United States

Department of

Agriculture

Forest Service

Pacific Northwest

Research Station

Research Paper

PNW-RP-574

November 2007

Growth of Douglas-fir Near Equipment

Trails Used for Commercial Thinning

in the Oregon Coast Range

Richard E. Miller, Jim Smith, Paul W. Adams, and Harry W. Anderson
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Authors

Richard E. Miller is an emeritus soil scientist and Harry W. Anderson is a forester, U.S. Department of Agriculture, Forest Service, Pacific Northwest Research

Station, 3625 93rd Ave. SW, Olympia, WA 98512-9393; Jim Smith is a forester,

Plum Creek Timberlands, L.P., Cottage Grove, OR 97424; and Paul W. Adams is

a professor, College of Forestry, Oregon State University, Corvallis, OR 97331.
Abstract

Miller, Richard E.; Smith, Jim; Adams, Paul W.; Anderson, Harry W. 2007.

Growth of Douglas-fir near equipment trails used for commercial thinning in

the Oregon Coast Range. Res. Pap. PNW-RP-574. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 33 p.

Soil disturbance is a visually apparent result of using heavy equipment to harvest

trees. Subsequent consequences for growth of remaining trees, however, are variable

and seldom quantified. We measured tree growth 7 and 11 years after thinning of

trees in four stands of coast Douglas-fir (Pseudotsuga menziesii var. menziesii

(Mirb. Franco)) where soil disturbance was limited by using planned skid trails,

usually on dry soils. The three younger stands had responded to nitrogen fertilizer

in the 4 years before thinning, but only one stand showed continued response in the

subsequent 7- or 11-year period after thinning. The most consistent pattern observed

was greater growth of residual trees located next to skid trails. The older stand also

showed greater growth in trees located next to skid trails, whereas tillage of skid

trails failed to benefit growth of nearby residual trees for the first 7 years after

tillage. We conclude that traffic that compacted soil only on one side of residual

trees did not reduce growth of nearby trees.

Keywords: Commercial thinning, coast Douglas-fir, equipment trails, tree

growth, mitigation.

Summary

Tree growth after commercial thinning is influenced by initial tree size, degree of

release when neighboring trees are removed, subsequent competition among residual trees, and soil disturbance from heavy equipment.

Our investigations at three locations in the Coast Range of western Oregon

sought to answer two questions: Did proximity to trails used by various groundbased logging vehicles affect subsequent growth of coast Douglas-fir (Pseudotsuga

menziesii var. menziesii (Mirb. Franco)) trees? At two of these locations, we posed

two additional questions: Did past nitrogen (N) fertilization continue to increase

growth or influence tree response to equipment traffic?

At Toledo South and Bone Mountain, we investigated response of 20-year-old

trees to thinning in plantations of uniformly spaced Douglas-fir. Four years before

these thinnings, a fertilization treatment was assigned randomly to 5 of the 10 plots

at Toledo South and to 3 of the 6 plots at both Bone Mountain A and B. Urea

fertilizer providing 200 lb N/acre was uniformly spread over assigned plots. These
plantations were commercially thinned after the 1993 growing season. Slash was

accumulated and retained on the traffic routes. At Toledo South, the first and every

seventh row were removed to provide routes for equipment traffic. In alternate rows

between these routes, all trees were cut and removed by extending the harvester

boom. No trees were cut in the remaining even-numbered rows, which included

rows beside the travel routes. Stand density was reduced from about 600 trees per

acre (TPA) to about 300 TPA. At Bone Mountain A and B, every fifth row of the

original planting was removed to provide traffic routes for a tracked harvester and

a forwarder. Some additional trees in intervening rows were cut and removed by

extending the harvester boom. Consequently, tree removal was not uniform as in the

purely row-thinning at Toledo South. The original density of 680 TPA was reduced

to about 260 TPA (62 percent cut).

After thinning, residual trees were identified in two categories: those located

beside traffic routes (position 1) and those located in the interior between traffic

routes (position 2). Because some within-row-trees were removed at Bone Mountain

A and B, residual trees in position 1 and position 2 had varying numbers of neighboring or competing trees, ranging from zero to four of the initial eight. On all

plots, the number of competing neighbors was fewer in rows beside the trails

(position 1) in these two plantations.

Effects of commercial thinning on mean tree breast-height (b.h.) growth (overbark cross-sectional or basal area) were estimated and compared by a fixed-effects

analysis of variance (ANOVA) or covariance (COVAR) analyses. An alpha value of

0.10 was used to judge statistical significance.

The position x fertilizer interaction was statistically nonsignificant at Toledo

South and Bone Mountain A and B. We infer that tree response to trails was independent of earlier fertilization. Residual effects of the earlier fertilization remained

statistically significant after thinning only at Bone Mountain A (12-percent increase

over control), but was not significant at Bone Mountain B and Toledo South (4 percent more and 1 percent less than nonfertilized, respectively). At Bone Mountain A,

however, inferences from statistical tests about continued response to earlier fertilization may be confounded by fewer residual trees on fertilized versus nonfertilized

plots.

Because slash was deliberately placed on equipment trails, rutting was infrequent and shallow (<3 in). Therefore, we assume that root crushing or severing was

not likely to have reduced growth of trees bordering trails. Bole damage also was

inconsequential. Trees next to trails at Toledo South averaged about 3 percent faster

b.h. growth than trees located farthest from the trails (p = 0.10). At Bone Mountain

A and B, b.h. growth of trees next to trails averaged about 14 and 11 percent more,
respectively, than growth of more distant trees. We speculate that this faster growth

is explained by greater release and fewer competing trees near position 1 trees, and

those advantages more than offset any negative effects of equipment traffic at Bone

Mountain A and B.

The Noti study is in a stand of 58-year-old Douglas-fir that averaged 206 TPA

before thinning. Thinning reduced stand density to 70 TPA (66 percent cut) and

produced harvested logs averaging 10 inches in diameter. Logging that began in

late July was completed by late August. Hot and dry weather reduced soil moisture

to less than 20 percent (weight basis) on all skid trails. Whole trees and logs were

skidded to roadside landings at the end of seven parallel, planned skid trails spaced

about 150 ft apart. In early September 1992, four of these skid trails were tilled

with a self-drafting, hydraulically controlled, winged-subsoiler mounted on a large

crawler tractor.

Shortly after logging and tillage, a total of 119 residual trees between and near

the seven tilled or nontilled skid trails were tagged and measured for diameter at

breast height, height, crown characteristics, and distance to neighboring trees, which

enabled computation of a competition index. Sampling provided about 40 trees in

each of three categories relative to skid trails: tilled, nontilled, and no trail (i.e.,

thinned but no trail influence). Basal area (BA) growth per tree in 1992 through

1999 was used as the dependent variable in growth analysis. Because tillage resulted

in noticeable damage to roots of adjacent trees, a damage index was used to evaluate

potential relation with observed growth responses. Soil bulk densities in skid trails

and around residual trees were measured with nuclear densiometers shortly after

logging, just after tillage, and after seven growing seasons.

Small differences in soil bulk densities were observed 7 years after treatment,

and less than 40 percent of the assumed rooting area of individual trees was affected

by equipment trails.

After covariance adjustment for differing starting BA, average growth of trees

near nontilled trails exceeded that of nontrail trees by about 13 percent and exceeded that of trees near tilled trails by about 22 percent. A second analysis was

necessitated because trees near nontilled trails averaged less and were less variable

in after-thinning competitive stress index than nontrail and tilled trees, which were

similar in both means and ranges of competitive stress index. Despite covariance

adjustment of observed means to mitigate this advantage, after-thinning growth of

nontilled trees still averaged 14 percent greater than nontrail trees and 6 percent

greater than tilled trees. The finding that growth was increased by proximity to

nontilled trails was consistent with results at Toledo South and Bone Mountain A
and B. We suspect that trafficking the clay loam soils at Noti when dry, and limiting

compacted area within the stand by using planned skid trails were effective management tactics at this site. Tillage provided no clear benefit to residual trees, at least in

the first 7 years after treatment.

