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CONTENTS

Page

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 

WHEN TO THIN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 

SPACING OF TREES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 

SELECTION OF LEAVE TREES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

STANDS PAST THE IDEAL STAGE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 

CONCLUSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
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lNTROUCTlON

Precommercial thinning is a practical means of substantially increasing the

production of usable wood. The larger the

trees must be to be merchantable, the 

greater are the gains from precommercial 

thinning.

This paper provides guidance on when 

and how to precommercially thin stands of

nearly pure Douglas-fir. Relative gains

from selected options are indicated; however, the complete information needed to

rank management alternatives is beyond

the scope of this paper. Conclusions and

recommendations given are my opinions,

based on observations and available data.

Although I believe the recommendations 

are sound, I am concerned more with 

presenting concepts than with specific 

recommendations.

The first part of this paper is concerned with thinning at the “ideal” stage––

the stage at which precommercial thinning

will have the greatest effect on subsequent

usable production. After this, some considerations regarding stands which have

passed this stage are discussed.

WHEN TO THlN

Ideally, stands should be thinned 

when leave trees are about 10 to 15 feet 

(3 to 5 m) tall and 10 to 15 years old (fig. 1).

Thinning should be delayed only long enough

for trees to express their growth and 

quality characteristics and to be above such

deterrents as brush competition and animal

browsing. Further delay will usually result in unnecessary loss in usable production and will often result in increased 

thinning costs.

A l0-year period of growth is generally sufficient to show potential growth

rate, stem and branching characteristics,

and susceptibility to various types of damage.

On the best sites, dominant trees will 

average about 15 feet (5 m) tall at this age, 

and the stands will be ready for thinning.

However, on poor sites, l0-year-old trees 

will be quite short and susceptible to damage;

therefore, thinning on the poorest sites 

should be delayed to about age 15 (fig. 1). 

Ages and heights of trees on intermediate 

sites will fall between these extremes.

The longer the delay of thinning after 

competition among trees begins, the greater 

will be the unrealized usable production. 

First, substantial growth will be added to 

trees which will never be used, and therefore, less growth will be added to trees

which subsequently will be used. Tree

growth begins to be reduced by competition

well before that competition becomes readily apparent; by the time lower branches 

start dying, competition is already quite

substantial. Second, trees in dense

stands are more susceptible to storm 

damage than are well-spaced trees in 

thinned stands, once the latter have adapted

to their increased growing space. The

older the stand when thinned, the longer 

the adaptation period.

Thinning can generally be done more

cheaply and efficiently when trees are small.

Excess trees can then be easily felled, with

few problems. Care should be taken to 

cut stumps below the lowest live branch,

if possible. If felling of excess trees is

delayed until trees become larger, it will 

create a larger amount of slash, which 

may have to be abated, and is likely to 

damage residual trees and to cause “shock.”

Killing these larger trees with chemicals,

rather than felling them, will avoid the 

slash and shock problems but often results 

in incomplete kill of unwanted trees or 

loss of some intended leave trees, or both.

In most situations, fill-in by 

natural seedlings after age 10 should be
inconsequential. The trees left after 

thinning will be in the period of most 

rapid height growth and will not give new

seedlings an opportunity to become competitors. Likewise, brush encroachment

2

generally should not be a severe problem.

Where severe brush problems exist prior 

to thinning, chemical control of the brush

should be considered.

Figure l.--Ideal time for precommercial thinning of 

Douglas-fir relative to height and age of leave trees.
SPACING OF TREES

The number of trees left after precommercial thinning should depend upon

when the first commercial thinning will 

be made––in terms of tree size at that 

time.l/ The larger the trees must be to 

support a commercial thinning, the fewer 

the trees left after precommercial thinning.

A guide to the desired number or 

spacing of trees is provided in figure 2. 

This format shows stocking at any point 

in time––n terms of stand average d.b.h. 

(Dq),2/ number of trees, and basal area. 

As long as there is no mortality, the 

development of a stand with increasing age

(increasing tree size) is depicted by a 

vertical progression upward. The 

“desired maximum stocking” curve shows

the basal area per acre and stand average 

d.b.h. which stands of various densities

(number and spacing of trees) can be expected to attain by the time another thinning is needed. It is applicable to all 

sites; only the timespan differs. The

position of this curve is based on experience with experimental plots, plus a

consideration of how stands develop.3/

Precommercially thinned stands will 

generally attain this level with no suppression-related mortality. Thus, the point

at which desired average diameter at the

time of commercial thinning intersects 

this “maximum” curve determines the 

number of trees to be left after precommercial thinning. For example, if average

1/ It the stand will be carried to final harvest 

with no commercial thinning, then the reader should

substitute “harvest” for “first commercial thinning”

throughout the paper.

2/ As used in this paper, average d.b.h. is the 

quadratic mean d.b.h., or d.b.h. of a tree of mean 

basal area.

