Is it true that ...

The prism is just used to select trees to measure?

NO, definitely not. There are two functions of the prism (or any angle gauge used in variable plot sampling, such as a relascope). Choosing trees to measure is the least important one.

The important function of the count of "in" trees with a prism is to calculate basal area. That is why "count" plots are so effective. The basal area is usually quite variable in stands, and the tree count alone is giving you that information. You do not have to measure a single tree to get the basal area of a stand.

This basal area measurement is extremely effective, and the genius of variable plot sampling is in how simple it is. Because we spend so much time measuring trees we begin to think that the data all comes from measuring trees, but this is dead wrong.

The second function of a prism is to select trees for measurement. These measurements establish the relationship of some item to the basal area of the tree. One example is the VBAR or "Volume to Basal Area Ratio," which is fairly consistent for gross volume, less so for net volume. Trees/square foot, $/square foot, etc, are all handled the same way. There are other ways of choosing these trees - using the prism is just a convenient way of doing it, not an essential one.

The final answer is the product of basal area times the relationship from the measured trees (VBAR, for instance). In practice, the basal area has a big effect on volume, numbers of trees, values, grades and all the rest - and that only comes from "tree counts."

It is easy to think that counting trees with a prism cannot be very important, because it is so simple. In fact, it is directly contributing to volume, value, and all of the other factors we associate with the trees themselves.

Choosing the trees to measure is the least important function of the prism.

Originally published April 1997

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