Thinning Systems for Western Oregon Douglas-fir Stands - [email protected] Home

EC 1132 • Reprinted July 2003 $2.00

W.H. Emmingham and D. Green

STAND MANAGEMENT

ContentsBasic stand growth ....................... 1

Thinning options ........................... 3Timing ........................................ 3High and low thinning .................. 3Intensity and frequency ................ 4

Stocking guides ............................ 5

How do you put it all together? ...... 5

What is best for you? ................... 6

Summary .................................... 7

For more information .................... 8

William H. Emmingham, Extensionsilviculturist emeritus; and DanielGreen, former Extension forester,Clackamas County; both of OregonState University.

Thinning is removing selected trees from a stand to allow others tocontinue growing. Ordinarily, a woodland manager uses a thinningsystem that encourages the remaining trees to grow in a manner

consistent with the manager’s objectives for those trees.This publication will help you understand how to thin Douglas-fir. It

also will help you choose the proper thinning system to achieve yourobjectives. You can apply the methods discussed here to all predominantlyeven-age and well-stocked Douglas-fir stands west of the Cascade crest inOregon.

Thinning is the best way to maintain maximum diameter and board-footvolume growth in Douglas-fir stands. It can produce income at 5- to 10-year intervals instead of at 30- to 50-year intervals without thinning. It alsocan lengthen the time span in which a stand produces income.

Basic stand growthA stand is a collection of living trees. It usually begins as hundreds of

small seedlings per acre of land.As these trees grow, they eventually occupy all the growing space,

crowd out lower growing plants, and compete with each other just ascarrots compete in a garden. Unless some of the trees die or are removed,others cannot continue to grow.

Certain trees dominate by slowly pulling ahead of their neighbors. Theybecome stand dominants or codominants (see Figure 1, page 2) and laterare harvested as crop trees.

Those that lose in this race for space, water, and light gradually fallbehind, becoming intermediate and eventually overtopped or suppressedtrees. They slowly die, fall to the forest floor, and rot.

Thinning removes trees before growth slows, thereby preventing mor-tality. It keeps crop trees growing rapidly.

It is easy to see the result of this process in the relative size of treecrowns (crown class) and stems within an undisturbed stand. Dominanttrees have the largest diameters and crowns, whereas suppressed trees aresmaller in diameter and shorter and have thin, short crowns.

Thinning Systems for Western OregonDouglas-fir Stands

Archival Copy. For current information, see the OSU Extension Catalog:https://catalog.extension.oregonstate.edu

2 THE WOODLAND WORKBOOK

Figure 1.—A typical Douglas-fir stand, with dominant (D), codominant (C), intermediate (I), and overtopped (O)trees. A wolf tree (W)—one that occupies more space than it warrants—also is part of the stand. The relativeamount of crown, height, and diameter of each tree determines its crown class.

Figure 2.—The stand in Figure 1 will look like this after a high thinning removes various dominant trees, leavingcodominants, dominants, intermediates, and overtopped trees to continue growing.

Figure 3.—The stand in Figure 1 will look like this after a low thinning removes all intermediate, most over-topped, and some codominant trees, leaving dominant trees to grow. Sometimes, a low thinning also removeswolf trees because they take up so much growing space.

D

D

D D D

D

D

D

D

D

O

O

O

O

O

O

O

O

O

O O

O

C

D

C

CCCCCI

D

I

O

CC

O

O

O

CD

C CD

C

O

Wolf DD

Wolf


STAND MANAGEMENT 3

In extreme cases of stand competition,growth in both diameter and height isrestricted and trees stagnate, staying virtu-ally the same size for years. A properthinning system prevents stagnation.Fortunately, Douglas-fir does not tend tostagnate, especially on more productivesites.

Find further discussion of thinning basicsin Extension publication PNW 184, Thin-ning: An Important Timber ManagementTool (see page 8).

Thinning options

TimingPrecommercial thinning is thinning

before trees are large enough to sell. Itsobjective is to give remaining young treesroom to grow as quickly as possible tomerchantable size. Precommercial thinningis necessary when young stands are over-stocked or when some trees are poorlyformed. (Young Douglas-fir stands areconsidered overstocked if they have morethan 350 to 400 trees per acre.)

