True Fir Spacing Trials— 

10-Year Results

Robert O. Curtis

United States 

Department of 

Agriculture

Forest Service

Pacific Northwest 

Research Station

General Technical

Report

PNW-GTR-749

April 2008
Author

Robert O. Curtis is an emeritus scientist, Forestry Sciences Laboratory, 

3625 93rd Avenue SW, Olympia, WA 98512-9193.

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Abstract

Curtis, Robert O. 2008. True fir spacing trials—10-year results. Gen. Tech. Rep. 

PNW-GTR-749. Portland, OR: U.S. Department of Agriculture, Forest Service, 

Pacific Northwest Research Station. 36 p.

Eighteen precommercial thinning trials were established in true fir-hemlock stands 

in the Olympic Mountains and the west side of the Cascade Range during the 

period 1987 through 1994. This paper updates a previous report, with results for the 

first 10 years after establishment. Results are given for (1) all trees, (2) the largest 

80 per acre of any species, and (3) those noble fir (Abies procera Rehd.) and Pacific 

silver fir (Abies amabilis Dougl. ex Forbes) included in the largest 80 per acre. 

Diameter growth of all species increased with increase in spacing. Height growth of 

Pacific silver fir decreased with increase in spacing. The largest 80 trees per acre of 

all species showed some increase in diameter and basal area growth with increased 

spacing, while height growth declined slightly and volume growth was nearly 

constant. Over time, these installations will provide a unique source of information 

on early development of managed stands of these species, for which little information is now available.

Keywords: True firs, Abies procera, Abies amabilis, precommercial thinning, 

spacing, noble fir, Pacific silver fir.
Contents

1 Introduction

2 Previous Research

2 Past Management

3 The Study

4 Objectives

4 Study Area

5 Study Design

6 Plot Establishment and Measurement

10 Remeasurement Schedule

11 Analyses

12 All Trees

12 Largest 80 Trees per Acre, All Species: Graphical Comparisons

17 Increment of Largest 80 Trees per Acre, All Species: Statistical Comparisons

20 Comparisons by Individual Species

25 Top Height (H80) in Relation to Breast-High Age, All Species Combined

25 Discussion

25 Installation (Block) Effects and Spacing Effects

26 All Survivors

27 Largest 80 Trees per Acre

28 Individual Species Comparisons: Trees Included in the Largest 80

30 Future Analyses

30 Future of the Study

34 Acknowledgments

34 Metric Equivalents

35 Literature Cited
Table 1—Areas of Abies amabilis zone forest in 

five national forests in the study area

National forest Abies amabilis zone

Percent Acres

Mount Baker-Snoqualmie 40.2 733,654

Olympic 29.0 190,544

Gifford Pinchot 42.7 587,631

Mount Hood 41.6 457,224

Willamette 38.9 686,856

Source: Henderson, J.A. Potential natural vegetation zones of 

Washington. Manuscript in preparation.

True Fir Spacing Trials—10-Year Results

Introduction

In the period 1987–94, 18 precommercial thinning trials were established in true 

fir-hemlock stands along the west side of the Cascade Range in Washington and 

Oregon and in the Olympic Mountains. Establishment of the study and growth during the first 5 years of observation were reported by Curtis and others (2000). The 

present report is essentially an update of that publication. It will begin by repeating 

much of the background information given in the previous report, to orient the 

reader. It then presents results for the first 10 years of growth and some additional 

comparisons not included in the earlier report.

The stands studied are within the Coastal True Fir-Hemlock type (Society of 

American Foresters type 226: Eyre 1980), primarily within the Abies amabilis zone 

(Franklin and Dyrness 1973) but also with two in the Tsuga mertensiana zone. The 

principal tree species are Pacific silver fir (Abies amabilis Dougl. ex Forbes), noble 

fir (A. procera Rehd.), and western hemlock (Tsuga heterophylla (Raf.) Sarg.). 

Other minor associated species include western redcedar (Thuja plicata Donn ex 

D. Don), Alaska-cedar (Chamaecyparis nootkatensis (D. Don) Spach), mountain 

hemlock (Tsuga mertensiana (Bong.) Carr.), subalpine fir (A. lasiocarpa (Hook) 

Nutt.), Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), and western white pine 

(Pinus monticola Dougl. ex D. Don).

True fir-hemlock forests at higher elevations in the Cascade Range and Olympic Mountains play major roles as protective cover for watersheds, as viewscapes 

and recreational areas, and have in the past been important as timber-producing 

forests. Large areas of Abies amabilis zone forest exist in the Mount BakerSnoqualmie (MBS), Gifford Pinchot (GP), Olympic (OLY), Mount Hood (MH), 

and Willamette (WIL) National Forests (table 1). There are large areas in the national parks in the region and limited areas on Washington Department of Natural 

Resources (WDNR) lands and private ownerships. There are also large areas on 
GENERAL TECHNICAL REPORT PNW-GTR-749

2

Vancouver Island and in the Coast Range of British Columbia that are important 

timber-producing areas.

Previous Research

Although there is a considerable body of information on stand development, yields, 

and site evaluation for low-elevation hemlock, there is very little information available on the true firs and high-elevation true fir-hemlock mixtures. 

Herman and others (1978) and Hoyer and Herman (1989) developed height 

growth and site index estimates for noble fir and Pacific silver fir, based on stem 

analyses of trees from unmanaged old-growth stands. These are only doubtfully 

applicable to young stands because of differences in stand establishment, early 

competition, climatic changes, and possible biases associated with changes in relative tree position over time.

Murray and others (1991) found that early height growth of young Pacific silver 

fir and noble fir established in clearcuts appeared to be considerably more rapid 

than predicted by the existing site curves. Harrington and Murray (1982) compared 

height-growth patterns in young Pacific silver fir, noble fir, and Douglas-fir. They 

point out that true firs characteristically have a period of slow juvenile growth, an 

extended period of almost linear rapid growth, and a final phase of appreciable 

growth extending to very advanced ages. Consequently, true firs often appear 

at an initial disadvantage in comparison with associated species, but may later 

equal or surpass their associates because of the long period of rapid and uniform 

height growth.

Oliver and Kenady (1982) provided a summary of existing information as of 

that date. The various U.S. Forest Service plant association guides have since presented several indicators of productivity by plant association (Brockway and others 

1983; Hemstrom and others 1982; Henderson and Peter 1984; Henderson and others 

1989, 1992; Logan and others 1987).

Past Management

Extensive harvesting in the true fir-hemlock type began in the early 1950s. Early 

operations commonly followed practices that had been generally successful in 

lower elevation Douglas-fir; namely, clearcut, burn the slash, and plant—often with 

Douglas-fir. A high proportion of these early plantations failed, and regeneration to 

true fir-hemlock took place by natural seeding over an extended number of years. 

The resulting stands were often patchy and contained considerable variability in 

tree sizes and ages. 

Little information 

exists on true fir 

stand development.
True Fir Spacing Trials—10-Year Results

Over subsequent years, better species selection combined with improved 

nursery, site preparation, and planting practices greatly improved survival and 

produced many successful plantations. Favored species for planting have been 

noble fir or noble fir-silver fir mixtures, with other species such as Engelmann 

spruce (Picea engelmanni Parry ex Engelm) or western white pine used on some 

frost-prone sites. Reduced use of fire favored survival of natural advance regeneration of Pacific silver fir and western hemlock. Numerous young true fir-hemlock 

stands now exist at higher elevations along the west side of the Cascade Range and 

in the Olympic Mountains.

Extensive precommercial thinning began in the late 1970s. There has been 

some question about the feasibility or desirability of future commercial thinning 

because many stands are on steep terrain and because these thin-barked species are 

highly susceptible to rot arising from logging injuries.

