For more accurate projections of both the global carbon (C) cycle and the changing climate, a critical current need is to improve the representation of tropical forests in Earth system models. Tropical forests exchange more C, energy, and water with the atmosphere than any other class of land ecosystems. Further, tropical-forest C cycling is likely responding to the rapid global warming, intensifying water stress, and increasing atmospheric CO2 levels. Projections of the future C balance of the tropics vary widely among global models. A current effort of the modeling community, the ILAMB (International Land Model Benchmarking) project, is to compile robust observations that can be used to improve the accuracy and realism of the land models for all major biomes. Our goal with this paper is to identify field observations of tropical-forest ecosystem C stocks and fluxes, and of their long-term trends and climatic and CO2 sensitivities, that can serve this effort. We propose criteria for reference-level field data from this biome and present a set of documented examples from old-growth lowland tropical forests. We offer these as a starting point towards the goal of a regularly updated consensus set of benchmark field observations of C cycling in tropical forests.
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Acknowledgements. This work was made possible by the support of the US Geological Survey John Wesley Powell Center for Analysis and Synthesis. The support from the Powell Center included funding the participation of Shinichi Asao. Deborah A. Clark was supported by US National Science Foundation LTREB grants DEB-1147367 and DEB-1357112. Tana E. Wood was supported by US Department of Energy, Terrestrial Ecosystem Sciences grant DE-SC0011806 and by the USDA Forest Service International Institute for Tropical Forestry in collaboration with the University of Puerto Rico. Xiaojuan Yang was supported by the Next-Generation Ecosystem Experiments-Tropics and the Biogeochemistry–Climate Feedbacks Scientific Focus Area (BGC Feedbacks SFA) of the US Department of Energy, Office of Science, Office of Biological and Environmental Research. Peter B. Reich was supported by the US Department of Energy, Office of Science (DE-SC0012677). Sasha Reed was financially supported by US Department of Energy, Terrestrial Ecosystem Sciences grant DE-SC0011806 and by the US Geological Survey Ecosystems Mission Area. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US Government. David B. Clark constructively commented on the paper.