Although a precommercial thinning canbe expensive, most foresters agree it iseconomically advantageous. Delayingprecommercial thinning delays the accumu-lation of merchantable board-foot volume,increases mortality, and increases risk ofstagnation.

The chief disadvantage of precommercialthinning is the expense. However, federalcost-share assistance might be available;see Extension publication EC 1119, Incen-tive Programs for Woodland Managementand Resource Conservation (see page 8).

A rule of thumb is to precommerciallythin stands to a 10- or 15-foot averagespacing by age 15 or before trees are 20 feettall. This allows trees enough room to reachcommercial size before competition slowsdiameter growth.

Some woodland owners have inherited orbought unthinned stands that are more than15 years old or greater than 20 feet tall butare not marketable. These owners mustdecide whether to wait for stands to reachcommercial size slowly or to do a late

precommercial thinning. If such stands areseverely overstocked and tending towardstagnation, owners should completeprecommercial thinnings.

Some woodland owners have foundmarkets for material that is too small forsaw logs. This allows recovery of someprecommercial thinning expense. Fence-posts, barn poles, and firewood are a few ofthe products.

Commercial thinning turns a profitimmediately. A good commercial thinningsystem improves the stand’s ability to grow,thereby producing further profit. It leaveshealthy trees with live crowns extendingmore than 30 percent of the length of thetrees’ height. Also, proper thinning does notinjure leave trees—those that remain afterthinning. Collectively, leave trees composethe growing stock.

Thinning shock, sunscald, and wind-throw are major reasons to avoid delayingthinnings.

Thinning shock When stands areallowed to compete excessively, the crownrecedes to the uppermost portion of the tree.When thinned, these small-crowned treesare not capable of using the added light andspace. In fact, they may experience thinningshock and stop growing for a few yearsafter thinning.

Sunscald Tree trunks exposed suddenlyto the sun after thinning might react with acondition called sunscald in which the hotsun on the south side of the tree kills theliving and growing portion of the treeimmediately under the bark. This results indefective logs and reduced growth andprofit from future harvests.

Windthrow Windthrow and breakagemight follow after heavy thinning in standsleft too long unthinned. The slender treesthat a late precommercial thinning leavesoften are bent or broken by wind or snow.

Thus, promptly thinning Douglas-firstands will prevent problems in addition topromoting rapid growth.

High and low thinningCommercial thinning strategies include

high (top or crown) thinning and lowthinning. They differ by the crown class oftrees removed. Each system has its owncharacteristics.



A high thinning removes dominant andcodominant trees but not to the point thatthe growing capacity of the stand is seri-ously reduced (Figure 2). You must care-fully choose the relatively few dominants toremove so you give other dominants,codominants, and even some intermediatetrees a better chance to continue growing.

High grading is high thinning carried toofar. It removes so many trees that futurestand growth and tree form are down-graded. High grading eventually results infinancial loss because it robs the growingstock to the extent that future growth isreduced excessively.

As compared with low thinning, highthinning has the advantage of increasing thevolume per tree cut. Because large trees arecut, the average stand diameter decreasesand leave trees have smaller live crownratios.

Proper high thinning, however, leavesmany stems per acre. Its chief advantagesare that it:• Allows early logging of marginally

commercial stands

• Reduces logging costs by taking only afew large trees per acre

• Makes more options available for futurethinningsThe major disadvantage of high thinning

is that people tend to get greedy and taketoo many dominant and codominant trees.This leads to thinning shock and poorgrowth response in the remaining trees andturns an intended high thinning into anobjectionable high-grade cut.

A low thinning removes the less com-petitive trees from a stand (Figure 3). Ittakes all overtopped and intermediate treesand some codominant trees. The remainingdominants and codominants, which had thebest crown positions, continue growing.Since slow-growing trees are removed, thestand continues to grow at a rapid rate.

To remove the same amount of board-foot volume, you need to remove moretrees in a low thinning than in a highthinning. Because you remove the lesscompetitive trees, you must take more ofthem to maintain or increase growth of theremaining dominant and codominant trees.

A common problem in low thinning isnot taking enough trees. Logging costs are

high because many small trees are removed.If the stand is large enough to be onlymarginally commercial, you may have towait several years to get a profitable lowthinning, but you could carry out a highthinning immediately.

