Comparison of methods for estimating forest overstory cover: differences among techniques

Abstract

Thirteen techniques for estimating forest overstory cover or mean crown completeness were tested for differences in angle of view of the technique and interaction with mean crown completeness or height to base of live crown. With increasing angle of view from common locations, mean estimates of mean crown completeness increased and the standard deviation decreased. For techniques with angles of view >30.degree. there was interaction among techniques with changing overstory cover. As mean crown completeness increased, the differences between wide- and narrow-angled techniques decreased and converged to 0 at a rate dependent on the angle of view. For most techniques the estimate of mean crown completeness increased with height to base of live crown. The more narrow the angle of view the greater was the effect of increasing height to base of live crown. Differences among techniques were those expected from basic trigonometry; they occur because wider angles of view are less likely to encounter only space without canopy. Attempts to develop relations between overstory cover and other factors (e.g., snow interception, understory growth) should use angles of view appropriate to the factor being studied.