Collecting Tree Survey Data

Each group is assigned a series of randomly generated points along a transect line passing through a stand of trees.

1. At each point, divide the working area into four quadrants by visualizing a line that is perpendicular to the transect line and passes through the point (see diagram).

2. Select the tree in each quadrant that is closest to the point.

3. Record the name of the species, its diameter (in inches) at chest height, and its distance (in feet) from the point. The data collection sheet will thus contain data for four trees at each point along the transect line (one tree from each quadrant).

Required Supplies

• Each group of students will need a Data Collection Sheet, a tape measure, and a tree identification manual.

Four Important Values:

• Relative Dominance (RDO)• Relative Density (RDE)• Relative Frequency (RFQ)• Importance Value (IV)

– calculated by adding together the previous three

Relative Dominance (RDO)

• The ratio of the total basal area of a single tree species to the total basal area of all tree species sampled.

• The basal area of a tree's trunk is roughly proportionate to the overall area of ground shaded by the tree's canopy.

• The RDO can be thought of as a measure of the ability of a tree species to dominate over other species by reducing the amount of sunlight the other species receive.

Relative Density (RDE)

• The relative density is the ratio of the total number of individuals of a single tree species to the total number of trees sampled.

• The RDE can be thought of as a measure of the ability of a tree species to propagate itself in the local environment.– A tree species with a high density value is very

abundant in the area being sampled, but not necessarily widely dispersed.  It may be clumped or clustered in one section of the transect line or evenly spread across the entire transect line.

Relative Frequency (RFQ)

• The relative frequency is calculated by comparing the frequency calculations for each tree species with the total frequency of all species.

• The frequency of each tree species is calculated by dividing the number of transect points at which that species occurred (in at least one quadrant) by the total number of transect points.

• The RFQ can be thought of as a measure of the ability of a tree species to disperse itself across the local environment.

Importance Value (IV)

• The importance value provides an overall estimate of the influence or importance of the tree species in the local community of organisms.

• Calculated by adding together the RDO, RDE, and RFQ for each species of tree.

• The importance value will be of greatest interest in future analyses of the tree survey data because it provides a way to plot over time the influence of a particular tree species on the local community.

Calculations in Detail: Step 1

• All point-to-tree distances in the samples are added together and divided by the total number of trees to obtain a mean point-to-tree distance.

mean distance  =  total distance / number of trees

Step 2

• The mean area covered on the ground per tree is obtained by squaring the mean distance.

mean area per tree  =  (mean distance)2

Step 3

• The total density of trees per acre is obtained by dividing the mean area per tree by 43,560 ft2 (the area of one acre). 

trees per acre  =  mean area per tree / 43,560 ft2

Step 4

• The basal area** of each tree is calculated from the diameter measurements by using the standard formula for the area of a circle

basal area(ft2) = pi x ½(diameter in inches)2/ 144

**Basal area is the area of a flat plane passed through the trunk of a tree at chest height.  Because the cross section of a tree is close to circular, its basal area can be computed from the standard formula for the area of a circle.

Step 5

• The relative dominance (RDO) is calculated for each species by dividing the total basal area (tba) of the species by the total basal area of all the trees and multiplying by 100.

RDO = (tba of a species/tba of all species) x 100

Step 6

• The relative density (RDE) is calculated for each species by dividing the number of individuals of a species by the total number of trees and multiplying by 100.

RDE = (total # of a species/total # of trees) x 100

• The density is calculated for each species by dividing the relative density by 100 and multiplying by the average number of trees per acre.

density  =  (RDE / 100) x trees per acre

Step 7

• The frequency of each species is calculated by dividing the number of transect points at which the species occurred by the total number of transect points sampled and multiplying by 100.

frequency  =  # points species occurred / total # of points 

• The relative frequency (RFQ) of each species is calculated by dividing the frequency of each species by the total frequency value for all species and multiplying by 100.

RFQ = frequency of species / total frequency of all species

Step 8

• Finally, the overall importance value (IV) is calculated by adding together the relative dominance, relative density, and relative frequency for each species.

IV  =  RDO + RDE + RFQ

• The importance value is expressed as a range from 0% to 300%.

Methodological Assumptions

• The underlying assumption of the point-quadrant is that individuals of all species are dispersed randomly.

• Although this assumption is most likely not true for all areas of the WSC, it does not seem to produce significant error, except in those instances where tree dispersion is obviously non-random (i.e. in areas like hedgerows where certain tree species were intentionally planted).

• Relative dominance and relative density are valid even if dispersion is not random.  The calculation of relative frequency is made invalid when non-random dispersion is identified.