We conclude that commercial thinning of these 20- to 60-year-old Douglas-fir

stands by using ground-based logging vehicles and planned skid trails usually on

dry soils did not reduce, and probably slightly increased tree growth of trail-side

residual trees. At the two locations where selected plots were fertilized before

thinning, tree response to adjacent trails was independent of the earlier fertilization

with 200 lb N/acre as urea.

Contents

1 Introduction

2 Methods

2 Toledo South and Bone Mountain

7 Noti

10 Results

10 Toledo South

14 Bone Mountain A and B

20 Noti

23 Discussion

25 Toledo South

25 Bone Mountain A and B

26 Noti

27 All Locations

27 Potential Effects of Equipment Trails

29 Continued Effects of Nitrogen Fertilization

30 Conclusions

30 Acknowledgments

31 Metric Equivalents

31 Literature Cited
1

Growth of Douglas-fir Near Equipment Trails Used for Commercial Thinning in the Oregon Coast Range

Introduction

Soil disturbance is a visually apparent result of using heavy equipment to harvest

trees. Typical types of soil disturbance from ground-based harvest include displacement, rutting, and compaction. Subsequent consequences of soil disturbance to

growth of trees, however, are variable and seldom quantified beyond short-term

effects. Despite a persistent need to further quantify tree response to soil disturbance, policies and practices intended to reduce or avoid soil disturbance from

logging have been widely adopted on both public and private forest lands in the

Pacific Northwest (Adams 2005).

Response of trees to soil disturbance is variable and dependent on soil, climate,

and perhaps tree species. Growth of Douglas-fir seedlings (Pseudotsuga menziesii

var. menziesii (Mirb. Franco)) planted in compacted and rutted skid trails at three

coastal Washington locations with moderate climate was compared to that of nearby

seedlings planted on nontrafficked soil. Difference in height growth and tree size in

the first decade were small and statistically not different (Miller and others 1996).

At eight Oregon Cascade locations with harsher climate, however, average height at

year 10 after planting was reduced by about 10 percent (Heninger and others

2002). In northern California with even drier climate, effects of controlled intensity of soil compaction on soil properties and response of young trees differed by

soil texture. Compaction of a coarse sandy loam improved young tree growth, but

compaction of a clay loam decreased growth, and compaction of a loamy soil had

no effect on growth (Gomez and others 2002).

Effects of soil disturbance on growth of residual trees after commercial thinning or other forms of partial harvest in the Pacific Northwest are less researched.

Froehlich (1979) reported that coast Douglas-fir having 10 percent or more increase in soil bulk density (BD) in 10 to 40 percent of their assumed rooting area

grew 14 percent less in basal area (BA) than residual trees on noncompacted soil.

Similarly impacted western hemlock (Tsuga heterophylla (Raf.) Sarg.) also averaged about 15 percent less basal area (BA) growth (Froehlich 1979). Douglas-fir

having larger increases in soil BD and greater percentage of root area affected grew

30 percent less than residual trees on nondisturbed soil. Other researchers did not

measure tree response, but speculated about growth consequences after commercial

thinning based on estimated percentage of the thinned stand in various soil disturbance classes (Murphy 1982), percentage of area with measured increases in soil

BD (McNeel and Ballard 1992), or numbers and severity of bole damage (Han and

Kellogg 2000, McNeel and Ballard 1992).

Response of trees

to soil disturbance

is variable and

dependent on soil,

climate, and perhaps

tree species.
2

GENERAL TECHNICAL REPORT PNW-GTR-574

We surmise that growth in thinned stands could be reduced initially where roots

of residual trees are crushed or severed by heavy equipment, and later where roots

encounter degraded soil conditions. Although proximity and severity of equipment

traffic are important variables affecting tree response after commercial thinning,

isolating effects of such traffic on individual tree growth is difficult. Growth after

thinning is also strongly influenced by initial tree size, degree of release when

neighboring trees are removed, and subsequent competition among residual trees

and other vegetation.

Our investigations at three locations in the Coast Range of western Oregon

sought to answer two questions: Did proximity to trails used by various groundbased logging vehicles affect subsequent tree growth? At two of these locations, we

posed two additional questions: Did past nitrogen (N) fertilization continue to

increase growth or influence tree response to equipment traffic?

Methods

Toledo South and Bone Mountain

At these two locations, we investigated former research plantations of uniformly

spaced Douglas-fir that were about 20 years old from seed (fig. 1). The sensitivity

of our research design was further enhanced at Toledo South because commercial

thinning systematically removed alternate rows of trees with no in-row removal.

Consequently, both release and subsequent competition for residual trees were

highly uniform.

The plantations were established in 1974 after clearcutting and intensive site

preparation on former Georgia Pacific Corporation land in the Oregon Coast

Range. The Toledo South plantation near Toledo, Oregon, is located on gently

sloping topography mapped as Elsie silt loam, a fine-silty mixed, mesic Typic

Haplohumult (USDA NRCS 1997). Elevation averages about 300 ft. The 50-year

site index (King 1966) averages 143 ft for this well-drained soil developing on

alluvium.

Bone Mountain plantations A and B are near Coquille, Oregon, on gently

sloping topography mapped as Preacher loam, a fine-loamy, mixed, mesic Typic

Haplumbrept, developing on marine sediments (Haagen 1989). Elevation averages

about 1,500 ft. The 50-year site index averages 127 ft.

At both locations, 1-year-old, container-grown seedlings were planted at a

precise 8- x 10-ft spacing (Toledo) or 8- x 8-ft spacing (Bone Mountain A and B),

Isolating effects

of such traffic on

individual tree

growth is difficult.
3

Growth of Douglas-fir Near Equipment Trails Used for Commercial Thinning in the Oregon Coast Range

Figure 1—Typical portions of the plantation at (A) Toledo South and

(B) Bone Mountain A after the 2004 growing season and 10 years

after commercial thinning. Note low-cut stumps between rows and

some within-row tree removal at (B).

A

BJim Smith Jim Smith

and within 10-acre areas surrounded by an 8-ft-tall fence to exclude large animals.

Seedlings were planted in blocks of 10 plots (Toledo) or 6 plots (Bone Mountain A

and B). Plots were 0.243 acre (Toledo) or 0.212 acre (Bone Mountain). Although

each plot contained about six randomly located seedlings from the same 30 half-sib

families, these 30 families differed among Toledo South, Bone Mountain A, and

Bone Mountain B. After planting, seedlings were carefully safeguarded from

weeds, disease, and small animals. This protection and some replacement planting

resulted in exceptionally uniformly spaced, well-stocked stands.
4

GENERAL TECHNICAL REPORT PNW-GTR-574

Fertilization—

After the 1989 growing season, a fertilization treatment was assigned randomly to

5 of the 10 plots at Toledo and to 3 of the 6 plots at both Bone Mountain A and B.

Urea fertilizer providing 200 lb N/acre was uniformly spread over assigned plots.

Cool weather and showers in the 2 weeks after fertilization probably minimized

potential loss of urea from volatilization.

Thinning—

After the 1993 growing season, the 20-year-old plantations were systematically

thinned. At Toledo South, a tracked harvester (John Deere 490) and wheeled

forwarder (1010 Timberjack) were used.1

 Every seventh row was removed to

provide routes for equipment traffic (in a direction diagonal to the original planting

rows and columns). Slash was accumulated and retained on the traffic route. Centerto-center distance between these routes was about 54 ft (6 rows x 9 ft). In alternate rows between these routes, all trees were cut and removed by extending the

harvester boom. No trees were cut in the remaining even-numbered rows, which

included two rows beside the travel routes and one interior row (fig. 2). Thereby,

stand density was reduced from about 600 trees per acre (TPA) to about 300 TPA.

We suspect that this operational thinning was conducted extra carefully because the

operator knew the plantation was special. Thinning occurred in April and surface

soil was moist.

At Bone Mountain A and B, every fifth row of the original planting was

removed to provide traffic routes for a tracked harvester (Timberjack 1270B) and

rubber-tired forwarder (Timberjack 1210B, fig. 3). Although less slash was accumulated and retained at Bone Mountain A and B than at Toledo South, ruts were

barely visible and less than 3 in deep. Center-to-center distance between these

routes was about 32 ft (4 rows x 8 ft). Because additional trees within the three

remaining interior rows were cut and removed, tree removal within the two trailside and one interior row was not as uniform as at Toledo South (fig. 2). The

original density of 680 TPA was reduced to about 260 TPA (62 percent cut).

