Site Index In Forestry: 11 Things (2023) You Ought To Know
Story by Erika Benson • Mar 26
If you work in forestry, it’s essential to understand terms, such as site index and site quality, that can help you make smart decisions when investing in land.
Site index and site quality are two of the factors that will ultimately assist you in determining:
The greatest potential timber volume
The time to harvest
The best way to yield an acceptable return on your investment
Although this industry is jargon-heavy and may feel overwhelming initially, terms such as site index are not that hard to understand.
Are you ready?
Let’s get started!
1. What is a site?
The term site refers to a geographic location that is considered homogenous in terms of its physical and biological environment.
A site in forestry is usually defined as a particular area that is characterized by a specific silviculture or dominant tree-type.
Sites can be classified into site types — often based on their similarities regarding climate, topography, soils, and vegetation.
2. What is a site index?
A site index is a forestry term used to describe the potential for forest trees to grow at a particular location or “site.”
It is therefore a measure of site productivity.
Similar to how farmers discuss soil productivity in terms of bushels per acre, forestlands can be compared using site indexes.
Based on a certain site index, foresters may make decisions about what species to grow and how intensely to manage those trees.
3. What “base age” is used in a site index?
A site index is the average total height of a particular species of trees in a forest stand at a given age.
This given index age is described as the “base age.”
In natural stands, it is usually age 50.
In planted stands, it is usually age 25.
Here’s an example.
A planted loblolly pine stand with a site index of 65 (base age 25 because it’s planted) would indicate that the average total height growth of the trees in that stand at age 25 would be 65 feet tall.
Note: a stand is a term in forestry that refers to a group of trees.
4. What is a site index curve?
Site index curves are developed by industry professionals based on the heights of different-aged trees of the same species from study areas throughout an entire region.
These curves allow landowners, buyers and foresters to estimate the future productivity of a stand of trees and compare it to other stands of the same species in the same region.
Essentially, the site index curve is a graph where the x-axis is the tree age and the y-axis is the tree height.
Each site index curve graph will also have several pre-plotted curves on it that represent various site indices.
So, if a loblolly pine tree has an average height of 70 feet, and its age is determined to be 60 years old, then you would define age 60 along the x-axis and 70 feet along the y-axis of a site index curve.
Once you find the point that represents an age of 60 and a height of 70 feet, you can then find the site index curve that intersects this point and follow it to estimate the future height of your loblolly pine at a given age (as shown in the video below).
This curve will also give you your tree’s site index.
5. How do you determine a stand’s site index?
To determine a forest stand’s site index, you must do the following:
Obtain the height and age of all of the dominant trees (i.e. those that are taller than the canopy) and/or codominant trees (i.e. those whose crown is at canopy height) of a particular species in your stand.
Plot each tree’s data on the species’ site index curve for your region.
Follow the site curve forward or backward to get the site index for each tree at the same base age (say 50 years).
The average site index for all of the trees at the same base age is the site index for the stand.
One key to remember is that the greater the site index, the greater the productivity of the site.
6. What is a site index used for?
The site index is a measure of site productivity.
It helps you understand the future potential of the trees on your land and also compare your land to other parcels in the same region.
Using the site index, you can:
Make a decision about whether to buy a parcel of timber land
Decide what trees to plant on your property
Determine how intensely you need to manage your trees
Estimate the future income of your timber
Figure out if you need to improve your forest management practices
7. What is site quality?
While many people use the terms site quality and site productivity interchangeably, site quality is a better baseline indicator.
Site quality refers to the inherent ability of a forest to produce biomass (in other words, to grow trees).
It is the “composite expression of a variety of physical and chemical attributes of a forested area.”
These attributes include:
Soil: soil depth, texture, and fertility
Topography: slope, aspect, and elevation
Climate: precipitation, temperature, and length of the growing season
Together, all of these elements combine to influence how well trees grow at any given site and, thus, how high your land’s site index is for a particular species.
If your site index is not where you want it to be, you should delve into each of these attributes to see how you may be able to improve growing conditions on your land.
8. What soil factors should I understand?
Here are some soil factors that have a major impact on the site index and site quality of a parcel of land.
