SOIL COMPACTION: CONCERNS, CLAIMS, AND EVIDENCE

ABSTRACT Soil resistance to penetration was measured in ten 7­

to 27-acre operational units in overstocked mixed-conifer stands at the Fritz Timber Sale in Northeast Washington. Different combinations of felling and yarding equipment were used to thin eight of these units; no combination was replicated. Two other units remained nonharvested controls. Using a recording penetrometer, resistance was measured to the 33-cm depth (13 inches) at ten stations on 5–17 100-foot long, randomly oriented transects in each unit. Ground-based harvesting equipment operated on and off designated trails. Although trails occupied 6– 57% of the harvested units, total area of strong compac­tion on these trails varied greatly (0–42%). Consequences of soil compaction for tree performance at this sale area are unknown. In fact, consequences of soil disturbance for trees have seldom been measured in the Northwest. At the relatively few places where trees were measured, response to compaction ranged from mostly negative through none to positive. Therefore, current claims about dire consequences of compaction for long-term site pro­ductivity must be based largely on limited sampling, as­sumptions about the consequences of compaction for tree performance, and speculation. We assert that uncertainty about the consequences of compaction and other forms of soil disturbance will remain until long-term tree perfor­mance is correctly measured over a wide range of regional soils and climatic.

Keywords: commercial thinning, harvesting equip­ment, soil bulk density, penetration resistance, penetrom­eter, Northeast Washington

Dick Miller and Harry Anderson

INTRODUCTION We suspect that many of you have observed soil distur­

bance by heavy equipment used to harvest trees. You and others are probably concerned that soil compaction, rut­ting, or displacement of topsoil could reduce future tree survival or growth. The scientific literature shares your con­cern (Greacen and Sands 1980; Froehlich and McNabb 1984; Geist et al. 1989; Page - Dumroese et al. 1993; Wronski and Murphy 1994).

In this presentation, we address concern about long-term soil capacity by supporting the following points:

Point 1. Yes, ground-based harvesting equipment used to thin overstocked forests disturbs the soil over much of the harvested area—to varying severity. To support this point, we will report results from a soil investigation in the recent Fritz Timber Sale on the Colville National For­est.

Point 2. But, the consequences of soil disturbance for subsequent tree performance have been measured only at a limited number of locations, and usually for short peri­ods.

Point 3. Yes, uncertainty about the actual consequences of compaction (and other forms of soil disturbance) will continue until tree performance is reliably measured over a wide range of regional soils and climatic conditions and for a long period of time. That is our concern and we will suggest ways to collect such direct evidence.

POINT 1. HARVESTING EQUIPMENT DISTURBS

THE SOIL (FRITZ TIMBER SALE)

Methods • Sale layout. The eight harvested units and the two

nonthinned control units were 7–27 acres in size (Figs. 1 and 2). The dots represent starting points for randomly oriented transects used to sample soil compaction.

Figure 1.—Units on flat terrain. Dots are starting points for transects.

Figure 2.—Units on steep terrain. Cots are starting points for transects.

Published in Small Diameter Timber: Resource Management, Manufacturing, and Markets proceedings from conference held February 25-27, 2002 in Spokane, Washington. Compiled and edited by D.M. Baumgartner, L.R. Johnson, and E.J. DePuit. Washington State University Cooperative Extension. (Bulletin Office, WSU, PO Box 645912, Pullman, WA 99164-5912. MISC0509. 268 pp.

98 Miller and Anderson

• Stand description. All units on flat terrain were in the subalpine-fir type with about 900–1100 stems per acre that averaged 5.0–5.9 inches d.b.h (Table 1). Of these trees, 24–57% were cut. Units on steep terrain gener­ally exceeded 30% slopes; the timber type was either Douglas-fir or lodgepole pine. Before-thinning density in steep units was about 440–800 trees per acre, less than in the flat units, but mean d.b.h. averaged about 1 inch larger. About 53–69% of these trees were cut.

• Soils. Units on flat terrain included several soil series, which ranged from low to high in their susceptibility to compression when subjected to a load (Table 2). High compressibility was related to finer soil textures (silt loams); conversely low compressibility was associated with sandy loam textures. Sandy loam textures were more prevalent in the steep units.

• Harvesting methods. Different combinations of felling or yarding equipment were used in each of the eight commercially thinned units; no combination was rep­licated (Table 3). Trees were felled by chain saw in only one unit; trees in the remaining units were felled by machines. Ground-based yarding equipment was used on flat units and on one of the four steep units. Desig­nated trails were spaced at either 40 or 130 feet (center­to-center distance). Corridors for skyline cables re-used some of the trails used earlier by either a feller-buncher (that bunched whole trees) or a tracked harvester that felled and processed cut-to-length logs for retrieval by cable. Both the harvester and the feller-buncher had extendable booms that enabled the operator to cut trees within a 30-foot radius.

