By Norma Kline, Oregon State University Extension Forester for Coos and Curry Counties

As with any specialized field, forestry has its own jargon. Some terms are precise and measurable while others describe broader concepts. This article covers a group of similar sounding terms (site quality, site productivity, site index, site class) which are all used to reference how well trees grow in a given area, or site. You may have observed that trees and understory vegetation grow better on some areas on your woodland than in other areas. These differences may seem more pronounced on steep areas with shallow soils where you might see less overall vegetation and slower tree growth.  In contrast, you might see fairly lush understory vegetation and rapid tree growth in flat areas near streams. You might even notice that the two areas have different species. The moist and deep soils near streams typically support mixed conifer and hardwood species including hemlock, western redcedar and alder and understory species including ferns, salal and red elderberry. In contrast, steep slopes with shallow soils might be dominated by Douglas-fir with sparse understory species including more drought tolerant species such as manzanita and silk tassel.

This variation in growth and species mixture is influenced by soil, topography and climate that combine to determine site quality. In other words, site quality refers to the amount of trees and associated vegetation that can grow in an area. Site quality and site productivity are typically used interchangeably. Although some argue that site quality refers to the natural condition of the area while site productivity can be influenced by fertilization or possibly irrigation.

Site quality is directly correlated to how tall trees can grow. Tree height is the foundation of a relative measure of site quality called site index. Site index is defined as the average height of dominant trees for a chosen base age. Site index graphs with nested growth curves represent tree heights at different ages. The benefit of using tree height to measure site index is that height is easily measured and is largely independent of stand conditions such as tree density. Site index is determined by taking the total height of dominant or codominant trees; the best growing and taller trees in a stand. It’s a good idea to measure a number of the dominant trees on your site to obtain a good indication of site index. These graphs might look complicated, but are actually quite straight forward to use.

Site index graphs are specific to a region and species. A selection of the most commonly used site index graphs used in Oregon can be found in the Basic Forest Inventory Techniques for Family Forest Owners publication referenced at the end of the article. Kings’ 1966 site index is a commonly used site index for Douglas-fir in southern Oregon’s coastal counties.

Figure 1 illustrates a simplified version of a site index graph for a fictitious fir species. Notice that the horizontal axis is labeled breast height age. Breast height age is determined with an increment core taken at 4.5 feet. Otherwise, if you know the age of a tree from planting records, subtract the number of years it took to grow to 4.5 feet to arrive at the breast height age. In this example, a site tree is known to be 55 years old from planting records, and it is estimated to have been 5 years old when it was 4.5 feet tall. Thus the breast height age is 50. The total height was measured and found to be 80 feet tall. To determine the site index, first locate the tree age on the horizontal axis and then find the height of the site tree on the vertical axis. Draw vertical and horizontal lines from those points. These two lines will intersect at or near a site index curve. The site index curve is labeled with the site index in feet. For our example, the site index for this tree is 80 ft. Notice that a 50 year tree on the highest curve (the best site) is 125 feet tall and a 50 year tree on the lowest curve is 70 feet tall. In other words, for the same amount of time, trees on our site grew slower than trees on better sites.  

If measuring site trees seems like a lot of work, don’t worry, there are indirect approaches to finding the site index for your property. The NRCS Web Soil Survey (WSS) is a great reference to find the site index for the different soil types on your property. Here is a quick tutorial in finding site index using the WSS.

• First, after you have opened the Web Soil Survey website (link in References), select the green Start WSS button.
• Zoom into your property and draw an Area of Interest (AOI) using the AOI tool located in the tool menu found at the top of the Area of Interest Interactive Map; double-click to end drawing and create AOI. Note: You have a choice of drawing a square or a polygon AOI.
• Select the Soil Data Explorer tab.
• In the menu located on the left, select Vegetative Productivity and scroll down to Forest Productivity (Tree Site Index).
• Click the button to expand this Forest Productivity menu and select the View Rating button.
• View the Site Index Rating in the table for each soil type (some soil types will not have a rating).

Next, determine how your site index fits into the site class rating designation. Site class ratings are a general scale of productivity. The Oregon Forest Management Plan Guidelines, referenced below, provides a table (Table 1, page 10) where site index is grouped by site class. The site indexes are referenced by species and base age. The base age can be found in the site index chart or noted on the soil survey record. For example, Kings’ 1966 Douglas-fir index is a 50 year base age site index. Once you have found the correct site index row, find the column that matches your site index height and read off the Site Class Rating.

Understanding the site class designations on your property will help you align management actions to the most suitable areas on your ownership. For example, you might have areas on your property rated as site class IV or V and other areas rated as site class II and III. Site classes II and III will be more productive for growing timber than classes IV or V. If you have diverse management objectives such as wildlife or recreation, you might focus activities related to those objectives in the lower site areas and focus timber management activities on the higher site areas. Site class ratings will also effect the timing of pre-commercial or commercial thinning operations. For example, the fast growing trees on higher sites would reach tree competition levels earlier than lower rated sites. And some sites may not even naturally grow trees. For example, coastal prairies typically have soil types and geology that limit tree growth (Figure 2).

The soil map boundaries are approximate, so you might find that your observations on the ground don’t correspond precisely to the Web Soil Survey.  Finally, site index information obtained from the Web Soil Survey may not be exact for a given location on your property.

References

Basic Forest Inventory Techniques for Family Forest Owners (Site index curves start on page 63). Download for free here:  https://pubs.extension.wsu.edu/basic-forest-inventory-techniques-for-family-forest-owners

Web Soil Survey: https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm

Oregon Forest Management Plan Guidelines: https://blogs.oregonstate.edu/forestplanning/templates/