Interactive Canopies for a Climate Model

Climate models depend on evapotranspiration from models of plant stomatal resistance and leaf cover, and hence they depend on a description of the response of leaf cover to temperature and soil moisture. Such a description is derived as an addition to the Biosphere–Atmosphere Transfer Scheme and tested by simulations in a climate model. Rules for carbon uptake, allocation between leaves, fine roots, and wood, and loss terms from respiration, leaf, and root turnover and cold and drought stress, are used to infer the seasonal growth of leaf area as needed in a climate model, and to provide carbon fluxes (assuming also a simple soil carbon model) and net primary productivity. The scheme is tested in an 11-yr integration with the NCAR CCM3 climate model. After a spinup period of several years, the model equilibrates to a seasonal cycle plus some interannual variability. Effects of the latter are noticeable for the Amazon. Overall, drought stress has nearly as large an effect on leaf mortality as cold stress. The leaf areas agree on average with those inferred from Normalized Difference Vegetation Index although some individual systems are either too high (grass and crops) or too low (deciduous needleleaf in Siberia) compared to the satellite data. Evergreen needleleaf forests have significantly smaller annual range and later phase than indicated by the data. The interactive parameterization increases temperatures and reduces evapotranspiration and precipitation compared to the control over the extratropical Northern Hemisphere summer. This interactive leaf model may serve not only to provide feedbacks between vegetation and the climate model, but also to diagnose shortcomings of a climate model simulation from the viewpoint of its impact on the biosphere.