Detection of Canopy Water Stress in Conifers Using the Airborne Imaging Spectrometer

Imagery was acquired by the Airborne Imaging Spectrometer (AIS-2) over adjacent plots of control (natural) and water-stressed canopies of Norway spruce and white pine, and analyzed for differences in near-infrared reflectance features. Water stress had been induced in the trees by severing the sapwood and was assessed with shoot water potential (ψ) and relative water content (RWC) measurements. Stressed Norway spruce was found to have approximately 20% greater relative reflectance in the 1.0–1.3 μm region compared to the control canopy in the image obtained at 0925 h 13 days after stress induction. The difference in ψ and RWC between stressed and control was measured at 1.2 MPa and estimated at 7%, respectively. However, the difference in reflectance decreased to insignificance in the image taken at 1219 h that same day when the differences in ψ and RWC were approximately 0.72 MPa and 5%. In white pine, no significant differences in reflectance between stressed and control canopies were found in images obtained 14 days and 20 days after treatment with estimated differences in ψ and RWC of 0.3 MPa and 6%. Because extensive ground data was required at times of AIS-2 overflights to detect these small reflectance differences, we believe that water stress in conifer canopies may not be routinely detectable at an operational landscape scale