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.
