Feature Article



Forest Inventory Answers the Question of What and How Much is Out There

By: Paul Doruska – Forest Measurements faculty member in the College of Natural Resources at the University of Wisconsin-Stevens Point

There are forests all across the United States that are used for a whole host of purposes, so there should be no doubt that the country’s forests are an amazing resource indeed. As with any resource, though, it is important to know how much of it is present and how might it be changing through time. Is knowing that possible for the country’s forests? 

Absolutely – through the USDA Forest Service’s Forest Inventory and Analysis (FIA) National Program (USDA Forest Service 2021). The FIA program covers all forested areas of the United States, not just Forest Service managed lands.

The FIA program started in the 1930’s, utilizes established plot locations across the country, and is implemented through regional offices covering the Pacific Northwest region, the Rocky Mountain region, the Southern region, and the Northern region, respectively. The Lake States area is part of the Northern region.  Three different types (or phases) of collected data form the basis of the FIA program (USDA Forest Service 2005). Phase 1 data are collected through aerial photography or remote sensing and are used to distinguish forested land from non-forested land. Phase 2 data include what most folks think about when thinking about forest inventories – including but not limited to individual tree measures, site quality, general land use, and estimates of growth and mortality. Interestingly, there is one phase 2 plot for every 6,000 acres (or about 9.4 square miles) of land. Phase 3 data relate to the functioning of forest ecosystems and include measures addressing soil conditions, down woody debris, wildlife habitat suitability and carbon cycling to list a few. There is one phase 3 plot for every 16 phase 2 plots, so there is one phase 3 plot for every 96,000 acres (or 150 square miles) of land.

Through the hard work of the Forest Service personnel involved along with that of contractors who also perform some of the inventories – the country has a whole host of forest inventory information at the ready and anyone can look up information via an FIA website (USDA Forest Service 2022). For example, one piece of information garnered through the FIA program is the amount of sawtimber volume present in an area or region, with those sawtimber volumes reported in board feet. Using board feet as the measure of sawtimber volume in this country is an interesting topic in and of itself – totally unrelated to the FIA program… 

A board foot of lumber, by definition, is 144 cubic inches of sawn wood – the equivalent of a piece of sawn wood that is 12 inches x 12 inches x 1 inch (Avery and Burkhart 1994), though please know one is not restricted to any of those dimensions when calculating board feet. So, a board foot of lumber is a board foot of lumber – right? Well, when in sawn board form the answer is generally yes, though shrinking during the drying process does come in to play a bit here. But is a board foot of lumber a board foot of lumber when it is still in a log or in a standing tree? The answer there is heck no! All kinds of factors come in to play the main one of which is touched upon here.

Did you know there are three different log rules (or ways of estimating the board feet of lumber present in a log or tree) that are primarily used across the United States (Avery and Burkhart 1994) though a host of others exist as well (Freese 1973)? Here in the Lake States region the Scribner log rule is the one primarily used whereas the Doyle log rule as well as the International log rule are used in other parts of the country. Now guess what – each log rule will estimate differing amounts of board feet of lumber present in any given log or tree and that is where the issues arise.

J.M. Scribner developed what has come to be known as the Scribner log rule in the mid 1840’s by determining how many one inch thick boards of varying widths could be cut from cylindrical logs (but are logs truly cylinders? What about taper?) of varying small end diameters while assuming a ¼ inch saw kerf (Avery and Burkhart 1994) or how much of the log becomes sawdust as the saw moves through it.   Edward Doyle developed what has come to be known as the Doyle log rule back in 1825 using what has turned out to be an algebraically incorrect formula applied to cylindrical logs while assuming a 4-inch slab deduction (the part of the log that is lost while squaring it up into cant form) and a 5/16 inch kerf (Avery and Burkhart 1994).  Judson Clark developed what has come to be known as the International log rule in 1906 and did account for log taper (to address that cylindrical assumption issue!) by using a fixed taper rate of ½ inch of log diameter for every 4 feet of log length. Two versions of this log rule exist - International ¼ assumes ¼ inch of kerf whereas International 1/8 assumes 1/8 inch of kerf (Avery and Burkhart 1994). The USDA Forest Services happens to use International ¼ rule within the FIA program. 

International is considered the most accurate of these three log rules and Doyle the least accurate. A phrase often associated with the Doyle log rule is “the Doyle log rule underscales small diameter logs (underestimates the board foot volume) and overscales large diameter logs (overestimates the board foot volume)”. That statement is true relative to the log size common when the rule was developed! Logs in the early to mid-1800’s tended to be 30 inches in diameter or so, much larger than many logs of today so in most modern applications of the Doyle log rule almost all logs are actually underscaled!

So how should we navigate these varying log rules? The key is to know how many board feet a given mill or sawyer actually cuts from various sized logs. With that information one can pretty easily develop mathematical conversion ratios that relate the actual sawn board foot volume to the scaled volumes, which are the estimated board foot volumes in logs or trees, from any one of those three log rules (such as International 1/4 from the FIA program) and that can address the log rule issue.  


References Cited:

Avery, T.E. and H.E. Burkhart. 1994. Forest Measurements 4th ed. McGraw Hill, New York. 408p

Freese F. 1973. A collection of log rules. Gen. Tech. Rep. FPL-01. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 65 p.

USDA Forest Service 2005. Forest Inventory & Analysis Fact Sheet Series: Phase 2 & Phase 3: Ground Measurements. Web https://www.fia.fs.fed.us/library/fact-sheets/data-collections/Phase2_3.pdf. (last accessed 3/24/22).

USDA Forest Service 2021. Forest Inventory and Analysis National Program. Web: https://www.fia.fs.fed.us/about/about_us/index.php. (last accessed 3/24/22)

USDA Forest Service 2022. Forest Inventory and Analysis Data and Tools. Web:

https://www.fia.fs.fed.us/tools-data/index.php. (last accessed 3/24/22)


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