8.1 Cover Overview

Video Presentation

Learning Guide

Introduction to Cover

Cover is the percentage of the ground surface covered by vegetation or other protective material. You can visualize cover as the shadow cast on the ground by vegetation when the sun is directly overhead. To illustrate, think of a beach with umbrellas scattered about on the sand.

Aerial view of beach umbrellas covering a sandy shore.  The surface area covered by the umbrellas is analogous to cover of plant canopies.

The proportion of the sandy surface in a specified area that is obscured from an aerial view by the umbrellas would be equal to the percent cover of umbrellas.

Just as there are different colors and sizes of umbrellas in the picture, so are there different species of plants on the landscape. Plant cover can be assessed by species, functional group, or all species combined as total vegetative cover.

Why Measure Cover?

Plant cover is an indicator of ecological processes and management effectiveness. For example, when plant cover is severely reduced, there is a high potential for soil erosion, loss of ecosystem resources and reduced grazing potential.  Plants with the greatest cover on a site also have a greater influence on successional processes such as immigration and establishment of new plants.

Because cover reflects the amount of soil, water, and nutrients that a plant can harvest and use to create biomass, it is a much better reflection of biomass than density or frequency. Cover is also generally easier to estimate than biomass. Cover provides more information on community composition than density or frequency because it equalizes the contribution of species that are small in size, but abundant in number with species that have large, but few individuals.

For example, Figure 1 shows a 10 – m² plot that includes a simple plant community with 2 species:  a shrub and a forb.  There are 3 shrubs and 45 forbs.  The density of forbs is 17 times greater than that of the shrubs, but canopy cover of the shrubs is 50%, while the canopy cover of the forbs is only 10% for the same 10- m² plot.

Figure 1.  Illustration of a simple plant community with 2 species that exhibit different density and cover values.

Cover is especially useful in evaluating hydrological processes.  For example, foliar cover influences the amount of rain that is intercepted; while ground cover, including litter, influences infiltration and potential erosion.

Cover can also be used to describe wildlife habitat in relation to thermal or hiding cover, as well as forage availability for livestock and wildlife. Basal cover is considered the most reliable measure of rangeland trend, particularly for herbaceous plants such as grasses or forbs, because basal area is less sensitive to short-term environmental conditions and grazing, but rather is an indicator of long-term change in species dominating the site.

Advantages of Assessing Cover

• Can be used to measure a variety of life-forms (e.g., moss, annual forbs, shrubs, trees).
• Easily visualized and intuitive measure that is strongly related to biomass and ecosystem processes.
• It is not necessary to distinguish between individuals.  Cover is often measured at the species level, but the number of individuals that produce the cover measurements does not matter.
• Provides different information about the plant community than density or frequency. Unlike density, there is no need to identify or count individual plants. Thus it is a popular way to assess herbaceous vegetation, particularly plants that are matted or where identifying individuals is challenging.
• Because it is expressed as a percent of area, cover measurements are applicable for nearly all types of plants.

Disadvantages of Assessing Cover

• Most changes in cover values (with the exception of basal cover) are highly dependent on climate. For example, a decrease in density of arrowleaf balsamroot, a perennial plant, is caused by mortality. On the other hand, cover values for arrowleaf balsamroot could decrease, but the cause could be mortality, grazing, or poor climatic conditions for plant production. Consequently, changes in cover values can be difficult to interpret.
• Most measures of cover (except basal cover) are affected by grazing or browsing by herbivores.
• Not always easy to estimate, and is difficult to determine the accuracy of estimates. This can lead to variation between observers looking at the same plot or plant community depending on the specific method used to assess cover.

Types of Cover

There are four types of cover that are measured on different parts of the plant or the soil surface. Each type of cover has different ecological and management implications, and varies in degree of difficulty for data interpretation.

Figure 3.  Comparison of canopy, foliar and basal cover.  The illustrations at the top show a side view from the ground level, and at the bottom show a bird’s-eye view, or view from above.   The orange highlights in the bird’s-eye view indicate the extent of the area included in measurements of the different cover types.  

1. Canopy Cover: The percentage of the ground covered by a vertical projection of the outermost perimeter of the natural spread of plant foliage (Figure 3).

• Included influence of the roots below the leaf/stem canopy
• Ignores small openings or gaps in the canopy.
• Vertical projection of the outermost perimeter of the natural spread of foliage of plants.
• For any area, the total canopy cover can exceed 100% because plants can overlap.

2. Foliar Cover : The percentage of the ground covered by a vertical projection of plant leaves or foliage (Figure 3).

• Small openings in the canopy or overlap within the plant are excluded.
• Highly susceptible to yearly fluctuations due to climatic or biotic factors.

3. Basal Cover: The percent of soil surface covered by plant bases (Figure 3).

• Compared to other estimates of cover, basal cover is:
• More stable from year to year, and less sensitive to climatic fluctuation.
• Not highly affected by utilization of grazing animals.
• Useful for trend comparisons, or for calculation of species composition.
• Less susceptible to bias, but can still be difficult to measure for forbs or grasses with a single, small stem.

4. Ground Cover = The percentage of the soil surface covered by protective material, such as standing vegetation (including live and dead plants), litter, rock, gravel, bedrock, and biological soil crusts (Figure 4).

• Unprotected bare ground or bare soil is not considered ground cover.  Ground cover plus bare ground always equals 100%.
• Most often used to determine the watershed stability of the site.
• Commonly included in measures of sparsely vegetated communities (i.e., deserts).

Figure 4.  Ground cover may include litter, rocks, basal cover and gravel.  All measures of ground cover summed with measurements of bare ground, always equal 100%.

Ground rules

A few important ground rules must be established before taking cover measurements.  These include:

1. Deciding level of plant identification (species, family, functional group, etc).
2. Determining details of what is considered cover (e.g. in canopy cover, will we incorporate the canopy area that composes the very outer edge of the plant, or the canopy that is most dense, excluding a few long stems or leaves?)

Measurement Methods

Numerous field methods have been used to estimate basal, canopy, foliar, or ground cover.  In general, these field methods are based on measurements that record cover using points, lines, or plots. In addition to field-based measurements, cover can be estimated using photographs and other types of remotely sensed images.

Points: All point methods involve examination of the material that a point intercepts.  Each interception is scored as a “hit” on a species, functional type, or ground cover category.

Lines:  Assessment of the proportion of a line that is intercepted by the cover category of interest.  In general, line intercept methods are best used to estimate canopy and basal cover.

Plots: The proportion of area in a plot or quadrat that is covered by canopy, foliage, or bases of plants. Plot-based measurements are commonly done by ocular estimation, but also include charting or mapping in permanent plots.

Photographs: Cover can be determined from ground-based and aerial photographs by super-imposing points and lines onto the image and determining the interception.  In addition, cover can be determined by drawing polygons around plant canopies or surface features in photographs.

Remotely Sensed Images:  The proportion of area covered by vegetation determined using computer software to analyze remotely sensed, satellite imagery. Evaluation of cover by species or functional type depends on the type of image and its level of resolution.