By Norma Kline, OSU Extension Forester for Coos and Curry Counties
This is the third article in the Writing a Forest Management Plan series. The objective of this series is to break the process of writing a forest management plan into simple steps intended to make it accessible and straightforward, regardless of your previous skills and knowledge. If you already have a management plan, use these steps to help review and update it.
The first article covers how to get a quick start on your management plan by using the online Landmapper tool to generate basic property information and maps. The second article details how to map forest cover (stands) based on stand age and species. Delineating forest stands will help landowners efficiently monitor their woodlands and plan for future actions. Links to the first two articles are provided at the end of this article.
In this article, we will describe how to take simple plot measurements to estimate diameter, height, and trees per acre. The Current Stand & Forest Health Conditions section in the Forest Management Plan template includes a Stand Characteristics table with space to enter age, diameter class, height, spacing, and tree species. It’s important to note that the forest management planning process does not require landowners to include a forest inventory or timber cruise for their property, although these might be desirable for other reasons based on your management objectives.
If you are interested in getting out on your property and learning a few basic measurement techniques, taking basic inventory plots is an activity that you and your family can accomplish with relatively inexpensive forestry tools. During this process, you may gain increased familiarity with your woodland, discovering interesting new features such as old roads, wildlife, or forest health issues. Alternatively, you may decide that hiring a forestry professional to conduct a forest inventory is the right choice for you.
The most common method used to gather forest inventory information is by measuring plots across each area of interest (a timber unit or stand). For the purposes of developing a management plan, we will focus on the process of taking simple fixed plots for a general forest inventory. You may have heard the terms “inventory” and “cruising” in the context of collecting tree measurement information. Forest inventory typically refers to taking forest measurements to gain a general understanding of the tree cover on a property. The number of plots per stand and the measurements included in an inventory will depend on the landowner’s objectives and how they intend to use the information.
On the other hand, a timber cruise is typically used to determine the volume of merchantable timber in a specific area, such as for a planned timber sale, or across an entire property to assess the value of an estate. The desired accuracy of a timber cruise is generally higher than that of an inventory and would typically involve more plots.
The objective of our forest inventory described here will be to obtain general information for each mapped stand. The number of plots you measure will depend on how you intend to use the information as well as how much energy and time you have. A common inventory design for landowners interested in getting an idea of stand density and general stand volume would be to measure between 15 to 30 plots per stand: 15 plots in a fairly uniform stand and 30 plots in a stand with a fair amount of variability. If this seems like more plots than you have time to measure and your main intent is to obtain a more general description of the stand, then measure at least one plot for every 10 acres with a minimum of 3 plots.
The next step is to determine the location of your plots and the measurement information you will collect. A systematic approach to taking plots is recommended to help prevent bias in sampling. Take your map of delineated stands and draw a grid on each stand. Figure 1 is an example of a simple inventory design created in CalTopo, which is a free online mapping platform. A grid of evenly spaced inventory lines was established to help position plots systematically. The inventory lines cross topographic features to ensure that plots are located on a variety of slope positions. Trees growing near rocky ridges tend to be smaller than those growing near a stream or bench area. Thus, for an unbiased sample, inventory lines should not be oriented to follow creeks or ridges.
Figure 1 was generated as a geospatial PDF in CalTopo for use on a handheld mapping app such as Avenza, which can be used to navigate to the plot center. Alternatively, the map can be printed and used along with a compass to navigate along the inventory lines. Another approach is to use a ruler to draw an inventory grid on a printed map. In the field, distances on the ground can be measured with a tape, paced (counting your steps), or by using an electronic rangefinder. Just remember that the distances between plots on the map are horizontal. Use a compass to take the bearings of the inventory lines. Even if you are using a smartphone for plot location, it’s always a good idea to carry a printed map and compass in the field.
| Plot Size (acres) | Radius (feet) |
| 1/5 | 52’8” |
| 1/10 | 37’2” |
| 1/20 | 26’4” |
| 1/30 | 21’6” |
| 1/40 | 18’7” |
Plot size will depend on how widely the trees are spaced. Choose a plot size that allows you to count an average of 5 to 10 trees per plot. Use the same plot size throughout an entire stand. Typical plot sizes are listed in Table 1. A good place to start is a 1/20-acre plot, which has a plot radius of 26 feet and 4 inches (see Table 1). You can test this out in a typical area of your stand by setting up a few preliminary plots to determine the number of trees per plot. If your average tree count exceeds 10 trees per plot, you might consider reducing the plot size.
It’s a good idea to establish a routine for collecting and measuring plot information. Once you locate the center of each plot, tie a piece of flagging to a stick placed in the ground. Measurements will be easier if you have a helper to hold the end of the tape over the plot center. Measure to the edge of the plot, making sure to keep the tape horizontal. Record the species, diameter, and height of each tree that falls within the plot. For trees that lie on the edge, estimate the center of the tree; if the center is outside the plot radius, it should not be measured (see Figure 2). Start recording trees from the same cardinal direction for each plot and work clockwise until you return to the starting point. You can also tie a piece of flagging at the starting point.
While there are several tools that can be used to measure the diameter and height of trees in your plot, an inexpensive woodland stick (also called a Biltmore stick) is a relatively simple way to get started. A cloth tape or a logger’s tape can be used to measure 100 feet from your reference tree, as well as the plot radius and the distance between plots.
To measure tree heights, stand 100 feet from the tree and hold the stick straight at 25 inches from your eye (see Figure 3). Height measurements will be more accurate on flat ground. For tree diameters, hold the stick 4.5 feet from the ground; this is known as diameter at breast height (DBH) (see Figure 4).
The live crown ratio of each tree is also important to note. This can be estimated (see Figure 5). Another piece of data to consider is the age of the trees. Most small woodland owners estimate the age of a stand using historical records or by counting rings on recently cut stumps, if present. Tree spacing, the average distance between trees, may also be of interest to you. Take some quick distance measurements between trees and record that information as well.
Other stand characteristics of interest include whether the stand has distinct layers of trees. For example, is there a primary layer of overstory trees with an understory (often composed of more shade-tolerant species)? This will be important if your objective is to develop or maintain wildlife habitat. Additionally, take note of any down logs and snags (dead trees). Your inventory data does not have to focus solely on living trees; knowing if you have other elements valuable for wildlife may also be useful.
Record your plot information in a weather-resistant field notebook. There are many options for field notebooks available from forestry supply companies. Write in the stand number, plot number, and column headings for the data you intend to measure (see Figure 6). It’s a good idea to attach your notebook to a field vest or jacket with a cord—you don’t want to lose your data!
As you navigate from plot to plot, take note of any forest health concerns, invasive plants, or unusual features that you find. These can be recorded in your field notebook, marked on your map, or noted using a mapping app on a smartphone to drop a pin, take notes, and even attach a photo.
The Basic Forest Inventory Techniques for Family Forest Owners (link below) is a useful resource if you would like to read more about designing an inventory.
In the next article, we will cover how to use the data you have collected to make some simple calculations.
References
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.
Oregon Forest Management Plan Template and Guidance. https://blogs.oregonstate.edu/forestplanning/templates/
Land Mapper. Automated and easy, produces many of the plan maps. https://landmapper.ecotrust.org/
Basic Forest Inventory Techniques for Family Forest Owners. https://pubs.extension.wsu.edu/basic-forest-inventory-techniques-for-family-forest-owners
Avenza. https://store.avenza.com/
CalTopo. https://caltopo.com
