Chapter 7 Vegetation Dynamics of Yellowstone's Grazing System

Theme Primary production in Yellowstone National Park is constrained by long winters to a short, intense growing season characteristic of high-altitude, temperate environments. In systems like this, vegetation phenology plays a governing role in terrestrial herbivore ecology by determining temporal fluctuations in the quantity and quality of forage. Phenology and productivity vary considerably at the patch level, and this spatio-temporal heterogeneity in the forage base provides much of the impetus for the distributions and movement patterns of the dominant herbivores that structure this system (Frank et al. 2002, Gates et al. 2005). Phenology and productivity are modulated by climate variability and disturbances, thereby providing the trophic link between these exogenous influences and animal populations. The growing collection of fine-scale, spatial datasets describing animal locations and landscape attributes, coupled with increasingly sophisticated modern scientific analyses, allow a fuller understanding of wildlife-vegetation interactions at the patch scale. Similarly, population studies also benefit from a better understanding of the variations in phenological patterns and overall productivity at regional scales. Using the normalized difference vegetation index (NDVI) derived from daily satellite imagery spanning July 1, 2000 to December 31, 2006, we generated phenological profiles that characterize the dynamic patterns of vegetation green-up and senescence at the large-patch scale ( ca. 250 m). We used these seasonal profiles to describe the vegetation dynamics of Yellowstone's grasslands and shrub-grasslands, paying particular attention to patterns of spring green-up.