Understanding these terms can enhance your knowledge of forestry and help you further analyze how to improve the productivity of your land.
Topsoil depth:
This is the depth of the uppermost soil layer, which is a critical factor affecting tree growth.
This is because topsoil is the highest in organic matter and nutrients and is usually well-aerated and drained.
Having good topsoil allows maximum root growth and root penetration.
Soil texture:
This term describes the proportion of sand, silt, and clay in the topsoil and subsoil layers.
The particular mixture of the soil matters because some soils are better suited than others for growing timber.
For example, sandier soils are typically well-drained and, thus, often lack nutrients because of a constant leaching process.
Subsoil consistence class:
Having a consistent subsoil layer is another essential factor in forest soil quality and productivity.
The combination of soil-sized particles and physical/chemical properties of each individual particle type in the soil determines its consistence class.
Limiting layers:
A layer that restricts the downward penetration of a tree’s root system will reduce tree growth in direct relation to the depth of the layer.
In some situations, a limiting layer may increase site productivity (i.e. on sandy soils where the layer may delay leaching of nutrients and increase moisture availability).
Fertility:
One of the most important but least understood elements of a forest ecosystem.
Be sure to do research on your specific site and trees to understand soil fertilitylevels and how to best support them.
Internal drainage:
Only a few tree species are able to grow in soils that are constantly wet.
Having internal drainage (by tilling, ditching, or adding bedding) can be a great way to improve your soil quality and productivity.
9. How do I use the site index to select tree species?
Before planting trees on your property, you can use site indices to help you decide which species will work best for you.
Here are the proper steps for planting and managing the proper tree species.
Determine your objective
Having a clear idea of what you’re looking to achieve is important.
For example, if you want to produce timber, then you’ll need to select a tree species that can economically produce timber products.
However, while people may think that forestry is all about producing timber, there are many other uses as well.
Wildlife, recreation, aesthetics, and other additional uses you may consider.
Select a reliable species
The species you select must be able to survive in your region.
If you are looking to grow trees for timber, do your research and select a species with a track record in terms of both growth and acceptance in the local market.
Choose a species with a high site index
If you are buying an investment property, keep the dollar amount in mind as you’re making decisions.
Your eye should ultimately be on the ROI!
What will your return on your investment be?
In many cases, you should consider selecting a species with the highest site index because that’s a solid indicator of growth and productivity.
You can use the USDA’s Web Soil Survey to compare the site index of various trees based on the known soil conditions of your lot (see the video below for a tutorial).
10. How productive are our forests?
A recent assessment of the status of timber resources has shown that forests in the Midwest are producing timber faster than they are losing it to mortality and land conversion or harvest of wood and paper products.
In fact, the volume of timber in the region has doubled in the last 50 years.
That said, people are consuming more wood than they are currently growing, harvesting, and processing in the region.
Moving forward, it’s up to us to learn how to sustainably manage the timber resources to meet the needs for wood and fiber in our region in a socially and environmentally responsible manner.
While the physical environment (climate, moisture, soil, and physiography) ultimately determines productivity, our forests could increase their productivity through investment in intensive plantation culture and existing silviculture tools.
This is why it is so important to have tools, such as the site index, to help measure forest sustainability and productivity.
11. How productive can our forests be?
Right now, not all forests are growing at their full capacity because of the lack of investment in more intensive forest management as well as a general failure on the part of forest landowners to use good forestry practices or management plans.
In order to increase forest productivity, it is important to focus on the biological growth potential of individual trees.
For example, improved seedlings can be used to supplement the natural regeneration of forests or to establish intensively managed plantations.
Scientists are constantly doing research on individual tree types and how to make them more productive.
Again, measurement tools such as the site index, can go a long way in helping foresters better manage their forests.
Final thoughts
Ultimately, the site index is a solid tool for a forestry management group or forestland owner who wants to buy or sell forest land and quantify the yields and economic returns of various forest practices and tree species.
The index is an essential baseline used to calculate and project the future growth of forest products and to estimate harvest volumes and income in the future.
Before purchasing land intended for forestry management, consult a professional to evaluate a given stand or property’s site index estimate.