An approach to spatially distributed modeling of net primary production (NPP) at the landscape scale and its application in validation of EOS NPP products

Substantial research seeks to improve estimates of eco-system processes and fluxes at a range of scales, notably from the stand scale (< l km(2)) usin g ecosystem physiology and eddy covariance techniques, to the landscape (si milar to 10(2) km(2)) and global (10(8) km(2)) scales using a variety of mo deling and data acquisition approaches. One approach uses remotely sensed e cosystem properties in the scaling process. This approach combines digital maps of key ecosystem properties such as land cover type, leaf area index, and/or canopy chemistry with quantitative models of biological processes ba sed on these ecosystem properties. Constraints on parametrizing models for global scale applications mean that relatively simple algorithms must be us ed which are based almost exclusively on satellite-derived inputs, for exam ple, the planned Earth Observation System (EOS)-MODIS Land Science Team mod el output. Presently, there are limited ways of validating these outputs. A t the landscape scale, the opportunity exists to combine remote sensing dat a with spatially distributed, process-based biogeochemistry models to exami ne variation in ecosystem processes such as NPP as a function of land cover type, canopy attributes, and/or location along environmental gradients. Th ese process models can be validated against direct measurements made with e ddy covariance flux towers and ground-based NPP sampling. Once ran and vali dated over local landscapes, these fine scale models may provide our best o pportunity, to provide meaningful evaluation (or "validntion" in some sense ) of simpler globally applied models. In this article, we I) provide a biol ogical framework that links ecosystem attributes and ecosystem carbon flux processes at a variety of scales, and summarizes the state of knowledge and models in these areas, 2) describe the need for developing NPP surfaces at a local landscape scale as a means of validating global models, in particu lar the MODIS NPP product, 3) describe the approach of the BigFoot project to performing such a validation exercise far a series of sites in North Ame rica, and 4) present an example using one such model (PnET-II) across diver se vegetation types in a heterogeneous landscape in central North America. (C) Elsevier Science Inc., 1999.