Comparison of five canopy cover estimation techniques 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 commonly used ground-based techniques for estimating overstory cover – line-intercept, spherical densiometer, moosehorn, and hemispherical photography – and cover estimates generated from crown radii parameters of the western Cascades variant of 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 were not related to stand-structure type (p = 0.33). As expected, estimates of cover increased and stand-level variability decreased with increasing angle of view among techniques. However, the moosehorn provided the most conservative estimates of vertical-projection overstory cover. Regression equations are provided to permit conversion among canopy cover estimates made with the four ground-based techniques. These equations also provide a means for integrating cover data from studies that use different techniques, thus aiding in the ability to conduct synthetic research. Ground-based measures are recommended for specific objectives. Because the FVS-estimated cover levels were consistently lower and more variable than most of the ground-based estimates (by up to 44, 17% on average), ground-based measures of canopy cover may be preferable when accuracy is an important objective.