Title: Determine trees per acre (TPA) from fixed plots.
Course: Forestry Management, Advanced (18502-II) > Section: Measuring and Marketing Forest Products

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?