MAPPING THE LAND-SURFACE FOR GLOBAL ATMOSPHERE-BIOSPHERE MODELS - TOWARD CONTINUOUS DISTRIBUTIONS OF VEGETATIONS FUNCTIONAL-PROPERTIES

Global land surface characteristics are important boundary conditions for global models that describe exchanges of water, energy, and carbon dioxide between the atmosphere and biosphere. Existing data sets of g lobal land cover are based on classification schemes that characterize each grid cell as a discrete vegetation type. Consequently, parameter fields derived from these data sets are dependent on the particular s cheme and the number of vegetation types it includes. The functional c ontrols on exchanges of water energy, and carbon dioxide between the a tmosphere and biosphere are now well enough understood that it is incr easingly feasible to model these exchanges using a small number. of ve getation characteristics that either are related to or closely related to the functional controls. Ideally, these characteristics would be m apped as continuous distributions to capture mixtures and gradients in vegetation within the cell size of the model. While such an approach makes it more difficult to build models from detailed observations at a small number of sites, it increases the potential for capturing func tionally important variation within, as well as between, vegetation ty pes. Globally, the vegetation characteristics that appear to be most i mportant in controlling fluxes of water, energy, and carbon dioxide in clude (1) growth form (tree, shrub, herb), (2) seasonality of woody ve getation (deciduous, evergreen), (3) leaf type (broadleaf, coniferous) , (4) photosynthetic pathway of nonwoody vegetation (C-3, C-4,), (5) l ongevity (annual, perennial), and (6) type and intensity of disturbanc e (e.g., cultivation, fire history). Many of these characteristics can be obtained through remote sensing, though some require ground-based information. The minimum number and the identity of the required land surface characteristics almost certainly vary with the intended object ive, but the philosophy of driving models with continuous distribution s of a small number of land surface characteristics is likely to be ap plicable to a broad range of problems.