Questions from the Field ...

"How should age be averaged?"

Good question. We don’t know. The question is certainly more difficult in stands of obviously different ages and sizes.

Is there a way of describing age that is built into the system you are using, such as the growth model or silvicultural rules? If so, you want to be aware of that. Inventory standards should always be changed carefully and slowly, if at all.

There may be an official company rumor about what “stand age” means, but is there any way to check it out and see if it is really true, or makes sense? Is it possible to check the previous work, and find out what age definition was actually used in the regression, model, or other research work?

The average tree age is not automatically 5 years older just because you return 5 years later. Trees come and go. For this reason, some people make a distinction between “stand age” and “time since stand establishment.” This usually considers the age of any planted stock as well as the planting date.

In inventory, our problem is often to estimate this “establishment date.” If it is not a planted stand, this means sampling for age. What an “average age” means, in a multiple age stand, is certainly a good question. For two distinct layers, as in a shelterwood, you can always keep both ages.

If you have your choice to describe stand age, you might consider using the tree age weighted by the tree basal area. This gives a heavier weight to larger trees. It’s pretty hard to believe that the age of a seedling should have the same effect on stand age as the largest tree in the stand.

You could also weight the tree age by volume, crown closure, or any number of other things. The idea would simply be to give more weight to trees which have a greater effect on the stand structure.

The mechanics are pretty simple. The table at below presents the weighted age of a set of trees chosen with a fixed plot.

Tree DBH (inches) Age BA Age*BA
1 12 30 0.79 24
2 35 45 6.68 301
3 18 35 1.77 62
4 44 50 10.56 528
5 15 28 1.23 34
Sum 21.0 948.4
Wt. Ave. 948.4 / 21 = 45.1

One advantage to using the tree basal area is that the DBH is easy to measure. A second advantage is that you automatically get the weighted average by taking a simple average of all the trees (or a random sample of trees) that are “in” with a prism.

What if you have both Variable and Fixed plots at the sample point? In that case, the weight is not the basal area of the tree itself, but the basal area per acre that the tree represents.

  • A 16 inch tree with a BAF 20, for instance, represents 20 square feet of basal area per acre at that sample point.
  • A 6 inch tree on a 1/50 acre plot surrounding the same sample point represents 50 trees per acre, each with [(3/12)2*3.14159] square feet of basal area for a total of 9.82 square feet.

These tree ages would therefore be weighted by 20 and 9.82 when the two plot sizes are combined at the same sample point to estimate stand age.

Originally published January 2001

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