Modeling the complex effects of climatic variation on tropical rainforest carbon cycling

Background/Question/Methods In old-growth moist tropical forest at La Selva Biological Station in Costa Rica, we used process-based modeling to address the question of how climate change might influence carbon, nutrient, and water cycles. We calibrated the CENTURY 4.5 model, using the monthly tropical forest version, and incorporating long-term weather and empirically based biogeochemistry data sets for this site. Using the model in an experimental mode, we explored effects on nutrient and water cycles under simulated El Niño Southern Oscillation (ENSO) events, including an El Niño year with the wet season characterized by very high rainfall and high temperature and the dry season by very low rainfall and high temperature. This was followed by La Niña, a year with average temperature but higher wet-season rainfall and lower dry-season rainfall. We also explored three climate-change scenarios: 1) minimum (night-time) air temperatures increasing; 2) dry season increasingly drier; and 3) annual rainfall amounts average, but events less frequent and more intense. Within each scenario, the experimental climate was imposed at five different levels for 500,000 years at each level, but ecosystem responses generally reached equilibria within 10 years. 

Results/Conclusions Simulated potential evapotranspiration (PET) was 8% higher during El Niño, but stream-flow was 41% higher, such that leaching of organic C and total N also increased. In general, net primary productivity (NPP) was less variable during ENSO, but biomass declined slightly. Soil C stocks remained constant, but N leaching losses rose 47%. Over a 2°C increase in night-time temperatures, ecosystem responses were strong, with biomass declining by nearly 25% and NPP by ~20%, with concomitant losses in soil C and N. Decline in biomass and in PET likely drove the simulated response of increased stream-flow. Mineralization rates of N and P declined with increasing temperature however, such that leaching losses also declined, despite higher stream-flow. Biomass and NPP reached their maxima at ~380 cm per yr, suggesting that light becomes relatively limiting at the higher rainfall levels typically experienced at La Selva. Soil C and N stocks also increased as rainfall decreased, as decomposition and N mineralization rates decreased. These simulation results suggest that tropical moist forest ecosystems will respond measurably in their cycling of carbon, nutrients, and water as climate change progresses, especially with rising minimum temperatures.