A common practice in the late 1970s and early 1980s was to thin at an early 

age to 350 to 400 stems per acre. Some have further reduced the number of leavetrees, with the expectation that no further stand entry might be feasible until final 

harvest. This raised concerns about the effects of number of residual trees on 

future stand development and average tree size at which precommercial thinning 

(PCT) should be done.

Today much of the land under the Northwest Forest Plan is in late-successional 

old-growth forest reserves (including wilderness). The rest is mostly in matrix 

or adaptive management areas. About 40 percent of these areas are in the Abies 

amabilis zone . In some areas the management goal is future timber production, 

whereas others are managed primarily for watershed, wildlife, or recreation values. 

Much of the area under the Northwest Forest Plan is currently managed for old 

growth protection, restoration of old growth or protection of old-growth-dependent 

species. Whatever the present or possible future management objectives, very little 

is known about the dynamics of young true fir forests.

The Study

In the period 1987–94, the Pacific Northwest Research Station (PNW) installed 

a series of 18 spacing trials in true fir-hemlock, with assistance from the Pacific 

Northwest Region of the U.S. Forest Service and the individual national forests 

concerned. The trials were established in stands considered ready for PCT under 

then-current operational practices. Seven were on the MBS, one on the OLY, five 

on the GP, four on the MH, and one on the WI National Forests.

1Henderson, J.A. 2007. Personal communication. Area Ecologist, Mount Baker-Snoqualmie 

and Olympic National Forests.

There are 

considerable areas 

of young true firhemlock stands.
Figure 1—Locations of study areas in 

western Washington and Oregon.

GENERAL TECHNICAL REPORT PNW-GTR-749

Objectives

Study objectives are (1) to determine the quantitative responses of Pacific silver fir, 

noble fir, and western hemlock to a range of precommercial thinning stocking levels; and (2) to obtain long-term growth data applicable to young managed stands, 

as a basis for estimates of development patterns and potential yields.

It is hoped that these trials will ultimately provide:

• Information on development of precommercially thinned stands needed 

for reliable estimates of productivity of young true fir-hemlock stands.

• Data useful, in combination with other data, for construction of stand simulators and yield projection systems for young-growth managed stands.

• A basis for choosing the optimum number of leave trees to meet different 

management goals.

• Information on successional and structural development over time for possible use in meeting landscape diversity, species habitat, and watershed 

management objectives.

Study Area

The study area consists of the true fir-hemlock type as it occurs on the MBS, OLY, 

GP, MH, and WIL National Forests. Emphasis is on stands located within the 

Pacific silver fir zone; however, several trials reach into the lower portion of the 

mountain hemlock zone.

For some years prior to the study, the 

national forests had been conducting an 

extensive precommercial thinning program in young true fir-hemlock. Choice of 

locations was therefore constrained by the 

fact that many stands otherwise suitable 

for the study had already been operationally thinned.

Sites selected (fig. 1) ranged from 

3,000 to 4,500 ft in elevation. Most of the 

plant associations found on the study sites 

have widespread distribution within the 

respective national forests, according to 

the various area plant association guides.

Quantitative 

information is 

needed on response 

to precommercial 

thinning.
Figure 2—Basic plot design.

True Fir Spacing Trials—10-Year Results

Study Design

The study uses a randomized block design, with six treatments per block. Where 

feasible, two plots of each treatment were established at a location for a total of 12 

plots per installation. Few relatively homogeneous areas large enough to accommodate 12-plot installations were found. Of the 18 installations established, 3 have 

12 plots and 15 have 6 plots.

Treatments—

The treatments consisted of five spacings, plus no treatment (NT). It was expected 

that the untreated plot(s) would be useful for demonstration purposes and possibly 

in analytical comparisons. The spacings and corresponding target values per unit 

area are as follows:

Spacing Trees per acre Area per tree 

Feet Number Square feet

7.9 700 62.2

10.1 430 101.2

12.8 265 164.6

16.4 163 267.7

20.9 100 435.5

Seven hundred and one hundred trees per acre were chosen as the limits of 

the range considered, and intermediate values were calculated by using a constant 

percentage of increase in area per tree.

Treatments were randomly allocated to plots within an installation.

Plot design—

Measurement plots were square, within a larger treatment plot (fig. 2), of 

the dimensions given in table 2. In the first installation, the treatment plot 

was 200 ft on a side. Thereafter, it was reduced to 180 ft (occasionally 160 

ft) because of the difficulty in fitting plots into available area. The plot sizes 

selected were thought sufficient to (1) give reasonably smooth diameter 

distributions, (2) give values reasonably stable in the presence of minor 

mortality, and (3) allow continuation of the plot to advanced ages, with or 

without one superimposed later thinning.

Measurement plot size was about 0.25 acre, with a slightly smaller plot 

(0.20 acre) for the unthinned plot and somewhat larger areas for the two widest spacings, so that a reasonable number of leave trees would be included. 

To facilitate control of leave-tree marking, the sides of the measurement plot 

were made multiples of the desired spacing, thus allowing subdivision of the 

plot into strips corresponding to the desired spacing. 
Table 2—Standard dimensions of plots used in true fir spacing trials

Treatment plot Measurement plot

Treatment Spacing Area Side Area Side Buffer width

Number/acre a Feet Acres Feet Acres Feet Feet

No thin – 0.7438 180 0.200 93.3 43.4

700 7.9 .7438 180 .242 102.7 38.6

430 10.1 .7438 180 .234 101.0 39.5

265 12.8 .7438 180 .241 102.4 38.8

163 16.4 .7438 180 .302 114.8 32.6

100 20.9 .7438 180 .361 125.4 27.3

aExpected number assuming no empty cells.

GENERAL TECHNICAL REPORT PNW-GTR-749

Plot Establishment and Measurement

Plot layout—

True fir-hemlock stands are characteristically patchy and variation is unavoidable, 

but plot locations were selected to be as nearly comparable in site conditions and 

initial stand conditions as feasible. Plots were either contiguous or quite close to 

each other. Obvious site changes and root rot pockets were excluded. Treatments 

were assigned randomly after the treatment plots were located on the ground but 

before the measurement plots were established.

After initial plot location and treatment assignment, the corners of treatment 

plots and interior measurement plots were established with staff compass and tape. 

All distances were measured horizontally. Measurement plot boundary closure was 

within 2 ft.

Initial tree measurements and marking—

Plots were subdivided with string at intervals equal to the desired spacing, and 

leave trees were selected as the best stem within each grid cell. To the extent possible, an effort was made to adjust all treated plots within an installation to nearly 

comparable species composition. Vigor was given precedence over strict adherence 

to spacing or species criteria. Occasional exceptionally large trees were excluded to 

make leave-tree diameter distributions and average diameters more comparable.

Because some cells within the grid lacked suitable leave trees, the specified 

numbers of leave trees per acre were not met exactly in all cases. 

All trees 0.6 and larger in diameter at breast height (d.b.h.) were tallied by 

species and 1-in d.b.h. classes at all measurement plots to be thinned. 
True Fir Spacing Trials—10-Year Results

Leave-tree measurements—

Trees were measured after the growing season. Diameters at breast height were 

measured to the nearest 0.1 in (by diameter tape) on all leave trees on thinned plots, 

and all trees 0.6 in and larger d.b.h. present at establishment on the unthinned plots. 

Heights and heights to live crown were measured to the nearest 0.1 ft (with height 

poles or—in later remeasurements—laser instrument). Heights were measured on 

all leave trees at the two widest spacings, on the first two of every three trees at the 

12.8-ft spacing, and on every other tree at the two closest spacings.

In unthinned plots, heights and heights to live crown were measured for at least 

40 trees, more in mixed-species stands. Trees were selected across the entire plot 

and across the full range of d.b.h. classes present, with about two-thirds of selected 

trees above the average diameter and one-third below for each species.

