Mechanisms of age-related changes in forest production: The influence of physiological and successional changes

Net primary production (NPP) declines as forests age, but the causal role of decreased gross primary production (GPP), or increased autotrophic respiration (R_a) is still a matter of debate. This uncertainty complicates predicted responses to future climate, as higher atmospheric carbon dioxide (CO₂) concentrations may amplify the carbon (C)-sink in temperate forests if GPP controls the decline in NPP, but increased temperatures may decrease this C-sink if R_a controls the NPP decline. We quantified NPP in forests dominated by loblolly pine (Pinus taeda) in North Carolina, USA that varied from 14 to 115 years old. We used a sap-flow approach to quantify summer canopy photosynthesis by pines and later-successional hardwood trees, and measured wood CO₂ efflux to investigate age-related changes in pine R_a. Despite increasing production by later-successional hardwoods, an 80% decline in pine NPP caused ecosystem NPP to decline with age by ~40%. The decline in pine NPP was explained by reduced stomatal conductance and photosynthesis, supporting the hypothesis that increasing hydraulic limitation and declining GPP drove the age-related decline of NPP in this species. The difference between GPP and NPP indicated that pine R_a also declined with age; this was corroborated by measurements of reduced stem CO₂ efflux with increasing age. These results indicate that C cycling in these successional temperate forests is controlled by C input from GPP, and elements of global change that increase GPP may increase the C-sink in aging warm-temperate pine forests.