Principles of Vegetation Measurement & Assessment
and Ecological Monitoring & Analysis

 

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Veg Sampling
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Nested Frequency Techniques

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Measuring Frequency

Overcoming Quadrat Size Issues

One of the greatest shortcomings of assessing vegetation with frequency is that this measurement is completely depended on quadrat size. Challenges with quadrat size include:

  • You can't really know until after a pilot study or after collecting data what an appropriate size will be.  You need a plot that samples a frequency between 20% and 80%.
  • Even if you select a quadrat size that works this year, the quadrat may become too big or too small as the vegetation community changes over time.
  • A quadrat that is "just" right for one plant may be too big or too small for another species of interest. (Sounds a bit like Goldilocks searching for the right chair).

To overcome this "right size" problem, rangeland scientists, (Smith, Bunting, and Hironaka 1986), proposed a Nested Frequency plot where 3 plots were nested within one another.
 

With a nested quadrat, all plants recorded in the smallest segment of the quadrat would automatically be known to occur in the larger quadrat. Each new plant occurring in successively larger quadrats, in the nested quadrate frame, is recorded. In the example below, the first section of the quadrat in which a plant occurs (1 = smallest, 2= medium, 3= largest) is recorded. When recording date with nested frequency plots, the smallest frame in which the plant occurs is recorded.

Sample 1:
Sample 2:
Sample 3:
Sample 4:
First record the smallest frame in which the plant occurs:
1 = smallest, 2= medium, 3= largest
  Sample 1 Sample 2 Sample 3 Sample 4
Red Flower 1 1 1 3
Blue Flower 1 3 3 -
Yellow Flower 2 2 1 2
Grass  3 1  1 1

Next summarize data  to examine occurrence in each plot size:

  Plot 1
(smallest)
Plot 2
(medium)
Plot 3
(largest)
  Hits % Freq Hits % Freq Hits % Freq
Red Flower 3 75% 3 75% 4 100%
Blue Flower 1 25% 1 25% 3 75%
Yellow Flower 1 25% 4 100% 4 100%
Grass  3 75% 3 75%  1 25%

** note, frequency count "Hits" is cumulative, if plant occur in 1 it also occurs in 2, etc.
The % Frequency = Number of Hits Total number of plots examined.. in this case 4 plots were examined.
 

Nested Frequency Method: Step-by-Step

  1. Create a nested quadrat that is a series of 3 to 5 quadrats nested within each other. A common frame size for herbaceous plants is 50 x 50cm, with four smaller quadrat sizes nested within the frame (5x5 cm, 25x25 cm, 25x50 cm, and 50x50 cm.

  2. Record all plants occurring in the smallest quadrat. Then, examine each successively larger quadrat and record new plants that are not already recorded as occurring in smaller quadrats.

  3. Remember, presence of a plant in a smaller quadrat means that it automatically occurs in larger quadrats.

  4. Estimate Frequency for each nested quadrat (small to large) separately.

  5. Look at data to see which plot size most appropriately estimates each important species. Recall that a good plot size would yield a frequency between 20 and 80%. The advantage of this technique is that one does not need to determine in advance which plot size is going to best represent each species.  You simply record occurrence in all quadrats and decide later which is best.

Give it try -

In the following example, Idaho Fescue (FEID) would be best sampled with a plot size 2 or 3.  Plot size 1 is too small because frequency should be > 20% and Plot size 4 is too big because frequency should be less than 80%. For Bottlebrush Squirreltail (SIHI), plot size 3 and 4 are the only plots that are large enough. 

In the example data sheet above, calculate the frequency and determine an appropriate plot size for:
►ELSP (Elymus spicatus) or Bluebunch Wheatgrass
►ERIOG (Eriogonum) or Buckwheat
►ROWO (Rosa woodsii) or Woods Rose
►CRAC (Crepis acuminata) or Tappertip Howksbeard

Click here for answers

Summary Questions

  1. What is the advantage of using a nested-quadrat approach for estimating frequency over a single plot frame?

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