Trends in satellite-derived phenology in grasslands and shrublands of Southern Colorado Plateau Network parks

Norris, Jodi

doi:10.36967/2312720

Purpose: The Southern Colorado Plateau Network (SCPN) used satellite-derived normalized difference vegetation index (NDVI) data over a 20-year period (2003–2022) to examine trends and patterns of variability in 12 grassland and shrubland study areas within SCPN park units. Methods: SCPN developed daily NDVI records for each study area, from which analyses and visualizations were derived. Raster graphs were created to facilitate data interpretation. Trends were calculated for each ranked day of the spring and monsoon seasons: For each study area, season, and year, each day was ranked from the highest NDVI value to the lowest. Total NDVI change was then estimated for the spring and monsoon season NDVI peaks (day-rank = 1) as well as all other non-peak day ranks for each season. The same analyses were also applied to subsets of pixels corresponding to more productive and less productive pixels within each study area. Results: The study areas had many “likely” trends in NDVI for ranked-daily values, with a smaller number of “very likely” and “highly likely” trends. Monsoon season trends were consistently negative or neutral for more northerly parks—Aztec Ruins National Monument (NM), Chaco Culture National Historical Park (NHP), and Glen Canyon National Recreation Area (NRA); and consistently positive or neutral for more southerly parks—Petrified Forest National Park (NP), Petroglyph NM, and Wupatki NM. Spring season trends varied spatially and included both positive and negative trends. Aztec Ruins NM and Glen Canyon NRA exhibited consistently negative or neutral trends for both seasons. Raster graphs revealed patterns including the variability in timing and intensity of seasonal greenness peaks and multi-year patterns. The raster graphs and associated data release can be used to evaluate the potential for the phenological patterns to correlate with on-the-ground conditions for wildlife resources that depend on vegetation. These data may also be used to parameterize models that relate climate to vegetation condition. Multi-year consecutive seasons of below-average NDVI were found in some records: At Wupatki NM, low NDVI from spring 2017 to spring 2021 corresponded in time with a widespread juniper dieback in 2021 in the adjacent pinyon-juniper woodlands of the area. Consecutive seasons of low NDVI also occurred in Petroglyph NM from 2008 to 2013 and at Aztec Ruins NM from 2020 to 2022. These records show the potential to link plant community stress to disturbance response thresholds, which is a subject of ongoing SCPN research.

Trends in satellite-derived phenology in grasslands and shrublands of Southern Colorado Plateau Network parks

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