Responses of a loblolly pine ecosystem to CO2 enrichment: a modeling analysis

The development of the Free-Air CO2 Enrichment (FACE) facilities represents a substantial advance in experimental technology for studying ecosystem re sponses to elevated CO2. A challenge arising from the application of this t echnology is the utilization of short-term FACE results for predicting long -term ecosystem responses. This modeling study was designed to explore inte ractions of various processes on ecosystem productivity at elevated CO2 on the decadal scale. We used a forest model (FORDYN) to analyze CO2 responses -particularly soil nitrogen dynamics, carbon production and storage-of a lo blolly pine ecosystem in the Duke University Forest. When a 14-year-old sta nd was exposed to elevated CO2, simulated increases in annual net primary p roductivity (NPP) were 13, 10 and 7.5% in Years 1, 2 and 10, respectively, compared with values at ambient CO2. Carbon storage increased by 4% in tree s and 9.2% in soil in Year 10 in response to elevated CO2. When the ecosyst em was exposed to elevated CO2 from the beginning of forest regrowth, annua l NPP and carbon storage in trees and soil were increased by 32, 18 and 20% , respectively, compared with values at ambient CO2. In addition, simulatio n of a 20% increase in mineralization rate led to a slight increase in biom ass 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 stor age by elevated CO2, is transient and (2) effects of elevated CO2 on ecosys tem processes-canopy development, soil nitrogen mineralization and root tur nover-have great impacts on ecosystem C dynamics. A detailed understanding of these processes will improve our ability to predict long-term ecosystem responses to CO2 enrichment.