Predicting long‐term carbon sequestration in response to CO 2 enrichment: How and why do current ecosystem models differ?

Large uncertainty exists in model projections of the land carbon (C) sink response to increasing atmospheric CO 2 . Free‐Air CO 2 Enrichment (FACE) experiments lasting a decade or more have investigated ecosystem responses to a step change in atmospheric CO 2 concentration. To interpret FACE results in the context of gradual increases in atmospheric CO 2 over decades to centuries, we used a suite of seven models to simulate the Duke and Oak Ridge FACE experiments extended for 300 years of CO 2 enrichment. We also determine key modeling assumptions that drive divergent projections of terrestrial C uptake and evaluate whether these assumptions can be constrained by experimental evidence. All models simulated increased terrestrial C pools resulting from CO 2 enrichment, though there was substantial variability in quasi‐equilibrium C sequestration and rates of change. In two of two models that assume that plant nitrogen (N) uptake is solely a function of soil N supply, the net primary production response to elevated CO 2 became progressively N limited. In four of five models that assume that N uptake is a function of both soil N supply and plant N demand, elevated CO 2 led to reduced ecosystem N 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 C sequestration. In addition, self‐thinning assumptions had a substantial impact on C sequestration in two models. Accurate representation of N process dynamics (in particular N uptake), allocation, and forest self‐thinning is key to minimizing uncertainty in projections of future C sequestration in response to elevated atmospheric CO 2 .