The extent to which terrestrial ecosystems will continue to accumulate carbon with CO2 enrichment and climate warming is a major source of uncertainty in earth system model projections of future climate. Among the most concerning scenarios is one where the recent acceleration of carbon accumulation in forest ecosystems proves to be a transient state of the system, such that this carbon returns to the atmosphere in the near future. Predicting the timescales over which carbon in plant biomass continues to accumulate in ecosystems or is released to the atmosphere, depends on demographic projections of recruitment, growth, and mortality processes at the population level and changing species composition at the community level. However, general rules for the population and community dynamics required to generate only transitory carbon accumulation in terrestrial ecosystems have not been explored. Here, we develop a simple, analytically tractable model of a forest ecosystem to identify the demographic and competitive processes favoring various timescales of carbon storage in terrestrial ecosystems.
Results/Conclusions
Results from a stage-structured, single species demographic model show that if the rate at which canopy trees mature to even larger canopy trees increases linearly with time owing to warming or CO2 enrichment, this sets in motion internal demographic dynamics that increase carbon accumulation in the short term, even if the larger canopy trees ultimately suffer greater mortality. By contrast, greater mortality in populations of these larger canopy trees reduces carbon storage in the long term, causing carbon initially stored in the ecosystem to return to the atmosphere in later years. Counterintuitively, in a two competitor version of the same forest model, the increased dominance of a larger species with greater mortality does not induce similarly transient carbon accumulation. This result arises because the superior demographic performance that makes for a dominant competitor also tends to favor both short and long term carbon storage in the ecosystem. Together, our results suggest that while demographic change can induce carbon accumulation in terrestrial ecosystems that is only transitory, these carbon dynamics may be less likely to arise from changing species composition.