Stand age estimates—

Ring counts were made at breast height (b.h.) and stump height on a sample of trees 

cut in thinning, excluding trees showing a suppressed core. There was considerable variation because of difference in species growth rates and because natural 

regeneration often occurred over several years. Regressions of ring count on d.b.h. 

were fit to all sample tree data from an individual installation. These equations 

were then used to estimate the average number of rings at breast and stump heights 

corresponding to mean diameter of leave trees on all thinned plots at an installation. Total age was taken as the estimated number of rings at stump + 3 (table 3) 

and checked against date of planting in cases of known planting date and good 

survival. Seedlings established as advance regeneration (principally Pacific silver 

fir) may have widely differing numbers of years in a suppressed condition and were 

not included in the above age estimates.

Description of ground vegetation—

All installations were classified by dominant plant association (tables 4 and 5), in 

most cases, by the area ecologist for the given national forest. Much more detailed 

descriptions were made for those installations located in the MBS and OLY National Forests (Curtis and others 2000), including ground vegetation species identification and inventory.

Pre- and postthinning conditions—

Estimated total stand ages at establishment ranged from 14 to 41 years. Elevations 

ranged from a low of 3,000 ft (Bonidu) to 4,500 ft (Alpine #4).

Pretreatment installation values (based on the untreated plots) for trees per acre, 

basal area, and quadratic mean diameter (QMD) are given in table 6. Initial trees 

per acre (excluding trees less than 0.6-in d.b.h, some of which became leave trees) 
Table 3—Basic descriptive information for the true fir spacing study installations

Installation

National 

foresta Plots

Year 

established

Stand 

origin

Breasthigh ageb Age at stump Total age Elevation Dom

as

No. Year - - - - - - - - - -Years - - - - - - - - - - - Feet

Alpine #4 MH 6 1990 Mixed 11 16 19 4,500 TSME/VBonidu OLY 6 1990 Natural 16 21 24 3,000 ABAM/Cabin #2 GP 6 1994 Planted 7 11 14 3,800 ABAM/Cat Creek GP 6 1990 Natural 13 20 23 4,300 ABAM/Crevice Cr MBS 6 1988 Mixed 22 26 29 3,000 ABAM/Cumberland MBS 6 1988 Natural 12 17 20 3,200 ABAM/Dog Creek MH 6 1991 Planted 13 17 20 3,600 ABAM/Evans Creek MBS 6 1989 Natural 32 38 41 3,700 ABAM/Haller Pass MBS 5 1989 Planted 24 29 32 4,200 ABAM/Iron Mt MBS 12 1987 Natural 19 25 28 3,500 ABAM/Job #2 GP 6 1992 Planted 8 14 17 3,700 ABAM/Marys Creek WIL 6 1994 Planted 8 12 15 4,000 ABAM/Memaloose MH 6 1992 Planted 13 21 24 4,000 ABAM/North Mt #2 MH 6 1993 Mixed 12 17 20 3,200 ABAM/Pointer #3 GP 12 1991 Planted 10 14 17 4,100 TSME/VRattrap Pass MBS 6 1988 Mixed 14 20 23 3,200 ABAM/Tonga Ridge MBS 6 1989 Mixed 20 26 29 3,800 ABAM/Twin #1 GP 12 1994 Planted 10 15 18 3,800 ABAM/aNational Forests (NF) as follows: GP = Gifford Pinchot, MH = Mount Hood, MBS = Mount Baker-Snoqualmie, OLY = Olympic, WIL = Willamette.bYears since attaining breast height.cPlant associations are given in table 3.

8
Table 4—Species names, true fir spacing trials

Species

Common name Botanical name Acronym

Trees

Pacific silver fir Abies amabilis Dougl. ex Forbes ABAM

Subalpine fir Abies lasiocarpa (Hook.) Nutt. ABLA

Noble fir Abies procera Rehd. ABPR

Alaska cedar Chamaecyparis nootkatensis (D. Don) Spach CHNO

Engelmann spruce Picea engelmanni Parry ex Engelm PIEN

Western white pine Pinus monticola Dougl. ex D. Don PIMO

Douglas-fir Pseudotsuga menziesii (Mirb.) Franco PSME

Western redcedar Thuja plicata Donn ex D. Don THPL

Western hemlock Tsuga heterophylla Tsuga heterophylla (Raf.) Sarg. TSHE

Mountain hemlock Tsuga mertensiana Tsuga mertensiana (Bong.) Carr. TSME

Shrubs and herbs

Vanilla leaf Achlys triphylla (Sm.) DC. ACTR

Dwarf Oregon grape Berberis nervosa Pursh BENE

Queencup beadlily Clintonia uniflora (Menzies ex J.A. & J.H. Schultes) Kunth CLUN

Bunchberry Cornus canadensis L. COCA

Cascades azalea Rhododendron albiflorum Hook. RHAL

Rhododendron Rhododendron macrophyllum D.Don ex G.Don RHMA

Alaska huckleberry Vaccinium alaskaense T.J. Howell VAAL

Big huckleberry Vaccinium menbranaceum D. Don ex G. Don VAME

Beargrass Xerophyllum tenax (Pursh) Nutt. XETE

Table 5—Plant associations in true fir spacing trials

Plant associationsa

Pacific silver fir/Alaska huckleberry/queencup beadlily ABAM/VAAL/CLUN

Pacific silver fir/big huckleberry ABAM/VAME

Pacific silver fir/big huckleberry/queencup beadlily ABAM/VAME/CLUN

Pacific silver fir/Cascade azalea ABAM/RHAL

Pacific silver fir/rhododendron/queencup beadlily ABAM/RHMA/XETE

Pacific silver fir/vanilla leaf/queencup beadlily ABAM/ACTR/CLUN

Pacific silver fir/Alaska huckleberry/bunchberry ABAM/VAAL/COCA

Pacific silver fir/rhododendron/dwarf Oregon grape ABAM/RHMA/BENE

Pacific silver fir/dwarf Oregon grape ABAM/BENE

Mountain hemlock/big huckleberry/queencup beadlily TSME/VAME/CLUN

a Sources: Brockway and others 1983; Henderson and others 1989, 1992; Logan and others 1987.

True Fir Spacing Trials—10-Year Results
Table 6—Pretreatment descriptive statistics for the true fir study 

installations, trees 0.6 in diameter at breast height or greater, 

all species

Installation Trees/acre Basal area QMDa QMD80b H80c

No. Ft 2 /acre - - - - Inches - - - - Feet

Alpine #4 1,337 39.4 2.32 4.10 17.5

Bonidu 2,343 100.0 2.80 5.91 30.0

Cabin #2 796 21.7 2.25 3.35 15.8

Cat Creek 1,995 30.0 1.66 3.38 18.6

Crevice Creek 2,390 183.5 3.75 8.88 42.3

Cumberland 2,716 65.0 2.09 3.99 22.3

Dog Creek 1,849 44.2 2.09 4.09 20.8

Evans Creek 2,675 181.4 3.53 8.83 49.1

Haller Pass 742 143.6 5.96 8.49 45.5

Iron Mountain 1,453 47.8 2.46 4.83 25.3

Job #2 783 26.4 2.49 3.71 18.1

Marys Creek 1,134 23.4 1.95 3.39 17.2

Memaloose 1,721 49.0 2.29 4.60 20.2

North Mountain 976 39.3 2.72 4.85 22.8

Pointer #3 543 14.3 2.20 4.31 19.6

Rattrap Pass 1,296 72.2 3.25 5.60 30.0

Tonga Ridge 1,465 137.6 4.15 7.99 37.8

Twin #1 386 9.8 2.16 3.63 17.2

aQuadratic mean diameter at breast height.

bQMD at year 0 of those trees included in the largest 80 per acre at year 10.

cEstimated mean height at year 0 of those trees included in the largest 80 per acre at 

year 10.

