Satellite Microwave Detection of Vegetation Phenology; Climate Constraints, Temporal Offsets and Post-Fire Recovery as Compared to Optical-Infrared Phenology Matthew O. Jones1,2, John S. Kimball1,2, Lucas A. Jones1,2, Kyle C. McDonald3,4 1The 3Jet University of Montana Flathead Lake Biological Station, Polson, MT 2Numerical Terradynamic Simulation Group, The University of Montana, Missoula, MT Propulsion Laboratory, California Institute of Technology, Pasadena, CA 4CUNY Environmental Crossroads Initiative and CREST Institute, City College of New York, New York, NY Email Contact: [email protected] Websites: www.umt.edu/flbs & www.ntsg.umt.edu Abstract The Vegetation Optical Depth (VOD) parameter from satellite passive microwave remote sensing provides an alternative means for global phenology monitoring that is sensitive to photosynthetic and non-photosynthetic vegetation canopy biomass and water content with minimal sensitivity to atmosphere and solar illumination constraints. The VOD record from the Advanced Microwave Scanning Radiometer for EOS (AMSR-E) displayed North America ecoregion start of season patterns and offsets (as compared to satellite optical-infrared remote sensing measures) that coincide with primary climate constraints (temperature and water) to vegetation growth. The VOD start of season generally preceded optical-infrared NDVI and LAI 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 VOD also displayed alternate canopy recovery trajectories versus NDVI recovery following large scale fire disturbance in boreal regions. VOD time series from the extreme 2004 fire year in Alaska and Canada showed an approximate two year recovery lag relative to NDVI; these results are consistent with greater microwave sensitivity to woody biomass regeneration which is expected to show a slower response than canopy greenness recovery. The AMSR-E VOD record provides new, independent phenological information for vegetation start of season measures as well as canopy post fire recovery of both photosynthetic and non-photosynthetic biomass, complementing phenological information from NDVI and LAI measures. Synergistic application of optical-infrared and microwave remote sensing data products are expanding the scope of observable vegetation phenology parameters and advance regional disturbance, carbon, water and energy cycle studies. This work was conducted at the University of Montana and Jet Propulsion Laboratory under contract to NASA (NNH07ZDA001N-TE). VOD Start of Season Offset in relation to Climate Constraints on Net Primary Productivityb AMSR-E Global Vegetation Optical Depth (VOD) c,d VOD Mean 2003-2010 VOD vs. NDVI Response to Extreme 2004 Wildfires in Alaska and Canadaa The summer of 2004 was one of the hottest and driest on record for interior Alaska resulting in a panoply of large-scale fires burning 2.7 million hectares of forest. MODIS NDVI and AMSR-E VOD time series were extracted over large scale fires (>1000km2) and unburned (control) pixels. The VOD record confirmed fairly rapid boreal vegetation recovery consistent with previous NDVI studies and showed an approximate 2 year recovery lag relative to NDVI. This lag is consistent with greater VOD sensitivity to photosynthetic and non-photosynthetic woody biomass, which is expected to show a slower response than canopy greenness recovery alone. VOD SOS earlier VOD SOS later No offset present. VOD SOS offset by ecoregion relative to NDVI Greenup date. VOD SOS vs. MODIS-for-NACP NDVI Greenup Date by ecoregion from 2003-07. Points are annotated by VOD SOS offset matching regions on the accompanying map. Daily 25 km resolution global EASE Grid brightness temperatures from AMSR-E are used to derive daily 10.7 GHz frequency VOD retrievals over a global domain. The VOD product integrates canopy attenuation related to vegetation canopy biomass and water content, while minimizing effects from subgrid scale open water variability and soil moisture. The AMSR-E VOD and global land parameter database is available online through the University of Montana (http://freezethaw.ntsg.umt.edu) and the NASA NSIDC DAAC (http://nsidc.org/data/nsidc-0451.html). VOD and NDVI Fire Anomalies Yearly summary statistics of NDVI and VOD fire anomalies relative to 2003(a) and unburned control pixels(b). Fires. Background is IGBP MODIS land cover. White box is zoomed region in yearly maximums figure. VOD Yearly Maximums Relative to 2003 Ternary plot of relative climate constraints on vegetation NPP by ecoregion; annotated by VOD SOS offset. Ecoregion level climate constraints on vegetation NPP (Nemani et al., Science 2003) Percent Tree Cover and SOS Offset Differences in VOD and NDVI Post-fire Recovery and Tree Cover Loss As an estimate of recovery, we determined post-fire years (2005-2010) that were significantly (p<0.05) less than the prefire (2003) year. NDVI statistically recovered prior to VOD for 12 of the 14 fires. NDVI values returned to pre-fire levels within 1 to 4 years (mean = 2.62 years) and 5 VOD values returned to pre-fire levels within 3 to 5 years (mean = 4 years). The ~2 year VOD recovery lag, relative to NDVI, is consistent with greater VOD sensitivity to photosynthetic and nonphotosynthetic woody biomass and is related to percent tree cover loss within fire perimeters. Plot (right) is normalized post-fire percent tree cover loss (PTC2003 – PTC2005 / PTC2003) versus the years to statistical recovery. Summary & References VOD SOS Phenology offsets follow the distribution of low temperature and water constraints to NPP. VOD provides an alternate land surface phenology data set that tracks seasonal changes in canopy water and biomass to complement the greenness measures provided by optical-infrared data. Tower Fluxes & VOD Start of Season VOD sensitivity to changes in canopy biomass provides a new methodology for tracking post-fire vegetation recovery. The less steep (versus NDVI) post-fire trajectory of VOD tracks both photosynthetic and nonphotosynthetic vegetation biomass. Synergistic use of optical-infrared and microwave satellite data following disturbance will advance our understanding of vegetation succession dynamics, aid in rehabilitation and restoration efforts and inform carbon budget models. aJones, VOD SOS temporal offset vs. MODIS mean percent woody tree cover by ecoregion. Ecoregions were classified as having a dominant water constraint (>50%) for plant growth. We used North America FLUXNET towers (n=33) from a range regional biomes. SOS was derived from tower Gross Primary Productivity (GPP) and Ecosystem Respiration (Reco) fluxes using TIMESAT and regressed against VOD ecoregion SOS. Yearly VOD maximums were calculated relative to 2003 (YEARmax - 2003max). All fires within the region are included. VOD pixels values show sensitivity to sub-pixel scale fires. M.O., Kimball, J.S. & Jones, L.A. Satellite microwave detection of vegetation response to the extreme 2004 wildfires in Alaska and Canada, in prep, Global Change Biology. bJones, M.O., Kimball, J.S, Jones, L.A., & McDonald, K.C. (2012). Satellite passive microwave detection of North America start of season. Remote Sens. of Environ., 115 cJones, M.O., Jones, L.A., Kimball, J.S., McDonald, K.C. (2011). Satellite passive microwave remote sensing for monitoring global land surface phenology. Remote Sens. of Environ, 123 dJones, L.A., Ferguson, C.R., Kimball, J.S., Zhang, K., Chan, S.K., McDonald, K.C., Njoku, E.G., & Wood, E.F. (2010). Daily land surface air temperature minima and maxima from AMSRE IEEE J-STARS, 3, 111-123. We thank the Ameriflux site principal investigators, R. Nemani, the Canadian National Fire Database and Alaska Interagency Coordination Center for the data used in these studies.