Controls on terrestrial carbon feedbacks by productivity and turnover of the vegetation and decomposing C pools in the CMIP5 ESMs

To better understand sources of uncertainty in modeled terrestrial carbon cycle feedbacks, we present an approach to separate the controls on productivity-driven from turnover-driven equilibrium carbon changes of both vegetation and decomposing carbon pools for a set of models participating in the CMIP5 1%/yr CO2 ESM experiments. We find that changes to the live vegetation pools are primarily explained by productivity-driven changes to equilibrium carbon stocks, with only one model showing large compensating change to vegetation turnover times. The changes in vegetation turnover in response to global change that are projected arise mainly from changes to allocation and fire regimes. For the decomposing carbon pools, the situation is more complex as the production-driven changes are not independent from turnover-driven changes. This dependence arises from the multi-pool representation of decomposition in the models, in which respiration responds more rapidly to perturbations than C stocks, and thus leads to transient but long-lived reductions in turnover times in all models in response to increases in productivity. This mechanism, which we refer to as "tau compression", masks much of the intrinsic response of turnover to changing climate. These patterns hold across the fully-coupled, biogeochemically-coupled, and radiatively-coupled experiments, demonstrating the importance of production changes in governing turnover changes, particularly for decomposing carbon pools, under global change scenarios.