Spherical densiometers produce biased estimates of forest canopy cover

We assessed suitability of convex and concave densiometers for measuring forest canopy cover in uneven-aged mature and selectively harvested grand fir-western larch (Abies grandis-Larix occidentalis) forests in northeast Oregon. Cover estimates from both instruments were similar, overestimated cover substantially, and were insensitive to substantial variations in forest cover. Spherical densiometers are not suitable for estimating forest cover for most applications in forest ecology and management. Precision of instruments with minimal bias, however, is relatively low. Workers should plan for greater sample sizes to achieve adequate precision if unbiased instruments or techniques are used to measure canopy cover.