Using NDVI and EVI to Map Spatiotemporal Variation in the Biomass and Quality of Forage for Migratory Elk in the Greater Yellowstone Ecosystem

Erica Garroutte,Rick Lawrence,Andrew Hansen

doi:10.3390/rs8050404

Erica Garroutte, Rick Lawrence + Show 1 more

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https://doi.org/10.3390/rs8050404

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Journal: Remote sensing	Publication Date: May 11, 2016
Citations: 109	License type: CC BY 4.0

Affiliation: Montana State University

Abstract

The Normalized Difference Vegetation Index (NDVI) and the Enhanced Vegetation Index (EVI) have gained considerable attention in ecological research and management as proxies for landscape-scale vegetation quantity and quality. In the Greater Yellowstone Ecosystem (GYE), these indices are especially important for mapping spatiotemporal variation in the forage available to migratory elk (Cervus elaphus). Here, we examined how the accuracy of using MODIS-derived NDVI and EVI as proxies for forage biomass and quality differed across elevation-related phenology and land use gradients, determined if polynomial NDVI/EVI, site, and season effects improved these models, and then mapped spatiotemporal variation in the abundance of high quality forage available to elk across the Upper Yellowstone River Basin (UYRB) of the GYE. Models with a polynomial NDVI effect explained 19%–55% more variation in biomass than the linear NDVI and EVI models. Models with linear season effect explained 14%–20% more variation in chlorophyll, 37%–69% more variation in crude protein, and 26%–50% more variation in in vitro dry matter digestibility (IVDMD) than the linear NDVI and EVI models. Linear NDVI models explained more variation in biomass and quality across the UYRB than the linear EVI models. The accuracy of these models was lowest in grasslands with late onset of growth, in irrigated agriculture, and after the peak in biomass. Forage biomass and quality varied across the elevation-related phenology and land use gradients in the UYRB throughout the season. At their seasonal peak, the abundance of high quality forage for elk was 50% greater in grasslands with late onset of growth and 200% greater in irrigated agriculture than in all other grasslands, suggesting that these grasslands play an especially important role in the movement and fitness of migratory elk. These results provide novel information on the utility of NDVI and EVI for mapping spatiotemporal patterns of forage biomass and quality.

Highlights

Remote sensing-based vegetation indices have gained considerable attention in ecological research and management as proxies for landscape-scale vegetation biomass and quality [1,2,3].Among the existing indices, the satellite-derived Normalized Difference Vegetation Index (NDVI)and the Enhanced Vegetation Index (EVI) are the most commonly used indices because of their simplicity, availability, and demonstrated utility in a variety of ecosystems [2,3,4]
We examined how the accuracy of NDVI and EVI models of biomass and quality differed across the landscape and used these models to map spatiotemporal variation in the biomass and quality of forage available to migratory elk across the Upper Yellowstone River
The specific objectives of this study were to: (1) examine how the accuracy of linear NDVI and EVI models of biomass and quality differed across elevation-related phenology and land use gradients throughout the growing season; (2) determine if including polynomial NDVI/EVI, phenology, land use, and season effects in NDVI and EVI models improved their estimates of biomass and quality; and (3) use these models and the nutritional requirements of elk [38] to map spatiotemporal variation in the abundance of high quality forage available to migratory elk across the Upper Yellowstone River Basin (UYRB)

Summary

Introduction

Remote sensing-based vegetation indices have gained considerable attention in ecological research and management as proxies for landscape-scale vegetation biomass and quality [1,2,3].Among the existing indices, the satellite-derived Normalized Difference Vegetation Index (NDVI)and the Enhanced Vegetation Index (EVI) are the most commonly used indices because of their simplicity, availability, and demonstrated utility in a variety of ecosystems [2,3,4]. 2016, 8, 404 leaf area, and structure of vegetation [5,6] Because of this link, NDVI and EVI have been used as proxies for the aboveground biomass [7], chlorophyll content [8,9], primary productivity [10], phenology [11], and quality [2] of vegetation in a variety of ecosystems. Once limited by field-based forage assessments, the availability of NDVI and EVI at large spatiotemporal scales has made them useful for understanding the role of forage patterns on the movement and population dynamics of herbivores [12,13,14]. This is especially true as climate warming and human land use increasingly alter the timing of growth and patterns of forage available to herbivores [12,15,16]

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