Spatio-temporal dynamics of forage quality in the greater Yellowstone ecosystem

Abstract

The health and movements of ungulates such as elk depend on the distribution and timing of high-quality forage. As climate change reshapes plant community composition, phenology, and nutrient dynamics, understanding its effects on forage resources and herbivore movements becomes increasingly important. This study integrated vegetation chemistry, field spectroscopy, and Harmonized Landsat Sentinel-2 (HLS) multispectral data to quantify forage quality across spatiotemporal gradients in the Greater Yellowstone Ecosystem (GYE). Specifically, I asked: (1) How does forage quality vary with phenology and elevation in the GYE? and (2) How accurately can forage quality be predicted using field spectroscopy compared to HLS multispectral models and indices? I also discuss how the distribution of high-quality forage may influence elk migration patterns. The study area encompassed 322 km² across a 1,524 m elevation gradient between Paradise Valley and the Lamar River Valley in Yellowstone’s Northern Range. Vegetation and spectral data were collected bi-monthly from June through September across low-, mid-, and high-elevation classes. Vegetation samples were analyzed for chlorophyll a, nitrogen (N), crude protein, carbon (C), non-structural carbohydrates, hemicellulose, cellulose, and lignin concentrations. Seasonal trend models were parameterized using Julian day and elevation class. Nitrogen concentrations declined significantly through the growing season (R² = 0.74) and varied among elevation classes. High-elevation sites maintained elevated nitrogen concentrations through August, while low elevations exhibited a late-season rebound and the highest nitrogen concentrations in September. Cellulose (R² = 0.43), lignin (R² = 0.51), and C ratios (R² = 0.52) increased significantly over time, whereas chlorophyll and hemicellulose displayed weak or inconsistent patterns. Field spectroscopy predicted nitrogen with moderate accuracy (R² = 0.20, RMSEP = 0.55), while HLS multispectral models showed similar but slightly lower performance (R² = 0.18, RMSEP = 0.64). All other vegetation traits were predicted poorly by both approaches. Overall, strong elevational and seasonal gradients in forage quality likely contribute to the dynamic nutritional landscape tracked by migratory ungulates in the GYE. Nitrogen was the most predictable forage-quality metric, while the limited performance of other traits highlights challenges in scaling vegetation nutritional quality using remote sensing approaches.