Refining Estimates of Tropical Forest Greenhouse Gas Exchange using Plant Traits

Tropical forests play a crucial role in regulating global concentrations of greenhouse gases, exchanging vast amounts of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) with the atmosphere. The importance of tropical forests as both a source and substantial sink of CO2 is well established, but we know much less about the contribution of tropical forests to global CH4 and N2O budgets. Quantifying CH4 and N2O exchange in tropical forests is important because, although wetland tropical forests are a large source of CH4, free-draining tropical soils are a source of N2O but a sink for CH4. Trees can also act as conduits for greenhouse gases, and our recent work demonstrated that tropical trees on free-draining soils can emit or take up both CH4 and N2O. However, the contribution of trees to tropical forest greenhouse gas budgets is entirely unknown and we do not know whether tropical forest trees on free-draining soils will act as an overall sink or source for CH4 and N2O. We urgently need to address this knowledge gap because tropical forests on free-draining soils cover vast areas of land and are being destroyed or disturbed by human activities at an alarming rate. To quantify the role of tropical trees in global greenhouse gas budgets, we need an approach that allows us to translate local or regional measurements into models that can represent tropical forests across much larger scales. Our project will develop such an approach by quantifying the emissions and uptake of CH4 and N2O by tropical forest soils and trees, and determining the extent to which these greenhouse gas fluxes are influenced by specific characteristics of tropical trees.
Trees can influence the production or consumption of greenhouse gases in the soil because root activity and plant litter inputs alter soil chemistry and microbial activity. Similarly, uptake or emission of greenhouse gases through stems, branches and leaves varies widely among tree species with distinct traits such as wood density, growth rate, or foliar chemistry. Our project will establish how such tree traits influence greenhouse gas fluxes by taking detailed measurements of CH4 and N2O fluxes from trees and the surrounding soil. In the first year of the project, we will determine the relationships between greenhouse gas fluxes and differences in soil chemistry or tree traits for six tree species at an intensive study site in tropical forest in Panama, Central America. In the second year of the project, we will expand our studies to multiple sites along a rainfall gradient to assess greenhouse gas fluxes from soils and trees for a larger range of species, and to identify the influence of changes in rainfall. Hence, our field measurements will provide the first ever comprehensive assessment of CH4 and N2O fluxes from tropical trees on free-draining soils, representing an entirely new component of the tropical greenhouse gas budget. In the final year of the project, we will use our data on variation within and among sites, species and individual trees to construct models that estimate total forest greenhouse gas fluxes using a combination of relevant plant traits and rainfall data. We will apply our models to 15 plots along the rainfall gradient to obtain estimates of forest greenhouse gas exchange across the landscape. By using tree traits rather than species identities, our approach will offer a way to estimate greenhouse gas uptake and emissions across wide areas of the tropics, which will improve the representation of tropical forests in global greenhouse gas budgets and inform impact assessments of tropical land-use change.