3/ Details and supporting evidence are on file,

Forestry Sciences Laboratory, Olympia, Washington.

3

d.b.h. of the stand must be 8 inches 

(20 cm) at the time of the commercial 

thinning, the stand should be precommercially thinned to about 400 trees per acre 

(1,000 per ha). If it needs to be only 6 

inches (15 cm), 610 trees per acre (1,520 

per ha) should be left; if it must be 10 

inches (25 cm), only 290 trees per acre 

(720 per ha) should be left. The average

spacing of these trees is closely approximated by D+2 feet; where D is the average 

d.b.h. at the time of the next (commercial)

thinning, not the precommercial thinning. 

If no commercial thinning is deemed 

practical, even fewer trees should be 

left––perhaps 150 to 200 per acre (370 to 

490 per ha).

This stocking guide is intended to be

used for spacing, not for absolute number

of trees per acre. Most stands contain 

openings not occupied by trees. Therefore, the average number of trees left per

acre should generally be less than called 

for by the stocking guide. One should not

maintain an overdense stand in one spot

to offset the lack of trees in another. As

a rough guide, the maximum number of 

extra trees left around an opening should 

be the number obtained by extending the

desired spacing pattern one more space

into the opening. The spacing between 

any two residual trees should be no less

than one-half the desired average spacing

(fig. 3). Thus, using these guides, we

might compensate for holes with a width 

of up to twice the desired average spacing

by leaving extra trees around the perimeter,

provided trees are present in the right

places. Any hole larger than this cannot 

be fully compensated for. Extra trees 

left to compensate for an opening should

be left only around the immediate perimeter of that opening. They should not be

crowded into other parts of the stand.

Elsewhere within the stand, an 

attempt should be made to maintain the
4

Figure 2.--Stocking-guide for precommercial thinning of Douglas-fir

(arrows show development of stands at selected spacings).
5

desired average spacing around each tree,

as nearly as possible. Thus, crowding on

one side of a tree should be compensated

for by extra space on another side. This 

condition can, of course, seldom be

achieved exactly. However, in no case

should several adjacent trees be crowded. 

The number of trees actually left per 

stocked acre should not be more than

10 percent greater than the number called 

for by the guide.

The spacing recommended in this 

guide is expected to allow all trees to 

reach merchantable size; the minimum

Figure 3.--Effect of stand openings on number of leave 

trees per acre (arrows show where extra trees could be

left on perimeter of openings).

merchantable size is assumed to be about 

25 percent smaller than the specified 

average diameter of the stand (Dq ). It 

is further expected that when the desired 

Dq is reached, most trees will still be 

growing well and will respond well to 

release. With more trees than this, 

growth of individual trees begins to be 

seriously retarded before trees become 

large enough to support commercial

thinning, and some trees will die. The

first commercial thinning will remove 

up to one-third of the trees, with average 

d.b.h. of cut trees about equal to average 

d.b.h. of the entire stand.
SELECTlON OF LEAVE TREES

Precommercial thinning at the ideal

stage should strive for uniform size of 

trees as well as spacing. One should

attempt to leave trees having diameters

within + 25 percent of the average for all

leave trees within that portion of the stand.

Thus, if average d.b.h. of all leave trees

is 2.0 inches (5.1 cm), the maximum 

range should be about 1.5 to 2.5 inches

(3.8-6.4 cm). If smaller trees are left,

they would generally be crowded out of the

stand. On the other hand, if larger trees

are left, they tend to grow at an excessive

rate and adversely affect their neighbors.

There is some question regarding 

the need to cut very small understory

trees. In some instances, they could be

left and would soon be crowded out of the

stand. However, if they have good root

systems, they could grow quite rapidly 

and become competitors. As a general 

rule, it is best to remove all unwanted

trees.

STANDS PAST THE lDEAL STAGE

Gains in usable yield from precommercial thinning are greatest if thinning 

is done at the ideal stage, and stands at

that stage generally should not be bypassed

in favor of thinning otherwise comparable

stands which have passed that stage.4/

However, there are still opportunities 

for substantial gains from precommercial thinning of somewhat older stands,

although a marginal point is soon reached,

beyond which precommercial thinning is 

no longer practical. This point will vary

with site quality, stand structure, expected markets for small trees, management objectives, and type of ownership.

4/ Gains in undiscounted dollar’s, and likely

even in discounted dollars, will also be greatest

with early precommercial thinning.

6

A primary gain from precommercial

thinning is a shortening of the time a stand

must be carried before it can be commercially thinned. Generally, the longer the

time required for trees to reach commercial size without thinning, the greater the

gain from precommercial thinning. Thus,

the larger the trees must be to support a

commercial thinning, the greater the gain.

Likewise, up to a point, the more slowly

the trees are growing, the greater is the

gain.

