Predicting Long-Term Carbon Sequestration in Response to CO2 Enrichment
Large uncertainty exists in model projections of the land carbon sink response to increasing atmospheric carbon dioxide (CO2). Free-Air CO2 Enrichment (FACE) experiments lasting a decade or more have investigated ecosystem responses to a step change in atmospheric CO2 concentration. To interpret FACE results in the context of gradual increases in atmospheric CO2 over decades to centuries, a recent study used a suite of seven models to simulate the Duke Forest and Oak Ridge FACE experiments extended for 300 years of CO2 enrichment. It also determined key modeling assumptions that drive divergent projections of terrestrial carbon uptake and evaluated whether these assumptions can be constrained by experimental evidence. All models simulated increased terrestrial carbon pools resulting from CO2 enrichment, though there was substantial variability in quasi-equilibrium carbon sequestration and rates of change. In two of two models that assume that plant nitrogen uptake is solely a function of soil nitrogen supply, the net primary production response to elevated CO2 became progressively nitrogen limited. In four of five models assuming that nitrogen uptake is a function of both soil nitrogen supply and plant nitrogen demand, elevated CO2 led to reduced ecosystem nitrogen losses and thus progressively relaxed nitrogen limitation. Many allocation assumptions resulted in increased wood allocation relative to leaves and roots, which reduced the vegetation turnover rate and increased carbon sequestration. In addition, self-thinning assumptions had a substantial impact on carbon sequestration in two models. Accurate representation of nitrogen process dynamics (in particular nitrogen uptake), allocation, and forest self-thinning is key to minimizing uncertainty in projections of future carbon sequestration in response to elevated atmospheric CO2.
Walker, A. P., S. Zaehle, B. E. Medlyn, M. G. De Kauwe, S. Asao, T. Hickler, W. Parton, D. M. Ricciuto, Y.-P. Wang, D. Wårlind, and R. J. Norby. 2015. “Predicting Long–Term Carbon Sequestration in Response to CO2 Enrichment: How and Why Do Current Ecosystem Models Differ?” Global Biogeochemical Cycles 29(4), 476–95. DOI: 10.1002/2014GB004995.