Alteration of forest succession and carbon cycling under elevated CO2.

Regenerating forests influence the global carbon (C) cycle, and understanding how climate change will affect patterns of regeneration and C storage is necessary to predict the rate of atmospheric carbon dioxide (CO2 ) increase in future decades. While experimental elevation of CO2 has revealed that young forests respond with increased productivity, there remains considerable uncertainty as to how the long-term dynamics of forest regrowth are shaped by elevated CO2 (eCO2 ). Here, we use the mechanistic size- and age- structured Ecosystem Demography model to investigate the effects of CO2 enrichment on forest regeneration, using data from the Duke Forest Free-Air Carbon dioxide Enrichment (FACE) experiment, a forest chronosequence, and an eddy-covariance tower for model parameterization and evaluation. We find that the dynamics of forest regeneration are accelerated, and stands consistently hit a variety of developmental benchmarks earlier under eCO2 . Because responses to eCO2 varied by plant functional type, successional pathways, and mature forest composition differed under eCO2 , with mid- and late-successional hardwood functional types experiencing greater increases in biomass compared to early-successional functional types and the pine canopy. Over the simulation period, eCO2 led to an increase in total ecosystem C storage of 9.7 Mg C ha(-1) . Model predictions of mature forest biomass and ecosystem-atmosphere exchange of CO2 and H2 O were sensitive to assumptions about nitrogen limitation; both the magnitude and persistence of the ecosystem response to eCO2 were reduced under N limitation. In summary, our simulations demonstrate that eCO2 can result in a general acceleration of forest regeneration while altering the course of successional change and having a lasting impact on forest ecosystems.