DISSERTATION RESEARCH: Drivers of Carbon Uptake in Tropical Forests: Implications for the Future of the Global Carbon Sink

世界海洋和森林每年吸收人类活动排放的二氧化碳(CO2)的一半。最近的研究表明，热带完好的森林可能占全球森林碳排放量的三分之一。这些森林的碳吸收被广泛归因于大气中二氧化碳浓度上升导致植物生长增加。二氧化碳是植物建造树叶、树根和木材的原材料，在温室和完好的森林中的实验表明，在更高的二氧化碳水平下，植物的生长往往会增加，这一效应被称为二氧化碳施肥。尽管气候模型预测森林将在本世纪继续吸收碳，但这种碳吸收的规模及其随时间的变化程度将取决于植物所需的其他资源的可用性。与肥力较低的土壤上的森林相比，在水分和阳光充足的高肥力土壤上的森林可能经历更大和更持久的增长，因为低肥力土壤上的森林缺乏足够的这些资源。这项研究的目的是在整个区域的范围内，如亚马逊盆地，估计资源可获得性对热带森林生长的制约程度。使用最新开发的从二维图像生成三维(3D)模型的计算机算法，可以从高分辨率卫星图像推导出热带森林的3D模型。热带森林的碳吸收可以通过计算两个时间段之间森林模型的差异来估计。这项工作将分析碳吸收随降雨、温度、海拔和土壤年龄等因素的变化，以估计资源可获得性在多大程度上限制整个热带地区的森林生长。关于资源限制严重程度的定量信息是必要的，以限制对本世纪森林可能吸收的碳量的估计，这对改善气候预测至关重要。此外，这项工作对减排政策的优化设计具有直接的指导意义。实现给定大气二氧化碳浓度的经济成本取决于预计碳林可以吸收多少碳。森林中的碳吸收受到的资源限制越多，减排幅度就越大，才能达到给定的大气二氧化碳浓度。

The world's oceans and forests annually absorb half of all carbon dioxide (CO2) emissions from human activities. Recent studies indicate that intact forests in the tropics may account for as much as one-third of the global uptake of carbon emissions by forests. Carbon uptake in these forests is widely attributed to increased plant growth as a result of rising atmospheric CO2 concentrations. CO2 is the raw material from which plants build leaves, roots, and wood, and experiments in greenhouses and intact forests demonstrate that plant growth often increases under higher CO2 levels, an effect known as CO2 fertilization. Although climate models project that forests will continue to absorb carbon over this century, the magnitude of this carbon uptake and the degree to which it changes over time will depend on the availability of other resources required by plants. Forests on higher fertility soils with sufficient water and sunlight are likely to experience larger and longer-lasting increases in growth than forests on lower fertility soils which lack sufficient levels of these resources. The objective of this research is to estimate the degree to which resource availability constrains forest growth in the tropics at the scale of entire regions, such as the Amazon Basin. Using newly developed computer algorithms that produce three-dimensional (3D) models from two-dimensional images, it is possible to derive 3D models of tropical forests from high-resolution satellite imagery. Carbon uptake by tropical forests can be estimated by calculating the difference in forest models between two time periods. This work will analyze variation in carbon uptake with factors, such as rainfall, temperature, elevation, and soil age, in order to estimate the degree to which resource availability limits forest growth throughout the tropics. Quantitative information about the severity of resource limitation is necessary to constrain estimates of how much carbon forests are likely to absorb over this century, which is essential to improving climate projections. Furthermore, this work has direct implications for the optimal design of emissions reduction policy. The economic cost of achieving a given atmospheric CO2 concentration depends on how much carbon forests can be expected to absorb. The more resource limitation constrains carbon uptake in forests, the steeper emissions reductions must be to achieve a given atmospheric CO2 concentration.