Moist tropical forests have huge amounts of carbon in their biomass and soils, and high rates of productivity: tropical forest growth, disturbance, and land use therefore have potentially large impacts on atmospheric CO2 levels. Accurate modeling of carbon uptake and release by forests is essential for global-change analyses, yet we do not even know how much CO2 tropical forests capture or produce. This project will make use of 19-yr-old replicated plantations in lowland Costa Rica to evaluate species-level controls over whole-forest carbon cycling. In comparisons among four species, three key plant traits that influence ecosystem-level carbon balance will be evaluated: 1) carbon use efficiency (CUE); 2) partitioning among plant parts that have differences in CUE (e.g., leaves, wood); and 3) tissue biochemistry, which influences carbon cycling rates.
The broader impacts of this project include training and international experience for both undergraduate and graduate students. Further development of the widely used Century model will offer a major advance in the ability to predict the effects of shifts in species composition on ecosystem-level carbon balances. Maintenance of this research site will provide infrastructure for other researchers, and a demonstration of restoration of ecosystem services. Results will be disseminated to the scientific community, and also to NGO's and land-holders in the area. Development of a user-friendly rainforest simulation model will enhance education opportunities for 29,500 K-12 students in the Miami, FL area, though a partnership with the Fairchild Tropical Botanical Garden.
Current climate-change models do not adequately represent the dynamic interactions that exist between vegetation, soils, the atmosphere, and climate. This research that was conducted in experimental plantations in Costa Rica that are dominated by single species addressed this deficit. Results of detailed plant physiological studies showed that individual tree species can differ in the balance between gaseous inorganic carbon (carbon dioxide) and solid organic carbon (organic matter). That is, the four tree species differed in the photosynthesis-respiration balance that influences atmospheric CO2 levels. Results of a field experiment involving addition to the soil of pure chemicals which are common constituents of dead plants showed that differences in detrital chemistry influenced how fast organic matter was decomposed and the extent to which it was stabilized. These species-level effects were incorporated into a terrestrial ecosystem model, to allow for integration of biological traits into the physical models that we currently depend upon for describing future climate scenarios. With the selection of individual species managed in so much of the world’s landscape, the choices made by land-use managers can feed back to atmospheric processes to drive climate changes. One of the most prevalent land-use changes within moist and wet tropical environments has been forest conversion to pasture. As a result, many formerly forested tropical landscapes currently support grasses. This research provides insight into the potential success of reforestation in such situations. Specifically we have found: (1) that very large trees can be grown quickly in the tropics, with proper management; (2) that very high rates of carbon sequestration can be maintained for at least two decades in former pastureland; (3) that single-species plantations can serve as nursery areas for local plant and animal species, and therefore promote biodiversity conservation despite the simple structure of the overstory; and (4) that soil fertility can be restored in degraded pasture sites. The success of this experiment has served to demonstrate that an alternative to cattle exists in the wet tropics, and that it offers many environmental and social benefits. Broader impacts included the development and dissemination of an ecosystem simulation model for use by middle-school students. This model was designed for hands-on investigation of relationships between tropical forest land use change, charismatic megafauna conservation, and atmospheric CO2 concentrations, and it was packaged with lesson activities to stimulate student exploration. It was used by more than 1250 students in the Miami-Dade County-area, as part of the Fairchild Tropical Botanical Garden’s 'Challenge' Program. Student training was also an important component of this project. Three graduate students, four undergraduates, one high school student, and two K-12 teachers were supported and mentored as a result of this award.