Thinning was completed in June and the surface soil was dry and dusty.

1

 The use of trade or firm names in the publication is for reader information and does not

imply endorsement by the U.S. Department of Agriculture of any product or services.

The 20-year-old

plantations were

systematically

thinned. At Toledo

South, no trees

were cut in the

remaining evennumbered rows.

At Bone Mountain

A and B, tree removal within the

two trailside and

one interior row was

not as uniform as at

Toledo South.
5

Growth of Douglas-fir Near Equipment Trails Used for Commercial Thinning in the Oregon Coast Range

Figure 2—Pattern of commercial thinning

at (A) South Tract and (B) Bone Mountain

A and B.

Figure 3—Rubber-tired forwarder used at Bone Mountain

A and B.

PLACEMENT ONLY

NEED HIGHER RESOLUTION
6

GENERAL TECHNICAL REPORT PNW-GTR-574

Tree measurements—

Diameter at breast height (d.b.h.) of all trees was measured periodically with a steel

tape to the nearest 0.1 in. At Toledo South, d.b.h. was measured after the growing

season in 1989 (before fertilization), 1993 (before commercial thinning), 1997, and

2000 (year 7 after thinning, year 11 after fertilizing, and year 27 after planting). At

Bone Mountain A and B, d.b.h. was measured after the growing seasons of 1989

(before fertilization), 1993 (before thinning), and 2004 (year 11 after thinning, year

15 after fertilizing, and year 30 after planting). Trees in the interior plot of each

whole plot were identified with numbered aluminum tags at breast height (b.h.) (4.5

ft) and were pruned to 6-ft height to provide safe access and more reliable measurement of d.b.h. Buffer areas around interior plots contained only one row of

nontagged trees.

Data summary—

After commercial thinning, residual trees were identified in two categories: those

located beside traffic routes (position 1) and those located in interior rows between

traffic routes (position 2). Because some within-row trees were removed at Bone

Mountain A and B, residual trees in position 1 and position 2 had varying numbers

of neighboring or competing trees, ranging from zero to four of the initial eight.

Number of competing neighbors was tallied as a measure of residual competition.

Tree diameters were examined on a plot-by-plot basis by plotting average annual

d.b.h. growth after thinning against d.b.h. before thinning. One tree at Toledo was

rejected as spurious. Two trees were rejected at Bone Mountain A and seven at Bone

Mountain B for two reasons: spurious growth related to forking below the b.h.

measurement point or excessive growth of some position 1 trees that was clearly

related to excessive release and isolation. Thereafter, d.b.h. growth was converted

to cross-sectional (BA) growth as the dependent variable in growth analyses.

Statistical analysis—

Effects of commercial thinning on mean BA growth per tree were estimated and

compared by analysis of variance (ANOVA) (fixed-effects, SE method) using a

split-plot, completely randomized design (SAS Institute, Inc. 1999). The large

experimental units are plots, and the whole-plot treatments are fertilized and

nonfertilized. The small experimental units are groups of trees in each plot that

were either adjacent to an equipment trail (position 1) or interior (position 2).

Both fertilized (whole-plot treatment) and position are fixed effects.

Residual trees were

identified in two

categories: those

located beside

traffic routes (position 1) and those

located in interior

rows between traffic

routes (position 2).
7

Growth of Douglas-fir Near Equipment Trails Used for Commercial Thinning in the Oregon Coast Range

Noti

The 18.5-acre study area was in a stand of 58-year-old Douglas-fir, located on

U.S. Department of the Interior Bureau of Land Management land near Noti,

about 10 mi west of Eugene, Oregon. The site is in the eastern portion of the

Oregon Coast Range at an elevation of about 625 ft, where a mild, temperate

climate averages about 50 in of precipitation annually. Soils in the study area

are derived from sedimentary rocks and are of a silty clay loam texture near

the surface, with increasing clay content with depth. The surface mineral layer

is highly porous, rich in organic matter, and of relatively low BD and strength.

The USDA soil classification is clayey, mixed, mesic Xeric Haplohumults, primarily Jory series with a small area of Bellpine at the upper portion of the unit

(Patching 1987). Slopes average 14 percent.

Our study area covers most of a 24-acre unit that averaged 206 TPA before

thinning (summer 1992). Thinning reduced stand density to 70 TPA (66 percent

cut) and produced harvested logs averaging 10 inches in diameter. Timber volume

removed averaged 12.7 mbf per acre Scribner scale (Hogervorst 1994).

Thinning—

Logging that began in late July was completed by late August. Hot and dry weather

reduced soil moisture to less than 20 percent (weight basis) on all skid trails. Trees

in skid trails were felled with a shear attachment on a crawler tractor (Case 1150C).

Other trees were manually felled toward the trails (i.e., felled to lead) using chain

saws. Although yarding of whole trees and logs was done primarily with a rubbertired skidder (John Deere 648D) with cable chokers, some skidding was done with a

crawler tractor (Case 850D). Static, unloaded ground pressures of the three vehicles

ranged from 39 to 54 kPa (5.6 to 7.8 psi). Logs were skidded to roadside landings

at the end of seven parallel, planned skid trails spaced about 150 ft apart, which

covered about 7 percent of the study area (fig. 4).

In early September 1992, four skid trails were tilled with a self-drafting,

hydraulically controlled, winged subsoiler (Davis 1990) mounted on a large

crawler tractor. The winged shanks are specifically designed to lift and fracture the

soil, thereby providing extensive soil loosening as deep as 20 to 40 in (Andrus and

Froehlich 1983). Because logging debris and other surface organic matter can

impede effective tillage, trail-clearing with a brush rake was necessary in some trail

locations before tillage.

The 18.5-acre study

area at Noti was in a

stand of 58-year-old

Douglas-fir.

Skid trails were

spaced about

150 ft apart, which

covered about 7

percent of the study

area.
8

GENERAL TECHNICAL REPORT PNW-GTR-574

Tree measurements—

Shortly after logging and tillage, a total of 119 residual trees between and near both

tilled and nontilled skid trails (fig. 4) were tagged and measured for d.b.h., height,

crown characteristics, and a competition index before and after thinning. Diameters

and distances of all trees within a 40-ft radius of each study tree were used to

calculate a d.b.h.-based competitive stress index (CSI) before and after thinning

(Arney 1973).

Sampling provided about 40 trees in each of three categories relative to skid

trails: tilled, nontilled, and nontrail (i.e., thinned but no trail influence). Sample

trees next to tilled and nontilled trails were selected intentionally to be close

enough to be influenced by the trail conditions; they averaged about 35 percent

of their lateral root extent within the adjacent trail area. Earlier compaction studies

(Froehlich 1979) suggested this is sufficient to impact residual tree growth significantly. Although sample trees were not chosen randomly, care was taken to select

trees in each of the three “treatment” categories that were similar and representative

of the thinned area as a whole.

Figure 4—Sample tree locations at Noti.
9

Growth of Douglas-fir Near Equipment Trails Used for Commercial Thinning in the Oregon Coast Range

Because tillage resulted in noticeable damage to roots of trees adjacent to trails,

a damage index was used to evaluate potential relation with observed growth responses. Measurements of the average diameter and length of severed and exposed

roots were thus combined with subjective judgment to place each tree into a low,

moderate, or high damage class.

Soil bulk density measurements—

Soil BD in skid trails and around residual trees was measured with nuclear

densiometers shortly after logging, just after tillage (Hogervorst 1994), and after

seven growing seasons (Martinez 2000). Three measurements were taken around

each nontrail tree within a vertical projection of crown diameter, whereas six

measurements were taken around the other trees, half of these on and half off the

trail (fig. 5). Because of the extended time (7 years) between initial and final soil

measurement, different densiometers were used in 1992 (Campbell Pacific Nuclear

Figure 5—Pattern of soil bulk density sampling at Noti.
10

GENERAL TECHNICAL REPORT PNW-GTR-574

MC-1 Stratagauge) and 1999 (Boart Longyear MC-3 Portaprobe). Although major

design differences (i.e., double- vs. single-probe) preclude direct, quantitative

comparisons of bulk density data from the two devices, some general contrasts or

trends should be discernible.

