NDVI, C-3 AND C-4 PRODUCTION, AND DISTRIBUTIONS IN GREAT-PLAINS GRASSLAND LAND-COVER CLASSES

The distributions of C-3 and C-4 grasses were used to interpret the di stribution, seasonal performance, and potential production of grasslan ds in the Great Plains of North America. Thirteen major grassland seas onal land cover classes were studied with data from three distinct sou rces. Normalized Difference Vegetation Index (NDVI) data derived from the National Oceanic and Atmospheric Administration (NOAA) Advanced Ve ry High Resolution Radiometer (AVHRR) sensor were collected for each p ixel over a 5-yr period (1989-1993), analyzed for quantitative attribu tes and seasonal relationships, and then aggregated by land cover clas s. Data from the State Soil Geographic (STATSGO) database were used to identify dominant plant species contributing to the potential product ion in each map unit. These species were identified as C-3 or C-4, and contributions to production were aggregated to provide estimates of t he percentage of C-3 and C-4 production for each intersection of the S TATSGO map units and the seasonal land cover classes. Carbon isotope v alues were obtained at specific sites from the soil organic matter of the upper horizon of soil cores and were related to STATSGO estimates of potential production. The grassland classes were distributed with b road northwest-to-southeast orientations. Some classes had large varia tions in C-3 and C-4 composition with high proportions of C-4 species in the south and low proportions in the north. This diversity of photo synthetic types within land cover classes that cross regions of differ ent temperature and precipitation results in similar seasonal patterns and magnitudes of NDVI. The easternmost class, 65, containing tallgra ss prairie components, bluestem, Indiangrass, and switchgrass, possess ed the highest maximum NDVI and time-integrated NDVI values each year. Grassland classes varied over 5 yr from a high integrated NDVI mean o f 4.9 in class 65 in the east to a low of 1.2 in class 76 (sand sage, blue grama, wheatgrass, and buffalograss) in the southwest. Although e nvironmental conditions varied widely during the 5 yr, the rankings of class performance were consistent across years for these NDVI metrics . Land cover classes were less consistent in time of onset, which was often earlier in areas in the north dominated by C-4 grasses than in a reas to the south dominated by C-4 grasses. At the level of seasonal l and cover classes, no significant relationship was found between the p roportions of C-3 and C-4 species and estimates of potential productio n derived from the STATSGO database or inferred from the seasonal patt erns of NDVI. The isotopic data from specific sites and the potential production data from STATSGO suggest similar patterns of high proporti onal production by C-4 species throughout the south and a decline in p roportional production north of the central Great Plains. The land cov er classes integrate ecosystem units that encompass a wide diversity o f species and C-3 and C-4 proportions and provide a classification tha t consistently captures significant ecosystem parameters for the Great Plains.