Bidirectional reflectance measurements were obtained on grazed, burned ungrazed, and unburned ungrazed tallgrass prairie in eastern Kansas. Observations were also made on experimental plots on which vegetation height and biomass were manipulated by mowing. Foliage biomass and productivity (including off-take estimates) were measured concurrently at all sites. While productivity of mowed or grazed sites was either equal to or greater than that on unmowed or ungrazed sites, individual or cumulative normalized difference vegetation index (NDVI) values tended to be positively correlated with biomass, not productivity. NDVI on grazed prairie was lower than on burned and unburned prairie during the first half of the growing season. After midseason, NDVI on grazed prairie was higher than on unburned prairie but no different than on burned prairie. Strong linear relationships among NDVI and canopy nitrogen and biomass existed early and late in the growing season but not at midseason.

These relationships suggest that plant physiological processes associated with regrowth following defoliation are dominant influences on reflectance early in the season, whereas the accumulation of senescent material is the dominant process affecting reflectance in the latter half of the growing season. Canopy temperature was related to canopy nitrogen and biomass at midseason. The use of NDVI to estimate plant productivity and vegetation-atmosphere exchange is complicated by changes in plant characteristics induced by grazing or mowing. Grazing tends to homogenize potential landscape-induced differences in vegetation activity. Factors otherwise useful in estimating plant production, such as burning treatment and soil depth, are not strongly correlated with aboveground biomass or NDVI under grazing conditions. Results presented here suggest that concurrent use of thermal information provided by some satellite sensors may improve this relationship. --American Geophysical Union 1992