Data summary—

Tree diameters of 1992 and 1999 were evaluated initially by plotting average annual

d.b.h. growth after thinning against d.b.h. before thinning. Two trees were rejected

as spurious. One tree grew much greater than the rest and the other tree showed

negative growth. For remaining trees, d.b.h. was converted to cross-sectional (BA)

growth as the dependent variable in growth analysis.

Statistical analyses—

Data were analyzed by analysis of covariance (ANCOVA) (SAS Institute, Inc.

1999). Note that trees sampling each treatment are located near specific trails (fig.

4). This means that trails are nested within treatments. There are four tilled trails,

which contribute 3 degrees of freedom (d.f.) to error. Similarly, three nontilled

trails contribute 2 d.f., and six nontrail areas between skid trails contribute 5 d.f.,

for a total of 10. An alpha value of 0.10 was used to judge statistical significance.

Results

Toledo South

Stand statistics before thinning—

Before commercial thinning at Toledo South, the 10 plots averaged 600 TPA, about

190 ft2 BA, and relative density (RD) of 68 (table 1). Thinning removed 50 percent

of these trees (51 percent of initial BA), and reduced mean RD to 33. Relative density (Curtis 1982) is an index of competition that can help predict mortality losses.

An RD value of 70 portends losses from overstocking. As anticipated, the rowthinning at South Tract removed and left similar TPA and BA/acre in both the

fertilized and nonfertilized plots.

Tree d.b.h. and sample size—

Before thinning, d.b.h. among position 1 trees (near future trails) averaged

7.7 in on both fertilized and nonfertilized plots (table 2). Position 2 trees (located

away from the parallel trails) similarly averaged 7.6 in on both fertilized and nonfertilized plots. After thinning, the number of position 1 trees (beside two trails)

was nearly twice that of position 2 trees located in one interior row (table 2).

Trees sampling

each treatment are

located near specific trails.
11

Growth of Douglas-fir Near Equipment Trails Used for Commercial Thinning in the Oregon Coast Range

Table 1—Mean live-stand statistics at Toledo South before and after commercial thinning in 1993

Before After Cut

Basal Basal Basal

Fertilizer Plot Trees area RD Trees area RD Trees area RD

Lb N/acre Trees/acre Ft2

/acre Trees/acre Ft2/acre Trees/acre Ft2/acre

0 1 605 214.7 75 292 105.1 37 313 109.6 38

2 630 202.1 73 292 96.4 34 337 105.7 38

4 626 186.3 68 317 87.0 33 309 99.3 36

8 572 168.8 62 284 84.0 31 288 84.8 31

10 605 192.7 70 309 92.3 34 296 100.3 36

Mean 608 192.9 70 299 93.0 34 309 99.9 36

Percent — — — — 51 52.0 51

200 3 601 205.7 73 317 104.9 38 284 100.8 35

5 593 200.2 71 296 98.4 35 296 101.7 36

6 568 167.8 62 284 88.1 32 284 79.7 30

7 597 177.3 65 305 87.1 32 292 90.2 33

9 597 181.1 66 284 84.1 31 313 97.0 35

Mean 591 186.4 67 297 92.5 33 294 93.9 34

Percent — — — — 50 50.0 51

All 600 189.6 68 298 92.8 33 302 96.9 35

Percent 100 100 100 50 49 49 50 51 51

N = nitrogen; RD = Relative density (Curtis 1982); — = not applicable.
12

GENERAL TECHNICAL REPORT PNW-GTR-574

Among the 10 plots, 303 measured trees were located in the rows beside trails and

147 were in the one noncut row midway between adjacent trails. This difference in

sample size is consistent with the thinning prescription and our field observation

that little or no within-row thinning had occurred. Therefore, position 1 and position 2 trees had similar numbers of competitors. Bole damage to residual trees was

infrequent and of minor severity.

Table 2—Sample size and arithmetic mean diameter at breast height

(d.b.h.) at Toledo South

D.b.h.b

Fertilizer Plot Positiona Sample size 1989 1993 2000

Lb N/acre – – – – Inches– – – –

0 1 1 27 7.5 8.4 10.8

2 15 7.5 8.5 10.8

2 1 27 7.0 7.9 10.1

2 18 7.0 7.8 10.2

4 1 26 6.8 7.6 10.1

2 17 6.5 7.3 9.6

8 1 31 6.2 7.1 9.5

2 14 6.2 7.1 9.6

10 1 30 6.8 7.7 10.1

2 16 6.5 7.5 9.4

Mean 1 34 6.9 7.7 10.1

2 16 6.7 7.6 9.9

200 3 1 38 7.3 8.2 10.4

2 9 7.0 8.0 10.2

5 1 34 7.1 8.0 10.4

2 10 6.9 7.8 10.1

6 1 30 6.2 7.3 9.7

2 15 6.4 7.6 10.0

7 1 32 6.1 7.2 9.6

2 17 6.2 7.3 9.6

9 1 28 6.5 7.7 10.2

2 15 6.4 7.5 9.9

Mean 1 32 6.6 7.7 10.1

2 13 6.6 7.6 10.0

N = nitrogen.

a 1 = beside trail, 2 = no trail.

b

 1989 = before fertilization, 1993 = before thinning.

Position 1 and 2

trees had similar

numbers of

competitors.
13

Growth of Douglas-fir Near Equipment Trails Used for Commercial Thinning in the Oregon Coast Range

Basal area growth after thinning—

Preliminary plotting of mean annual BA growth per tree in the 7 years after thinning suggested that growth was related to mean d.b.h. at thinning (fig. 6). Each

plotting point (a specified plot and position) is the mean of 9 to 34 trees (table 2).

Because before-thinning growth was influenced by fertilization, we did not use

covariance analysis to test differences among adjusted treatment means. To do so

would have removed some of the effects of the fertilizer treatments.

Mean BA growth per tree was similar on previously fertilized and nonfertilized

plots (p = 0.60, table 3). Note that in the first 4 years after fertilization (and before

thinning), fertilized plots at this location averaged about 11 percent greater BA

growth per acre than nonfertilized (p = 0.07, Miller and others 2001). Because the

P x F interaction was statistically nonsignificant (p = 0.29, table 3), we infer that

tree response to trails was independent of earlier fertilization. Trees next to trails

averaged about 3 percent faster growth in cross-sectional area at b.h. than trees

located farthest from the trails (p = 0.09).

Figure 6—Mean annual basal area (BA) growth of individual trees in a 7-year period after commercial thinning at Toledo South, by position (beside equipment trail = position 1 or interior =

position 2) and by previous fertilization treatment (fert = 200 lb N/acre and nonfert = 0 lb N/

acre), related to mean diameter at breast height (d.b.h.) before thinning.

Tree response to

trails was independent of earlier fertilization.
14

GENERAL TECHNICAL REPORT PNW-GTR-574

Table 3—Statistical significance of prior fertilization, proximity to equipment trails, and their interaction effect on 7-year mean annual basal area

(BA) growth per tree after commercial thinning at Toledo South

Factor D.f. F-value P

Main plots: 0.21

Fertilizer (F) 1 0.30 0.601

Plots within F (error 1) 8 — —

Split-plots:

Position (P) 1 3.79 .088

P x F 1 1.28 .290

Error 8 — —

Total 19

Adjusted treatment means and 90-percent confidence interval

Mean basal area growth Relative

Fertilizer: (ft2

/year) percentage

0 N 0.0339 (0.0327–0.0351) 100

200 N 0.0334 (0.0322–0.0346) 99a

Position:

1 (Beside trail) 0.0344 (0.0333–0.0355) 100

2 (Mid row) 0.0329 (0.0317–0.0340) 97

N = nitrogen; 200 N = 200 lb N/acre; — = not applicable.

a

 In years 1-4 after fertilization, response in gross BA per acre to 200 pounds nitrogen

per acre averaged 11 percent (p = 0.07) or 6.3 percent, if adjusted for difference in

prefertilization growth (p = 0.25) (Miller and others 2001).

