Ranking Species Contribution to Forest Community Composition:
Calculation of Importance Values

One measure of the relative dominance of species in a forest community is called the Importance Value (IV). Importance values rank species within a site based upon three criteria:

1) how commonly a species occurs across the entire forest;
2) the total number of individuals of the species; and
3) the total amount of forest area occupied by the species.

Importance value = Relative frequency + Relative density + Relative dominance

To obtain importance values one must first determine the frequency, density, and dominance of each species within the community through a forest inventory.

Frequency: the percentage of inventory points occupied by a given species, a measure of species distribution across the site

Density: the average number of individuals per unit area (per acre or hectare)

Dominance: the average dominance each species within the study area is estimated by its total basal area per unit area (ft2 per acre or m2 per hectare )

Basal Area (BA): The cross sectional area of each tree stem measured at 4.5 ft (1.37 m) above the ground. This value is normally obtained from diameter and can be calculated using one of the following equations:

BA in ft2 = dbh2 (inches) * 0.005454
BA in m2 = dbh2 (cm) * 0,00007854

To compare communities that may differ in size, or that were sampled at different intensities, importance values are calculated using relative rather than absolute values.

Relative frequency: Number of occurrences of 1 species as a percentage of the total number of occurrences of all species.

Relative density: Number of individuals of one species as a percentage of the total number of individuals of all species

Relative basal area: Total basal area of one species as a percentage of the total basal area of all species.

To calculate importance values you need to obtain inventory data of the forest community. One way to do this is to establish a series of inventory plots of a specified size. Circular plots work well because it is easy to measure plot radius from the center of the circle and determine which trees are within the plot boundary. The plots size you choose may depend upon the average size of the trees in your study area. Choose a larger plot size if the trees are large, and a smaller plot size if most of the trees are small.

Within each plot you should record the following information for each tree stem*:

1) Species name
2) Diameter at breast height (dbh): 1.37 m (4.5 ft)

* Only those stems taller than breast height (1.37 m; 4.5 ft) are generally included. If you are only interested in larger trees you can choose to limit your inventory to those stems over a specified diameter

Importance Values Calculations:

Frequency: The number of plots in which a given species is found divided by
the total number of plots sampled.

Density: The total number of individuals tallied for a given species divided by the total area of the measured plots (in acres or hectares)
(=trees per acre or trees per hectare)

Dominance (Basal Area): sum of the basal area of each tree of a species from all plots divided by the total area of all of the measured plots (= in ft2 per acre or m2 per hectare).

Tree Basal Area:
stem dbh (in) * dbh (in) * 0.005454 = BA in ft2 per tree
stem dbh (cm) * dbh (cm) * 0.00007854 = BA in m2 per tree.

Relative frequency: Frequency of a given species divided by the sum of the frequencies of all of the species * 100.

Relative density: Density of a given species divided by the sum of the densities of all of the species* 100.

Relative Dominance: Basal area of a given species divided by the sum of the basal areas of all of the species * 100.

Importance Value: Relative frequency + Relative Density + Relative Basal Area for each species

(The maximum importance value for any one species is 300 (100 + 100 + 100).

After calculations are complete species can be ranker from high to low for comparison with other sites

Note: Importance values can also be calculated on a 200 scale. When this is done the frequency calculation is left off and only relative dominance and relative density are included.
For values in metric: Multiply trees per acre by 2.47 to get trees per ha
Multiply ft2 per acre by 0.23 to get m2 per ha

References

Curtis, J.T. and McIntosh, R.P. 1951. An upland forest continuum in the priarie-forest border region of Wisconsin. Ecology 31: 476-96.

Kent, M. and Coker, P. 1992. Vegetation Description and Analysis, A Practical Approach. John Wiley and Sons, NY, pp 167-169.

