Summarizing values to Describe Plant
communities
Assessing Similarity
In vegetations studies it is often desirable to compare two plant communities
and determine how similar they are. This can be accomplished with a
similarity index. Measures of similarity can be used to examine:
- Differences between two sites on a landscape or management units.
- Differences between similar sites in different units under different
management practices.
- Changes that may have occurred because of a natural or human caused
disturbance (e.g., similarity between burned and unburned sites).
- Variation between different study times on the same site. (e.g.,
determine how similar the communities is to what is was 10 years ago)
- Comparison of a site to desirable state or described "referent."
Calculating a Similarity Index
The first step in making a reasonable comparison is to collect data of
similar units and scale on two sites or times. In the example used in the
previous lesson, it may be useful to describe, in numbers, the similarity
between the clay loam and the sandy loam site in terms of shrub density.
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Where:
nc
= number of common species between sites
this number is the lowest value
among the compared sites
n1
= number of individuals of site 1
n2
= number of individual of site 2 |
For example, the similarity between the Clay Loam and Sandy Loam Site
in South Texas from the previous lesson?
Shrub |
Clay Loam |
Sandy Loam |
Number in common |
blackbrush |
156 |
65 |
65 |
guajillo |
176 |
55 |
55 |
catclaw acacia |
43 |
45 |
43 |
granjeno |
56 |
32 |
32 |
whitebrush |
25 |
67 |
25 |
kidneywood |
15 |
25 |
15 |
elbowbush |
1 |
70 |
1 |
wolfberry |
0 |
28 |
0 |
shrubby bluesage |
0 |
40 |
0 |
|
472 |
427 |
236 |
Similarity = (2 x 236) / (472
+427) = 52.5% similarity between sites.
What is a referent?
Similarity indexes are often applied to see how similar an existing community
is to a desired or historically relevant state. To accomplish this
comparison on must create what is called a referent
or a state
to which comparisons are going to be made. The value of a referent lies in several important
features:
- Is the referent state clearly described?
- Is it realistic for the soil,
climate, and disturbance regimes on the site?
- Is the referent state relevant in
terms of specific uses of values for the site?
The more clearly the referent is
described, the more effectively the current state of the community can be
measured against it. If a land manager can quantify how close or far they are
from the desired state, they can derive management plans to achieve the desired
state.
In a sagebrush-juniper community (such as pictured above), the following
comparison might be made:
% Composition based on biomass |
|
Plant |
Current |
Referent |
Similarity |
Wheatgrass |
21 |
50 |
21 |
Native Forbs |
12 |
20 |
12 |
Sagebrush |
10 |
20 |
10 |
Juniper |
38 |
10 |
10 |
Annual grass |
19 |
0 |
0 |
Total |
100 |
100 |
53% |
Bottom Line ~~ the current site is 53%
similar to the desired referent or desired state. This comparison could be made over
years as management strategies attempt to bring the plant community closer to
the referent state. Changes in similarity between the two states will
suggest whether management is working to change the community closer to the
desired state or not.
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