Comparisons among five canopy-cover estimating methods in five Douglas-fir/western hemlock structure types in the western Oregon Cascades

Estimates of forest canopy cover are widely used in forest research and management, yet methods used to quantify canopy cover and the estimates they provide vary greatly. Four ground-based techniques for estimating overstory cover -line- intercept, spherical densiometer, moosehorn, and hemispherical photography-and cover estimates generated using the Forest Vegetation Simulator (FVS) were compared in five Douglas- fir/western hemlock structure types in western Oregon. Differences in cover estimates among the ground-based methods did not depend on the structure type in which they were measured (p=0.33). As expected, estimates of cover increased and within-stand variability decreased with increasing angle of view among techniques. However, the moosehorn provided the most conservative estimates of vertical-projection overstory cover. The FVS-generated cover was consistently lower (by up to 44%, 17% on average) than the ground-based estimates and is not advised as a substitute for ground-based measures in these forest types. Regression equations are provided to allow conversion among canopy cover estimates developed with the four ground-based methods.