Near-infrared discrimination of leafless saltcedar in wintertime Landsat TM

To test a hypothesis that leafless riparian canopies enable accurate multi-spectral discrimination of saltcedar (Tamarix ramosissima Ledeb.) from other native species, winter Landsat TM5 data (16 November 2005) were analysed for a reach of the Arkansas River in Colorado, USA. Supporting spectroscopic analysis confirmed that saltcedar could not easily be discriminated from other riparian vegetation using TM5 data when in-leaf, but bare branches could be easily distinguished due to much lower reflectance than other riparian cover. Use of TM Band 4 (B4) allowed differentiation of wintertime saltcedar into four qualitative density classes judged from high-resolution low-oblique aerial photography: high (76%-100%), medium (51%-75%), low (16%-50%), and none (0%-15%). Spectral overlap was removed from the B4 saltcedar classification using TM Band 5 (B5) thresholds to eliminate low-reflectant wet areas and higher-reflectant multi-year darkened weed canopies. The accuracy of a classification algorithm that used B5 thresholds followed by a B4 density slice was judged against high-resolution aerial photography as providing 98% discrimination of saltcedar cover from other riparian cover and about 90% discrimination of the qualitative density classes. Applying this method to the 2835 km(2) riparian corridor study area, 1298 km(2) (45.78%) was identified as containing saltcedar, with over 43% having medium or greater density.