Background/Question/Methods Anthropogenic emissions of carbon dioxide (CO₂) are causing unwanted climate change, but the rate of such change is partially determined by how much CO₂ terrestrial ecosystems can absorb into plant biomass via photosynthetic pathways. Tropical forests are notably important to this process at the global scale, as they store substantial amounts of carbon (C) and exchange more CO₂ with the atmosphere than any other terrestrial biome.As with any ecosystem, rates of tropical forest C uptake and storage are mediated by climate, but the role of precipitation remains poorly understood and oft-debated. Some pan-tropical analyses suggest a uni-modal pattern between tropical forest above-ground net primary production (ANPP) and precipitation, with peak rates near 2500 mm/yr, and declining values in the wettest of regions. However, this downturn appears to be driven by plant-soil nutrient feedbacks that may not be common in lowland systems. Using meta-analysis, we assembled a new and expanded tropical carbon dynamics database (~656 sites) to re-visit the potential response of tropical forest C dynamics to shifts in climate.  

Results/Conclusions

We found that the direction and magnitude of climate-forest C relations diverge significantly for different types of tropical forests. First, we demonstrate that rainfall seems to ANPP and biomass in lowland forests receiving rainfall in excess of 2500 mm/yr, whereas montane systems exhibit the opposite pattern. Second, using broad indices of soil nutrient status, we find suggestions that some variation in these divergent patterns may be explained by interactions between nutrient availability, soil structure and temperature. The results of our analyses suggest divergent trajectories for C uptake and storage for different tropical forests in response to climate change, and suggest that commonly used relationships between ANPP and rainfall may not be correct. Finally, we integrate our plot-based meta-analysis with simulations using the CASA biosphere model to explore both current and potential future pattern in tropical ANPP in relation to changes in precipitation.