Sample Exercise

Comparison of Species Importance Values on two Different Landscape positions on the Cumberland Plateau

Objectives:

1) To calculate species importance values
2) To use importance values to characterize forest communities on two landscape positions: convex ridge and concave drainage

Supplies per group:

map, compass, Dbh tape (cm)
ruler, center pins, metric tape
clip board, pin flags

Field Methods

1) Each of 3 groups will inventory 6 plots…three ridge and three drainage landscape positions. (The overall class plot total will be 9 ridge and 9 drainage plots.)

2) At each inventory point mark plot center with a flag.

3) Use the engineer’s tape to identify and tally trees within a radius of 5.7 m from plot center. (This is the radius of a 0.01 ha plot.) Beginning from an imaginary N line, work clockwise around the plot. For each tree, identify the species and diameter to the nearest 0.1 cm. Only include individuals that are Ž1.37 m tall. Exclude dead stems. It may help to place a pin flag at the base of the first tree you measure so that you do not measure it again when you complete the circle.

Data Entry and Computations:

1) Enter all of the data in a spread sheet with columns that are similar to those in Table 1. Use abbreviations for species names to facilitate data entry. Use the first two letters of the genus and the first two letters of the specific epithet ( Liriodendron tulipifera = LITU; Quercus alba = QUAL) Be careful with spelling!

 Table 1. Sample table for data entry for 1 plot with 6 trees on the ridge position. Landscape Position Plot Species DBH(cm) Tree BA(m2) R 1 ACRU 3.5 0.000962 R 1 ACRU 38.1 0.114009 R 1 QUAL 4.1 0.001320 R 1 QUAL 3.2 0.000804 R 1 QUAL 25.3 0.050273 R 1 QUAL 30.1 0.071158

2) Use the Sort function in your spreadsheet program to sort the data by landscape position, species, and plot. This will allow you to determine importance values separately for the two different landscape positions.

For each landscape position:

3) Determine frequency: Add a new column to the spreadsheet and title it "Freqency". Count the number of plots in which each species is found. Divide this number by the total number of plots surveyed and record it next to the first listing for a species in a new spreadsheet column.

4) Determine density: Add a new column to the spreadsheet and call it "Density" . Count the number of trees per species, and divide by the total area surveyed* and add this to the new column). * area surveyed = # plots per landscape position* 0.01 ha each

5) Determine dominance (basal area): Add a new column to the spreadsheet entitled "Tree BA". Calculate BA from diameter using the equation:
BA (m2 ha-1) = dbh (cm)2 * 0.00007854

Sum the basal area for each tree species and divide by the total area surveyed* and add this to a new column (See Table 1……"BA per ha").

6) Copy all of the data to a new worksheet. (Use the "Paste Special" option to copy the values and not the formulae) Sort the data by any of the three last columns and delete all rows except those that have data in the last three columns. Next delete the following columns: Plot, dbh, and Tree BA. Calculate the sum of the Freq., Density, and BA columns.

7) Add four more columns labeled Relative frequency, Relative Density, Relative Dominance, and Importance Value to the chart. Use the formulas listed earlier in this exercise to calculate the Relative frequency, Relative Density, Relative Dominance, and Importance Value for the first species on the list. Copy this formula down to get the values for the rest of the species.

8) Sort the data so that each chart lists the species in descending order of Importance Values.

Data Analysis

1) How do the two landscape positions differ in their ability to support forest tree growth?

2) Using the differences you have identified in question 1, compare the ranking of the trees in the two landscape positions. Use information from Silvics of North America about the growth needs of species to discuss any differences you found in species dominance on the two sites.

Silvics of North America can be found online in its full text version at the following web site: (http://www.na.fs.fed.us/spfo/pubs/silvics_manual/table_of_contents.htm).

Silvics of North America: 1. Conifers. R.M. Burns, and B.H. Honkala (tech. coords.). Agriculture Handbook 654. U.S. Department of Agriculture, Forest Service, Washington
Silvics of North America: 2. Hardwoods. R.M. Burns, and B.H. Honkala (tech. coords.). Agriculture Handbook 654. U.S. Department of Agriculture, Forest Service, Washington