Narasimhan, R, Stow, D (2010). Daily MODIS products for analyzing early season vegetation dynamics across the North Slope of Alaska. REMOTE SENSING OF ENVIRONMENT, 114(6), 1251-1262.
Monitoring the growth and distribution of Arctic tundra vegetation is important for understanding changes in early growing season conditions in Arctic ecosystems in response to a warming climate. The primary objective of this study is to examine the utility of computed Daily Moderate Resolution Imaging Spectroradiometer (MODIS) Normalized Difference Vegetation Index (NDVI) products relative to 16-day maximum value composite (MVC) datasets for observing early season green-up dynamics of Arctic tundra vegetation across the North Slope of Alaska. Greening in the Arctic typically occurs shortly after snowmelt and can potentially be captured by using satellite observations that are available on a daily basis. Daily MODIS Snow Cover products were employed to retrieve dates of complete snowmelt (DOCS) for 2003-2005 for pixels that were cloud free at the time of complete snowmelt. Given the sparseness of cloud-free observations in both space and time, early season NDVI trajectories for cloud-free pixels were derived using daily MODIS data based on two approaches: a chronosequence (temporally continuous but aspatial) and a pixel trajectory (temporally discontinuous but spatial explicit) approach. On average during the three-year period, 12.5% of the North Slope region was cloud free at the time of complete snowmelt and a majority of these cloud-free pixels (65%) were associated with the Coastal Plain province. In contrast, the Foothills region was relatively less cloudy from the time following complete snowmelt until peak greenness (56%) than the Coastal Plain province (61%). As a result, vegetation communities that lie mostly in the Foothills province such as shrub tundra and moist acidic tundra classes had more cloud-free observations available to characterize NDVI trajectories using the pixel trajectory approach. Complete snowmelt in the North Slope generally occurred between day of year (DOY) 140 and 170 over the three years with areas covered by the shrub tundra vegetation community (Foothills province) experiencing snowmelt first in all three years with mean DOCS ranging from DOY 148 in 2004 to DOY 158 in 2003. For approximately two weeks following complete snowmelt (Phase I, a period of rapid NDVI increase), the Daily NDVI derived trajectories were substantially different from the MVC NDVI trajectories. Early season integrated NDVI (ESINDVI) values computed for Phase I were 7% higher using the Daily NDVI approaches relative to those derived from the MVC MODIS data for the North Slope region. Following this initial period, until peak greenness (Phase 2, a period of gradual NDVI increase), the Daily and MVC trajectories were similar in shape and magnitude. This study demonstrates the utility of the Daily MODIS Snow product for assessing cloud cover and snowmelt patterns and Daily MODIS NDVI data for observing and detecting sharp and rapid changes in early season vegetation phenology as seen during Phase I. (C) 2010 Elsevier Inc. All rights reserved.