Principles of Vegetation Measurement & Assessment
and Ecological Monitoring & Analysis

 

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Veg Sampling
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Point Intercept Techniques to estimate cover

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measuring cover

Basics for Point Measurements

A point is a very very small "plot" that is measured to determine what plant or soil attribute occurs at that point. Assessment points are usually accomplished with a "pin" or long metal wire that is sharpened to create a "point." However, there are many variations on the point theme including, laser points, and cross-hairs.

Cross-Hair Sighting Device

10-point Point Frame


Cross-Hair Point Frame

No matter how the point is created, the idea is the same... at each point the species, vegetation type (i.e., grass, forb, or shrub), or ground cover (i.e., rock, bare ground, biotic crust, etc.) that intersects the point is recorded as a "hit." The formula to calculate cover is simply the proportion (%) of hits for a species of vegetation type:

For example, from Measuring and Monitoring Plant Populations (Elzinga et al. 1998; Fig 8.10, pg 182)


Ground, basal, canopy, and foliar cover can all be measured by point methods, depending on the rules established to guide decisions what constitutes a hit. It is generally easier to determine if a point hit the base of plant (i.e., basal cover) or a leaf (i.e., foliar cover) than if a point is in the canopy of a plant. Therefore, points are seldom used to estimate canopy cover, though it is possible.

For example, in this 10-point sample, what is the cover of grass estimated as ground, basal, foliar, or canopy cover?
Type of Cover 1 2 3 4 5 6 7 8 9 10 % Cover
Ground No No No No No No No No No Yes 10% grass (90% bare ground below leaves)
Basal No No No No No No No No No Yes 10% grass
Foliar Yes Yes No Yes No No No No No Yes 40% grass
Canopy Yes Yes No Yes No Yes Yes Yes No Yes 70% grass

Points to Estimate Ground Cover

Ground cover is most often and easily measured with a point technique. It is a quick and objective way to assess how much plant cover is on a site or landscape.

One common technique used on grasslands and shrublands for estimating ground cover is called the "Step-Point" method. This involves selecting a point to begin, setting a transect direction with a compass bearing, then following the bearing and recording what occurs at regular intervals along the transect.

These regular intervals are created by counting a number of paces between "hits" which are usually a pin that is set to the ground at specific "point" on the technicians boot.

For example, one might head off on a 120 bearing across the landscape. At every step-point, the field technician records a "hit" as a plant, biotic soil crust, or bare ground.

Data from 350 step-points might look like this:

Ground Cover Hits Cover
Plant 77_ 22%
Boitic soil Crust 119_ 34%
Bare Ground 154_ 44%

Total

350_ 100%

Layers of Vegetation - A Challenge and Opportunity

Measuring vegetation would be much easier (and more boring) if it occurred in just one horizontal plane.  But, this is seldom the case. The layers of vegetation that occur in the real world create a problem and an opportunity when conducting point measurements to estimate cover.  There are basically two solutions:

1) Ignore layers - Make a rule to ignore anything except the top or bottom layer of cover. Making a rule like this is okay as long as it meets your objectives, is consistently followed, and stated in the results and methods. For example:

  • When conducting a ground cover survey, the vegetation hanging over the ground is ignored.
  • When conducting a study of foliar cover or canopy cover only the top layer of vegetation is usually be counted. Any vegetation below the top layer or an the ground would be ignored.

2) Include Layers - Record hits of vegetation encountered as the point is lowered. This point-hit method is much more time consuming but gives a richer 3-dimensional view of the site or landscape. For example, in the following four points, hits would be recorded as such:

Point Ground Level Veg Layer 1 Veg Layer 2 Veg Layer 3  
1 Bare Ground Wheatgrass Bitterbrush Pinyon Pine  
2 Sagebrush Pinyon Pine      
3 Wheatgrass Rabbitbrush      
4 Litter Wheatgrass      

Data summary: Ground Cover = 25% bare ground, 50% perennial vegetation, and 25% litter
Vegetative Cover (of 8 hits): 38% wheatgrass (3 hits), 25% Pine (2 hits), 12 % Bitterbrush, 12% Sagebrush, 12% Rabbitbrush.

 

Analysis of this multi-layer method can be difficult. But, the field data would look like this:

----->

 

For more information about this method, read pages 73 through 77 of Sampling Vegetation Attributes.

If you are not familiar with this Dot-Count Method it works like this:
   

Points Can Be Combined with Other Methods

The use of points to estimate cover is so simple and flexible that it can be used independently, as described above for the step-point method, or in combination with other methods. For example:

  • Points Along a Line - Transects or lines are used in many vegetation assessment protocols. Points are often recorded at regular intervals along a transect that has been laid for other techniques such as to estimate cover, frequency, structure, or biomass.

  • Points on Plots - Once a quadrat is place for biomass or frequency assessments, it is easy to examine what is directly below each corner or point on the plot perimeter.

Summary Questions

  1. To which of the four types of cover (basal, canopy, foliar, or ground) can point intercept methods be applied?

  2. How can one deal with layers of vegetation when conducting a field survey with point intercept methods?

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