Determining Frequency

How to calculate frequency?

As with any vegetation sampling activity, the first step is to design a protocol that can accomplish the assessment or monitoring goal within available time and resources. Once this is set, then frequency measures can be accomplished by random, systematic, or subjective locations of plots. A stratified approach might also be advantageous. 

Step 1: Count and record the number of individuals of each species in a each plot. In the example below, 100 plots were examined.

Step 2: Calculate frequency of each plant.

• In the example data sheet above, 100 plots were examined so total number of plots in which a plant occurred = % Frequency.

• Two important details:
  1) The number of times a plant occurs in a plot are often called "hits." In the example above, there was 1 hit of juniper (bottom of the list). 
  2) Plant names are often recorded with species codes, such as in the data sheet above.  In most cases these are 4-letter codes with the first 2 letters of the genus and first 2 letters of the specific epithet.  Therefore, BOCU is Bouteloua curtipendula or sideoats grama.  KOCR = Koeleria cristata or prairie junegrass. To find the species codes (i.e., symbol) for a plant or the name of the plant associated with a code visit www.plants.usda.gov.

• On the data sheet above what is the frequency for the following plants:
  Frequency of BOHI2 (Bouteloua hirsuta or Hairy grama) = ________.
  Frequency of SIHI (Sitanion hystrix or Squirreltail) = ________.
  Frequency of AMPS (Ambrosia psilostachya or Cuman ragweed) = ________.
  Frequency of ERIOG (Eriogonum or buckwheat) = ________.
  »» Click here for answers

Is a species present or absence?

It seems obvious, but conducting a frequency assessment requires clearly specifying what constitutes a plant species being present in the quadrat or plot. Any of the following criteria will work as long as the approach is documented and consistently followed:

• Rooted Frequency - the plant of interest must be rooted inside the sample plot in order to be counted.

• Shoot Frequency - any runners or shoot of a plant is rooted in the plot but the whole plant does not need to be rooted in the plot. This is the most commonly recorded measure of frequency.

• Mere Presence - all plants, runners, and shoots rooted in the plot plus overhanging leaves or shoots that lie inside the plot boundary are counted as a hit.

Quadrat Size Matters!

If the sample plot is too small, the chances of the plant species of interest being recorded is small, causing a low frequency. If the quadrat is too large, the plant species of interest will be in almost all the plots. This will cause a high frequency and will not show the plant distribution in a community.

In other words, plant frequency is completely affected by the size of the quadrat.  This effect of plot size is so profound that some ecologists, like my colleague Dr. Stephen Bunting, don't recognized frequency as a true plant attribute. Rather, Dr. Bunting thinks of frequency as a joint characteristic of plant and quadrat.  Thus, when assessing frequency it is very important to select an appropriate plot size that adequately samples each plant of interest.  Yes, this will occasionally require that several different plot sizes be used if several plant are being examined.  This problem is solved by a nested frequency plot, discussed in the next lesson.

The appropriate quadrat size depends on the size and density of the plant. 

• It is difficult to detect an increase in a plant's abundance if it occurs in nearly all the plots. General rule: If frequency is 90 to 100% for the plant species of interest, reduce the quadrat size.

• Likewise, it is difficult to record a decline in a species if the initial frequency of the plant is very low. General rule: If the plant species of interest is not present in most of the samples taken, increase plot size.

A few more guidelines for selecting an appropriate quadrat size:

• Plot size should be such that frequency for the important species fall between 20% and 80%. Some researchers suggest the resulting frequency should be between 30% and 70%.

• Plot size may change depending on species measured. Plot size should be 1 to 2 times as large as mean area of the most common species.

• Include maximum number of species possible. In other words, the plot should be big enough to include as many of the species on the site as possible.

• There is a math equation to help determine size of plot using a logarithmic relationship between frequency and density (refer to Bohnam 1989).

• Suggested empirical sizes from Cain and Castro 1959:
  ► Moss layer 0.01- 0.05 m²
  ► Herb layer 1-2 m² (commonly used units are 1 m²)
  ► Tall herbs and low shrubs 4 m²
  ► Tall shrubs and low trees 10 m²
  ► Trees 100 m²

Summary Questions

In the following plots, what is the frequency of "Crimson Bell"?  What is the frequency of "Wild Yellow Daisy"?

Plants of Interest = and

Advanced Questions:

1. The term "frequency" has different meetings to different disciplines. For example, in fire ecology and management, the term "Fire Frequency" means something a bit different than how we have used it in this lesson. Visit this link to the Fire Regime Condition Class Definition to determine how frequency is used in defining wildland fire regimes.  How similar or different is it from how the term frequency was defined in this module?

References

• Bonham, C.D. 1989. Frequency 90-96p. Measurements for Terrestrial Vegetation. John Wiley and Sons Inc. New York, NY pp. 90-96.
• Cain, S.A. and G.M. DE O. Castro. 1959. Manual of Vegetation Analysis. Harper, NY pp. 325.
• Daubenmire, R. E. 1968. Plant communities: A Textbook of Plant Synecology. Harper and Row, New York. pp. 300.