Changes in Vegetation Cover of Yellowstone National Park Estimated from MODIS Greenness Trends, 2000 to 2018

Abstract

Trends and transitions in the MODerate resolution Imaging Spectroradiometer (MODIS) Normalized Difference Vegetation Index (NDVI) time series at 250-m resolution were analyzed for the period from 2000 to 2018 to understand recent patterns of vegetation change in Yellowstone National Park (USA). Statistical change in the NDVI time series was detected using the “Breaks for Additive Seasonal and Trend” (BFAST) method. This structural change analysis showed that around 30% of abrupt negative shifts in NDVI over the years 2000 to 2018 could be explained by the impacts of recent wildfires. At least one breakpoint could be detected at 12% of the 250-m MODIS pixel locations within the entire YNP study area since the year 2000, but that the majority (about 70%) of NDVI breakpoints detected in vegetation greenness could be not be explained by the impacts of recent wildfires. Positive growing season NDVI trends were detected across most young forests and regrowing (from recent fire disturbance) woodland cover. Evidence further suggested that the 1988 wildfire burns did not pre-dispose vegetation cover in YNP to a higher number of abrupt negative shifts in NDVI since the year 2000. The wildfires of 1988 were associated with significantly higher NDVI recovery trends over the recent 18-year MODIS time series, compared to areas unburned in 1988. Locations on the Northern Range that showed the highest greening trends since 2000 were commonly located in sagebrush steppe-dominated vegetation communities growing at lower than 2500 m elevation. Results from NDVI trend analysis supported the hypothesis that years with relatively high snowpack water content, such as 2007–2008 and 2010–2011, were most closely associated with abrupt negative shifts in NDVI, but no findings strongly supported the supposition that vegetation cover in YNP is changing in greenness in association solely with warming in surface air temperatures or with extreme drought periods across the study region over the past two decades.