Table 1.—Site and stand characteristicsa

Before Unit Mean thinning Cut

No. Area slope Forest type Trees Dqb Trees BA

Ac. % ac-1 in. —%—

Flat terrain:

2 27 15 Subalpine fir 1101 5.2 24 34 3 24 7 Subalpine fir 1080 5.9 49 56 4 20 13 Subalpine fir 879 5.0 33 46

19 17 10 Subalpine fir 1025 5.9 57 54 Control 7 12 — — —

Steep terrain:

8 8 31 Douglas-fir 802 6.1 53 56 9 12 36 Douglas-fir 438 7.1 69 54

16 16 33 Lodgepole pine 701 6.4 64 56 17 18 25 Lodgepole pine 537 6.4 66 55

Control 17 40 Lodgepole pine — — — —

• Theoretical coverage of trail/corridors. Designated trails were about 14 feet wide. Therefore, with a center­to-center spacing of 40 feet, designated trails would cover about 35% of the harvested area (Table 4). At this trail spacing, both the feller-buncher and harvester could fell nearby trees, yet remain on the trail. With a designated trail spacing of 130 feet, however, the edge­to-edge distance between trails was 116 feet (130 mi­nus 14 feet). This would require equipment operators either to move off designated trails or leave about a 60­foot wide portion nonthinned. Note that fewest trees were cut on Unit 2, where chainsaws were used to fell trees.

• Measurement of soil strength (resistance to cone pen­etration). Resistance to penetration was measured to the 33-cm depth (13 inches) at ten systematically lo­cated stations on 100-foot long, randomly oriented transects (Fig. 3). There were 5–17 transects in each unit. At nearly all stations, three profiles of soil resistance were registered by a Rimik CP-20 cone penetrometer. Data from these subsamples were averaged to obtain a station mean for each 1.5 cm depth. Post-harvest mea­surements were made that same summer or fall shortly after each unit was harvested in 1998 on steep terrain and in 1999 on flat terrain. The two control units were sampled concurrently. The location of each station rela­tive to skid trails was documented: Code 2 = skid trial rut, Code 6 = beside or between ruts, and Code 0 = non-trail location. Some Code 6 sampling points could have been on displaced soil.

• Measurement of soil bulk density. Fine-soil bulk den­sity near the midpoint of the 0- to 7.5-cm depth was measured before (summer 1997) and after harvest (fall 1999). Soil cores (68.7 cm3, 5.4 cm diameter) were col­lected at systematic locations along transects but only within units on flat terrain.

a Trees 1.0-in. d.b.h. and larger. Source: Camp, A. Nonpublished data, Pacific Northwest Research Station. Wenatchee Forestry Sciences Lab.

b Dq = quadratic mean diameter = diameter of tree of average basal area; BA = basal area

Miller and Anderson 99

Table 2.—Soil series in flat and steep terrain, by unit; based on a soil survey by Zulauf and Starr (1979).

Unit Parent material

No. Area Series name Cap Base Compressibilitya

%

Flat terrain:

2 60 Neuske silt loam silty till silty till H 40 Scar sandy loam till till L

3 45 Nevine loam ash compact till M 30 Scar sandy loam till till L 20 Gahee loam ash outwash L 5 Neuske silt loam silty till silty till H

4 75 Neuske silt loam silty till silty till H 25 Scar sandy loam till till L

19 100 Scar sandy loam till till L Control 50 Nevine loam ash compact till M

50 Gahee loam ash outwash L

Steep terrain:

8 100 Merkel sandy loam ash granitic till L 9 50 Merkel sandy loam ash granitic till L

50 Rock land — — L 16 50 Merkel sandy loam ash granitic till L

50 Rock land — — L 17 80 Merkel sandy loam ash granitic till L

20 Nevine loam ash granitic till M Control 100 Merkel sandy loam ash granitic till L

a H=high, M=moderate, L=low

Table 3.—Harvesting equipment; designated trails corridors were 14-feet wide with center-to-center spacings of 40 or 130 feet.

