Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/27/2007
Publication Date: 3/1/2007
Citation: Boyd, C.S., Bates, J.D., Miller, R.F. 2007. The influence of gap size on sagebrush cover estimates using line intercept technique. Rangeland Ecology and Management 60(2):199-202. Interpretive Summary: Sagebrush is an important cover and food source for many sensitive wildlife species (e.g. sage-grouse) and sagebrush cover is often estimated using the line intercept method, however, protocols for line intercept field methodology have not been standardized. Sagebrush cover is spatially discontinuous and a minimum gap size should be set for field measurement (breaks in cover larger than the set gap size are excluded from measurement); we tested the influence of 5, 10, and 15cm gap sizes on sagebrush canopy cover estimates. Canopy cover estimates differed by up to 2.4 canopy cover units on sites ranging from 11 to 18% live canopy cover. These differences are small in magnitude and would not likely change a gross assessment of vegetation status, however, where accurate and/or precise measures of canopy cover are needed, specific gap sizes should be set at the outset of data collection efforts.
Technical Abstract: Sagebrush cover is often estimated using the line intercept method. However, a lack of standardized protocols can lead to variable estimates of sagebrush canopy cover. Our objectives were to determine the influence of gap size on 1) sagebrush canopy cover estimates, 2) time needed to read a transect, and 3) between observer variability in sagebrush canopy cover estimates. We utilized 5, 10, and 15cm gaps, and defined a “gap” as a lack of continuous live or dead shrub canopy. In instances where a segment of dead cover was less then the gap size and adjoined live cover, the dead cover was measured as live. We evaluated canopy cover at 6 Wyoming big sagebrush (Artemisia tridentata Nutt. ssp. Wyomingensis Beetle & A. Young) sites in southeast Oregon. At each site, 4, 2-person teams measured sagebrush canopy intercept along 50-m transects. Each transect was read by multiple teams to allow for assessment of between observer variability. Intercept values were converted to % canopy cover and we used analysis of variance to determine the influence of site and gap size on 1) time to measure the transect, and 2) live, dead and total sagebrush canopy cover. Between observer variability was highest at the intermediate gap size (i.e. 10cm). Transect measurement time did not differ by gap size (P = 0.132). Total cover estimates were not related to gap size (P = 0.270). Live canopy cover estimates increased (P < 0.001) from 12.1 to 14.5% with increasing gap size, and cover of dead material decreased (P = 0.015) from 4.4 to 3.2%. These differences are small in magnitude and would not likely change a gross assessment of vegetation status. However where accurate and/or precise measures of canopy cover are needed specific gap sizes should be set at the outset of data collection efforts.