If precommercial thinning is delayed,

trees will be substantially older when they

reach the desired size for commercial 

thinning. First, leave trees will be smaller

at the time of precommercial thinning than

they would be at that age if the stand had

been thinned earlier. Second, they will

continue to grow more slowly for the next

few years than they would if the stand had

been thinned earlier. I am reluctant to

give specific estimates but have done so 

for the purpose of illustrating a concept.

These estimates are based on the assumption that the stands, before thinning, are

“normal.”5/ Obviously, the ages for 

both unmanaged stands and stands receiving precommercial thinning will vary 

somewhat with initial stand density, among

other factors.

Estimated gains in time are shown 

for three commercial thinning options: 

thin when average d.b.h. of merchantable trees is (I) 8 inches (20.3 cm), (II) 

9 inches (22.9 cm), or (III) 10 inches 

(25.4 cm) (table 1). If minimum merchantable size is 25 percent smaller than

5/ Normal by site and age, according to 

Richard E. McArdie, Walter H. Meyer, and 

Donald Bruce, The yield of Douglas-fir in the 

Pacific Northwest, U.S. Department of Agriculture Technical Bulletin 201 (revised 1961), table 

2. (Reprinted by Oregon State University Book 

Stores, Inc., Corvallis, Oregon, 1971.)
7

these stand averages––i.e., 6.0, 6.75, 

and 7.5 inches (15.2, 17.1, and 19.0 cm), 

the corresponding numbers of merchantable trees for each of the three options 

will be about 400, 335, and 290 per acre

(990, 830, and 720 per ha), respectively. 

For precommercially thinned stands, this 

is the total number of trees in the stand.

For unmanaged stands, many additional

trees will die without reaching merchantable size. For each option, gains with 

(1) precommercial thinning at the ideal

time and (2) precommercial thinning when

leave trees are 30 feet (9.1 m) tall are

shown; for sites where trees are more 

than 20 years old when they reach a height

of 30 feet (fig. 4), precommercial thinning

at age 20 is also included. Stand ages

associated with these gains are shown in

table 2.

Gains from delayed precommercial

thinning in stands where leave trees are 

up to 30 feet (9 m) tall or 20 years old,

whichever comes sooner, are still substantial, although much less than gains

from earlier thinning. Gains from thinning after about age 20 decline very rapidly. This is the approximate age at 

which current annual height growth culminates. The differences in gains for the

three commercial thinning options are 

due to differences in merchantability 

standards. If smaller trees could be 

used, then gains from precommercial 

thinning would be less than in option I.
8

On the other hand, if commercial thinning

required larger trees than in option III,

gains from precommercial thinning would

be even greater. Where delayed precommercial thinning might not be justified if 

6-inch (15-cm) trees are considered merchantable, it might be justified if trees

must have a d.b.h. of 8 inches (20 cm) 

or larger to be merchantable.

Percentage gains in mean annual

increment (m.a.i.) of usable volume due 

to precommercial thinning are closely 

related to these gains in time and age at
9

first commercial thinning; the ratio of 

percentage-gain to years-gained tends to

increase with age at first commercial 

thinning.6/ Thus, for any of these commercial thinning options, percentage 

gains from precommercial thinning within

the above limits increase markedly with

decreasing site quality. The resulting

absolute gains in usable m.a.i. also increase with decreasing site quality––at

6/ This conclusion has been derived from both

theoretical considerations and preliminary estimates

of yields attainable with thinning.

least through site IV; gains on site V may 

be about equal to those on site IV.

If precommercial thinning is not done

until leave trees are about 30 feet (9 m) tall,

it will not be practical to create as uniform

a stand as could be done with earlier thinning. Dominant and codominant trees will

then have an average d.b.h. of about 4 

inches (10 cm) or more, and some will be

more than 25 percent larger than this average. When these trees are close to merchantable size, they should generally be

retained in the stand until at least the first

commercial thinning, unless they are 

wolf-trees.
10

CONCLUSlON

Ideally, precommercial thinning

should be done when leave trees are about

10 to 15 feet (3 to 5 m) tall and 10 to 15

years old. The spacing of residual trees

should depend on how large the trees will

be when the first commercial thinning 

will be made; the larger the trees at that

time, the fewer should be left, as per the

stocking guide. The thinning should create

a uniform tree size.

If this ideal stage has passed, there

are still worthwhile opportunities for precommercial thinning; but gains will be less.

Gains from precommercial thinning in

stands where leave trees are up to 20 

years old or 30 feet (9 m) tall, whichever

comes first, can still be quite substantial.

The maximum age or tree size at which 

precommercial thinning is practical

depends on several factors. Probably 

the most critical of these is the size that

trees must attain before a commercial 

thinning will be made.

The need for precommercial thinning

is greatest on poor-quality sites, where it

may make the difference in whether or not

a crop of merchantable trees is produced

within an economic time frame. On better

sites, a satisfactory crop can be produced

without precommercial thinning.