Some tests of film types for remote sensing of purple loosestrife,Lythrum salicaria, at low densities

Abstract

Purple loosestrife,Lythrum salicaria, is an exotic plant that has proven to be extremely competitive with native wetland and riparian zone species. Current control measures are limited and most effective when infestations are detected at an early stage. The goal of this project was to provide a detection tool (remote sensing) for relatively small loosestrife infestations. Small format (35 mm) aerial photography was acquired over seven sites in central Washington during the peak bloom period ofL. salicaria. The sites were selected to provide a variety of typical background plants and a range of infestation sizes, from a few isolated individuals to large, monospecific stands. All sites were photographed at approximately 1∶5000 (large) and 1∶12,000 (small) scale with Fuji Velvia 50 and Kodak Kodachrome 64 color transparency films. Sixteen sample slides were ranked by 46 interpreters according to the ease of finding loosestrife flowers. As expected, the interpreters ranked massive loosestrife stands as the most easy to identify. However, a large scale image of single loosestrife flowers was ranked almost as easy to interpret as the high altitude imagery of massive stands. Loosestrife was most difficult to detect on imagery from sites with few loosestrife plants or with large amounts of dead plant material below the flower heads. Differences in film type were significant, ranking in order of decreasing ease of interpretation: Fuji Kodachrome. Remote sensing using small format aerial photography can be successfully applied to discriminate flowering loosestrife plants from other background wetland vegetation. Color slide transparencies taken at 1∶5000 or larger scale can show flowers in sufficient detail for detection and mapping of isolated plants. Screening of interpreters can enhance the probability of proper interpretation and detection. Fuji Velvia 50 or other films that provide greater saturation of green colors can offer an advantage in detection.