Daubenmire Cover Class Method
The Daubenmire cover class method was developed in the 1950s by Daubenmire, an ecologist who worked in the forests and rangelands of northwest USA. It is accepted as a relatively accurate method that has been used extensively to determine cover in rangeland situations.
The technique involves visually designating one of the 6 cover classes (Table 1) to each quadrat. Each species within the quadrat is usually assessed separately. Canopy cover is typically considered, simply because ground cover and basal cover are difficult to estimate if obscured by other plants. Originally, 20 cm x 50 cm quadrats were used, but sample unit size and sample unit shape should be selected to suit the vegetation patterns at the site. Sections of the quadrat frame can also be delineated into various colors, to provide a visual guideline of the area that would represent cover in 10% or 25% increments.
Table 1. Daubenmire Cover Classes.
Cover Class Range of Cover Midpoint of Class (%) (%) 1 0 - 5 2.5 2 5 - 25 15.0 3 25 - 50 37.5 4 50 - 75 62.5 5 75 - 95 85.0 6 95 - 100 97.5
Once sampling is completed, species cover can be estimated by multiplying the number of times a class was recorded by the midpoint of that cover class, adding the results for each class, and calculating an average by dividing by the total number of quadrats sampled. Data is usually collected from many quadrats (n$20) located along a transect, so that the transect is the sample unit. Therefore, data must be collected from several transects to determine the precision of the sample, for statistical analysis of cover data.
This method recognizes the difficulty in accurately assigning an exact percent cover value to each quadrat, since even highly experienced workers are unlikely to be able to visually estimate closer than about 5% cover! Assigning broad cover classes provides an equally accurate result as long as the data follows a normal distribution around the midpoint within each class. The narrower upper and lower classes of the Daubenmire scale protect against skewed data in extremely sparse or dense vegetation.
Ranking data into broad classes is also relatively rapid procedure, since observers are not required to spend as much time contemplating quadrat cover to the nearest percent. In fact, rapid evaluation of each quadat is the key to success with this approach, since a large sample is less sensitive to the occasional incorrect ranking. However, the method is still subject to personal bias, and sampling accuracy and precision will benefit if time is devoted to training observers.
References and Further Reading
Bailey, A.W., and C.E. Poulton. 1968. Plant communities and environmental relationships in a portion of the Tillamook burn, Northwest Oregon. Ecology 49:1-13.
Bureau of Land Management. 1996. Sampling vegetation attributes. Interagency Technical Reference, B.M./RS/ST-96/002+1730. pp 55-63.
Cook, C.W., and J. Stubbendieck. (eds) 1986. Range research: Basic problems and techniques. Society for Range Management, Denver, CO. pp 56-57.
Daubenmire, R.F. 1959. Canopy coverage method of vegetation analysis. Northwest Science 33:43-64.
Hanson, H.C., and E. Dahl. 1957. Some grassland communities in the mountain-front zone of northern Colorado. Vegetatio 7:249-270.