THE EFFECT OF PHENOLOGICAL STAGE ON DETECTABILITY OF YELLOW HAWKWEED (HIERACIUM-PRATENSE) AND OXEYE DAISY (CHRYSANTHEMUM LEUCANTHEMUM) WITHREMOTE MULTISPECTRAL DIGITAL IMAGERY

Many upland pastures and forest meadows in the western United States c ontain significant infestations of yellow hawkweed and oxeye daisy. Do cumentation of infestations is necessary in order to plan and assess c ontrol tactics. Previous work with an airborne charge coupled device ( CCD) with spectral filters indicated that flowering yellow hawkweed wi th at least 30% cover was detectable at 1 m resolution. A single image of a large area may not capture all plants in the flowering phase and multiple images are costly. The objective of this paper was to assess the accuracy of images recorded at different phenological stages. We compared three methods of classification: unsupervised classification of a three principal component analysis image, supervised classificati on of a three principal component analysis image, and supervised class ification of a composited image consisting of four bands and normalize d difference near infrared (NIR)/red band. Regardless of the classific ation method, images of yellow hawkweed and oxeye daisy in full bloom had lower classification error than at early bloom or post bloom. The percent error for yellow hawkweed classification was about twice as hi gh at post bloom as at full bloom, but varied slightly depending on th e method of classification and cover class. The ability to detect disc rete colonies of yellow hawkweed was not affected by phenological stag e, but the ability to measure the area of each cluster differed among stages. Less than one-third fo the pixels classified as yellow hawkwee d or oxeye daisy in the early bloom image remained in the same class i n the full bloom image. About half the pixels in the full bloom image remained in the 90 to 100% cover class at the post bloom image. Season al growth of the grasses masked some yellow hawkweed and oxeye daisy p lants, and accounted for differences in classification among phenologi cal stages.