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Spectral Sensing of Foliar Water Conditions in Conifer Needles

Explore relationships between water content and reflectance in conifer needles, assessing various indices. Study paves way for aerial imaging of water status. Findings relate spectral indices to water content and potential.

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Spectral Sensing of Foliar Water Conditions in Conifer Needles

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  1. 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

  2. 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

  3. system

  4. 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

  5. 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

  6. protocol

  7. protocol

  8. data

  9. data

  10. data

  11. data

  12. 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)

  13. analysis

  14. analysis 970nm

  15. analysis 1200nm

  16. analysis 860-1240 860+1240 NDWI

  17. analysis 880-660 880+660 NDVI

  18. analysis 970nm 1200nm NDVI NDWI

  19. 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)

  20. 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|)

  21. results

  22. 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.

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