warming Climate

森林植被在北极内部和向北极扩展是未来几十年北极陆地表面可能发生的深刻变化之一。森林植被的扩展对北极系统具有重大影响，因为它可能对气候造成正反馈和负反馈，并改变关键自然资源的可用性。尽管近几十年来，北极地区的森林扩张和向北极地区的森林扩张一直很普遍，但越来越多的证据表明，在森林扩张的地区，对气候变暖的非线性反应可能普遍存在。特别是，最近在北极南部发现了大面积的“褐化”（NDVI下降趋势）。本研究的目的是评估树木生长、气候和NDVI之间的关系，以确定对温度的非线性响应（即“褐变”响应）的原因，并更好地了解未来变暖对森林植被向北极扩展的生产力和动态的可能影响。提出的研究测试了三个假设。首先，假设北极南部的“褐变”反应是对干旱胁迫引起的温度的非线性反应。研究人员建议通过编制一个环北极树木年轮年表数据库来验证这一假设，该数据库代表了北极所有主要树种和地区。这些年表中的大多数都可以在现有的数据存档中随时获得；他们建议在项目的第二年和第三年进行实地考察，以更新两个关键领域的年表。他们将使用回归树分析来模拟气候-树的生长关系。其次，他们假设NDVI的趋势与树木生长的趋势是一致的：他们期望干旱引起的对温度的阈值反应可以解释树木生长和NDVI数据。他们将通过比较AVHRR-NDVI记录和树木年轮数据来验证这一假设。最后，他们假设未来温度和降水将继续影响NDVI和北极森林生产力的趋势。他们将在有限数量的地点测试这一假设，使用定制版本的生物群落- bgc模型，一种生态生理森林生长模型。这项建模工作将使他们能够以定性的方式探索未来几十年南北极地区空间变化的树木生长（和NDVI）对碳动态的影响。这项研究将产生第一个环北极地区树木生长对气候响应的分析，解决北极森林对气候的非线性响应的普遍性（及其原因）这一关键问题。这将为了解北极南部土地覆盖未来可能的变化轨迹提供重要的见解：如果对温度的非线性响应占上风，那么未来森林向北极扩张的确定性似乎远不如响应主要是线性的情况。其次，我们提出的研究将把站点水平的气候响应与碳动态联系起来，使我们能够确定，至少对于一部分站点，非线性气候响应是如何改变森林碳通量的。

The expansion of forest vegetation within and into the Arctic is one of the profound transformations that the Arctic land surface is likely to undergo in the coming decades. The spread of forest vegetation has significant ramifications for the arctic system, as it is likely to cause both positive and negative feedbacks on climate, and to alter the availability of crucial natural resources. Although forest expansion within and into the Arctic has been widespread in recent decades, there is growing evidence that non-linear responses to warming may prevail within areas of expanding forest. In particular, large areas of 'browning' (declining trends in NDVI) have recently been identified in the southern Arctic. The goal of this research is to assess the relationships among tree growth, climate, and NDVI in order to identify the causes of non-linear responses to temperature (i.e., the 'browning' response) and better understand the likely effects of future warming on the productivity and dynamics of forest vegetation expanding into the Arctic. The proposed research tests three hypotheses. First, it is hypothesized that the 'browning' response in the southern Arctic represents a non-linear response to temperature induced by drought stress. The researchers propose to test this hypothesis by compiling a circum-arctic database of tree-ring chronologies, representing all major Arctic tree species and regions. Most of these chronologies are readily available in an existing data archive; they propose fieldwork in years 2 and 3 of the project to update chronologies from two key areas. They will use regression tree analysis to model the climate-tree growth relationships. Second, they hypothesize that trends in NDVI are consistent with trends in tree growth: they expect that a drought-induced threshold response to temperature explains both tree growth and the NDVI data. They will test this hypothesis by comparing the AVHRR-NDVI record with the tree-ring data. Finally, they hypothesize that temperature and precipitation will continue to influence trends in NDVI and arctic forest productivity in the future. They will test this hypothesis at a limited number of sites using a customized version of the Biome-BGC model, an ecophysiological forest growth model. This modeling exercise will allow them to explore, in a qualitative fashion, the implications of spatially variable tree growth (and NDVI) for C dynamics in the southern Arctic during the next several decades.The research should yield the first circum-arctic analysis of tree growth response to climate, addressing the critical question of the prevalence (and causes) of non-linear responses to climate in Arctic forests. This will provide important insight into the likely future trajectory of land cover change in the southern Arctic: if non-linear responses to temperature prevail, then future expansion of forests into the Arctic seems far less certain than if responses are primarily linear. Second, the proposed research will link the site-level climate responses to C dynamics, allowing us to determine, at least for a subset of sites, how non-linear climate responses alter forest C flux.