II. Procedures

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, or if you are interested in seedlings..

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

Density: Divide the total number of individuals tallied for a given species by the total area of the measured plots (in acres or hectares) to give number of trees per acre or hectare for each species

Dominance (Basal Area): Sum the basal area of each tree of a species (from all plots) and divide 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: Divide the frequency of each individual species by the sum of the frequencies of all of the species and multiply by 100 to obtain the Relative Frequency for each species..

Relative Density: Divide the density of a each individual species by the sum of the densities of all of the species and multiply by 100 to obtain the Relative Density for each species.

Relative Dominance: Divide the total basal area of each individual species by the sum of the basal areas of all of the species and multiply by 100 to obtain the Relative Dominance for each species.

Importance Value: Add together the Relative Frequency, Relative Density, and Relative Dominance for each species to obtain the Importance Value for each species.

(Note: 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 if you only have the data from one plot. When this is done the frequency calculation is omitted 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