Terrain and Forwarding-yarding unit Ground-based Cable system

No. Area Tree felling Processing to logs Equipment Spacing Skylinea Corridor spacing

ac ft ft Flat (7-15 % mean slopes):

2 27 Chain saw Harvester b Forwarder c 130 - - - ­3 24 Harvester Harvester Forwarder 40 - ­4 20 Feller-buncher c Whole tree Skidder c 130 - - - ­19 17 Feller-buncher Harvester Forwarder 130 - - - ­

Steep (25-36 % mean slopes):

8 8 Harvester f Harvester - - 40 Uphill 80 9 12 Harvester Harvester Forwarder c 40 — - ­16 16 Feller-buncher d Whole tree - - 40 Downhill 40 17 18 Harvester Harvester - - 40 Downhill 80

a Skagit model 333 yarder, adapted with a third drum; Christy haul-back carriage b Tracked Kabelco model 200 single-grip harvester with Kato 500 saw head; cut logs to length (CTL) c Rubber-tired Valmet model 892 forwarder (14-ton capacity) d Tracked Timbco model 445 B feller-buncher with Quadco Hot-Saw felling head on an extendable boom e Rubber-tired Cat model 518 skidder with swinging grapples f Tracked Valmet model 500T single-grip harvester (with tilting cab); cut logs to length (CTL)

100 Miller and Anderson

Table 4.—Designated trails: theoretical coverage

Width

Spacing between

Centerline Edges

Needed

reach

Percentage of harvest area

(width ÷ CL distance)

- - - - - - - - - - - - - - - Feet - - - - - - - - - - - - - - ­

14 40 26 13

130 116 58

%

35

11

Figure 3a.—Designated trail with slash place by a tracked- Figure 3b.—Soil strength was measured by a cone penetrometer. harvester.

Figure 3c.—The general terrain of the Fritz Timber Sale.

Results (from Fritz) • Percentage of area in trails, by equipment combina­

tions. We have two independent estimates of the com­bined area in designated and supplemental trails (Table 5). The first is based on the percentage of all sampling stations (10 per transects) that fell in or near trail ruts (Codes 2 and 6). By this estimate, 6–57% of the unit areas were in trails. The second estimate was based on a similar number, but different 100-foot long transects in each unit. Where these transects crossed machine trails, the intercepted distance was expressed as a per­centage of total transect length (Tepp, in review). Trails occupied 12–38% of the harvested area based on this second estimate.

Miller and Anderson 101

• Bulk density in surface soil of flat terrain (trail vs. non-trail portions). Among the four harvested units on flat terrain, fine-soil bulk density on trails averaged 3–14% greater than that in non-trail portions (Table 6). Note that the USFS. Northwest Region’s standard for judging compaction as detrimental is a 20% or more increase in BD of soils derived from volcanic ash or pum­ice (USFS 1998). By this standard, average compaction on the trails in flat units was not detrimental. Because some of the non-trail portions also could have been compacted, we calculated a 20% increase in the mean before-harvest BD as our threshold standard. Based on this standard, 15% of unit 19 had “detrimental” com­paction.

Table 5.—Percentage of thinned area in trails, by unit.

Terrain Harvesting equipment Trail Penetrometer Monitoring transects a

and unit spacing transects Trails Difference

No. % of stations No. % of total length

Flat:

2 Chain saw, harvester, forwarder 130 13 21 13 12 -10 3 Harvester, forwarder 40 14 57 14 29 -13 4 Feller-buncher, whole-tree skidder 130 10 17 10 28 3 19 Feller-buncher, harvester, forwarder 130 8 39 8 38 -1

Steep:

8 Harvester, uphill skyline 40 10 6 14 19 13 9 Harvester, forwarder 40 15 18 16 28 10 16 Feller-buncher, whole-tree, downhill 40 14 27 14 25 -1 17 Harvester, downhill skyline 40 18 13 17 30 17

a Total transect lengths per unit range from 800 to 1700 feet. Adapted from Tepp (in review).

Table 6.—Fine-soil bulk density after harvest in the 0- to 7.5-cm mineral soil depth in trails (ruts and adjacent soil) vs. other portions, in flat terrain.

Visual strata

Trails Non-trail Difference

Unit No.

Equipment Sta. No.

Mean SE a

— Mg m-3 — % Sta. No.

Mean SE a

— Mg m-3 — % Absol

Mg m-3

Rel %

2

3

4

19

Chain saw, harvester, forwarder

Harvester, forwarder

Feller-buncher, whole-tree skidder

Feller-buncher, harvester, forwarder

25

74

29

30

0.851

0.695

0.816

0.823

0.000

0.015

0.044

0.046

0.0

2.2

5.4

5.6

100

57

65

48

0.782

0.676

0.701

0.763

0.024

0.020

0.025

0.029

3.1

3.0

3.6

3.8

0.069

0.019

0.115

0.060

8

3

14

7

a SE = standard error of mean; derived from nested ANOVA (2-stage sampling)

102 Miller and Anderson

• A few 70-year-old trails of a former fire-salvage sale were readily identifiable in some units on steep terrain by paucity of vegetation and shallow ruts. Lateral berms were absent or indistinct, so topsoil displacement was less evident. Soil in trails remained compacted, espe­cially below the 5-cm depth (Fig. 4); however, maxi­mum resistance was less than 1500 kPa. Note that 2000– 3000 kPa is generally considered detrimental to root growth (Powers et al. 1998).

