Is it true that...
There are too Many Trees in the Border Half-plots?
At a recent cruiser gathering, I heard that "border plots are not reliable because there are too many trees when border (half) plots were put near the edge." Really? How interesting.
The statement was that "there were about 6 trees in the full sweeps, but also 6 in the half plots near the edge as well." When the half plots were doubled, then it gave 12 trees, which was obviously too much."
What are the alternatives? Putting in full sweeps (which is obviously biased) or moving the plot to the interior of the stand (another biased procedure). Not too attractive. How could the border plots give bad answers? Without looking at the stand, it is unfair to say that this is impossible - but is it likely?
The presumption was that the trees are more or less evenly spaced within the stand. In the interior of the stand, the half of the plots toward the stand center would pick up 3 trees (assuming an average of 6 for a full sweep).
When you are near the edge, with the same size and spacing of trees, wouldn't you expect that you would see about 3 trees on the inside-stand side of the sample plots. Of course you would, if the tree spacing and size were the same! Yet the statement was that half plots near the edge had twice as many as the interior, even though the basal area and volume were assumed to be the same.
It boggles the imagination to think that this would happen. It makes no sense. What, then, is the alternative explanation? If these cruisers are really finding twice the tree count on the inside half of the plot it is because the trees near the edge are NOT the same.
Suppose that we are dealing with a typical situation. There is the usual variability in spacing, tree size and so on. What if there was an 18% larger diameter on each of the trees near the edge - both large and small ones. Would you notice it? Perhaps not. What would be the effect on the basal area? Almost 40%, as a matter of fact.
Suppose that these trees, which are quite variable in their spacing, were only 80% as far apart as in the interior. That might not be obvious either. The combined effect would be that the basal area near that edge would be
(1.182 * 1.202) = 2 or 200%
larger than the interior. This larger count is because there really is more basal area there, as you might expect if the area is picking up more light and has been growing better. If the stand edge is there because you are bordering on really poor ground, you might expect the opposite effect - smaller trees with slightly wider spacing.
The point is - you do not get increased tree counts without increased basal area. You may not notice the difference in stand characteristics, but the prism will pick it up.
Do we know that this is the case with the area this cruiser described? No, we do not. Is there any reason to suspect that there is a real problem here? No. Is there any problem with using half plots near the border? Only a small bias which might be positive or negative, depending upon the exact tree spacing.
An increased tree count is certainly not cause for concern (and I suspect that a "doubling" was probably a slight exaggeration). Moving plots to the interior of the stand, which might be quite different, certainly is a cause for concern.
It does not take much of a change to be noticed by the prism, even if your eye does not pick it up.
Originally published July 2004
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