A technique for monitoring ecological disturbance in tallgrass prairie using seasonal NDVI trajectories and a discriminant function mixture model

Natural and anthropogenic disturbance in tallgrass prairie communities can induce changes in plant species composition, including shifts in the relative abundance of C-3 and C-4 lifeforms. The asynchronous seasonality in greenness exhibited by C-3 and C-4 species enables monitoring of their relative abundance using temporal trajectories of sensor-derived vegetation indices, such as NDVI. We else close-range measurements made over 22 experimental plots at the Konza Prairie Research Natural Area (KPRNA) to evaluate seasonal trajectories in NDVI as a function of C-3/C-4 ratio. The NDVI data were collected from each plot at approximately 10-day intervals throughout the 1995 growing season. Metrics for summarizing the temporal behavior of NDVI trajectories were derived from two transformations of the data: 1) a conventional approach plotting NDVI against day of year and 2) an alternative approach plotting normalized cumulative integrated NDVI against growing degree clay. Discriminant function mixture models derived from each set of metrics were used together with species composition data from the experimental plots to derive relative C-3/C-4 abundances. Results show that both methods can classify the majority of cases into their correct C-3/C-4 abundance category; however, the transformation using normalized integrated NDVI against growing degree day was a significantly better discriminator (p=0.0102). The techniques presented are effective for monitoring relative C-3/C-4 abundance in tallgrass prairie; however, more investigation is needed to assess their performance at different spatial resolutions and in different geographic setting. (C) Elsevier Science Inc., 1997.