Conifer moisture stress within moisture regimes considered normal was remotely sensed using Near-infrared (NIR, 0.7-1.3 microns) and Middle-infrared (MIR, 1.3-2.5 microns). The role of water content on spectral reflectance was examined using a radiometer. Light microscopy was used to investigate relationships between reflectance and leaf cellular anatomy.

Eastern white pine (Pinus strobus ) seedlings were exposed to water stress naturally in a greenhouse and artificially in a laboratory. Using the moisture stress index (MSI = MIR/NIR) and a leaf water content index, small differences in seedling relative water content were detected within the range of normal waster stress. Under greenhouse conditions, high correlations (r > .90) between MSI and xylem pressure potential were observed. In the laboratory experiment, RWC changes as slight as 3 percent withing the "normal" range of moisture conditions were remotely detected. MSI and LWCI explained 98 percent of the variance in relative water content. MSI mimicked LWCI because of a flat NIR response to normal range of water stress. No significant cellular changes were observed microscopically from relative water content of 90 to 80 percent. The slope of the spectral relationship changed dramatically at -1.2 Mpa water potential; this may be related to stomatal closure.
 

 email: pwolter@sparkie.nrri.umn.edu