The chief advantages of low thinningare:• Little delay in growth response

• Little risk of windthrow or thinningshock (because the trees that remain arethose that were exposed to nearly full sunbefore thinning)If special products are your objective,

you can combine high and low thinnings.For example, managing for poles and pilingrequires frequently thinning trees fromeither high or low crown classes. A com-mon practice is to do a high thinning as thefirst commercial thinning following with alow thinning later in the life of the stand.

You have a great deal of flexibility inthinning strategy as long as it improvesyour stand.

Intensity and frequencyThinnings vary in intensity or number of

trees removed. A thinning that removesonly a few trees is referred to as light; aheavy thinning removes more trees.

Frequency, or how often you thin, isanother major difference in thinnings. Youcan thin several times or only once duringthe life of an individual stand. Increasingfrequency may increase injury to remainingtrees as well as increase soil compaction onthe site. Thinning at 5-year intervals isconsidered high frequency; a 20-yearinterval is low frequency.

Frequency and intensity apply equally tohigh and low thinning systems. Considerthem a unit. To maintain an acceptablelevel of growing stock, you must balancethe two—if you increase intensity, reducefrequency.

In summary, thinning systems vary withrespect to:• Timing—precommercial or commercial?

• Strategy—which crown classes toremove?

• Intensity—how many trees to remove?


STAND MANAGEMENT 5

These factors influence astand’s response in terms ofvolume growth rate, type ofproduct produced, extent ofdefect, species composition,and, ultimately, the dollarsreturned from harvestedtimber.

Stocking guidesThe stocking guidelines in

Table 1 are averages based onmeasurements taken in hun-dreds of Douglas-fir stands.Well-stocked stands haveaverage spacing and trees peracre that range between theunderstocked and overstockedlimits. Research has shown thatthis is the approximate size anddistance at which the full potential of a siteis captured on fast-growing crop treeswithout tree mortality.

Before you thin, compare the presentcondition of your stand to your target stand.Determine your target stand by looking atthe overstocked column in Table 1. If youwant your stand to grow to 16 inches indiameter, leave 160 trees per acre at about17-foot spacing.

For example, a stand with a 14-inchaverage diameter is considered well stockedif it has 90 to 200 trees per acre. The aver-age acceptable distance between trees is15 to 22 feet. If the stand is near the limit of200 trees per acre, you should thin it soon.If it is near the 90-trees-per-acre limit, itwill be years before thinning is advisable ornecessary.

When using the stocking guidelines, it isimportant to realize that thinning changesthe average diameter of trees in a stand. Alow thinning removes small trees, increas-ing the average stand diameter.

In contrast, high thinning reduces theaverage stand diameter because it removeslarge trees. This change in average diametercan affect the number of trees and spacingyou use as a thinning target.

For example, consider a 14-inch standwith 200 trees per acre. A high thinning thatreduces the average diameter by 2 inchesshould leave no fewer than 120 trees per

acre. On the other hand, a low thinning thatincreases average diameter by 2 inchescould leave as few as 75 trees per acre.

You have considerable flexibility in thefrequency and intensity of cuts. If cuts arefrequent, restrict intensity. Infrequent cuts,however, can be more intense.

The time it takes to grow from under- tooverstocking depends on the productivityof a site. On a less productive site, thismight take 20 years; on a very productivesite, perhaps only 5 years. Nevertheless, therelationship holds for all sites on whichDouglas-fir can produce a fully stockedstand.

How do youput it all together?

Imagine that you have a 10-year-oldstand on high ground with more than1,200 stems per acre and an average diame-ter of 2 inches. If you do nothing, the standwill be overstocked long before the treeshave 8-inch diameters. Growth will slow,and trees will die.

Unfortunately, markets for small mate-rial are extremely limited. Therefore, youdecide to do a precommercial thinning.

From the stocking table, you see thatleaving 280 trees per acre will allow trees

Table 1.—Trees per acre and spacing limitsfor even-age Douglas-fir stands*

8 300 12 500 910 200 15 390 1112 120 19 280 1314 90 22 200 1516 75 24 160 1718 60 27 125 1920 48 30 100 2122 42 32 90 2224 35 35 75 24

Understockedstands have

Average Fewer Wider More Narrowerstand trees spacing trees average

diameter per acre than about per acre spacing than(inches) than (feet) than about (feet)

Overstockedstands have

* Trees per acre and spacing for well-stocked stands fall between the under-stocked and overstocked limits.



to reach 12 inches in diameter before thestand is overstocked. Therefore, youremove three of every four trees, resultingin a 12- or 13-foot spacing. This spacingwill produce fast growth on individualtrees, and the stand will become marketableas quickly as possible.

