Investigation of wildfire impacts on land surface phenology from MODIS time series in the western US forests

摘要

Land surface phenology (LSP) characterizes the timing and greenness of seasonal vegetation growth in satellite pixels and it has been widely used to associate with climate change. However, wildfire, causing considerable land surface changes, exerts abrupt changes on the LSP magnitudes and great influences on the LSP long-term trends, which are poorly investigated. This study for the first time conducted a systematic analysis of the wildfire impacts on LSP by investigating 838 forest wildfires occurred from 2002 to 2014 across the western United States. Specifically, we derived three LSP timing metrics that are the start (SOS), end (EOS), and length (LOS) of growing season and two LSP greenness metrics that are seasonal greenness maximum (GMax) and minimum (GMin) from daily time series of 250-m MODIS two-band enhanced vegetation index (EVI2) during 2001-2015. Burned area and burn severity were obtained from the Monitoring Trends in Bum Severity project. The results showed GMax and GMin were decreased at an extent of 0.063 and 0.074 EVI2, respectively. LSP timings presented diverse responses to wildfire occurrences. Absolute abrupt shift of > 2 days in SOS appeared in 73% of burned areas with 40% advances and 33% delays, the shift in EOS occurred in 80% of burned areas with 33% advances and 47% delays, and the shift in LOS occurred in 85% of the burned areas with 36% shortening and 49% lengthening. Moreover, the LSP changes were significantly influenced by burn severity with the largest impact on LSP timing at the moderate burn severity and on LSP greenness at the high burn severity. Finally, the phenological trends from 2001 to 2015 differed significantly between burned and unburned reference areas and the trend difference varied with the wildfire occurrence year. Overall, this study demonstrated that wildfires exert complex and diverse impacts on LSP timing and greenness metrics and significantly influence LSP trends associating with climate change. The approach developed in this study provides a prototype to investigate LSP responses to other land disturbances associated with natural processes and human activities on the landscape.