Spatiotemporal variation in grassland biomass and quality across the upper Yellowstone River basin : variation across phenology and land use gradients and validation of remote sensing vegetation indices

Abstract

Spatial and temporal heterogeneity in forage biomass and quality is known to play an integral role in the movement and population dynamics of migratory ungulates. Once limited by field-based forage assessments, the Normalized Difference Vegetation Index (NDVI) and the Enhanced Vegetation Index (EVI) have gained considerable attention as proxies for landscape-scale forage biomass and quality at fine temporal scales. In the Greater Yellowstone Ecosystem (GYE), these indices have become especially important for understanding how potential advances in the timing of spring green-up due to climate change and human land use may be influencing the forage patterns available to migratory elk (Cervus elaphus). Given this concern, more information is needed on how the forage biomass and quality available to elk varies across elevation-related phenology and land use gradients and on the reliability of using NDVI and EVI to map forage patterns across the GYE. Using 250m2 MODIS NDVI and EVI and field estimates of grassland biomass and quality, we examined how the rate and magnitude of seasonal variation in forage biomass and quality differed across elevation-related phenology and land use gradients, assessed how the accuracy of NDVI and EVI as proxies for forage biomass and quality differed across the landscape, and then mapped spatiotemporal variation in the abundance of high quality forage for elk across the Upper Yellowstone River Basin (UYRB). We found that: (1) Grasslands with late onset of growth and irrigated agriculture had a faster rate of growth and a greater seasonal magnitude of biomass and quality for elk than all other grasslands; (2) 250m2 NDVI and EVI explained minimal variation in grassland biomass and quality across the UYRB; and (3) the accuracy of NDVI and EVI differed across elevation-related phenology and land use gradients in the UYRB. These results provide novel information on the rate and magnitude of seasonal variation in forage biomass and quality and on the reliability of using NDVI and EVI to map the forage patterns available to migratory elk.