Responses of a loblolly pine ecosystem to CO(2) enrichment: a modeling analysis

The development of the Free-Air CO(2) Enrichment (FACE) facilities represents a substantial advance in experimental technology for studying ecosystem responses to elevated CO(2). A challenge arising from the application of this technology is the utilization of short-term FACE results for predicting long-term ecosystem responses. This modeling study was designed to explore interactions of various processes on ecosystem productivity at elevated CO(2) on the decadal scale. We used a forest model (FORDYN) to analyze CO(2) responses-particularly soil nitrogen dynamics, carbon production and storage-of a loblolly pine ecosystem in the Duke University Forest. When a 14-year-old stand was exposed to elevated CO(2), simulated increases in annual net primary productivity (NPP) were 13, 10 and 7.5% in Years 1, 2 and 10, respectively, compared with values at ambient CO(2). Carbon storage increased by 4% in trees and 9.2% in soil in Year 10 in response to elevated CO(2). When the ecosystem was exposed to elevated CO(2) from the beginning of forest regrowth, annual NPP and carbon storage in trees and soil were increased by 32, 18 and 20%, respectively, compared with values at ambient CO(2). In addition, simulation of a 20% increase in mineralization rate led to a slight increase in biomass growth and carbon storage, but the simulated 20% increase in fine root turnover rate considerably increased annual NPP and carbon storage in soil. The modeling results indicated that (1) stimulation of NPP and carbon storage by elevated CO(2) is transient and (2) effects of elevated CO(2) on ecosystem processes-canopy development, soil nitrogen mineralization and root turnover-have great impacts on ecosystem C dynamics. A detailed understanding of these processes will improve our ability to predict long-term ecosystem responses to CO(2) enrichment.