Net primary productivity (NPP) is enhanced under future atmospheric [CO2] in temperate forests representing a broad range of productivity. Yet questions remain in regard to how elevated [CO2]-induced NPP enhancement may be affected by climatic variations and limiting nutrient resources, as well as how this additional production is distributed among carbon (C) pools of different longevities. Using 10 years of data from the Duke free-air CO2 enrichment (Duke FACE) site, we show that spatially, the major control of NPP was nitrogen (N) availability, through its control on canopy leaf area index (L). Elevated CO2 levels resulted in greater L, and thus greater NPP. After canopy closure had occurred, elevated [CO2] did not enhance NPP at a given L, regardless of soil water availability. Additionally, using published data from three other forest FACE sites and replacing L with leaf area duration (LD) to account for differences in growing season length, we show that aboveground NPP responded to [CO2] only through the enhancement of LD. For broadleaf forests, the fraction of aboveground NPP partitioned to wood biomass saturated with increasing LD and was not enhanced by [CO2], whereas it linearly decreased for the conifer forest but was enhanced by [CO2]. These results underscore the importance of resolving [CO2] effects on L to assess the response of NPP and C allocation. Further study is necessary to elucidate the mechanisms that control the differential allocation of C among aboveground pools in different forest types.
