Airborne detection of southern pine beetle damage using key spectral bands

Damage by the southern pine beetle (SPB) (Dendroctonus frontalis Zimm.) occurs frequently in the southeastern United States and can result in tree death over large areas. A new technique for detection of SPB activity was tested for shortleaf pine (Pinus echinata Mill.) in the Caney Creek Wilderness, Ouachita National Forest, Arkansas. Digital images with 1-m pixel resolution were acquired from a light aircraft in 6- to 10-nm bandwidths centered at wavelengths of 675, 698, and 840 nm. The 675-nm band was selected to yield a maximum contrast between yellow or brown versus green foliage. The 698-nm band was selected based on its high sensitivity to leaf chlorophyll content to enable detection of less severe chlorosis in more recently damaged trees. The 840-nm band was used as a reference band that is not sensitive to chlorophyll. Images acquired within each band were calibrated to percent reflectance based on the known reflectances of a gray scale placard located on the ground. Individual trees with yellow to brown foliage were easily located in the 675- and 698-nm images. Milder chlorosis in more recently damaged pines was detected by a normalized difference vegetation index (NDVI) that was derived from 698- and 840-nm reflectances. Although statistically significant, the contrast of recently infested trees versus undamaged trees was generally visually poor in NDVI or color composite images. This was apparently a result of the inherent variability in leaf chlorophyll content throughout the forest. The increased reflectance near 700 nm characteristic of recent damage likely would be resolved more easily in pine plantations of low species diversity. Images of a NDVI that was based on 675- and 840-nm reflectances produced the strongest contrast between heavily damaged and undamaged trees.