Principles of Vegetation Measurement & Assessment
and Ecological Monitoring & Analysis


Veg Sampling
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Estimating Similarity

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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.

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? 


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


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.

Summary Questions

  1. How might similarity be used as part of a monitoring protocol?

  2. What is a "referent state" and how might this be determined or defined?

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