Estimating Forest Productivity Over Regional and Multi-Decadal Time Scales Using Carbon Isotopes in Tree Rings

陆地生态系统是全球碳循环的关键组成部分，需要更好地了解它们在碳循环动态中的作用，以预测未来的碳循环。目前的生态系统碳循环模型对陆地碳循环对气候变化的响应给出了高度可变的预测。限制生态系统碳预算预测是一个重要问题，因为人们有兴趣管理森林，以增加碳汇的强度，以减少人为碳排放。需要关于森林生态系统生产力和森林与大气之间的净碳交换的空间明确数据，这些数据跨越几十年到百年的时间段，以限制模型参数和评估模型的性能。但是，这样的记录并不多见。目前的观测和遥感数据仅限于过去几十年，树轮宽度记录与当代净碳通量测量结果的相关性相对较差。在这项研究项目中，研究人员假设，树木年轮中的碳同位素代表了森林生态系统生产力和森林与大气之间的净碳交换。这一假设是基于碳同位素理论和对位于涡旋协方差二氧化碳通量塔足迹内的树木年轮的碳同位素测量。这个项目将检验两个相关的假设。首先，树木年轮中的碳同位素可以很好地估计森林生产力和净生态系统碳交换。这将通过将树木的同位素测量结果与附近长期运行的涡旋协方差通量塔的测量结果进行比较来确定。第二个假设将调查代表森林生产力和生态系统交换的树轮记录中碳同位素记录的时间变异性是否在相隔数百公里的两个通量塔之间是一致的。这将通过对两个通量塔之间的九个地点的树木年轮进行抽样，并分析这些地点的树木年轮同位素记录和其他几个气候和森林生产力变量的空间连贯性来确定。如果这些假设得到支持，就可以建立一个特定于生态系统的森林生态系统生产力和净碳交换记录网络，以对下一代地球系统和碳循环模型进行参数化。该项目的科学教育影响将延伸到大学以外，开发供中学使用的学习模块，利用宾夕法尼亚州立大学建立的NSF GK-12项目，为代表不足的群体提供服务。该项目将确定树木年轮中的碳同位素是否记录了森林和大气之间的二氧化碳交换。当代的森林-大气交换测量只能提供大约20年的时间，因此很难测试用于预测未来森林和其他生态系统的碳收支的模型，并确定它们对未来气候变化的可能贡献。该项目将展示利用树木年轮中的碳同位素在地方和区域范围内估计一个世纪或更长时期的森林生产力和森林净碳储量的潜力。对区域百年尺度森林生产力和净碳储量的更准确估计将为气候和碳管理相关政策决策提供信息，以减少人为碳排放的碳固存努力。

Terrestrial ecosystems are a key component of the global carbon cycle and a better understanding of their role in carbon cycle dynamics is needed to project the future carbon cycle. Current ecosystem carbon cycle models give highly variable projections for the response of the terrestrial carbon cycle to changing climate. Constraining ecosystem carbon-budget projections is an important issue given the interest in managing forests to increase the strength of the carbon sink to mitigate anthropogenic carbon emissions. Spatially explicit data on forest ecosystem productivity and net carbon exchange between the forest and atmosphere that span multi-decadal to centennial time periods, are needed to constrain model parameters and to evaluate model performance. But, such records are sparse. Current observational and remotely sensed data are limited to the last few decades, and tree-ring width records have shown relatively poor correlation with contemporary net carbon flux measurements. In this research project, the investigators postulate that carbon isotopes in tree-rings represent a proxy of forest ecosystem productivity and net carbon exchange between forests and the atmosphere. This hypothesis is based on carbon isotope theory and carbon isotope measurements from tree rings located in the footprint of an eddy covariance carbon dioxide flux tower. This project will test two related hypotheses. The first is that carbon isotopes in tree rings provide a good estimate of forest productivity and net ecosystem carbon exchange. This will be determined by comparing isotope measurements from trees with measurements from nearby long-running eddy-covariance flux towers. The second hypothesis will investigate whether the temporal variability of the carbon isotope record in the tree ring records representing forest productivity and ecosystem exchange are coherent between two flux towers separated by several hundred kilometers. This will be determined by sampling tree rings at nine sites between two flux towers and analyzing the spatial coherence of the tree-ring isotope record and several other climate and forest productivity variables across the sites. If the hypotheses are supported, a network of ecosystem-specific records of forest ecosystem productivity and net carbon exchange could be established to parameterize the next generation of earth system and carbon cycle models. The science education impact of this project will extend beyond the university by developing learning modules for use in secondary schools that service underrepresented groups using an established NSF GK-12 project at Penn State.This project will determine if carbon isotopes in tree rings record the exchange of carbon dioxide between forests and the atmosphere. Contemporary forest-atmosphere exchange measurements are only available for approximately twenty years, making it difficult to test models used to project the carbon budget of forests and other ecosystems into the future and to determine their likely contribution to future climate change. This project will demonstrate the potential of using carbon isotopes in tree rings to estimate forest productivity and net carbon storage by forests over time periods of a century or more at local to regional scales. More accurate estimates of regional century-scale forest productivity and net carbon storage will inform climate and carbon management-related policy decisions regarding carbon sequestration efforts to mitigate anthropogenic carbon emissions.