Because the trees are not crowded, theyshould grow at about three annual rings perinch of radius. In 10 to 12 years, the aver-age stand diameter should be 12 inches.The stand will have grown to the point ofoverstocking, and it should be thinned atthat time.

You decide to remove one-third of thetrees, leaving about 200 trees at a spacingof about 15 feet. If trees have an averagegrowth rate of five rings per inch, thesetrees will need another 8 years to reach anaverage diameter of 15 inches.

At that time, you remove 100 15-inchtrees in a profitable commercial thinning.This leaves about 100 trees per acre. Youallow these trees to grow until they reach a20-inch average diameter, the point ofbeing overstocked. At that time, you har-vest the rest of the trees and plant a newstand.

In another example, imagine you havethree stands, each averaging 12 inches indiameter. Stocking is 100, 200, and 400trees per acre. What do you do?

First, it is important to thin the densestand down to about 120 trees per acre(18-foot spacing); otherwise, trees will diesoon.

The stand with 200 trees per acre is ingood shape—for now—but you should thinit after it grows 3 inches in diameter.

The stand with only 100 trees per acrewill have room to grow for years before thetrees average 20 inches in diameter. Thenyou would most likely harvest all the treesand plant a new stand.

What is best for you?Many factors determine which thinning

system is best for you. Of critical impor-tance is the condition of your stand and thesteepness of your ground.

Your management objectives and finan-cial situation have a strong influence onwhat you should and can do. For example,you’ll need to consider whether you need aperiodic income from your woodlandproperty and whether you can afford out-of-pocket expenses.

Other factors include the time you haveto devote to planning and thinning, yourskill level for doing part or all of the work,and your aesthetic tastes (light thinnings areless noticeable than heavy thinnings).

Both high and low thinnings requireskill, experience, and judgment. However,some foresters believe damage is morelikely during high thinning. A novicethinner might do a low thinning to be morecautious.

A forest’s appearance is important tomost landowners. Both high and lowthinnings can be done in a way that leavesthe forest neat and attractive. However,each system produces a different result.High thinning creates greater diversity oftree size but relatively few large trees. Lowthinning produces a more uniform forestbecause big trees are left to grow bigger.

Regardless of whether high or low,intense thinnings tend to create a moreopen, sunny, brushy forest than do lessdrastic thinnings. Infrequent thinningsallow the forest more time to return to anundisturbed state than do frequent entries.Either of these characteristics might influ-ence you toward one thinning systeminstead of the other.

Fragile areas (e.g., streambanks and wet-lands) and areas with access problems caninfluence your selection of a thinningsystem. You might use intense, infrequentthinnings to reduce the number of timesyou disturb a fragile site. In a similar case,you might decide infrequent logging entryis the best way to reduce the environmentalimpact of a temporary stream crossing.

Stands on very steep slopes (greater than60 percent) require expensive cable log-ging. They are difficult to thin becauselogging can damage residual trees. The best


STAND MANAGEMENT 7

strategy might be a heavy precommercialthinning followed by final harvest. (A16-foot spacing would produce 170 treeswith 14- to 16-inch diameters for finalharvest.)

On moderately steep ground (40 to60 percent slopes), infrequent thinningallows you to take enough volume to makecable yarding profitable. If your ground isgentle (less than 25 percent slope), you canchoose from an array of thinning systems.

In young, marginally commercial stands,an intense high thinning might be necessaryto cut enough big logs to pay for the thin-ning operation. This could improve thestand if most of the trees cut are defectiveor of poor quality.

A high thin can turn into a high gradewhen it removes most of the trees capableof good growth. In older stands, you mightuse either a high or low thinning system ora combination of both.

The types of products you can marketmight affect your choice of thinning sys-tem. A high price for poles of a certainlength may dictate taking those trees thatqualify.

Damage to trees from insects, disease,and weather also can influence thinning. Ifwind and ice damage large trees, a highthinning would be logical.

