Climate or disturbance: Temperate forest structural change and carbon sink potential
Structural changes in forests have long-term implications for the terrestrial carbon sink, but remain a challenge to predict due to the complex interactions of succession, disturbance, and climate sensitivity. Estimates of existing forest structure and biomass are improving globally; however, vegetation models continue to show substantial spread in predictions of future land carbon uptake and the roles of forest structural change and demography are increasingly being recognized as important. Our study empirically unravels some of the fundamental processes that influence structural change and carbon storage in forests, and investigates interactions of changing climate with disease and other disturbance agents. To quantify successional structural change we use the US Forest Service (USFS) Forest Inventory and Analysis (FIA) database to map how stem density, radial tree growth, and net primary productivity (NPP) change as a function of mean diameter and relative density (stocking) over time for a broad range of common temperate forest densities.
Results show temperate forest mean tree size, density and carbon change in a coherent, cyclic pattern predicted by successional theory and indicate significant sensitivity to climate anomalies. For example, in the eastern US above average temperature (+1.0oC) was associated with a -29% (-0.55±0.08 Mg C ha-1 yr-1) reduction in net primary productivity attributed to higher rates of disease (+31%) and weather disturbance (+42%). Projections of future C sink potential suggest vegetation carbon would be lowest on managed lands (72±2.2 Mg C ha-1) and highest when larger trees survive in undisturbed conditions (152.7±20.6 Mg C ha-1). Results provide robust comparisons for global vegetation models, valuable projections for carbon mitigation efforts, and context for management.