Determination should include using the following steps:

Determine the expansion factor (the number of trees per acre a given plot tree represents) by taking the denominator of the plot size (e.g., 20 for a 1/20th acre plot) and dividing by the number of plots expansion factor = plot size denominator/plots

Add up the total number of trees for all the plots in the stand
Multiply the total number of plot trees by the expansion factor to determine total TPA for the stand

TPA = trees x expansion factor

Example: Suppose you acquired data on two 1/20th acre plots

(i.e., you sampled a tenth of an acre total, or 1/20 + 1/20)

Suppose that between the two plots there were 11 total trees:

- The expansion factor would be 20 / 2 = 10.0 trees per acre. Thus, each plot tree represents 10 trees in the stand.
- There are 11 total trees for all plots.
- Multiply 11 (total number of trees) by the expansion factor of 10 to get 110 TPA for the whole stand. Credit: Lesson 10: Basic Inventory Calculations, Virtual Cruiser Vest, The Rural Technology Initiative, Washington State University.

Teacher Resource: Lesson 10: Basic Inventory Calculations, Virtual Cruiser Vest, The Rural Technology Initiative, Washington State University

Process/Skill Questions:

Why is it important to determine how dense the trees are in a stand?
Why are inventory calculations important?
What is basal area and why is basal area important when determining inventory calculations?