By Norma Kline, OSU Extension Forester for Coos and Curry Counties

This is the fourth article in the Writing a Forest Management Plan series. The objective is to simplify the process of creating a forest management plan, making it accessible and straightforward regardless of your prior skills and knowledge. If you already have a management plan, use these steps to review and update it.

The first articles cover generating basic property information and maps from web-based resources, mapping forest cover (stands) based on stand age and species, and taking simple plot measurements. Links to the first three topics are at the end of this article.

In this article, we will explain how to use plot measurements to calculate average diameter, height, and trees per acre. This information can be included in the “Stand characteristics” table located in the “Current stand & forest health conditions” section of the Forest Management Plan Template. While the planning process does not require landowners to include a forest inventory or timber cruise unless they are seeking funding for NRCS Cost Share Programs, inventory information can help describe the stand more clearly and assist landowners in considering possible management actions to achieve their goals and objectives.

The “Stand characteristics” table includes areas to enter information on stocking, diameter, and height class. Stocking describes how well the site is utilized by the trees. Under-stocked stands will typically have openings filled with brush species, Figure 1. Over-stocked stands will be extremely dense with little to no vegetation growing on the forest floor, Figure 2. In over-stocked stands, trees may die from competition.

Trees per acre (TPA) is a useful measure of stocking when considered alongside the stand’s age and growth stage. For example, a healthy mature stand will have substantially fewer trees than a newly planted reforestation unit.

To calculate the average TPA for a stand, multiply the number of trees in each plot by the expansion factor (the plot size denominator). For example, using a 1/20th acre fixed plot size gives an expansion factor of 20. Then, sum the TPA for all plots in the stand and divide by the total number of plots. For stocking surveys of newly planted stands, it is recommended to take more frequent, smaller plots. A typical reforestation stocking survey consists of 1/50th acre (16.7 ft. radius) plots, with at least one plot for every two acres.

Here is an example of the trees per acre calculation, using the plot data located at the end of this article:

Step 1. Multiply the number of trees in each plot by the expansion factor.

             Plot 1. 7 trees. 7 x 20= 140 trees per acre (TPA).

             Plot 2. 9 trees. 9 x 20 =180 TPA

             Plot 3. 8 trees. 8 x 20 =160 TPA

Step 2. Add the TPA for all the plots in the stand and divide by the total number of plots.

            The average TPA for this stand is 480 divided by 3 = 160 TPA.

The next steps are to calculate the average DBH and average height for your stand. First calculate the average DBH for each plot, then divide by the total number of plots. In our three plot example, this would be (11.3 in. + 11 in. +11.25 in. )/3 =11.18 in.  Use the same process to calculate the average height. The average height for our example is 90.06 ft.

The process of taking plots and summarizing information provides woodland owners insights into stand development and forest health conditions. This information can be used to assess whether a stand is on a desired trajectory and to determine if management actions are needed. Additionally, including stand averages in addition to written descriptions of each stand in your management plan will help document the stand for future reference. It’s recommended to update your forest management plan every 10 years, in which time you may consider taking additional plots to continue assessing and monitoring your woodland. Including photos of each stand in this section will be helpful for future reference as well.

In the next article, we will discuss additional information required to complete the “Stand characteristics” table, including a description of the stand’s growth stage, spacing, vertical structure (layers) and forest health observations.

Example Plot Data

Plot #	Tree #	Species	DBH (Inches)	Height (Feet)	Live Crown Ratio
1	1	DF	11	90	30
	2	DF	14	95	60
	3	DF	13	95	50
	4	DF	10	85	30
	5	DF	9	80	25
	6	DF	10	90	35
	7	DF	12	90	45
2	1	DF	9	85	25
	2	DF	8	85	25
	3	DF	14	95	50
	4	DF	16	100	60
	5	DF	15	100	55
	6	DF	8	90	25
	7	DF	9	90	30
	8	DF	10	95	30
	9	DF	10	95	30
3	1	DF	16	100	50
	2	DF	13	90	45
	3	DF	13	90	45
	4	DF	10	85	30
	5	DF	9	85	25
	6	DF	9	85	25
	7	DF	11	90	35
	8	DF	9	80	30

References

Oregon Forest Management Plan Template and Guidance. https://blogs.oregonstate.edu/forestplanning/templates/

Writing a Forest Management Plan: Start with the Easy Sections. https://blogs.oregonstate.edu/southcoastfieldforest/2024/02/15/writing-a-forest-management-plan-start-with-the-easy-sections/

Writing a Forest Management Plan: Mapping Stands.

Writing a Forest Management Plan: Mapping Stands

Writing a Forest Management Plan: Measuring Plots to Describe Your Woodland.

Writing a Forest Management Plan: Measuring Plots to Describe Your Woodland