Cover is the vertical projection of plant material onto the ground when viewed from above. It is usually expressed as a percentage value. For example, 18% cover indicates a birds-eye-view would reveal 18% of the surface area as vegetative material with the remaining 82% as bare ground. In some forestry situations cover is expressed on an area basis, such as square meters/hectare or square feet/acre.
Cover is one of the most commonly measured attributes in range inventory and monitoring programs. Many of the methods to determine cover can be conducted simply, quickly, and with acceptable levels of accuracy and precision, making it a favored attribute to survey large areas.
Cover can be measured for the entire vegetation in an area, or can be applied to individual species. In many areas, overlapping plants can mean more than one species contributes to cover. Decisions involving which species to record depend upon the objectives of the study, and should be clearly described as ground rules during the planning stage. Usually, if ground cover is the attribute of interest only the uppermost component is recorded, whereas basal cover is identified by the species occurring at the soil surface.
Species composition may also be determined from cover data by calculating the relative contribution of each species to the total cover at the site. Sometimes species composition is obtained by recording all leaf area so that underlying species are not excluded, and this approach can cause the value for total cover to be greater than 100%!
Cover is regarded as an important indicator of ecological and management processes within the vegetation, though many of the direct relationships still have to be quantified.
- Ecological indicators - cover is a characteristic expression of ecological dominance, or the degree of influence a particular species exerts in the vegetation. Although ecological dominance is also a function of species biomass, cover values (eg., leaf area index) provide a close reflection of biomass rankings while being easier to determine. Furthermore, cover allows the species of various life forms to be evaluated together on a comparable basis, in contrast to other attributes such as density and frequency. Canopy cover is usually most meaningful when considering ecological processes, particularly when woody species are included, because measurements based on basal cover do not adequately reflect the extent of their influence within the vegetation.
- Management indicators - cover provides a variety of interpretations of direct concern to rangeland management, including erosion potential, the value of wildlife habitat, availability of forage, and trends in range condition. Ground cover is considered the best indicator of protection of the landscape against erosion, whereas canopy cover is commonly used to describe wildlife habitat or related to forage availability. Basal cover provides the most reliable measure for monitoring range trend (particularly when focusing on herbaceous components), because it is less sensitive to fluctuations caused by current seasonal conditions or immediate grazing history.
Terminology Related to Cover
Measurement Techniques and Statistical Analysis
References and Further Reading
(Note: pdf files require Adobe Acrobat (free) to view)
Bonham, C.D. 1989. Measurements for terrestrial vegetation. John Wiley Sons, New York, NY. pp 96-135.
Cooper, C.F. 1959. Cover vs. density. Journal of Range Management 12:215. (pdf)
Greig-Smith, P. 1983. Quantitative plant ecology. Academic Press, New York, NY. pp 5-9.
Mueller-Dombois, D., and H. Ellenburg. 1974. Aims and methods of vegetation ecology. John Wiley Sons, New York, NY. pp 80-92.