Principles of Vegetation Measurement & Assessment
and Ecological Monitoring & Analysis

 

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What is Cover?

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Readings & Instructions

  1. This lesson provides an introduction to what the cover is and how it is measured to assess vegetation.
  2. Read through the Text and work through the Summary Questions at the end of the text.
What this Reading Covers

What is Cover?

Cover is the area of the ground surface covered by vegetation or other coverages including rocks, litter, moss, or bare ground.  Cover is expressed as a % of area. For example, if you look directly over the quadrat below, you might estimate that the area is covered by 35% Grass, 12% White Flower, and 15% rock. Then, bare ground (the area not covered by vegetation or rocks) would be 38%.

Grass:

White Flower:

Rock:

Cover is assessed to describe which plants dominate the water and nutrient resources on a site.  Vegetative cover is also an important characteristic that affects hydrological attributes of the site. Cover can be measured for the entire vegetation in an area (e.g., "total vegetative cover was 45%"), or can be applied to individual species (e.g., "sagebrush cover was 10 to 15%").

Four Types of Cover

Basal Cover = The proportion of the plant that extends into the soil.
Compared to other estimates of cover, basal cover is:

  • more stable from year to year and less sensitive to changes due to climatic fluctuation.
  • not affected by utilization by grazing animals.
  • usually used for trend comparisons or for calculation of species composition.
  • can be difficult to measure for forbs or grasses with a single, small stem.

Canopy Cover = An estimate of the area of influence of the plant.

  • Included influence of the roots below the leaf/stem canopy
  • Ignores 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.

Foliar Cover = A vertical projection of exposed leaf area. The cover would equal the shadow cast if the sun was directly overhead.

  • Small openings in the canopy or overlap within the plant are excluded.
  • Highly susceptible to yearly fluctuations due to climatic or biotic factors.
  • Can be difficult to measure except for some growth forms. Easiest to measure for forbs, shrubs or succulents with large leaves (i.e., cactus).
  • Leaf Area Index is common way of measuring of foliar cover

Comparison of Basal, Canopy & Foliar Cover

Ground Cover = Cover of the soil surface with plants, litter, biotic soil crust (i.e., mosses or lichens), rocks, or gravel.

  • Most often used to determine the watershed stability of the site.
    Commonly included in measures of sparsely vegetated communities (i.e., deserts).

Three Approaches to Estimate Cover

Many different field methods have been invented to estimate basal, canopy, foliar, or ground cover. However, all field methods are based on 3 basic approaches that involve recording the occurrence of vegetative cover on Points, Lines, or Plots.

  • Points = Examination of many points on a site to estimate the proportion of "hits" or occurrences for a plants species or type. (Covered in Lesson 8_3)
  • Lines = Assessment of the distance along a line that is intercepted by a plant species or type. (Covered in Lesson 8_4)

  • Plots or Quadrats = The proportion of area in a plot that is covered by a plant species or type. (Covered in Lesson 8_5)

Summary Questions

  1. How does canopy cover differ from foliar cover?

  2. How is ground cover related to hydrological properties on a site?

Advanced Questions:

1. Canopy cover is often thought to reflect which plants in the community that control the resources on a site.  What is meant by this statement?

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