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 (Ra) 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 Ra 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 Ra. 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 Ra 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.