GENERAL TECHNICAL REPORT PNW-GTR-749

10

ranged from a low of 386 trees per acre at Twin #1 to a high of 2,716 trees per acre 

at Cumberland Creek. Basal area per acre ranged from 10 ft2 at Twin #1 to 180 ft2

at Crevice Creek and Evans Creek. The QMD ranged from 1.66 in at Cat Creek to 

5.96 at Haller Pass.

Prethinning and postthinning composition by species, expressed as percentages of total basal area, are shown in table 7. Shifts in overall species composition 

caused by the thinning treatments were generally minor.

Remeasurement Schedule

All plots were remeasured at 5 and 10 years after establishment. Thereafter, they 

were placed on a 10-year remeasurement schedule, because of limitations of 

personnel and funding. Ingrowth trees 1.6 in diameter or greater were tagged 

and recorded.
Table 7—Mean percentage species composition of thinned plots by installation before thinning 

and immediately after thinning

Pretreatment Posttreatment

Installation ABAMa ABPRb TSHEc Other ABAM ABPR TSHE Other

Percent

Alpine #4 58.6 38.6 1.4 1.4 41.7 56.7 – 1.6

Bonidu 65.4 – 32.0 2.6 72.3 – 26.2 1.5

Cabin #2 1.6 86.4 .5 11.5 1.5 92.0 – 6.5

Cat Creek 75.0 4.3 2.1 18.6 80.4 10.7 4.1 4.8

Crevice Creek 24.5 – 73.9 1.6 27.6 – 72.3 .1

Cumberland 63.5 – 33.4 3.1 59.5 – 40.5 –

Dog Creek – 66.0 26.5 7.5 – 70.2 26.3 3.5

Evans Creek 80.2 .2 12.3 7.3 82.8 .1 10.9 6.2

Haller Pass – 91.8 .7 7.5 – 94.6 .2 5.2

Iron Mountain 84.6 – 14.2 1.2 86.4 – 13.0 .6

Job #2 9.3 80.5 .1 10.1 4.5 90.6 – 4.9

Marys Creek 3.3 67.1 11.4 17.9 1.7 80.1 5.4 12.8

Memaloose 5.4 93.5 .3 .8 9.4 90.6 – –

North Mountain 66.1 2.0 12.9 19.0 71.3 2.1 12.5 14.1

Pointer #3 31.2 66.7 .2 1.9 14.5 85.2 .3 –

Rattrap Pass 11.0 – 82.5 6.5 12.0 – 86.3 1.7

Tonga Ridge 83.7 11.7 4.3 .3 83.2 14.7 2.1 –

Twin #1 1.3 82.9 .4 15.4 .9 90.4 .1 8.6

aAbies amabilis

bAbies procera

cTsuga heterophylla

True Fir Spacing Trials—10-Year Results

11

Analyses

All comparisons made here are based on those initially tagged trees that survived to 

the 10-year remeasurement. Ingrowth, if any, is omitted. Trees that died during the 

10-year period of observation are omitted. Mortality over the period was generally 

slight, and on thinned plots, statistics for survivors at year 10 are not appreciably 

different from those for all leave trees. “Top height” is here defined as the mean 

height (H80) of trees included in the 80 largest (by d.b.h.) stems per acre.

Three principal analyses were made, for:

• All surviving trees, all species included.

• All surviving trees of all species that at year 10 were included in the largest 80 (by d.b.h.) stems per acre (i.e., the top height component).

• Possible effects of spacing on growth of those noble fir and Pacific silver 

fir that were included in the largest 80 trees per acre.
Table 8—Comparison of target numbers of 

leave trees with mean numbers at year 10

Nominal spacing Target Survivors, year 10

Feet - - - - - Number per acre - - - - - - 

No thinning – 1,575

7.9 700 598

10.1 430 387

12.8 265 256

16.4 163 158

20.9 100 94

GENERAL TECHNICAL REPORT PNW-GTR-749

12

All Trees

For each plot within each treatment, summary values were calculated for:

• Number of survivor trees present at year 10.

• Basal area of these trees at years 0 and 10.

• Quadratic mean diameter of these trees at years 0 and 10.

• Volume of these trees at years 0 and 10. 

Volume computations used the equations of Browne (1962). His equation 

for “coast balsam species” was used for both noble fir and Pacific silver fir.

Mean numbers of survivors present at year 10, by treatments, are shown 

in table 8.

Analyses for the “all survivors” component given below are in terms of 

simple graphics showing mean values. Trends are self-evident, without further 

statistical analyses.

Means of survivor basal areas, QMDs, and volumes by treatment, across all 

installations, are shown in figures 3 to 5. Means and standard deviations of basal 

area increments, QMD increments, and volume increments are given in table 9. 

Largest 80 Trees per Acre, All Species: Graphical Comparisons

The data show strong effects of spacing on growth in diameter, basal area, and 

volume when all trees present are considered. But many of these trees will not 

survive to maturity. Alternative comparisons are made here based on the 80 largest 

(by diameter) trees per acre. This number is somewhere near the number of trees 

expected to be present in the overstory as stands mature and is probably a number 

that can be expected to survive over a long period at the lowest stocking of 100 

residual trees per acre, and it provides reasonable samples for computation of 

plot values.
0

20

40

60

80

100

120

140

160

180

200

No thin 7.9 10.1 12.8 16.4 20.9

Spacing (feet)

Basal area (ft 2 / acre)

Year 0

Year 10

0

1

2

3

4

5

6

7

8

9

10

No thin 7.9 10.1 12.8 16.4 20.9

Spacing (feet)

QMD (inches)

Year 0

Year 10

Figure 3—Basal areas of all survivors at years 0 and 10, by treatment.

Figure 4—Quadratic mean diameters (QMD) of all survivors at years 0 and 10, 

by treatment.

True Fir Spacing Trials—10-Year Results

13
0

500

1000

1500

2000

2500

3000

3500

No thin 7.9 10.1 12.8 16.4 20.9

Spacing (feet)

Volume (ft 3 / acre)

Year 0

Year 10

Table 9—Means (and standard deviations) of 10-year increments

Spacing Basal area QMD Volume Height

Feet Ft 2/acre Inches Ft 3 /acre Feet

All surviving trees

Not thinned 101.2 (33.5) 1.94 (0.81) 2,093 (1032) —

7.9 77.3 (28.6) 2.62 (0.68) 1,619 (982) —

10.1 65.7 (25.7) 2.96 (0.89) 1,508 (1092) —

12.8 54.4 (20.4) 3.38 (0.86) 1,154 (682) —

16.4 43.6 (19.0) 3.82 (0.97) 899 (527) —

20.9 30.0 (13.3) 4.22 (1.20) 622 (381) —

Largest 80 trees per acre

Not thinned 21.0 (8.7) 3.28 (0.95) 521 (269) 17.35 (3.79)

7.9 22.1 (9.8) 3.53 (0.95) 548 (342) 16.92 (4.33)

10.1 24.6 (7.5) 3.93 (0.98) 571 (299) 16.05 (3.75)

12.8 24.4 (8.9) 3.95 (0.91) 583 (322) 16.49 (5.02)

16.4 26.5 (9.5) 4.14 (0.93) 526 (309) 15.34 (3.92)

20.9 27.3 (11.6) 4.36 (1.22) 588 (334) 15.47 (4.35)

QMD = quadratic mean diameter at breast height.

Figure 5—Volumes of all survivors at years 0 and 10, by treatment.

GENERAL TECHNICAL REPORT PNW-GTR-749

14
0

5

10

15

20

25

30

No thin 7.9 10.1 12.8 16.4 20.9

Spacing (feet)

10-year basal area increment (ft 2 / acre)

Figure 6—10-year basal area increment 

of largest 80 trees per acre, years 0 to 10, 

by treatment.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

No thin 7.9 10.1 12.8 16.4 20.9

Spacing (feet)

10-year QMD increment (inches)

Figure 7—10-year quadratic mean 

diameter (QMD) increment of largest 80 

trees per acre, years 0 to 10, by treatment.

