Productivity and turnover controls on terrestrial carbon feedbacks in the CMIP5 ESMs
We present an approach to separate the controls on productivity-driven from turnover-driven carbon changes of both live vegetation and dead soil and litter carbon pools, and apply this method to the set of models participating in the CMIP5 carbon cycle feedback experiments. The purpose of this approach is to better partition the uncertainty of carbon cycle feedbacks to ecological driving processes: changes to productivity, changes to turnover of live carbon via changes in mortality or allocation, and changes to turnover of dead carbon via changes to heterotrophic respiration rates. We apply this analysis to both the climate-driven responses and the CO2 concentration-driven responses.
We find that changes to the live pools are primarily explained by productivity-driven changes, with only one model showing reductions in live C turnover times. For dead carbon pools, the situation is more complex as all models predict a strong reduction in turnover-driven dead C in response to increases in productivity-driven dead C. This responses arises from the common representation of a broad spectrum of decomposition turnover times via a multi-pool approach, in which flux-weighted turnover times are faster than mass-weighted turnover times, and which leads to a shift in the distribution of carbon among dead pools in response to changes in inputs, and therefore a transient but long-lived reduction in turnover times in response to increases in productivity. Since this behavior is superficially similar to priming processes, but occurring without the mechanisms responsible for priming, we call the phenomenon "false priming". These patterns hold across the fully-coupled, biogeochemically-coupled, and radiatively-coupled 1%/year increasing CO2 experiments. We disaggregate inter-model uncertainty in the C responses to initial turnover times and productivity and fractional changes in turnover and productivity, and find that for both the live and dead C pools, inter-model spread in C changes arising from initial conditions is dominated by model disagreement on turnover times, whereas inter-model spread in carbon changes from fractional changes to these terms is dominated by model disagreement on changes to productivity in response to both warming and CO2 fertilization. However, the lack of changing turnover time control on C responses, for both live and dead C pools, in response to the imposed forcings may indicate a common lack of process representation behind changing turnover times (e.g. allocation and mortality for live C; permafrost, microbial dynamics, and mineral stabilization for dead C) rather than a true estimate of the uncertainty in these processes.