NARROW-BAND REFLECTANCE IMAGERY COMPARED WITH THERMAL IMAGERY FOR EARLY DETECTION OF PLANT STRESS

A field experiment compared plant stress detection by narrow-band refl ectance and ratio images with thermal infrared images. Stress was indu ced in a mixed stand of 5 year old loblolly pine (Pinus taeda L.) and slash pine (Pinus elliottii Engelm.) by a soil application of diuron ( DCMU) on 22 August followed by bromacil on 19 September, 1994. Herbici de-induced stress was first indicated on 24 and 26 September by signif icant (p less than or equal to 0.05) decreases in photosynthesis and t he ratio of variable to maximum fluorescence (F-v/F-m), respectively. Stress was first detected remotely on 5 October by 694 +/- 3 nm reflec tance imagery and its ratio with reflectance at 760 +/- 5 nm (p less t han or equal to 0.05). This reflectance increase was detected at least IG days prior to the first visible signs of damage, as quantified by the CIE color coordinate u', that occurred between 21 and 26 October. Reflectance images at 670 +/- 5 nm, 700 +/- 5 nm and 760 +/- 5 nm firs t detected stress on 21 October 12 October and 20 December, respective ly. Canopy temperature as indicated by imagery in the 8 to 12 mu m ban d never differed significantly between herbicide-treated and control p lots. This resulted from the close coupling of leaf temperatures with air temperature, and the tendency of wind and environmental moisture t o equalize temperatures among treatments. The high sensitivity to stre ss of reflectance imagery at 694 +/- 3 nm supports similar conclusions of earlier work, and indicates that imagery in the 690 to 700 nm band is far superior to thermal imagery for the early and pre-visual detec tion of stress in pine.