Evaluation of modeled carbon fluxes for a slash pine ecosystem: SPM2 simulations compared to eddy flux measurements

A process-based model (SPM2) was developed to simulate net hourly exchanges of carbon for slash pine (Pinus elliottii var. elliottii) ecosystems in no rth-central Florida, and simulated values were compared to those obtained u sing eddy covariance measurements above the canopy. The SPM2 simulated rela tively low hourly rates of net carbon exchange at high irradiance during th e day, especially during the month of May. This discrepancy contributed the most to differences between the two approaches, and suggests that the mode l underestimates the maximum rate of leaf-level net photosynthesis during t he period of rapid growth in the spring. The SPM2 also simulated lower net carbon release during the nighttime across the range of air temperatures, r elative to eddy covariance measurements. As a result of these discrepancies , accumulated daytime and nighttime sums were quite different between the t wo approaches. However, once results were aggregated over 24 hr, difference s between simulations and measurements were insignificant at all but the hi ghest and lowest daily net carbon gain values, Annual estimates for 1996 we re similar: 693, 719, and 745 g C m(-2) yr(-1) for the SPM2 model, eddy cov ariance measurements and mass balance measurements, respectively. This mode l evaluation has lead to specific research questions for guiding further mo del development.