Increased leaf area index and efficiency drive enhanced production under elevated atmospheric [CO₂] in a pine-dominated stand showing no progressive nitrogen limitation

Enhancement of net primary production (NPP) in forests as atmospheric [CO₂] increases is likely limited by the availability of other growth resources. The Duke Free Air CO₂ Enrichment (FACE) experiment was located on a moderate-fertility site in the southeastern US, in a loblolly pine (Pinus taeda L.) plantation with broadleaved species growing mostly in mid-canopy and understory. Duke FACE ran from 1994 to 2010 and combined elevated [CO₂] (eCO₂) with nitrogen (N) additions. We assessed the spatial and temporal variation of NPP response using a dataset that includes previously unpublished data from 6 years of the replicated CO₂ × N experiment and extends to 2 years beyond the termination of enrichment. Averaged over time (1997–2010), NPP of pine and broadleaved species were 38% and 52% higher under eCO₂ compared to ambient conditions. Furthermore, there was no evidence of a decline in enhancement over time in any plot regardless of its native site quality. The relation between spatial variation in the response and native site quality was suggested but inconclusive. Nitrogen amendments under eCO₂, in turn, resulted in an additional 11% increase in pine NPP. For pine, the eCO₂-induced increase in NPP was similar above- and belowground and was driven by both increased leaf area index (L) and production efficiency (PE = NPP/L). For broadleaved species, coarse-root biomass production was more than 200% higher under eCO₂ and accounted for the entire production response, driven by increased PE. Notably, the fraction of annual NPP retained in total living biomass was higher under eCO₂, reflecting a slight shift in allocation fraction to woody mass and a lower mortality rate. Our findings also imply that tree growth may not have been only N-limited, but perhaps constrained by the availability of other nutrients. The observed sustained NPP enhancement, even without N-additions, demonstrates no progressive N limitation.