Using Aerial Photography for Detecting Blackbrush (Acacia Rigidula) on South Texas Rangelands

Abstract

Blackbrush (Acacia rigidula) is a native shrub found on a variety of soil types in south Texas and northern Mexico. It often becomes a serious management problem on rangelands, especially where associated species have been removed. During late February to late March it produces small cream to light yellow flowers that encompass the entire plant giving it a conspicuous appearance. Field spectroradiometric plant canopy measurements showed that blackbrush in flower had significantly higher (p=0.05) visible light (0.45- to 75-um waveband [WB) reflectance that did 6 other associated plant species or mixtures of species. The conspicuous light yellow color of blackbrush in flower made it distinguishable from other plant species on both conventional color (0.40- to 0.70-um WB) and color-infrared (CIR) (0.50- to 0.90-um WB) aerial photos. However, conventional color photography was superior to CIR photography because blackbrush had a more distinct image on color photography and it could also be identified on smaller scale photos. Microdensitometric measurements made on conventional color film showed that blackbrush had significantly lower optical counts than those of associated species. These results show that aerial photography may be a useful tool to distinguish blackbrush from other plant species in late winter or early spring to locate its endemic areas, monitor its spread, and delineate areas needing control.