Bone Mountain A and B

Stand statistics before and after thinning—

Before commercial thinning, the six plots in Bone Mountain A averaged 678 TPA,

about 153 ft2 BA per acre, and RD 61 (table 4); those at Bone Mountain B averaged

677 TPA, about 164 ft2

 BA, and RD 64 (table 5). At Bone Mountain A, thinning

removed 60 percent of the trees, 56 percent of the initial BA, and reduced RD to 26.

Thinning at Bone Mountain B, was slightly heavier, removing 62 percent of the

trees, 60 percent of the initial BA and also reducing RD to 26.

At both Bone Mountain A and B, previously fertilized plots had similar livestand statistics before thinning. Percentage cut (TPA, BA, RD) averaged slightly

heavier among fertilized plots at Bone Mountain A, and slightly lighter at Bone

Mountain B. In both plantations, after-thinning growing stock averaged slightly

greater among fertilized plots (tables 4 and 5). Slightly greater growing stock could

In both plantations,

after-thinning growing stock averaged

slightly greater

among fertilized

plots.
15

Growth of Douglas-fir Near Equipment Trails Used for Commercial Thinning in the Oregon Coast Range

Table 5—Mean live-stand statistics at Bone Mountain B before and after commercial thinning in 1994

Before After Cut

Basal Basal Basal

Fertilizer Plot Stems area RD Stems area RD Stems area RD

Lb N/acre Trees/acre Ft2/acre Trees/acre Ft2/acre Trees/acre Ft2/acre

0 3 670 164.4 63 250 67.6 25 420 96.8 38

4 675 161.7 63 241 61.7 24 434 100.0 39

5 689 171.2 66 245 69.2 26 443 102.0 40

Mean 678 165.8 64 245 66.2 25 432 99.6 39

Percent — — — — 64 60.0 61

200 1 675 151.0 60 274 59.7 24 401 91.2 36

2 675 161.1 63 278 71.8 27 396 89.3 35

6 679 172.4 66 255 65.8 25 425 103.6 40

Mean 676 161.5 63 269 65.8 26 407 94.7 37

Percent — — — — 60 59.0 59

All 677 163.6 64 257 66.0 26 420 97.2 38

Percent 100 100 100 38 40 41 62 60.0 59

N = nitrogen; RD = relative density (see Curtis 1982); — = not applicable.

Table 4—Mean live-stand statistics at Bone Mountain A before and after commercial thinning in 1994

Before After Cut

Basal Basal Basal

Fertilizer Plot Stems area RD Stems area RD Stems area RD

Lb N/acre Trees/acre Ft2

/acre Trees/acre Ft2/acre Trees/acre Ft2/acre

0 1 693 158.1 61 302 75.0 29 392 83.1 33

2 670 145.8 58 288 71.1 27 382 74.7 31

6 660 157.3 61 255 66.7 25 406 90.6 36

Mean 674 153.7 60 282 70.9 27 393 82.8 33

Percent — — — — 58 54.0 55

200 3 670 157.8 62 250 63.9 24 420 93.9 37

4 684 151.1 60 259 63.7 25 425 87.4 35

5 689 149.9 60 259 63.4 24 429 86.5 35

Mean 681 152.9 61 256 63.7 24 425 89.3 36

Percent — — — — 620 60.0 59

All 678 153.3 61 269 67.3 26 409 86.0 35

Percent 100 100 100 40 44 43 60 56 57

N = nitrogen; RD = relative density (see Curtis 1982); — = not applicable.
16

GENERAL TECHNICAL REPORT PNW-GTR-574

reduce after-thinning BA growth per tree, but increase subsequent BA growth per

acre among fertilized plots compared to nonfertilized.

Tree d.b.h. and sample size—

After commercial thinning in 1993, d.b.h. at Bone Mountain A for position 1 and 2

trees averaged about 6.8 in for both fertilized and unfertilized plots (table 6). At

Bone Mountain B, d.b.h. averaged 6.6 in for position 1 and 6.8 in for position 2;

mean d.b.h. in both positions was slightly smaller in fertilized plots (table 7).

Although removal of every fifth row left two position 1 rows for each position 2

row (a 2:1 ratio), sample size at Bone Mountain A totaled 101 trees in position 1

and 127 in position 2. At Bone Mountain B, totals were 103 and 115 trees for positions 1 and 2, respectively. At both plantations, the smaller number of residual trees

in position 1 indicates greater within-row release.

Table 6—Sample size after thinning and arithmetic mean diameter at

breast height (d.b.h.) by plot position and year at Bone Mountain A

D.b.h.

Fertilizer Plot Positiona Sample size 1989 1993 2004

Lb N/acre – – – – Inches – – – –

0 1 1 18 5.3 6.8 10.7

2 21 5.1 6.5 10.0

2 1 19 5.1 6.6 10.4

2 20 5.4 6.9 10.5

6 1 15 5.5 7.0 11.3

2 25 5.4 6.8 10.3

Mean 1 17 5.3 6.8 10.8

2 22 5.3 6.7 10.3

200 3 1 18 5.2 7.0 11.5

2 17 5.1 6.8 10.6

4 1 18 5.2 6.8 11.1

2 20 5.1 6.9 10.7

5 1 13 4.9 6.5 10.9

2 24 5.1 6.8 10.6

Mean 1 16 5.1 6.8 11.2

2 20 5.1 6.8 10.6

All All 1 17 5.2 6.8 11.0

2 21 5.2 6.8 10.5

N = nitrogen.

a

1 = beside trail, 2 = no trail.

At both plantations,

the smaller number

of residual trees in

position 1 indicates

greater within-row

release.
17

Growth of Douglas-fir Near Equipment Trails Used for Commercial Thinning in the Oregon Coast Range

Table 7—Sample size after thinning and arithmetic mean diameter at

breast height (d.b.h.) by plot position and year at Bone Mountain B

D.b.h.

Fertilizer Plot Positiona Sample size 1989 1993 2004

Lb N/acre – – – – Inches – – – –

0 3 1 20 5.5 6.8 10.7

2 20 5.4 6.8 10.2

4 1 20 5.5 6.8 10.8

2 19 5.4 6.6 9.9

5 1 14 5.6 6.8 11.1

2 17 5.9 7.4 11.1

Mean 1 18 5.5 6.8 10.9

2 19 5.6 6.9 10.4

200 1 1 18 4.6 6.1 10.0

2 20 4.6 6.1 9.8

2 1 20 5.0 6.5 10.4

2 19 5.5 7.1 10.7

6 1 11 5.2 6.7 10.9

2 20 5.3 6.8 10.4

Mean 1 16 4.9 6.4 10.4

2 20 5.1 6.7 10.3

All All 1 17 5.2 6.6 10.7

2 19 5.4 6.8 10.4

N = nitrogen.

a

1 = beside trail, 2 = no trail.

For the six plots at Bone Mountain A, average number of residual competitors

for position 1 trees ranged from 1.6 to 2.2, but for position 2 trees ranged from

3.0 to 4.0. Corresponding numbers at Bone Mountain B were 1.5 to 2.1 for position 1, and 3.2 to 3.7 for position 2. Because the two positions clearly have distinct, nonoverlapping mean numbers of competitors, no valid adjustment for

different tree numbers by covariance analysis is possible.

