The Influence of Drought on Spring Vegetation Green-up

Abstract:

Herbivore species such as elk and deer depend on the availability of herbaceous plants and deciduous shrubs for forage. These vegetation types are most nutritious for herbivores during the early part of the growing season, so characterizing spring vegetation phenology over decades can provide crucial information towards understanding how shifts in climate could affect animal behavior and health. Many studies have shown that spring vegetation growth is sensitive to temperature, but less research exists on the influence of drought on phenology. We tested hypotheses on the interactions of recent drought and the phenology of forage utilized by herbivores across the state of Wyoming, USA. Phenological indicators, including the start of season time (SOST), the time of maximum change (in greenness response) (MCT), length of the green-up window (GUW: days from SOST to time of peak greenness), and early spring window (ESW: days from the SOST to the MCT) developed from Moderate Resolution Imaging Spectroradiometer satellite imagery at 250-m resolution, provided broad coverage of the temporal and spatial characteristics of green-up cycles. Gridded precipitation data generated with the Precipitation-elevation Regressions on Independent Slopes Model (PRISM) were used to characterize drought conditions at a coarser spatial scale. We found evidence to prove an initial hypothesis that drought advanced spring development. Spring drought conditions were statistically related to advanced vegetation green-up in SOST and MCT variables, especially across higher elevations and in forested land cover, as well as in some shrublands and grasslands. We did not find evidence that drought made green-up occur faster (based on GUW and ESW variables), although the ESW showed slight acceleration across the northern third of Wyoming. We are further investigating whether the phenological signal of vegetation in more arid areas has been too subtle to detect a significant response to drought.