Estimating canopy cover in forest stands used by Mexican spotted owls: Do stand-exam routines provide estimates comparable to field-based techniques?

Canopy cover has been identified as an important correlate of Mexican spotted owl (Strix occidentalis lucida) habitat, yet management guidelines in a 1995 U.S. Fish and Wildlife Service recovery plan for the Mexican spotted owl did not address canopy cover. These guidelines emphasized parameters included in U.S. Forest Service stand exams, and canopy cover typically is not sampled in these inventories. Algorithms exist to estimate canopy cover from stand-exam data, but the accuracy of resulting estimates is unknown. We compared existing field data on observed canopy cover within forest stands used by radio-marked Mexican spotted owls with estimates derived from those analysis routines. Based on arbitrary criteria for minimum canopy cover, we also estimated proportions of these stands that would be misclassified by derived estimates. Canopy-cover estimates derived from stand-exam data differed widely from observed canopy cover in many stands, and derived estimates frequently misclassified stands based on canopy-cover criteria. These algorithms performed worst in mesic mixed-conifer forest, the forest type in which spotted owls occur most commonly. We conclude that existing algorithms for estimating canopy cover from stand-exam data are not useful in forest habitat for Mexican spotted owls.