Dr. Ray L. Winstead
Professor of Biology, Indiana University of Pennsylvania

Quantitative Vegetation Analysis


         This lab will demonstrate one method to obtain and analyze data to determine several parameters (or characteristics) of tree populations. For example, measures of density, average distance between trees, and the average area occupied by a tree will be calculated for each tree species. These types of parameters are important in comparing community growth and differences in community structure between two areas.

Point- Quarter Plotless Method

Procedure:

1. Proceed into the forest along a designated line (e.g., by compass) and choose a first point so that it is well within the forest. You should choose successive points along the line far enough apart to prevent sampling the same trees. Pace out or measure a certain distance between points, e.g., 20 yards.

2. Mark each point on the ground and divide the working area into four quarters or quadrants by visualizing a second line through the point and perpendicular to the first line, i.e.

Transect Line ****+****+****+****+****+****+****+****

3. At each point select the closest tree in each of the four quadrants. Record the species of each tree. Then measure and record the distance of each tree from the point (in meters or in feet). (If you use feet, you may round off your measurements to the nearest foot.) Obtain the diameter of the tree (round off to the nearest cm or inch) about 4 1/2 feet up from the ground. Sample only those trees with a diameter of at least 10 cm or 4 inches. Note that four trees are measured at each point. Record the data on the attached data sheet. Note that the information from the four trees at one point is entered horizontally across the page.

Analysis:

         For each species calculate the following and record the results on the Summary Sheet.

1. FREQUENCY = total number of points of occurrence for specified species / total points sampled.

2. AVERAGE DISTANCE (d) from point to tree = (sum of all distances for that one species)/ (total number of distances for that one species)

3. AVERAGE AREA occupied by tree = d2

4. DENSITY or number of trees:
        per hectare = 10,000 / d2         (d in meters)
        per acre = 43,560 / d2         (d in feet)

    (One hectare is about 2 1/2 acres.)

5. RELATIVE DENSITY = (Total number of trees of a species / total number of trees of all species) x 100

6. The BASAL AREA of each tree is the circular area of the tree trunk where you measured its diameter. Determine the basal area of each tree using the formula (pi)r2. Recall that (pi) = 3.14 and r is the radius, which is one-half of the measured diameter. Add up the basal area of all the trees of a species to determine the TOTAL BASAL AREA for the species.



 
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