True Fir Spacing Trials—10-Year Results

15

Mean 10-year increments in basal areas (B80), QMDs (QMD80), heights (H80), 

and volumes (V80) for all installations combined are shown in figures 6, 7, 8 and 9 

and table 9, for thinned and unthinned treatments. There appear to be considerable 

increases in basal area increment and diameter increment with increasing spacing 

(decreasing number of trees), little effect of spacing on volume increment, and a 

slight decrease in height increment with increasing spacing.

Figures 10, 11, 12, and 13 show individual installation trends in basal area 

increment, QMD increment, height increment, and volume increment in relation to 
10

11

12

13

14

15

16

17

18

No thin 7.9 10.1 12.8 16.4 20.9

Spacing (feet)

10-year height increment (feet) 300

350

400

450

500

550

600

650

No thin 7.9 10.1 12.8 16.4 20.9

Spacing (feet)

10-year volume increment (ft 3 / acre)

Figure 8—10-year height increment of largest 80 trees per acre, years 0 to 10, 

by treatment.

Figure 9—10-year volume increment of largest 80 trees per acre, years 0 to 10, 

by treatment.

GENERAL TECHNICAL REPORT PNW-GTR-749

16

the overall trends represented by the heavy line, for thinned treatments only. There 

are large differences in curve elevations among installations, and in some installations there are also considerable irregularities in trend with spacing. Overall, the 

plots suggest that the individual installation trends could be regarded as straight 

lines more or less parallel to the overall mean but different in elevation.
0

10

20

30

40

50

60

5 7 9 11 13 15 17 19 21 23

Spacing (feet)

10-year basal area increment (ft 2 / acre)

Iron Mountain

Crevice

Rattrap

Cumberland

Tonga

Haller

Evans Creek

Bonidu

Cat Creek

Alpine

Dog Creek

Pointer

Job

Memaloose

North Mountain

Cabin

Twin

Marys Creek

Overall mean

Figure 10—10-year basal area increment of largest 80 trees per acre, years 0 to 10, by installation.

True Fir Spacing Trials—10-Year Results

17

Increment of Largest 80 Trees per Acre, All Species: 

Statistical Comparisons

Basal area increment (∆B80) of largest 80 trees per acre, all species—

Individual installation curves of ∆B80 over spacing differ widely in curve elevation 

(fig. 10). Slopes also differ somewhat among installations. Although much variation 

is present, in general ∆B80 increased with spacing (table 9).

An overall regression not including the effect of blocking provided 

the equation:

∆B80 = 19.83 + 0.3831(spacing) (1)

with R2 = 0.035. This closely approximates the heavy line in figure 10.

Statistical significance of an average slope, specifically the hypothesis that 

a linear trend over spacing differs from zero, was then tested using orthogonal 

contrasts for linear trend across the levels of the spacing factor and accounting for 

installations as a blocking factor, using the GLM (general linear models) procedure 

of SAS (SAS Institute 1999). The test showed a significant linear trend across the 

levels of spacing (p < 0.01). Installation and spacing together accounted for 0.85 of 

the total sum of squares.

Spacing affects 

development of 

dominant trees.
0

1

2

3

4

5

6

7

8

5 7 9 11 13 15 17 19 21 23

Spacing (feet)

10-year QMD increment (inches)

Iron Mountain

Crevice

Rattrap

Cumberland

Tonga

Haller

Evans Creek

Bonidu

Cat Creek

Alpine

Dog Creek

Pointer

Job

Memaloose

North Mountain

Cabin

Twin

Marys Creek

Overall mean

Figure 11—10-year increment in quadratic mean diameter (QMD) of largest 80 trees per acre, years 0 to 10, by installation.

GENERAL TECHNICAL REPORT PNW-GTR-749

18

There is a real effect of spacing across installations.

Quadratic mean diameter increment of largest 80 (∆QMD80) trees

per acre, all species—

Figure 11 and table 9 indicate that on average, diameter increment increased 

with spacing, as would be expected. Again, there is a great deal of variation 

among installations.

An overall regression not including the effect of blocking provided 

the equation:

∆ QMD80 = 3.28 + 0.05249(spacing) (2)

with R2 = 0.06.

This closely approximates the heavy line connecting spacing means in 

figure 11.

A similar analysis to that used for basal area increment, including installation 

as a blocking factor, showed a significant linear trend with spacing (p < 0.01). 

Installation and spacing together accounted for 0.83 of the total sum of squares.
0

5

10

15

20

25

30

5 7 9 11 13 15 17 19 21 23

Spacing (feet)

10-year height increment (feet)

Iron Mountain

Crevice

Rattrap

Cumberland

Tonga

Haller

Evans Creek

Bonidu

Cat Creek

Alpine

Dog Creek

Pointer

Job

Memaloose

North Mountain

Cabin

Twin

Marys Creek

Overall Mean

Figure 12—10-year height increment of largest 80 trees per acre, years 0 to 10, by installation.

True Fir Spacing Trials—10-Year Results

19

Top height increment (∆H80) of largest 80 trees per acre, all species—

Overall means (fig. 8, 12, table 9) suggest a slight decline in ∆H80 with 

increasing spacing.

An overall regression not including the effect of blocking provided 

the equation:

∆H80 = 17.32 - 0.09644(spacing) (3)

with R2 = 0.01.

An analysis similar to the preceding ones, including installation as a blocking 

factor, showed a significant trend of height increment with spacing (p = 0.046). 

Installation and spacing together accounted for 0.80 of the total sum of squares.

Thus, 10-year height increment of the largest 80 trees decreased with increase 

in spacing, although this effect was not large (fig. 12).

Volume increment (∆V80) of largest 80 per acre, all species—

Figure 9 suggests that there is little if any overall relationship of 10-year ∆V80 to 

initial spacing in thinned plots. As with other variables, there is a great deal of 

variation among and within installations (fig. 13). 
0

200

400

600

800

1000

1200

1400

1600

5 7 9 11 13 15 17 19 21 23

Spacing (feet)

10-year volume increment (ft 3 / acre)

Iron Mountain

Crevice

Rattrap

Cumberland

Tonga

Haller

Evans Creek

Bonidu

Cat Creek

Alpine

Dog Creek

Pointer

Job

Memaloose

North Mountain

Cabin

Twin

Marys Creek

Overall Mean

Figure 13—10-year volume increment of largest 80 trees per acre, years 0 to 10, by installation.

GENERAL TECHNICAL REPORT PNW-GTR-749

20

An analysis similar to that for basal area and for diameter increment, including 

installation as a blocking factor, showed no significant effect of spacing on volume 

increment. Although 0.90 of the sum of squares was accounted for, this was almost 

entirely due to installation.

The overall mean, ∆V80 = 572 ft3/ac, closely approximates the spacing means 

as shown by the heavy line in figure 13.

Comparisons by Individual Species

The above comparisons take no account of differences in species composition 

among installations and among plots within installations. It seemed of interest to 

compare growth by species, where the data allow. The dependent variables used 

here are means of those trees of the given species that are included in the largest 80 

per acre.

Noble fir—

Data used were from those installations having noble fir present in the largest 80 

component on at least 4 of the 5 thinned plots. 
0

5

10

15

20

25

30

0 5 10 15 20 25

Spacing (feet)

10-yr height increment (feet)

Haller

Alpine

Dog Creek

Pointer

Job

Memaloose

Cabin

Twin

Marys Creek

Overall mean

Figure 14—10-year height increment of noble fir included in the largest 80 trees per acre present at year 10, by 

installation and treatment.

True Fir Spacing Trials—10-Year Results

21

Figure 14 shows the relation of unweighted means of noble fir 10-year height 

increment to spacing, for the nine installations meeting the above criteria. The 

overall means seem to show a slight decline with increasing spacing although there 

is a great deal of variation present. Differences in curve elevation are presumably 

due mainly to site differences, which we cannot express numerically until stands 

develop further. Much variation also arises from the fact that the sample available 

on each treatment within an installation can range from 1 tree to a possible maximum of 29 trees in the widest spacing.