The type of thinning system you choosealso will influence the way you regeneratea stand of Douglas-fir. A heavy, highthinning often promotes natural seeding inwestern redcedar and western hemlock.You can encourage this by managing aforest with continued thinning and no finalclearcut.

Low-intensity low thinnings at frequentintervals lead to a clearcut, and you willneed to replace the stand with young andvigorous trees.

When you fully understand these impli-cations, you can choose a thinning systemthat delivers the kinds of benefits you seekfrom your forestland at the times you needthem.

SummaryThe thinning concepts described here

are not difficult, but they may be confusingas you read them for the first time and tryto relate them to each other. Here is areview of the key concepts.

Crown class• Dominants—Larger-than-average trees

with crowns that extend above a stand’scrown level.

• Codominants—Medium-size trees thatform the general level of crown cover.

• Intermediate—Trees that are shorterthan dominants and codominants;crowns are below or extend into thecrown cover formed by the larger trees.

• Overtopped—Small trees with crownsbelow crown-cover level.

Types of thinning• Precommercial thinning—Removing

small trees in a young stand to reducecompetition and to accelerate growth ofremaining trees to the point that theyhave commercial value. Consider thisfirst thinning an investment, becausewhat you spend now will pay off later.

• Commercial thinning—A profitableoperation that removes trees from adeveloping stand to give the remainingtrees more growing space. It improvesindividual tree growth.

• High thinning—Removing large domi-nant and codominant trees, therebyreleasing the remaining trees to growmore rapidly.

• Low thinning—Removing small,noncompetitive trees (overtopped,intermediate, and some codominants)so remaining trees can continue rapidgrowth.

Thinning factors• Intensity—The number of trees you

remove. It can be light (removing onlya few trees) or heavy (removing a lot oftrees).



• Frequency—How often you thin.It can be high (5-year intervals) orlow (20-year intervals).

The present condition of your stand, yourmanagement objectives and constraints, andthe amount of time and skill you can devoteto managing your stand are importantfactors in determining the best thinningsystem for you. Thinning systems varyaccording to:• Frequency of cutting

• Intensity of cutting

• Proportion of crown class removed(high or low thinning)

Many combinations of frequency andintensity will keep your stands wellstocked. Do remember that intensity andfrequency must balance each other—ifintensity is high, keep frequency low.

High thinning tends to produce greaterincome in early logging entries than doeslow thinning. Low thinning tends to pro-duce greater income in later logging entriesand creates a forest with greater standingtimber volume and, therefore, greater value.

Because of these differences, highthinnings appeal to landowners who need alarger cash flow in the present decade thanin the decades to come. Low thinningsappeal to landowners who want to deferincome into future decades.

For more informationPNW 184, Thinning: An Important Timber

Management Tool. 8 pages.

EC 1119, Incentive Programs for WoodlandManagement and Resource Conserva-tion. 4 pages.

The above publications are availablefrom:

Publication OrdersExtension & Station CommunicationsOregon State University422 Kerr AdministrationCorvallis, OR 97331-2119541-737-2513fax [email protected]

Please contact the office or visit the Website at http://eesc.oregonstate.edu to learnabout publication availabilities, prices,quantity discounts, and shipping andhandling charges.

The Woodland Workbook comprises some 60 publications prepared by Oregon State University Extension forestersspecifically for owners and managers of private, nonindustrial woodlands. Workbook publications contain information oflong-range and day-to-day value for anyone interested in wise management, conservation, and use of woodland proper-ties. Publications are available for purchase separately. A special three-ring binder with tabbed dividers also is availablefor purchase. For information about how to order, and for a current list of titles and prices, contact the OSU ExtensionService office that serves your county. Or, visit the Extension Web site at http://eesc.oregonstate.edu/ then Publications& Videos, then Forestry.

This publication was produced and distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914.Extension work is a cooperative program of Oregon State University, the U.S. Department of Agriculture, and Oregoncounties.

Oregon State University Extension Service offers educational programs, activities, and materials—without regard torace, color, religion, sex, sexual orientation, national origin, age, marital status, disability, and disabled veteran orVietnam-era veteran status—as required by Title VI of the Civil Rights Act of 1964, Title IX of the Education Amend-ments of 1972, and Section 504 of the Rehabilitation Act of 1973. Oregon State University Extension Service is an EqualOpportunity Employer.

Published 1984. Reprinted July 2003.


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