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Spectral sensing of foliar water conditions in conifer needles. Hugh Stimson 1 , David Breshears 2 , Shawn Kefauver 1 , Susan Ustin 1 1 University of California, Davis 2 Los Alamos National Laboratory. Ecological Society of America Annual Meeting, Savannah Georgia USA, August 7 th 2003.
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Spectral sensing of foliar water conditions in conifer needles Hugh Stimson1, David Breshears2, Shawn Kefauver1, Susan Ustin1 1 University of California, Davis 2 Los Alamos National Laboratory Ecological Society of America Annual Meeting, Savannah Georgia USA, August 7th 2003
goals • find relationships between water and reflectance at the needle level • assess a suite of reflectance indices • lay groundwork for pending aerial imaging of water status
protocol • sample gradient • 17 trees selected • selected from visual estimate for wide water range • water content • 3 twigs per sample • 1st whorl of needles removed • weighed, dried to constant weight, reweighed • water potential • N = 15 • average of 2 twigs per tree • measured in pressure bomb
protocol • spectral reflectance • same 3 twigs as water content • subset of needles removed, racked onto vinyl mounts • measured with ASD full-range field spectrometer via Licor integrating sphere • Measured within 15 minutes of fridge, 5 of instrument calibration • integration time 10s
analysis direct water measurements v. spectral indices 980nm water absorption 1200nm water absorption normalized differential water index normalized differential vegetation index Water content (% foliar mass) Water potential (MPa)
analysis 970nm
analysis 1200nm
analysis 860-1240 860+1240 NDWI
analysis 880-660 880+660 NDVI
analysis 970nm 1200nm NDVI NDWI
results spectral indices v. water content 970 nm NDWI R2=0.91 p<0.001 R2=0.96 p<0.001 1200 nm NDVI R2=0.59 p<0.001 R2=0.93 p<0.001 foliar water content (%mass)
results spectral indices v. water potential 970 nm R2=0.37 p=0.03 NDWI R2=0.45 p=0.006 1200 nm NDVI R2=0.03 p=0.5 R2=0.35 p=0.02 water potential (|MPa|)
results • The foliar water content of Piñon pine needles can be reliably predicted from their reflectance. • A weaker, indirect, relationship exists between water potential and reflectance. • The most robust measure is NDWI, closely followed by NDVI and 970nm absorption.