Because the number of available trees of the species differs widely among plots, 

weights equal to the square root of the number of trees were used in calculating an 

overall regression, not including blocking as a factor. This weighted regression was:

∆H80 = 20.3578 - 0.12092 (spacing), (4)

with R2 = 0.03.

An analysis similar to that for the largest 80 of all species, including installation 

as a blocking factor, showed a marginally significant linear trend with spacing

(p = 0.055). Installation and spacing together accounted for 0.73 of the total sum 

of squares.
0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

0 5 10 15 20 25

Spacing (feet)

10-year QMD increment (inches)

Haller

Alpine

Dog Creek

Pointer

Job

Memaloose

Cabin

Twin

Marys Creek

Overall mean

Figure 15—10-year diameter increment of noble fir included in the largest 80 trees per acre present at year 10, by 

installation and treatment.

GENERAL TECHNICAL REPORT PNW-GTR-749

22

Thus, although the data suggest that there may still be some reduction in height 

increment with increase in spacing, this is relatively small and much less than at the 

5-year analysis (Curtis and others 2000).

Figure 15 shows diameter increment by installation and treatment. An analysis 

similar to that for height increment showed that average slope was positive and 

significantly different from zero (p < 0.01).

Pacific silver fir—

Figure 16 shows trends of ∆H80 over spacing, for those installations having silver 

fir present in at least four of the five treatments. The heavy line represents overall 

means and suggests that height increment decreases with increasing spacing, 

although there is considerable variation in trends among installations and among 

spacings within installations.

An overall weighted regression of height increment on spacing, not including 

the block effect, gave:

∆H80 = 15.1715 - 0.2322(spacing), (5)

with R2 = 0.10.
0

5

10

15

20

25

0 5 10 15 20 25

Spacing (feet)

10-yr height increment (feet)

Iron Mountain

Crevice Creek

Cumberland

Tonga

Evans Creek

Bonidu

Cat Creek

Alpine

North Mountain

Overall mean

Figure 16—10-year height increment of Pacific silver fir included in the largest 80 trees per acre present at year 10, by 

installation and treatment.

True Fir Spacing Trials—10-Year Results

23

An analysis similar to that for the largest 80 trees of all species, including 

installation as a blocking factor, showed a significant linear effect of spacing on 

height increment (p < 0.01). Installation and spacing together accounted for 0.86 of 

the total sum of squares. 

Clearly, height increment has been less at the wider spacings, and the effect is 

greater than is the case with noble fir.

Figure 17 shows trends of diameter increment with spacing, by installation. An 

analysis similar to that for height increment showed that average slope was positive 

and significantly different from zero (p < 0.01). 

Western hemlock—

Only five installations had sufficient hemlock present in the largest 80 trees per acre 

across spacings to allow any comparisons. The small amount of data does not allow 

any clear conclusions, but there is no indication of any relation between height 

increment and spacing. The data do indicate that diameter increment of hemlock 

included in the 80 largest trees per acre increased with spacing.

Wide spacing 

reduced silver fir 

height growth.
0

1

2

3

4

5

6

7

0 5 10 15 20 25

Spacing ( feet)

10-year QMD increment (inches)

Iron Mountain

Crevice Creek

Cumberland

Tonga

Evans Creek

Bonidu

Cat Cr

Alpine

North Mountain

Overall mean

Figure 17—10-year diameter increment of Pacific silver fir included in the largest 80 trees per acre present at year 10, 

by installation and treatment.

GENERAL TECHNICAL REPORT PNW-GTR-749

24

Comparative growth by species—

Average 10-year height growth of trees included in the largest 80 per acre, by species, was calculated as arithmetic means of the plot values for those plots included 

in the previous comparisons. These values were:

Standard 

∆H80 deviation Range

Feet

Noble fir 18.7 3.55 11.4-24.9

Pacific silver fir 12.3 3.61 6.2-20.2

Western hemlock 16.8 1.67 14.0-20.2

A high proportion of the noble fir was of plantation origin, whereas many silver 

fir and all of the hemlock were from natural seeding. These differences in origin 

may accentuate the growth differences, but it is clear that growth of dominant noble 

fir has been superior to growth of comparable silver fir.

Noble fir height 

growth exceeds 

associated species.
0

10

20

30

40

50

60

70

0 5 10 15 20 25 30 35 40 45

Breast-high age

Height of 80 largest trees (feet)

Iron Mountain

Crevice

Rattrap

Cumberland

Tonga

Haller

Evans Creek

Bonidu

Cat Creek

Alpine

Dog Creek

Pointer

Job

Memaloose

North Mountain

Cabin

Twin

Marys Creek

Figure 18—Mean height of 80 largest trees per acre (H80) of all species in relation to age at breast height (age since 

reaching breast height).

True Fir Spacing Trials—10-Year Results

25

Top Height (H80) in Relation to Breast-High Age, 

All Species Combined

Figure 18 shows the values of H80 at years 0 and 10 after establishment, in relation 

to breast-high age (years since attaining 4.5 feet height). It is evident that there are 

wide differences in curve elevation and slope, which presumably are largely an 

expression of site differences although possibly somewhat modified by differences 

in species composition.

Discussion

Installation (Block) Effects and Spacing Effects

Comparison of individual installation trends in basal area increment, QMD increment, and volume increment of thinned plots with overall means shows wide 

differences (figs. 10 through 13) in position of individual installation curves relative to the mean curve, and considerable differences among treatments (spacings) 

within installations. Differences among installations are far greater than differences 
GENERAL TECHNICAL REPORT PNW-GTR-749

26

among spacings. This is true for both all trees and for the largest 80. Installation 

differences arise from several causes:

• Inherent differences in environment and in site productivity.

• Differences in species composition.

• Differences in stage of stand development (height, basal area, QMD, age) 

at which installations were established.

Although inherent site differences are probably the largest component of these 

differences, differences in stage of development at plot establishment may also be a 

factor. The true firs typically have slow early development followed by a period of 

accelerating growth. Some installations were established during the early phase of 

slow height growth, whereas others were well into the phase of accelerated growth. 

An additional though probably minor factor that may contribute to deviations 

from the overall trend within an installation is that, although treatments are defined 

in terms of number of residual trees, and for plotting purposes the corresponding 

spacing is here treated as equivalent, the actual number of trees is not identical with 

the target number. Generally, it is somewhat less although not very different (table 

8). The principal cause, aside from minor errors in plot establishment and occasional mortality, is the existence of empty cells in the grid used to select leave trees 

in these often somewhat patchy stands. The differences are thus greater at narrow 

spacing than at wider spacing.

Now, to summarize the results of analyses to date:

All Survivors

Trends for all survivors are generally similar in kind to those found in early results 

from many similar experiments with other species. Namely:

• Basal area increment declines with increasing spacing.

• Diameter increment increases with increasing spacing.

• Volume increment decreases with increasing spacing.

The marked increase in QMD (fig. 4) with increasingly wider spacing is in 

part the result of the removal of generally smaller trees in thinning—the so-called 

“chainsaw-effect.” 

The lower basal area and volume increments at wider spacing simply reflect 

the fact that at 10 years after treatment, the wider spacings are still quite open and 

do not fully occupy the site. Presumably this will change over time. Much of the 

basal area and volume growth at close spacing is on trees that will not survive to 

maturity. Therefore, the above trends may not be very meaningful for long-term 

management.
0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

0 5 10 15 20 25

Spacing ( feet)

Ratio of increment to increment of 7.9-ft spacing

Basal area

QMD

Volume

Height

Figure 19—Basal area, quadratic mean diameter (QMD), volume, and height of the largest 80 trees per acre expressed 

as ratios to the corresponding value for the 7.9-ft spacing.

