Improving the predictive ability of terrestrial biosphere models

Dan Scott

A recent analysis of terrestrial carbon dynamics in the Northeastern US has shown how the Ecosystem Demography model version 2 (ED2) can be jointly constrained against eddy-flux measurements and forest-inventory measurements to yield accurate short-term and longer-term ecosystem dynamics.

The ED2 model was initialized with the observed canopy composition structure in the footprint of the Harvard Forest flux tower, and then fitted simultaneously to the 1995 and 1996 hourly, monthly and yearly CO2 and ET flux data, and to the observed rates of deciduous and coniferous tree basal area growth and mortality in these years. Prior to the optimization, the model significantly underestimated the seasonal cycle of Net Ecosystem Productivity and significantly over-estimated rates of tree growth and mortality. After fitting, the model accurately captured the observed CO2 fluxes, ET fluxes, and canopy growth and mortality dynamics over timescales spanning hours to decades.

We then evaluated the performance of the optimized ED2 model at a different site in the Northeastern US, Howland forest. As before, the model was initialized with the observed canopy composition in the tower footprint, but the model parameters were not re-optimized. Despite the markedly different forest composition between the Howland and Harvard Forest sites (conifer-dominated as opposed to mixed-hardwood) there was a substantial improvement in the model's predictions of the 5-year CO2 flux record, as well as measured tree growth and mortality dynamics at Howland. The optimized values all fell within specified acceptable ranges for each parameter. Changes in parameters responsible for the improved goodness-of-fit include: an increased maximum photosynthetic rate of hardwoods, a marked increase in the rate of fine root turnover, and a decrease in the carbon allocation to fine roots in conifer species.

To further investigate these results, refine approximation to daytime respiration rates, and improve our understanding of soil organic matter decomposition, we are currently working on increasing the resolution of ED's carbon and oxygen accounting schemes. By tracking carbon and oxygen isotopes we can better constrain carbon and oxygen fluxes in the model, exploiting recent advances in mass spectrometry and the additional data thus generated.