LANDSCAPE-LEVEL INTERACTIONS BETWEEN TOPOEDAPHIC FEATURES AND NITROGEN LIMITATION IN TALLGRASS PRAIRIE

Transects across watersheds with varying fire histories and remotely-s ensed data were used to study vegetation-resource interactions in a ta llgrass prairie in Kansas. Paired plots (fertilized, control) were est ablished along these transects and sampled for grass and forb biomass during the 1989 and 1990 growing seasons. Fertilization resulted in si gnificant production responses in grass and total biomass on the west slopes of the annually burned (1D) and infrequently burned (N4) waters heds for both years (p = 0.05). In 1989, fertilization also produced a significant increase in grass biomass on the west slope of the unburn ed transect (p = 0.05), however, total production was not significantl y increased. East slopes were insensitive to nitrogen additions. Diffe rences in production response along these transects were assessed by t esting the interaction between fertilization response and slope positi on. Significant interactions occurred on both 1D and N4, but only in 1 990. Production data for both years were also compared to Normalized D ifference Vegetation Index (NDVI) values derived from thematic mapper (TM) images for 1989 and 1990. When differences among transects or wat ersheds were statistically different, a positive relationship between NDVI and biomass was observed. NDVI values accurately reflected the sp atial patterns of production along these transects for both years alth ough not necessarily the magnitude.