Basal area growth after thinning—

Average BA growth per tree in the 11 years after thinning was consistently greater

for position 1 trees at both locations (fig. 7). Based on ANOVA, the P x F interaction for both Bone Mountain A and B was statistically nonsignificant, indicating that

tree growth response to trail position was independent of the earlier fertilizer treatment (tables 8 and 9). Position 1 trees averaged greater growth than position 2 trees

at both Bone Mountain A and B, 14 and 11 percent, respectively. Greater growth at

For both Bone

Mountain A and

B, tree growth

response to trail

position was independent of the

earlier fertilizer

treatment.
18

GENERAL TECHNICAL REPORT PNW-GTR-574

Figure 7—Mean annual basal area (BA) growth of individual trees in an 11-year period after commercial thinning, by position (beside equipment trail = position 1 or interior = position 2) and

previous fertilization treatment (fert = 200 lb N/acre and nonfert = control), related to mean diameter

at breast height (d.b.h.). A = Bone Mountain A; B = Bone Mountain B.
19

Growth of Douglas-fir Near Equipment Trails Used for Commercial Thinning in the Oregon Coast Range

Table 8—Statistical significance of prior fertilization, proximity to equipment trails, and their interaction effect on 7-year mean annual basal area

growth per tree after commercial thinning at Bone Mountain A

Factor D.f. F-value P

Main plots:

Fertilizer (F) 1 7.78 0.049

Plots with F (error 1) 4 — —

Split-plots:

Position (P) 1 14.13 .020

P x F 1 0.13 .734

Error 2 4 — —

Total 11

Treatment means and 90-percent confidence interval

Annual basal area growth Relative

Fertilizer: (ft2

/year) percentage

0 N 0.0333 (0.0313-0.0354) 100

200 N .0372 (0.0351-0.0392) 112

Position:

1 (Beside trail) .0378 (0.0358-0.0399) 100

2 (Mid row) .0327 (0.0306-0.0347) 86

N = nitrogen; 200 N = 200 lb N/acre; — = not applicable.

Table 9—Statistical significance of prior fertilization, proximity to equipment trails, and their interaction effect on 11-year mean annual basal

area growth per tree after commercial thinning at Bone Mountain B

Factor D.f. F-value P

Main plots:

Fertilizer (F) 1 0.43 0.547

Plots within F (error 1) 4 — —

Split-plots:

Position (P) 1 20.15 .009

P x F 1 2.46 .192

Error 2 4 — —

Total 11

Treatment means and 90-percent confidence interval

Annual basal area growth Relative

Fertilizer: (ft2

/year) percentage

0 N 0.0340 (0.0309-0.0371) 100

200 N 0.0327 (0.0296-0.0357) 96

Position:

1 (Beside trail) 0.0353 (0.0330-0.0377) 100

2 (Mid row) 0.0313 (0.0290-0.0337) 89

N = nitrogen; 200 N = 200 lb N/acre; — = not applicable.
20

GENERAL TECHNICAL REPORT PNW-GTR-574

b.h. by position 1 trees at both Bone Mountain A and B, however, is at least partially explained by greater release, hence fewer competitors around position 1 trees.

Clearly, inferences about position are confounded by differing competition from

neighboring trees. To some nonmeasureable extent, greater growth of position 1

trees in both plantations relates to fewer competitors near position 1 trees.

Residual effects of the earlier fertilizer treatment remained significant at Bone

Mountain A (12-percent increase over control), (p = 0.05, table 8), but was not

significant at Bone Mountain B (4 percent less than control p = 0.55, table 9). Note

that in the first 4 years after fertilization (and before thinning), response at Bone

Mountain A also was greater than that at B. Fertilized plots at Bone Mountain A

initially averaged 12.6 percent greater BA growth per acre than nonfertilized (p =

0.01) and those at Bone Mountain B initially averaged 9.2 percent more (p = 0.07,

Miller and others 2001). Inferences about continuing response to past fertilization

at Bone Mountain A and B, but not Toledo South, may be confounded by differences in numbers of cut and residual trees (table 4). At Bone Mountain A, fertilized

plots averaged 256 TPA residual versus 282 on nonfertilized plots. This suggests

that faster mean growth by fertilized trees at this location could in part, be explained by wider spacing. Conversely at Bone Mountain B, fertilized plots averaged

269 TPA residual versus 245 TPA on nonfertilized. Thus, absence of continuing

per-tree response of fertilized trees at Bone Mountain B could be explained by

closer spacing than on nonfertilized plots.

Noti

Stand statistics before thinning—

Before thinning in summer 1992, the nearly pure Douglas-fir stand at Noti averaged 58 years old. Small numbers of similar age western hemlock contributed to

stand density that averaged 206 TPA. Thinning reduced stand density to 70 TPA

(66 percent cut).

Soil density—

Bulk density near nontrail trees averaged 0.89, 0.98, and 1.05 Mg/m3

, near the 4-,

8-, and 12-in depth, respectively. Shortly after logging, soil BD at the 4- and 8-in

depths in nontilled skid trails averaged 18 and 11 percent greater, respectively, than

near nontrail trees. At the 12-in depth, skid trails averaged slightly less density (3

percent). Shortly after tillage, BD in tilled trails averaged 6 percent greater than

near nontrail trees at the 6-in depth, but 2 and 4 percent less dense at 8- and 12-in

depths, respectively. Although tillage loosened the soil, roots of many adjacent

sample trees were damaged.

Greater growth

of position 1 trees

relates to fewer

competitors.

Shortly after logging,

soil BD in nontilled

skid trails was

greater than near

nontrail trees.
21

Growth of Douglas-fir Near Equipment Trails Used for Commercial Thinning in the Oregon Coast Range

Seven years after logging, average soil BDs at the 0- to-10-in depth were

similar among several sampling positions (fig. 8). Means did not differ significantly. In nontilled trails, densities averaged about 4 percent greater than in tilled

trails, where BD averaged only 1 percent greater than nontrail areas (statistical tests

not shown). Tillage with the winged subsoiler appeared to reduce soil BD slightly,

both immediately after logging and 7 years later. Because of differing densiometers

and depths of sampling, however, we have no reliable evidence for natural recovery

in the nontilled trails 7 years after logging.

Basal area growth after thinning—

Analyses of tree-growth relations were based on the average BA growth for study

trees associated with each trail treatment. As tabulated, number of subsample trees

was similar among treatments, but tilled trees had the largest average d.b.h. both

before and 7 years after thinning:

D.b.h.

Treatment Sample size Before After

– – – Inches – – –

No trail 39 15.3 16.6

Nontilled 41 15.4 16.9

Tilled 37 16.7 18.0

Figure 8—Means and standard deviations of soil bulk density 7 years after commercial

thinning at Noti.

Because of different

methods, we have

no reliable evidence

for natural recovery

in the nontilled

trails 7 years after

logging.
22

GENERAL TECHNICAL REPORT PNW-GTR-574

Covariance analysis indicated that after-thinning growth was related to tree

BA before thinning (p = 0.001), and that the three treatments differed in average

7-year BA growth per tree (p = 0.02, table 10). After covariance adjustment,

growth of nontilled trees averaged 13 percent greater than that of nontrail trees and

22 percent greater than that of tilled trees. The 90-percent confidence interval (CI)

for tilled and nontilled means do not overlap.

A second covariance analysis using after-thinning CSI as the covariate confirmed an anticipated negative relation between tree growth and local competition

after thinning (p = 0.06, table 11). As tabulated below, trees beside nontilled trails

averaged less competition than control trees or those beside tilled trails:

Competitive Stress Index

Treatment Mean Coefficient of Variation Minimum Maximum

Nontilled 191 17.8 150 284

Tilled 212 19.7 133 291

No trail 217 19.8 135 319

After adjustment to a common CSI, growth of trail-side trees averaged 8 to 14

percent greater than control trees (table 11), but adjusted treatment means were not

statistically significant (p = 0.48).

Table 10—Statistical significance of no trail, nontilled, and tilled

skid trails on mean basal area (BA) growth per tree after commercial thinning at Noti, using tree BA before thinning as a

covariate

Factor D.f. M.s. F-value P

Regression 1 305.90 118.6 0.001

Treatment (T) 2 9.86 3.82 .025

Trail within T (error 1) 10 2.27 — —

Error (2) 103 2.58 — —

Total 116

Adjusted means and 90-percent confidence intervala

Treatment Mean basal area Relative

growth (in2/year) percentage

Nontilled 5.86 a 5.35-6.34 113

No trail 5.20 ab 4.75-5.64 100

Tilled 4.72 b 4.23-5.20 91

— = not applicable.

a

 Treatment means followed by the same letter are not significantly different

based on Bonferroni test (p ≤ 0.10).

After covariance

adjustment, growth

of nontilled trees

averaged greater

than tilled or

nontrail trees.
23

Growth of Douglas-fir Near Equipment Trails Used for Commercial Thinning in the Oregon Coast Range

Percentages of estimated root zone trafficked by equipment were similar for

trees next to nontilled or tilled trails (fig. 9), and trees along both tilled and

nontilled trails averaged similar mean rates of growth after thinning. A subsample

of trees with obvious root damage was evaluated separately, but this revealed no

unique pattern of poorer growth with increasing severity of root damage (p = 0.89,

table 12).

