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北极陆地表面在今后几十年可能经历的深刻变化之一。森林植被的蔓延对北极系统有着重大影响，因为它可能对气候产生积极和消极的反馈，并改变关键自然资源的可用性。虽然近几十年来北极内部和向北极地区的森林扩张十分普遍，但越来越多的证据表明，在森林扩张的地区，对变暖的非线性反应可能占主导地位。特别是，最近在北极南部发现了大面积的“布朗宁”（归一化差异植被指数下降趋势）。这项研究的目标是评估树木生长、气候和NDVI之间的关系，以确定对温度的非线性响应的原因（即，"布朗宁“反应），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。该研究测试了三个假设。首先，据推测，在北极南部的“布朗宁”的反应是一个非线性响应干旱胁迫引起的温度。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测试这一假设，该数据库代表了所有主要的北极树种和地区。这些年表大多在现有的数据档案中很容易找到;他们建议在项目的第二年和第三年进行实地工作，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与树木生长的关系。其次，他们假设，在NDVI的趋势是一致的树木生长的趋势：他们预计，干旱引起的阈值响应温度解释树木生长和NDVI数据。他们将通过比较AVHRR-NDVI记录和树木年轮数据来检验这一假设。最后，他们假设温度和降水量将继续影响未来NDVI和北极森林生产力的趋势。他们将使用定制版本的Biome-BGC模型（一种生态生理学森林生长模型）在有限数量的地点测试这一假设。这种建模工作将使他们能够探索，在一个定性的方式，空间可变的树木生长（和NDVI）的影响，在北极南部的C动态在未来几十年的研究应该产生第一次环北极分析树木生长对气候的响应，解决的流行率（和原因）的关键问题，在北极森林气候的非线性响应。这将为了解北极南部土地覆被变化的未来可能轨迹提供重要的见解：如果对温度的非线性反应占主导地位，那么未来森林向北极的扩展似乎远不如主要线性反应那么确定。第二，拟议的研究将链接到C动态的站点级气候响应，使我们能够确定，至少对于一个子集的网站，非线性气候响应如何改变森林碳通量。

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.