Evaluation of a technique for measuring canopy volume of shrubs

Abstract

Cover methods quantify vegetative communities in only 2 dimensions. The addition of height measurements to cover data, resulting in canopy volume estimates, provide a more practical level of description for shrub communities. We evaluated a technique to estimate canopy volume of shrubs that used a formula [2/3piH (A/2 x B/2)] derived from the basic ellipsoid volume formula. Objectives of this study were to determine if there were significant differences among means of repeated observations on sample units: (1) among observers; (2) within observers; and (3) between sample periods when using this technique. At 2 locations in Wyoming, 10 planeleaf willow (Salix planifolia var. planifolia Pursh) plants along each of 5 randomly established transects were sampled during 2 consecutive periods by 4 observers. Differences among observers were significant at both sites (P < 0.05). However, within observer variation between sample periods was not significant (P > 0.05) at either site. Mean canopy volume did not vary significantly (P > 0.05) between sample periods when averaged across observers. Estimated sample sizes ranged between 2 and 31 transects depending on the desired sampling precision and confidence level. The average time per transect among all observers decreased from 13 minutes (SD = 3.7) in sample period 1 to 9 minutes (SD = 1.3) in sample period 2. Using this method, managers can better describe and monitor trends in the structural diversity of shrub communities. This canopy volume technique can be applied with minimal training and is precise, efficient, and repeatable.