Age-driven decline of forest productivity and respiration

Background/Question/Methods: The traditional view of forest dynamics suggests the decline in net primary productivity (NPP) in ageing forests due to stabilized photosynthesis and continuously increased autotrophic respiration (R). The alternative conceptual model which proposed a decline in R with forest age, also suffered from a shortage of belowground data. We analyze data from a recently assembled global database of carbon fluxes, as well as from a sample chronosequence dataset in northern deciduous forests in Wisconsin and Michigan.

Results/Conclusions: Our results substantiate the age-driven decline in NPP, but in contrast to the traditional view, both gross primary production (GPP) and R decline in ageing forests. The decline in NPP in ageing forests is primarily driven by GPP, which decreases more rapidly with increasing age than R does, but the ratio of NPP/GPP remains approximately constant within a biome. These results imply that dynamic forest ecosystem models should revisit the age effect on production and respiration.