Panama

Methane (CH4) is an important greenhouse gas that is ~25 times more powerful than CO2 at trapping the Sun's energy. There is therefore considerable interest in the processes involved in CH4 production, principally in waterlogged soils in wetlands, and the processes that lead to its emission to the atmosphere. This study is concerned with processes that enhance the amount of CH4 emitted to the atmosphere, in particular, a novel mechanism for transferring CH4 from soil to the atmosphere. It is generally thought that CH4 produced in waterlogged soils is emitted by a combination of three processes: 1) by diffusion through water-filled pores, 2) by abrupt release of bubbles, and 3) through internal spaces in the stems of grass-like plants which are adapted to live in waterlogged soils. We propose that the stems of wetland trees also provide an important conduit for the transfer of CH4 from wet soils to the atmosphere, a possibility that to date has been almost entirely overlooked. This project builds on published data gathered by this team which showed that mature temperate wetland alder trees indeed emit CH4 via their trunks, a finding that is corroborated by one other recent study of ash trees in Japan. This is an important finding because wetlands are the largest single source of CH4 emissions to the atmosphere and 60% of these ecosystems are forested. We now have additional unpublished data that was collected in the spring of 2011 (10 weeks before the call deadline) which show that tropical peat swamp forest trees in Borneo emit 65% off all ecosystem methane emissions and twice as much as emissions currently quantified from the peatland surface. At present, researchers working in forested wetlands typically measure only CH4 emitted from the soil surface and thus we assert that the total amount of CH4 being released from these ecosystems is being grossly underestimated. This oversight in the past may also explain why different ways of estimating CH4 emissions for a region rarely agree. Estimates of CH4 emission obtained from satellite or atmospheric measurements are often greater than estimates based on observations made at ground level. This is particularly evident in forested tropical areas. Our finding that trees enhance venting of CH4 from soil is a possible explanation to account for the discrepancy, in part, because soils in many of the forested areas are flooded either seasonally and in many cases permanently, which means an abundance of CH4 should be present in soils. We suggest that there are two ways by which CH4 produced in wet soils may be transported and emitted through trees: i) as a gas through air-filled tissue in trees that has formed as an adaptation to enable transfer of oxygen from the atmosphere to the tree's roots which are growing in oxygen-poor waterlogged soil, and ii) dissolved in sap and then liberated to the atmosphere when tree water is lost by transpiration through pores in tree stems and leaves. In the proposed study we will measure CH4 emissions from tropical wetlands, principally in Borneo but also in Panama using techniques to help distinguish the tree emission routes and establish their contribution to ecosystem methane flux as measured using larger scale micro-meteorological methods. We will also measure the ratio of two naturally occurring 'versions' (isotopes) of carbon: the relatively rare heavy isotope carbon-13 and the lighter more common carbon-12. The ratio of these isotopes of carbon in CH4 in the soil and in tree emissions provides valuable information about how CH4 is produced and how it moves through the tree. Ours will be the first multi-year study of tropical wetland tree emissions which should, for the first time, establish the true contribution of these ecosystems to the atmospheric methane concentration.

Abstracts are not currently available in GtR for all funded research. This is normally because the abstract was not required at the time of proposal submission, but may be because it included sensitive information such as personal details.

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.

At least 50% of earth's plant and animal species is found in tropical rainforests, but this rich biodiversity is under threat from deforestation and climate change. Ecologists are interested in understanding why these habitats are so diverse, and how their diversity will change in the future. One leading explanation for high plant biodiversity in tropical forests is the Janzen-Connell Effect. This theory suggests that pests such as plant-feeding insects and fungal diseases can help maintain tropical biodiversity if (1) they specialise on particular plant species, and (2) they cause 'density-dependent' mortality (i.e., they kill more seeds and seedlings where these are locally abundant). This pest pressure acts as a negative feedback mechanism, putting locally rare plant species at an advantage and preventing any one species from reaching high abundance. Recent research shows that this form of density-dependence from both insects and fungi plays a key role in the maintenance of plant diversity in the tropics. We now want to discover how this process changes under different climatic regimes. Wetter tropical forests have more plant species than drier forest, and we will test the theory that more intense density-dependent pest pressure in these places is a factor behind these differences. We will also investigate whether future changes to the climate (higher or lower rainfall) are likely to alter the strength of the Janzen-Connell Effect, and consequently plant diversity. Our work will take place in Panama, where we will take advantage of a steep gradient in rainfall and soil humidity from the dry (Pacific) coast to the humid (Atlantic) coast to test our hypotheses. We will carry our experiments in the field and in controlled nursery conditions that manipulate the density of seeds and seedlings and the presence of fungal pathogens and plant-feeding insects, and we will analyse long-term data and build mathematical models to explore whether and to what extent climate change will alter tropical plant diversity.