A Great Practical Edge-effect Solution for Typical Cruising Situations
There was a recent article (August, 2004, at website (http://126.96.36.199/pubs/7909 ) in the journal Forest Science that was really useful – although perhaps not very readable, which is a common situation in Forest Science. I would like to bring it to your attention, if you have not already heard about it from the OSU short course or my book (which is the only discussion in a forest measurements book so far). The article was about the “Walkthrough Method”. This is one of those happy times when a practical problem produces a practical solution from the forest research community. The authors deserve real credit for one of the best ideas in a long time.
The method works with curved borders, small patches inside the stand, typed-out roads, and all those situations that make most other edge-effect methods impractical or make them incorrect. Try it. You’ll like it. It works with Variable or Fixed Plots. It is easy to teach, and easy to learn.
Every plot follows the same procedure. For any “in” tree, measure the distance (“d” in the illustration below) from the sample point to the tree, then duplicate that distance on the other side. If you fall outside the sample area at the end of this duplicated distance, record that tree twice. In most cases, you can simply estimate the distance, or the situation is obvious when you are not even near the sample area border. This double counting makes it simple to do the computations, and computer programs do not require any modifications. For count trees, you would just count the tree twice. For measured trees, record it as two identical measured trees.
You will only have to check a few trees. Most of the situations will be obvious. One advantage is that you do not actually have to go beyond the sample area border (a good thing with rivers, cliffs, and swamps), you just need to know that you would fall outside the sample area at that distance. The idea is shown in the illustration at the right.
What is really happening here? The key to the system is that any part of the tree circle around each tree that falls outside the sample area is “duplicated” by rotating it 180 degrees. If the sample point falls into the duplicated part, you count the tree twice (that is what the process of “walking through” the tree is detecting). The number of times you count it twice makes up for the number of times a sample point would not fall into the part of the tree circles lying outside the area being sampled. The geometry may be a bit odd, but the field procedures are really simple.
It is hard to imagine a simpler process, or a more flexible one. If the sampled area is really narrow, or when you are in corners of the tract, the process is not exactly correct – but much of the bias that is caused by edge effect is accommodated even in these circumstances. Reports from the field have been very positive, and the procedure is fast becoming a standard for companies and government organizations.
Originally published December 2005
Regular Article Index