Abstract
To assess how montane meadow vegetation recovered after a wildfire that occurred in Yosemite National Park, CA in 1996, Google Earth Engine image processing was applied to leverage the entire Landsat Thematic Mapper archive from 1985 to 2012. Vegetation greenness (normalized difference vegetation index (NDVI)) was summarized every 16 days across the 28-year Landsat time series for 26 meadows. Disturbance event detection was hindered by the subtle influence of low-severity fire on meadow vegetation. A hard break (August 1996) was identified corresponding to the Ackerson Fire, and monthly composites were used to compare NDVI values and NDVI trends within burned and unburned meadows before, immediately after, and continuously for more than a decade following the fire date. Results indicate that NDVI values were significantly lower at 95% confidence level for burned meadows following the fire date, yet not significantly lower at 95% confidence level in the unburned meadows. Burned meadows continued to exhibit lower monthly NDVI in the dormant season through 2012. Over the entire monitoring period, the negative-trending, dormant season NDVI slopes in the burned meadows were also significantly lower than unburned meadows at 90% confidence level. Lower than average NDVI values and slopes in the dormant season compared to unburned meadows, coupled with photographic evidence, strongly suggest that evergreen vegetation was removed from the periphery of some meadows after the fire. These analyses provide insight into how satellite imagery can be used to monitor low-severity fire effects on meadow vegetation.
Highlights
The evaluation of vegetation change over time can be conducted in a variety of ways, from traditional field observations to monitoring with terrestrial or space borne Earth Observation Systems (EOS) [1]
We learned that NDVI values were significantly lower (p < 0.05) for burned meadows following the fire date
By having a near-continuous Landsat record to evaluate seasonal difference we determine that burned meadows consistently exhibited lower monthly NDVI in the dormant season more than a decade following fire (p < 0.05) and a more prominent trend towards declining greenness overall (p < 0.10)
Summary
The evaluation of vegetation change over time can be conducted in a variety of ways, from traditional field observations to monitoring with terrestrial or space borne Earth Observation Systems (EOS) [1]. Satellite data provide repeated measurements, but the length of the temporal record, pixel and spectral resolution, and the revisit rate dictate the type of surface processes that can be analyzed [2]. Spatial, and spectral resolutions, satellite EOS provide tools to evaluate vegetation attributes over time [3,4]. Vegetation estimates from moderate/low resolution MODIS (250/500 m) and AVHRR (1.1 km) sensors have been extensively and successfully used to measure and interpret vegetation dynamics [5,6].
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