DISSERTATION RESEARCH: Investigating the resiliency of the savanna-forest biome to environmental change

今天，北美中西部的非农业土地主要是封闭式森林，但不到200年前欧洲人定居之前的数据表明，该地区同时存在开阔的稀树草原和封闭式森林。森林的这种变化通常归因于火灾发生和频率的变化，但对树木年轮的分析表明，其他环境变化，如二氧化碳上升和天气模式的变化，可能导致了森林和稀树草原的变化。在这篇博士论文中，改良赠款(DDIG)奖励资金将用于测量树木年轮碳稳定同位素，以估计稀树草原和森林地点网络的水分利用效率的年度趋势，以确定树木生长变化的机制，以及在历史调查和年轮宽度数据中观察到的趋势。更好地了解过去导致森林覆盖率变化的许多因素，将有助于理解预期的未来环境变化可能如何影响中西部森林：更热的温度和干旱压力可能会导致树木衰退，将今天的森林推向稀树草原状态。或者，二氧化碳的增加可能会通过促进树木生长和有利于森林扩张来增强抗旱能力。中西部地区的这些替代轨迹在预测未来植被分布方面造成了不确定性，并使当地土地和森林管理者难以制定有效的资源管理计划。奖励资金将用于检验这样一个假设，即大气中二氧化碳的增加推动了树木生长的过去变化，并改变了森林对未来变化的适应能力。这项研究促进了对环境变化如何影响中西部森林弹性和生长的科学理解，并更广泛地确定了树木生长对环境变化具有弹性的条件。该奖项建立在一个地点网络的基础上，研究人员在这些地点制定了1850年至2015年在封闭森林和开阔草原上的橡树年轮生长记录。从这些地点年轮生长的年度趋势来看，研究人员已经确定了自1950年以来树木生长增加的地方，以及生长与气候的关系随着时间的推移而变化的地方。这样的变化与二氧化碳驱动的树木水分利用效率(树木每单位所需水分的碳吸收)的提高是一致的，从理论上讲，这可以推动树木生长的增加，并提高树木的抗旱性。然而，仅靠生长数据不能确定二氧化碳施肥是否是导致这些变化的机制。因此，研究人员将使用树木年轮中与水分利用效率相关的稳定碳同位素来确定二氧化碳上升在多大程度上可以解释树木生长的变化。总体而言，该项目将测试二氧化碳增强是否在稳定中西部森林覆盖趋势方面发挥了作用，并将为二氧化碳增强树木生长所需的条件提供科学见解。

Today, closed forests dominate the non-agricultural land in the North American Midwest, but data from before European settlement, less than 200 years ago, indicate that both open savanna and closed forests co-existed in the region. Such changes in forests are often attributed to changes in fire occurrence and frequency, but analyses of tree rings suggest other environmental changes such as rising CO2 and changes in weather patterns that may have contributed to shifts in forests and savannas. In this Doctoral Dissertation Improvement Grant (DDIG) award funds will be used to measure tree ring carbon stable isotopes to estimate annual trends in water use efficiency across a network of savanna and forest sites to determine the mechanism for changes in tree growth and the trends observed in historical survey and ring-width data. A better understanding of the many factors responsible for changing forest cover in the past will help in understanding how anticipated future environmental changes might affect Midwestern forests: Hotter temperatures and drought stress could drive tree decline, pushing today's forests towards the savanna state. Alternatively, increases in CO2 may confer drought resilience by enhancing tree growth, and favoring forest expansion. These alternative trajectories for the Midwest create uncertainty in predicting future vegetation distribution and make it difficult for local land and forest managers to develop effective resource management plans. The award funds will be used to test the hypothesis that increased atmospheric CO2 has driven past changes in tree growth and altered forest resilience to future changes. This research advances scientific understanding of how environmental changes affect Midwestern forest resilience and growth, and more broadly identifies the conditions where tree growth is resilient to environmental changes. This award builds on a network of sites where the researchers have developed annual tree ring growth records from 1850-2015 for oak trees in both closed forest and open savannas. From the annual trends in tree ring growth across these sites, the researchers have identified places where tree growth has increased since 1950 and where the growth-climate relationships change over time. Such shifts are consistent with CO2 driven enhancement of tree water use efficiency (carbon uptake by the tree per unit of water needed), which can theoretically drive increases in tree growth and improve drought tolerance in trees. However, growth data alone cannot determine if CO2 fertilization is the mechanism for these changes. Thus, researchers will use stable carbon isotopes in tree rings, which are related to water use efficiency, to determine the extent to which rising CO2 can explain changes in tree growth. Overall, this project will test whether CO2 enhancement played a role in stabilizing trends in Midwest forest cover, and will provide scientific insight into the conditions necessary for CO2 enhancement of tree growth.