Principles of Vegetation Measurement & Assessment
and Ecological Monitoring & Analysis

 

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Why Measure Cover?

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measuring cover

Advantages of Measuring Cover

Vegetation can give important indicators of Ecological processes occurring on a site and can also be a valuable Management indicator for monitoring.
(
This distinction is adopted from an on-line text book by Susan Muir, New Mexico State University, and Mitchel P. McClaran, University of Arizona)

Ecological Indicator - Vegetative cover (particularly canopy cover) is an ecological indicator of what species are dominating the site.

  • The area covered by a species is related to the control that species has over solar, water, soil, and nutrient resources on a site. It is true that a plant's biomass is more clearly related to the ecological dominance of the plant than cover. However, cover reflects the amount of soil, water, and nutrients that the plant can harvest and use to create biomass. Therefore, cover closely reflects biomass but it is generally easier to estimate.

  • Plants with the greatest cover on a site have a greater influence on succession processes such as immigration and establishment of new plants.

  • Cover is useful for characterizing ecosystems across life forms. Cover is expressed as a % of area. Therefore, the meaning of cover is the same for grasses, forbs, shrubs, and trees. Cover allows species of different life forms or morphologies to be evaluated on a comparable basis, in contrast to other attributes such as density or frequency.

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

Cover gives different information about a plant community than density. In the example below, cover has decreased overtime and bare soil has increased. Density has also increased in this example because larger plants have fragmented into smaller ones. A measure of plant age, reproductive status, and vigor would be needed to determine if this is a downward trend.

 

Management Indicator - 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 a useful indicator of how well a site is protected against erosion.
  • Canopy cover or vertical cover are commonly used to describe wildlife habitat in relation to thermal or hiding cover.
  • Cover variables also relate to forage availability for livestock or wildlife habitat.
  • Basal cover is considered the most reliable measure for monitoring rangeland trend, particularly when examining trends in herbaceous plants such as grasses or forbs. If there are changes in basal cover overtime, it likely indicates real changes in the plants dominating the site. Basal cover is often assessed over time because it is not highly influenced by:
    seasonal patterns (i.e., spring vs. fall)
    yearly precipitation pattern
    immediate grazing history (i.e., grazed or not grazed)

Pros & Cons of Assessing Cover

Advantages:

  • Can be used to measure a variety of life-forms (e.g., moss, annual forbs, shrubs, trees).
  • Strongly related to biomass and ecosystem processes.
  • Though cover is often estimated by species, it is not necessary to count the number of individuals within a species which can be difficult and time consuming.
  • Can easily be used to measure plants, mosses, or lichens at the ground surface.

Disadvantages:

  • Most measures of cover (except basal cover) vary greatly depending on climatic conditions.
  • Most measures of cover (except basal cover) are affected by grazing or browsing by herbivores.
  • Cover is not always easy to estimate, and it is difficult to determine the accuracy of estimates. This can lead to variation between observers when looking at the same plot or plant community.

Summary Questions

  1. What is the best measure of cover for determining trend?

  2. Why is cover a good estimate of what plants are dominating a site?

  3. When would it be better to measure cover than density?

Advanced Questions:

  1. Read Monitoring Vegetation Cover by Sheila Barry. (University of California Davis Cooperative Extension). What time of year would it be best to estimate cover if your goal for estimating cover was to determine bioiversity of a site?

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