True Fir Spacing Trials—10-Year Results

27

Largest 80 Trees Per Acre

Trees included in the largest 80 per acre at this stage are mostly dominants, and 

most can be expected to survive for many years. Trends shown by this stand component are therefore much more meaningful in stand management.

For this more restricted stand component, spacing still had considerable effects 

on growth, although these were of much smaller magnitude than those for all trees 

and were not always in the same direction. Namely:

• Basal area increment increased somewhat with increase in spacing.

• Diameter increment increased with spacing.

• Height increment declined with increase in spacing.

• Volume increment had little relation to spacing at this point. 

These relationships are illustrated in figure 19, which expresses values relative 

to values of the variable in question for the 7.9-ft spacing. 

At 10 years after thinning, these stands are still relatively open. As they develop 

and competition intensifies, it can be expected that effects of spacing on diameter, 

basal area, and volume growth will increase.
GENERAL TECHNICAL REPORT PNW-GTR-749

28

Reduction in height increment after thinning in other species has sometimes 

been referred to as “thinning shock.” The term seems inappropriate in the present 

case, because the trees have been growing vigorously. The effect of reduced height 

increment on volume increment has apparently been offset by increased basal 

area increment.

Individual Species Comparisons: Trees Included in 

the Largest 80

The samples available for individual species comparisons are much smaller, both in 

number of installations with trees of the species present in all or most treatments, 

and in number of sample trees available by treatment and installation.

Noble fir—

Data suggest a small decline in height increment with increase in spacing, although 

the effect is much less than that found at the 5-year measurement (Curtis and 

others 2000).

Trends of diameter growth in relation to spacing are fairly consistent and 

indicate a moderate increase in diameter growth with increase in spacing.

Pacific silver fir—

Data show a statistically significant decline in height increment with increased 

spacing. This was also present at the 5-year measurement. (A similar decline was 

noted in Husted and Korelus 1982).

There is also a fairly strong trend of increasing diameter growth with 

increasing spacing.

Western hemlock—

With only five installations available for comparisons, little can be said beyond the 

apparent facts that (1) there is no indication of a trend of height increment in relation to spacing, and (2) there is a clear increase in diameter increment with spacing.

Future Analyses

Periodic increment in basal area (and QMD and volume) should be a function of 

some combination of (1) spacing; (2) initial postthinning condition, perhaps expressible by top height, basal area, or age; and (3) top height increment. Regressions 

involving these variables do indeed account for a substantial part of the variation. 

But, they are not readily interpretable because of the interrelationships among these 

variables. Purely empirical predictive equations for the first 10 years of growth do 

not seem useful.
True Fir Spacing Trials—10-Year Results

29

Site quality—

It is obvious from the various graphs presented that there is a great amount of variability in the data. Differences among installations are far greater than those among 

spacings within installations and represent differences in site productivity and age, 

possibly somewhat modified by differences in species composition.

Site productivity differences would be best represented by comparative heights 

at a specified reference age, interpretable as site index values. At present there is 

no satisfactory basis for calculating these. It is clear (fig. 18) that there are wide 

differences in height development among installations. Although site curves exist, 

the curves now available (Herman and others 1978, Hoyer and Herman 1989) 

were derived from stem analyses of old trees whose conditions of origin and early 

development may have been considerably different from those of the present. They 

may not be a satisfactory representation of development of these young stands. 

When 20-year data become available (2014 as now scheduled), heights at a 

common breast-high age of 25 years will be available for all installations, via 

reasonable interpolation or extrapolation. These could be used as site indices in 

analyses and should account for a large part of the differences among installations. 

They should provide quantitative values useful as predictors of growth and directly 

interpretable by plant association, and possibly in relation to other environmental 

variables. It will also then be possible to compare actual observed development of 

these young stands with that predicted by the existing site curves and to determine 

applicability of the existing curves to young stands. With four points available for 

growth curves, it should be possible to compare observed growth rates in basal 

area, QMD, height, and volume at this common age with site index, initial spacing, 

and alternative measures of stocking.

Stand structure and understory development—

Curtis and others (2000) gave some limited information on differences in understory vegetation composition and development at several of the older installations. 

There has been some further data collection by Henderson on the MBS and OLY 

Forests, but as yet no subsequent published analysis. Further measurements of 

ground vegetation and associated species may also lead to better understanding of 

the effect of environmental factors (geology, soils, precipitation, and temperature 

regimes) beyond simple site index estimates. Data are being collected on development of tree species ingrowth, although with only 10 years of record, there has been 

no analysis of this aspect as yet. 

2Source: Henderson, J.A. Potential natural vegetation zones of Washington. Manuscript 

in preparation.

Installations differ 

widely in productivity.
GENERAL TECHNICAL REPORT PNW-GTR-749

30

A very important aspect of the study is the information it can provide on successional trends and the effects of early stocking control on later stand development 

and stand structure. Early stocking control clearly accelerates the development of 

large trees, and different residual numbers of trees have a strong influence on the 

amount of ingrowth and its rate of development, and on composition and vigor 

of understory vegetation. Ingrowth at the wider spacings has been abundant, and 

will likely eventually produce layered stands with structures differing considerably from those developing at close spacings. These differences are likely to be 

important for wildlife, recreation, and timber production objectives. No quantitative 

comparisons have been made here because of the short (10-year) period of record, 

but development of ingrowth should be addressed when the 20-year measurements 

become available.

As might be expected, the differences in residual stand density have strong 

effects on composition and vigor of understory vegetation. There are also strong 

effects on huckleberry (Vaccinium spp.) production (important to recreationists 

and local native American communities). Differences in understory composition 

and stand structure will need evaluation from the standpoints of wildlife biology, 

biodiversity, and old-growth restoration. 

Some of these differences are illustrated by the Iron Mountain installation, 

which is the only one for which we have 20-year measurements at this time. The 

unthinned plots are extremely dense, with little understory remaining (fig. 20). The 

intermediate spacing is approaching full canopy cover while retaining understory 

vegetation (fig. 21). The very wide spacings are still quite open and have a dense 

shrub understory (fig. 22) and a developing younger cohort of conifers (fig. 23).

These subjects should be addressed for the overall series at some time in the future. It will require a major effort in addition to the normal stand remeasurements, 

and will need active participation by a plant ecologist. At this time, personnel and 

funding limitations prevent addressing these questions, but they should have high 

priority if and when resources become available.

Up to this point, mortality has been minor and has not been considered in these 

analyses. Mortality will probably not remain minor in the future, and will also need 

attention in the 20-year analysis.

Future of the Study

Although the number of installations is only about half that originally planned, 

the study still constitutes a unique source of information on growth patterns and 

development of true fir-hemlock stands over a range of densities. Its value will 

increase rapidly with advancing age. The data should be extremely valuable as a 

Spacing affects 

understory 

development.
Figure 20—Unthinned 

plot at Iron Mountain 

at the 20-year 

remeasurement.

Figure 21—

10.1-ft spacing at Iron 

Mountain at the 20-year 

remeasurement.

True Fir Spacing Trials—10-Year Results

31
Figure 22—Wide spacing (16.4 ft) at Iron Mountain at the 20-year remeasurement has 

developed large trees and a dense shrub understory (mostly Vaccinium alaskaense).

GENERAL TECHNICAL REPORT PNW-GTR-749

32

part of that needed to construct reliable growth models for these species that will be 

applicable to a range of management regimes and objectives.

There are a million acres or more of young (<70 years) Pacific silver fir zone 

forests in Oregon and Washington (see footnote 1). Under current management 

direction, many of these forests are being managed as old-growth reserves, yet 

there is almost no information about successional development of these forests 

or about how they could be managed to restore old-growth conditions, if desired. 
Figure 23—20.9-ft spacing at Iron Mountain at the 20-year remeasurement, showing shrub understory and 

developing younger cohort of conifers.

