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

北极圈内和北极圈内森林植被的扩展是北极陆地表面在未来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具有重大影响，因为它可能对气候造成正负反馈，并改变关键自然资源的可获得性。尽管近几十年来，北极地区的森林扩张很普遍，但越来越多的证据表明，对气候变暖的非线性反应可能会在森林扩张的地区盛行。尤其值得一提的是，最近在北极南部发现了大面积的“褐化”(NDVI下降趋势)。这项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反应(即褐变反应)的原因，并更好地了解未来变暖对扩展到北极的森林植被的生产力和动态的可能影响。这项拟议的研究测试了三个假设。首先，假设北极南部的褐变反应是由干旱胁迫引起的对温度的非线性反应。研究人员建议通过编制一个代表北极所有主要树种和地区的年轮年表的环北极数据库来检验这一假设。这些年表中的大多数都可以在现有的数据档案中随时获得；它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-树木生长关系。其次，他们假设NDVI的趋势与树木生长的趋势是一致的：他们预计，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。他们将通过将AVHRR-NDVI记录与树木年轮数据进行比较来检验这一假设。最后，他们假设温度和降水将继续影响未来NDVI和北极森林生产力的趋势。他们将在有限的几个地点使用定制版本的Biome-BGC模型来测试这一假设，BGC模型是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空间可变的树木生长(和NDVI)对未来几十年北极南部碳动态的影响。这项研究将首次对树木生长对气候的响应进行环北极分析，解决北极森林对气候的非线性响应的流行(和原因)这一关键问题。这将提供对北极南部土地覆盖变化未来可能轨迹的重要洞察：如果对温度的非线性反应占上风，那么未来向北极扩张森林的可能性似乎远低于主要是线性反应的情况。其次，拟议的研究将把立地层面的气候反应与碳动态联系起来，使我们能够确定，至少对一部分地点而言，非线性气候反应如何改变森林的碳通量。

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.