Discussion

Before seeking alternative explanations, we conditionally inferred from these data

that cumulative impacts of various ground-based logging equipment in planned skid

trails did not reduce b.h. growth of trail-side trees in the subsequent 7 to 11 years,

and may have slightly increased average growth by about 3 to 15 percent among

the three locations.

We assessed two alternative explanations for equal or greater b.h. growth of

individual trees:

1. Position 1 trees may have had greater growth potential as evidenced by larger

d.b.h. before thinning.

2. Position 1 trees may have had fewer competitors after release than position

2 trees.

Table 11—Statistical significance of tree location (no trail,

nontilled, and tilled skid trails) on mean basal area growth per

tree after commercial thinning at Noti, using after-thinning

competitive stress index as a covariate

Factor D.f. M.s. F-value P

Regression 1 19.40 3.62 0.060

Treatment (T) 2 3.92 0.73 .484

Trail within T (error 1) 10 5.24 — —

Error (2) 103 5.36 — —

Total 116

Adjusted means and 90-percent confidence intervala

Treatment Mean basal area growth Relative

(in2

/year) percentage

Nontilled 5.78 a 4.99-6.57 114

Tilled 5.46 a 4.73-6.19 108

No trail 5.07 a 4.39-5.75 100

— = not applicable.

a

 Treatment means followed by the same letter are not significantly different

based on Bonferroni test (p ≤ 0.10).
24

GENERAL TECHNICAL REPORT PNW-GTR-574

Figure 9—Mean annual basal area (BA) growth over 7 years related to percentage of estimated

root zone impacted at Noti.

Table 12—Statistical significance of root-damage class after

tillage of skid trails on basal area growth per tree after commercial thinning at Noti, using percentage area of root zone

that was trafficked as a covariate

Factor D.f. M.s. F-value P

Regression 1 3.72 0.76 0.392

Root damage class 2 0.56 .11 .893

Trail within class (error 1) 9 3.36 — —

Error (2) 24 4.91 — —

Total 36

Adjusted means and 90-percent confidence intervala

Damage class Mean basal area growth Relative

(in2/year) percentage

Slight 5.33 a 4.44-6.22 100

Moderate 5.46 a 4.46-6.40 102

Severe 5.89 a 4.36-7.42 110

— = not applicable.

a

 Treatment means followed by the same letter are not significantly different

based on Bonferroni test (p ≤ 0.10).
25

Growth of Douglas-fir Near Equipment Trails Used for Commercial Thinning in the Oregon Coast Range

Toledo South

Trees next to trails averaged about 3 percent faster growth at b.h. than trees located

farthest from the trails, and this difference was statistically significant (p = 0.09).

Greater growth potential?—

Because the four treatment groups (position 1, position 2, 0-N, and 200-N) were

similar in starting mean d.b.h., we reject this explanation.

Fewer competitors?—

This is unlikely. Because only neighboring rows of trees were cut, both position 1

(near trail) and position 2 trees were released to the same extent. No trees were cut

within residual rows.

Opinion—

Theoretically, position 1 trees had two unique and potentially growth-reducing

influences: nearby soil compaction and shallow rutting, and greater risk of bole

damage from moving booms or extracted trees. We suspect that soil compaction

was mitigated by slash deliberately placed on the trails; minimal bole damage was

observed. We infer that equipment traffic did not affect tree growth at Toledo

South. Bole damage during stem extractions was insignificant. Placing logging slash

on all trails before trafficking probably mitigated soil compaction and rutting near

trail-side trees.

Bone Mountain A and B

Breast-high growth of trees next to trails averaged about 14 percent more than

growth of more distant trees at Bone Mountain A (p = 0.02) and 11 percent more

at Bone Mountain B (p = 0.01) (tables 8 and 9).

Greater growth potential?—

Trees in position 1 at Bone Mountain A had after-thinning mean d.b.h. similar to

those in position 2. At Bone Mountain B, however, d.b.h. after thinning averaged

6.6 in for position 1 and 6.8 in for position 2. We infer that some of the observed

greater growth of position 2 trees at Bone Mountain B could be explained by somewhat larger tree size at start of the growth period.

Fewer competitors?—

In contrast to the strict row-thinning at Toledo South, some in-row trees were

removed in both Bone Mountain plantations, especially in rows beside equipment

routes. On all plots, position 1 trees consistently had fewer competitors than did

Equipment traffic

did not affect tree

growth at Toledo

South.
26

GENERAL TECHNICAL REPORT PNW-GTR-574

position 2 trees. We suspect that much of the 14- to 11- percent greater growth of

position 1 trees is explained by their having fewer competitors, hence more growing

space.

Speculation—

Benefits of greater release and fewer competing trees near position 1 trees more

than offset any negative effects of equipment traffic at Bone Mountain A and B.

As at Toledo South, placing logging slash on trails before trafficking mitigated

soil compaction and rutting.

Noti

Patterns of tree response near skid trails at Noti were consistent with those at

Toledo South and Bone Mountain A and B. Moreover, diameter before thinning

and local competition are important variables for predicting growth response of

individual trees to thinning.

Greater growth potential?—

Trees next to tilled trails initially averaged larger d.b.h. than nontrail and nontilled trees. Despite this indication of greater growth potential, observed mean

growth of tilled trees averaged less than mean BA growth of both nontilled and

nontrail trees (table 10). After covariance adjustment to mitigate these initial differences in starting d.b.h., average growth of trees near nontilled trails exceeded that

of trees near tilled trails by about 22 percent and exceeded that of nontrail trees by

about 13 percent.

Less competition?—

After-thinning growth was related to CSI, which was computed for each subject tree

from d.b.h. and distances to neighboring trees. As expected, growth increased with

decreasing CSI. Fortuitously, trees near nontilled trails averaged less CSI and less

variable CSI after thinning than nontrail and tilled trees, which were similar in both

mean values and variation. Despite covariance adjustment that used observed mean

CSI to mitigate this advantage, after-thinning growth of nontilled trees still averaged 14 percent greater than nontrail and 6 percent greater than tilled trees (table

11).

Inferences—

That growth was increased by proximity to nontilled trails was surprising, yet

consistent with results at Toledo South and Bone Mountain A and B. Large differences in growth among treatments owing to soil compaction was unlikely, because

Placing logging

slash on trails

before trafficking

mitigated soil compaction and rutting.
27

Growth of Douglas-fir Near Equipment Trails Used for Commercial Thinning in the Oregon Coast Range

only small differences in soil BD were observed 7 years after treatment, and less

than 40 percent of the assumed rooting area of individual trees was affected by

equipment trails.

We suspect that trafficking the clay loam soil when dry, and limiting the extent of compacted area within the stand by using planned skid trails were effective tactics at Noti. Because increases in soil BD on trails were small, and trees

near tilled trails averaged no or little response to thinning, tillage provided no

clear benefit to residual trees, at least in the first 7 years after thinning and tillage.

Because of differing densiometers and depths of sampling, we have no reliable

evidence for natural recovery from compaction in nontilled trails 7 years after

logging. Published estimates of the time required for natural recovery of soils from

compaction after forest harvesting differ considerably. Early studies examined only

recovery of surface horizons, and more recent studies have included both surface

and subsurface horizons. Determining the time for soils to recover their original

BD retrospectively is especially problematic. Perhaps the greatest difficulty in

assessing recovery rates is that compaction on trails is often preceded or accompanied by removal of surface soil. As a result, retrospective comparisons of BD in the

surface soil of heavily trafficked areas versus that in nearby nontrafficked areas

may actually compare compacted subsoil versus noncompacted surface soil of

inherently lower BD. Given this invalid comparison, rate of apparent recovery is

underestimated (Miller and others 2004).

All Locations

Further monitoring of soil conditions and tree growth on these sites could help us

discern the duration of these interim patterns. An even greater need, however, is to

conduct similar studies at other locations to determine where local site and treatment conditions have significant effects on residual tree growth. As experienced in

this investigation, experimentally uncontrolled factors can confound interpretation

of statistical tests.

