Satellite passive microwave detection of North America start of season

The start of season (SOS) phenological metric indicates the seasonal onset of vegetation activity, including canopy growth, photosynthesis and associated increases in land-atmosphere water, energy and carbon (CO2) exchanges influencing weather and climate variability. Satellite optical-infrared (IR) remote sensing is responsive to vegetation greenness and SOS, but measurement accuracy and global monitoring are constrained by atmosphere cloud/aerosol contamination and seasonal decreases in solar illumination for many areas. The vegetation optical depth (VOD) parameter from satellite passive microwave remote sensing provides an alternative means for global phenology monitoring that is sensitive to vegetation canopy biomass and water content, and insensitive to atmosphere and solar illumination constraints. A global VOD record from the Advanced Microwave Scanning Radiometer for EOS (AMSR-E) was used to estimate North America SOS patterns and annual variability at the ecoregion scale. The SOS metrics were derived for a four year (2004-2007) record using TIMESAT and AMSR-E 10.7 GHz frequency VOD retrievals composited to 4-day median time series. The VOD SOS corresponded favorably with MODIS-for-NACP NDVI (0.73<0.81; p<0.01) and LAI (0.66<0.89; p<0.01) greenup dates, and stand level SOS estimates derived from flux tower gross primary production (r(2) = 0.61, p<0.01) and ecosystem respiration (r(2)=0.44, p<0.01) estimates. The VOD SOS was temporally offset from MODIS greenup and tower SOS metrics by up to 4-7 weeks (RMSE) and the offset patterns coincided with the primary climate constraints (temperature and water) to vegetation growth. The VOD SOS generally preceded greenup in cold temperature constrained ecoregions and followed greenup in warmer, water limited ecoregions, with delays increasing for areas with greater woody vegetation cover. The AMSR-E VOD record captures canopy biomass changes independent of NDVI greenness or LAI measures, providing new and complementary phenological information for regional carbon, water and energy cycle studies. (C) 2012 Elsevier Inc. All rights reserved.