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Evaluation of the SPUR Model for Grasslands of the Northeastern United States

USDA-ARS, Pasture Systems and Watershed Management Research, University Park, PA 16802

^* Corresponding author (Email: wsl{at}psuvm.psu.edu).

Computer models that simulate grassland production must be valid for the ecosystem in which they are used. Our objective was to develop a grassland production data base and to use the data base to calibrate and validate a grassland production model for the northeastern United States. The SPUR (Simulation of Production and Utilization of Rangeland) model was evaluated for its ability to simulate aboveground biomass production in two physiographic provinces of northeastern USA. The model was calibrated for orchardgrass and switchgrass using one year of plant, soil, and weather data from four study sites. Calibrations mainly involved adjustment of photosynthetic rate, root-to-shoot ratio, temperature for root-to-shoot translocation, and rooting depth. Once calibrated, the model was validated by simulating aboveground biomass production for two ensuing years on the four sites. Two years of simulated biomass for the grasses agreed well with the observed biomass on all four study sites with r² values of 0.48 for orchardgrass and 0.70 for switchgrass. The largest discrepancies between simulated and observed biomass occurred early in the spring at low levels of N fertilization. In its present form, SPUR can be used to develop production indices for cool- and warm-season grasses on different soils.

Received for publication February 8, 1993.

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