True Fir Spacing Trials—10-Year Results

33

There is very little research about how to restore targeted biodiversity to this 

landscape, about how these young stands might differ from their predecessors, or 

how they may develop under possible future climatic change. Future measurements 

of projects such as this one are needed to meet the substantial information gap for 

management of these forests.

At present, we lack quantitative and site-specific information on growth rates, 

patterns of development, potential yields, and effects of active management. This 

is—to our knowledge—the only study of the kind in existence. Without long-term 

growth data such as this study can provide, there is little basis for evaluating the 

productive potential of such stands under management or for construction of management guides or growth models applicable to any kind of active management. 

Additionally, these installations provide an opportunity to assess the effects of a 

wide range of tree densities on composition and growth of understory vegetation; 

which are important in these upper elevation forests, where berry picking, hunting, 

and wildlife and wildflower viewing are significant recreational activities.

The present publication is a progress report detailing the present status of the 

study and a few preliminary comparisons. A much more thorough analysis will 

These installations 

provide a unique 

information source.
GENERAL TECHNICAL REPORT PNW-GTR-749

34

only become possible with completion of the 20-year remeasurements, expected 

in 2014. It is therefore important that these remeasurements be carried out 

as scheduled.

There are major obstacles at present. Aside from immediate limitations in 

personnel and funding, the recent deemphasis of timber production as an objective 

on the national forests means a loss of the strong support that originally made the 

study possible. Another obstacle, potentially serious, is that the current program 

of road closures may make some installations inaccessible. Yet, all history indicates that policies and needs will change over time. Although some people tend to 

dismiss the importance of these high-elevation forests for timber production, these 

forests include a wide range of potential productivity and some stands are highly 

productive. There is no substitute for reliable information on development patterns 

and silvicultural responses as a basis for future decisions on management options.

We cannot anticipate all future information needs, but all past experience 

indicates that long-term studies such as this have great value for a variety of 

purposes, including some not envisioned at the time of establishment. It is therefore 

important that measurement of this study be continued at regular intervals for the 

next several decades. 

Acknowledgments 

Helpful reviews and suggestions were provided by Jan Henderson (Mount BakerSnoqualmie National Forest), Rolfe Gersonde (Seattle Public Utilities), and Dave 

Peter (PNW Research Station). Tim Max (PNW Research Station) provided guidance on the statistical analysis. Connie Harrington (PNW Research Station) provided photographs. Grace Haight and Joe Kraft assisted in final preparation of the 

manuscript and figures.

Metric Equivalents

When you have: Multiply by: To find:

Inches (in) 2.54 Centimeters

Feet (ft) 0.3048 Meters

Miles (mi) 1.609 Kilometers

Square feet (ft2) 0.929 Square meters

Acres (ac) 0.405 Hectares

Trees per acre 2.471 Trees per hectare

Square feet per acre (ft2/ac) 0.229 Square meters per hectare

Cubic feet per acre (ft3/ac) 0.07 Cubic meters per hectare
True Fir Spacing Trials—10-Year Results

35

Literature Cited

Brockway, D.G.; Topik, C.; Hemstrom, M.A.; Emmingham, W.H. 1983.

Plant association and management guide for the Pacific silver fir zone: Gifford 

Pinchot National Forest. R6-Ecol-130a-1983. Portland, OR: U.S. Department of 

Agriculture, Forest Service, Pacific Northwest Region. 122 p.

Browne, J.E. 1962. Standard cubic-foot volume tables for the commercial species 

of British Columbia, 1962. Victoria, BC: British Columbia Forest Service. 107 p.

Curtis, R.O.; Clendenen, G.W.; Henderson, J.A. 2000. True fir-hemlock spacing 

trials: design and first results. Gen. Tech. Rep. PNW-GTR-492. Portland, OR: 

U.S. Department of Agriculture, Forest Service, Pacific Northwest Research 

Station. 35 p.

Eyre, F.H. 1980. Forest cover types of the United States and Canada. Washington, 

DC: Society of American Foresters. 148 p.

Franklin, J.F.; Dyrness, C.T. 1973. Natural vegetation of Oregon and Washington. 

Gen. Tech. Rep. PNW-8. Portland, OR: U.S. Department of Agriculture, Forest 

Service, Pacific Northwest Forest and Range Experiment Station. 417 p.

Harrington, C.A.; Murray, M.D. 1982. Patterns of height growth in western 

true firs. In: Oliver, C.D.; Kenady, R.M., eds. Proceedings of the biology and 

management of true fir in the Pacific Northwest symposium. Institute of Forest 

Resources, Contrib. 45. Seattle, WA: University of Washington, College of Forest 

Resources: 209–214.

Hemstrom, M.A.; Emmingham, W.H.; Halverson, N.M.; Logan, S.E.; Topik, 

C. 1982. Plant association and management guide for the Pacific silver fir zone, 

Mt. Hood and Willamette National Forests. R6-ECOL-100-1982A. Portland, OR: 

U.S. Department of Agriculture, Forest Service. Pacific Northwest Region. 

104 p.

Henderson, J.A.; Lesher, R.D.; Peter, D.H.; Shaw, D.C. 1992. Field guide to the 

forested plant associations of the Mt. Baker-Snoqualmie National Forest. Tech. 

Pap. R6 ECOL TP 028-91. Portland, OR: U.S. Department of Agriculture, Forest 

Service, Pacific Northwest Region. 196 p.

Henderson, J.A.; Peter, D.H. 1984. Preliminary plant associations and habitat 

types of the Mt. Baker Ranger District, Mt. Baker-Snoqualmie N.F. Portland, 

OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Region. 

60 p.
GENERAL TECHNICAL REPORT PNW-GTR-749

36

Henderson, J.A.; Peter, D.H.; Lesher, R.D.; Shaw, D.C. 1989. Forested plant 

associations of the Olympic National Forest. Tech. Pap. R6 Ecol. 001-88. 

Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest 

Region. 502 p.

Herman, F.R.; Curtis, R.O.; DeMars, D.J. 1978. Height growth and site index 

estimates for noble fir in high-elevation forests of the Oregon-Washington 

Cascades. Res. Pap. PNW-243. Portland, OR: U.S. Department of Agriculture, 

Forest Service, Pacific Northwest Forest and Range Experiment Station. 15 p.

Hoyer, G.E.; Herman, F.R. 1989. Height-age and site index curves for Pacific 

silver fir in the Pacific Northwest. Res. Pap. PNW-RP-418. Portland, OR: U.S. 

Department of Agriculture, Forest Service, Pacific Northwest Research Station. 

33 p.

Husted, L.; Korelus, V. 1982. Management of true fir species for timber 

production—south coastal British Columbia and the Olympic Peninsula. In: 

Oliver, C.D.; Kenady, R.M., eds. Proceedings of the biology and management 

of true fir in the Pacific Northwest symposium. Institute of Forest Resources, 

Contrib. 45. Seattle, WA: University of Washington, College of Forest Resources: 

277–283.

Logan, S.E.; Hemstrom, M.A.; Paviat, W. 1987. Plant association and 

management guide—Willamette National Forest. R6-Ecol 257-A-86. Portland, 

OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Region. 

223 p.

Murray, M.D.; Coble, D.; Curtis, R.O. 1991. Height growth of young Pacific 

silver fir and noble fir established on clearcuts in the Pacific silver fir zone of 

western Washington. Canadian Journal of Forest Research. 21: 1213–1221.

Oliver, C.D.; Kenady, R.M., eds. 1982. Proceedings of the biology and 

management of true fir in the Pacific Northwest symposium. Institute of Forest 

Resources, Contrib. 45. Seattle, WA: University of Washington, College of Forest 

Resources. 344 p.

SAS Institute. 1999. SAS/STAT users guide. Version 8. Cary, NC: Statistical 

Analysis Systems Institute, Inc. 3884 p.


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