I. Background
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): Basal area is a unit of tree size that is determined
from stem diameter. It is equal to the cross sectional area of a tree
stem measured at 4.5 ft (1.37 m) above the ground. This value is normally
obtained by measuring diameter 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 make it easier
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.
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