|
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.
- 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
section 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.
Sample 1:
 |
Sample 2:
 |
Sample 3:
 |
|
DATA: |
|
|
|
Frequency Count (of 3 Plots) |
| |
Sample 1 |
Sample 2 |
Sample 3 |
Quad 1 (sm) |
Quad 2 (med) |
Quad 3 (lrg) |
|
Red Flower |
1 |
1 |
1 |
3 |
3 |
3 |
|
Blue Flower |
1 |
3 |
3 |
1 |
1 |
3 |
|
Yellow Flower |
2 |
2 |
1 |
1 |
3 |
3 |
|
Grass |
3 |
1 |
1 |
2 |
2 |
3 |
|
** record the quadrat in which the plant first occurs:
1 = smallest, 2= medium, 3= largest |
** note, frequency count is cumulative, if plant occur in 1 it
also occurs in 2, etc. |
|
Nested Frequency Method
- Create a 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 (5x5cm, 25x25cm, 25x50cm, and 50x50cm).
- Record all plants in smallest quadrat, then in each successively larger
quadrats
- Presence of a plant in smaller quadrat = presence in larger quadrat. Only need
to record species in larger quadrats that did not occur in smaller quadrats.
- Estimate Frequency for each nested quadrat (small to large) separately.
- 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.
In the following example, Bottlebrush Squirreltail (SIHY) would be best
sampled with a plot size 1 or 2. For Cheatgrass (BRTE), plot size 1 is the only
size that is small enough. For Hoods Phlox (PHHO), none of plot is big
enough. Even plot size 4 yields only 5% frequency.
|
