Long-term effects of elevated carbon dioxide, soil nutrition, water availability and disturbance on carbon relations in a loblolly pine forest

Elevated [CO2] is expected to increase carbon sequestration in forest ecosystems. However, responses to elevated [CO2] are likely to be mitigated by nutrient and water availability, and disturbance events caused by climatic change. The interactions of these crucial factors on growth, phenology and carbon storage of loblolly pine and hardwood trees were assessed using 8-10 years of data from the Duke University free-air CO 2 enrichment (FACE) site.; There was no evidence that elevated [CO2] changed the carrying capacity of the Duke FACE site, by allowing an increase in the size of the average tree at a given stand density. Nor were mortality rates significantly different under elevated [CO2] (2.4% yr-1 for pines, 1.2% yr-1 for hardwoods). Reconstruction of leaf area ( L) showed that dynamics of pine L production and senescence were unaffected by elevated [CO2], while the magnitude of pine and total canopy L (pine + hardwood) was increased (on average 16% and 14% respectively). Spatially the major control of L and thus net primary productivity (NPP) was N availability, and temporally NPP was controlled by soil moisture (theta), with elevated [CO 2] reducing sensitivity to theta across N gradients. Although elevated [CO2] resulted in a sustained treatment-level increase in plant biomass production (average 273 g C m-2 yr-1 or 28% greater stand level NPP) and led to increases in live plant (average 234 g m-2 yr-1 or 36% greater accumulation) and ecosystem C (average 90 g m-2 yr-1 or 10% greater accumulation), it did not change the distribution of C in these pools.; Additionally, elevated [CO2] reduced the carbon transfer resulting from a severe ice storm (26.5 +/- 3.3 Tg of carbon transferred from living to dead biomass pools across the southeastern US) to a third, also resulting in a smaller reduction in biomass production the following year.; This research suggests that the occurrence and magnitude of response of pine forests to elevated [CO2] will be highly dependent on the availability of N and water, with low N levels possibly precluding any enhancement of L and NPP. Such findings contribute to the development of a mechanistic basis for making projections of how forests will function under future atmospheric and climatic conditions.