Potential Effects of Equipment Trails

Soil in logging trails can be impacted several ways and with varying, hence poorly

predictable, consequences for soil properties and subsequent tree growth. The

consistent pattern among the four stands was that residual trees near equipment

trails grew faster than those farther away. We consider additional explanations for

this unexpected outcome.

Tillage provided no

clear benefit to

residual trees.
28

GENERAL TECHNICAL REPORT PNW-GTR-574

Soil disturbance—

At our study locations, soil disturbance was largely compaction. Rutting and displacement were minimal at Toledo South and Bone Mountain A and B, because

slash from log processing was deliberately placed on equipment trails. At Noti, soil

disturbance, including compaction, on trails was minimized by dry-season logging

even though traffic was concentrated on designated trails.

Effects of logging traffic on soil properties and tree growth are complex and

depend on several factors, including (1) depths of slash and the forest floor; (2) soil

texture (percentage of sand, silt, clay) and coarse fragments (gravel, cobbles, rock);

(3) soil moisture content; (4) vehicle pressure, vibration, and traction; and (5) subsequent rate and effectiveness of natural remediation (freezing-thawing, wettingdrying, soil organisms, and vegetation). For example, coarse-textured soils dry

rapidly, but they still can be susceptible to compaction via vehicle vibration. More

importantly, compaction of coarse-textured soils can increase soil moisture-holding

capacity by converting large pores to smaller pores, which retain moisture longer

thereby benefiting tree growth in drier climates (Gomez and others 2002, Powers

and Fiddler 1997). Although conversion to smaller pores in the fine-textured soils

at our study sites (silt loam to clay loam textures), would not seem beneficial, a

recent study (Ares and others 2005) reported both increased water-holding capacity

and seedling growth on a silt loam soil (Boistfort series) in the Washington Coast

Range where climate is more mesic than at study sites in northern California.

Slash—

Thinning slash was deliberately concentrated on equipment trails to cushion the

physical impacts of equipment at Toledo South and Bone Mountain A and B. This

mitigating practice has been shown to reduce compaction on similar soils (Allen and

others 1997), and is likely to be most effective where slash remains deep enough to

cushion impacts after repeated passes of equipment. Subsequently, slash can moderate soil temperature, reduce loss of soil moisture from evaporation, and reduce

vegetative competition.

Slash can also affect subsequent nutrient availability for tree and vegetative

growth. Initially, slash in contact with soil can reduce the amount of N available to

plants by temporary incorporation into woody portions or microbes (N-immobilization). Subsequent decomposition eventually recycles most of this N for plant

uptake. Such recycling of N in slash placed on equipment trails is more likely to

have enhanced tree growth at Bone Mountain A, because we measured response

after an 11-year period after thinning (compared to 7 years at Toledo South).
29

Growth of Douglas-fir Near Equipment Trails Used for Commercial Thinning in the Oregon Coast Range

Moreover in earlier trials, trees at Bone Mountain A and B responded more

strongly to N fertilization than at Toledo South (Miller and others 2001).

Continued Effects of Nitrogen Fertilization

Four years before commercial thinning, half of the plots in the plantations at

Toledo South and Bone Mountain A and B were fertilized with 200 lb N/acre. In

the first 4 years after fertilization, fertilized plots at Toledo South averaged about

6 percent greater BA growth per acre than nonfertilized plots (Miller and others

2001). At Bone Mountain A, fertilized plots averaged 12.6 percent greater BA

growth per acre than nonfertilized plots; response at Bone Mountain B averaged

9.2 percent (Miller and others 2001).

Where the 1994 thinnings had cut and left the same number and size of trees in

both fertilized and nonfertilized plots, any difference in growth could result from

continuing response to fertilizer, possibly from improved nutrition and by the continued benefit of larger or more efficient crowns of fertilized trees. We examined

our results at each plantation to decide if those after-thinning conditions had been

achieved. At Toledo South, a slightly smaller mean percentage of the original BA

was cut in the fertilized plots (50 vs. 52), and the remaining BA per acre averaged

about the same as in nonfertilized plots (table 13). In years 5 through 11 after

fertilization, previously fertilized trees averaged 1 percent less growth at b.h. than

nonfertilized. Moreover, response in BA per acre averaged -1.6 percent in the six

fertilized plots (table 13). We infer no detectable continued response to urea

fertilizer at this location.

At Bone Mountain A, a slightly larger mean percentage of the original BA was

cut in fertilized plots (58 vs. 54 percent) and the residual stand averaged less BA

(63.7 vs. 70.9 ft2

) (table 13). This should favor growth of individual trees but not

growth per acre on the fertilized plots. Although response per tree averaged 10.5

percent, response per acre averaged only 0.3 percent because fewer trees remained

on the fertilized plots (256 vs. 282 percent) (table 13). Evidence for continued

response to urea is inconclusive.

At Bone Mountain B, mean percentage of cut BA and the remaining BA are

similar for fertilized and nonfertilized plots (table 13). Although response per tree

in the 5- to 15-year period after fertilization averaged -3.8 percent, BA response

per acre averaged 5.6 percent. We conclude that continued response in the 11-year

after-thinning period is minimal and inconclusive.
30

GENERAL TECHNICAL REPORT PNW-GTR-574

Conclusions

• Commercial thinning of these 20- to 60-year-old Douglas-fir stands by

using ground-based logging vehicles and planned skid trails usually on dry

soils did not reduce, and probably slightly increased tree growth of trailside residual trees.

• At three plantations where half the plots were fertilized 4 years before

thinning, tree response to adjacent trails was independent of the addition of

200 lb N/acre as urea. Little evidence exists for continued response to urea

in the after-thinning period (5 to 11 years).

• Quantifying the practical consequences of soil disturbance for subsequent

tree growth is a complex challenge involving many interacting factors.

Further research at other locations is warranted.

Acknowledgments

We thank Plum Creek Timberlands L.P. (for tree data, professional assistance,

and financial support); Steve Duke, biometrician, for advice about statistical

testing; Johan Hogervorst and Jesus Martinez-Ben, former Oregon State University

graduate students who investigated the Noti location; and Ron Heninger, Pat

Cunningham, Doug Robin, Robert Curtis, and M. Conner Fristoe for technical

review of our draft manuscript.

Table 13—Mean annual growth per tree extended to annual growth per acre in the 11-year period after thinning

of three plantations

BA Growth Growth Response per

Location Fertilizer cut left per treea Treesb per acre tree acre acrec

Lb N/acre Percent Ft2/acre Ft2 No./acre Ft2/acre – – – Percent – – –

Toledo South 0 52 93.0 0.0338 299 10.11 — — —

200 50 92.8 .0335 297 9.95 -0.9 -1.6 6.3

Bone Mountain:

A 0 54 70.9 .0333 282 9.39 — — —

200 58 63.7 .0368 256 9.42 10.5 0.3 12.6

B 0 60 66.2 .0340 245 8.33 — — —

200 59 65.8 .0327 269 8.80 -3.8 5.6 9.2

— = not applicable.

a From tables 3, 6, and 7.

b

 From tables 1 and 4.

c In year 1 through 4 after fertilizer (Miller and others 2001, table 14).
31

Growth of Douglas-fir Near Equipment Trails Used for Commercial Thinning in the Oregon Coast Range

Metric Equivalents

1 inch (in) = 2.54 centimeters

1 square inch (in2

) = 6.45 square centimeters

1 foot (ft) = 0.3048 meter

1 mile = 1.6 kilometers

1 pound per square inch (Psi) = 6.90 kPa

1 pound per acre (lb/acre) = 1.12 kilograms per hectare

1 pound per square inch (lb/in2

) = 70.3 grams per square centimeters (g/cm2)

1 pound per cubic foot (lb/ft2

) = 0.0160 gram per cubic centimeter or megagrams

per cubic meter (Mg/m3

)

1 square foot (ft2

) = 0.093 square meter

1 acre = 0.4047 hectare

1 pound (lb) = 454 grams

1 tree per acre (TPA) = 2.47 trees per hectare

1 thousand broad feet (mbf) = about 4.5 cubic meters

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