Tree Transects
(Adapted from: Alan Berkowitz Institute of Ecosystem Studies, Millbrook, New York)
A current major ecological concern is the fragmentation of forest habitat that is occurring in many parts of the world. Forest fragmentation decreases the amount of interior forest habitat and increases the amount of edge habitat. This is a particular threat to those species that require conditions that are only found in the interior of forests.
The objective of this exercise is to compare how the tree species vary from the edge to the interior of a forest by sampling tree saplings and adults along a transect. A transect is a straight line, along which ecological measurements are made.
Materials
- 1 measuring tapes at least 5m long
- 1 12m long transect rope with marks at 1m, 6m and 11m
- 1 meter stick
- flexible measuring tape for measuring tree circumference (optional)
- tree and shrub identification books and keys
- stakes for anchoring transect
- data sheets and clip board
Procedure
Three Steps:
Step 1: Locating and Running Transects
- Start by finding an area of forest that has a clear edge (along a road, for example) and choose a random point along the edge to start your transect.
- Start the transect approximately 2m into the forest. Anchor the transect at the 2m point and run it into the forest, as perpendicular to the forest edge as possible.
- You will be sampling a the 1m, 6m and 11m points along the transect. (See diagram 1.)
Diagram 1: Locating the Transect
Step 2: Collecting Data at Each Sampling Point
- Think of each point as the center of a "plus" sign. The transect line is one part and an imaginary line through the sampling point at right angles to the tape is the other. This identifies four "quadrants" for sampling. You can lay the meter stick perpendicular across the transect line to help you visualize the quadrants.
- Find and identify the adult (dbh more than 10cm) and the sapling tree (dbh less than 10cm) closest to the sampling point in each of the four quadrants. NOTE: dbh means diameter at breast height, or 4.5' above the ground. (See diagram 2.)
- Record the distance from the sampling point to each tree in meters.
- Repeat this procedure at each sampling point along the transect.
Diagram 2: Measurements to be taken at each sample point (1m, 6m, and 11m) along the transect.
Step 3: Interpreting Your Data
- Calculate the average distance from the sampling point to the four adult and the four sapling trees. Then, square this distance to get the average amount of space (in square meters) per tree in that part of the forest. The inverse of this number is the average number of trees per square meter in this part of the forest!
- Record the diversity or species richness of adult and sapling trees in each of the three parts of the forest. This is simply the number of different species you recorded at each spot.
- Compare the species composition of each type of tree in each part of the forest, and for adult versus sapling trees. This could be the proportion or percentage of each species, etc.
Questions to think about and discuss: Do different tree species occur along the edge versus the interior of a forest? Does the total number of tree species differ in different parts of a forest stand? Does the mix of young versus old trees differ from edge to interior?
Write up your experiment in a brief report. The report can be written in a standard form (Introduction, Methods, Results, Discussion). For some more detail on writing scientific reports the following site is useful: http://www.unc.edu/depts/wcweb/handouts/lab_report_complete.html