Figure 4.—Average soil resistance in steep unit 17, by location of sampling station (3 subsamples per station). Note the residual mean compaction in 70-year-old trails of a former salvage sale.

• Soil resistance increased after recent thinning in the Fritz Sale (Figs. 5 and 6). The increase was greater in the finer textured soils of the flat terrain than in sandier textures on slopes.

* Figure 5. Unit 3 had designated trails at 40-foot spac­ing; therefore, the harvester could readily cut trees in the intervening portions and pile logs along the trail. A rubber-tired forwarder transported these logs to the landing. Note the much greater resistance in the ruts (tracks) especially at 5-cm and lower depths. Note also that below 20 cm, average resistance exceeded 2000

Figure 5.—Average soil resistance in flat unit 3 after harvester-for­warder combination and 40-foot trail spacing (center-to-center). Note greater resistance in the tracks than in adjacent soil, some of which could be displaced berm.

kPa. Our sample size was large, 32–58 sampling sta­tions per coded condition.

* Figure 6. Trails in unit 4 were designated at 130-foot intervals. Therefore, the feller-buncher had to leave designated trails to fell intervening trees, which were then yarded as whole trees with a rubber-tired skidder. About 75% of the unit was Neuske silt loam, a highly compressible soil. Note the large difference in soil re­sistance at stations associated with trails (ruts and adjacent soil) compared with off-trail stations. Note also that resistance generally exceeded 2000 kPa at lower depths, but this root-restricting resistance was close to the surface in the trails.

Figure 6.—Average soil resistance in flat unit 4 after combination of felling by a feller-buncher and whole-tree skidding; 130­foot trail spacing. Note resistance at stations in trails (ruts and nearby soil) exceeded 2000 kPa close to surface.

* Note that different combinations of equipment were assigned to each unit. This lack of true replication pre­cludes statistical testing to indicate which equipment provides the least impact on soil. Although we used different methods to assess soil compaction after har­vesting (Landsberg et. al., pending review), we pro­vide only the post-harvest comparisons in this pre­sentation.

* Trail vs. nontrail. This method of comparison equals the usual, retrospective (after-harvest) monitoring in which one samples soil on trails and compares these estimates of soil resistance or bulk density to corre­sponding estimates from non-trail portions. When interpreting retrospective results, one must verify or assume (1) that trails were placed on soils representa­tive of the remaining portions (soils were similar), (2) that soil moisture conditions on and off trails were similar when sampled, hence (3) that differences can be explained by equipment impact (a typical moni­toring question). Based on this after-harvest moni­toring, we note that average resistance in trails of only one of eight units exceeded the proposed standard defining detrimental resistance (Table 7). Of the sta­tions located on trails, 0–70% had penetration resis­tance equal to or exceeding 2000 kPa on the 15- to 25-cm depth. This equated to as much as 40% of Unit 3 being detrimentally compacted (Fig. 7).

Miller and Anderson 103

Table 7.—Average difference in after-harvest soil resistance on trails (ruts and adjacent soil) versus non-trail portions, by unit and depth in mineral soil.

Terrain Surface soil (0-10 cm) Standard zone a (15-25 cm)

and Equipment Trail Non-trail Difference Trail Non-trail Difference Unit

Flat: ———— kPa———— % ———— kPa ——— — %

2 Chain saw, harvester, forwarder 1682 852 830 97 1926 1720 206 12 3 Harvester, forwarder 1257 1092 165 15 2258 2192 66 3 4 Feller-buncher, whole-tree skidder 1890 890 1000 112 2976 1803 1173 65 19 Feller-buncher, harvester, forwarder 1514 1058 456 43 2198 1714 484 28

All Mean 1586 973 613 63 2340 1857 483 26

Steep:

8 Harvester, uphill skyline 627 717 -90 -13 794 888 -94 -11 9 Harvester, forwarder 887 816 71 9 1416 989 427 43 16 Feller-buncher, whole-tree, 790 754 36 5 1058 850 208 24

downhill skyline 17 Harvester, downhill skyline 1046 757 289 38 1396 986 383 35

All Mean 838 761 77 10 1166 928 238 26

a Dr. Robert Powers (PSW Research Station, USFS) has proposed to the U.S. Forest Service (Pacific Southwest Region, Region 5) that detrimental soil damage be defined as a 500 kPa or more increase in soil strength (15- to 25-cm depth in mineral soil).

Figure 7.—Percentage of harvested area in trails (ruts and nearby soil) and with pen­etration resistance of 2000kPa or more in the 15- to 25-cm depth of trails, by unit number. Additional non-trail area could have compacted soil.

Conclusions from the Fritz Timber Sale 1. Among the eight thinned units that we investigated, 6–57% of the harvested area was in designated and supplemen­

tal trails. Supplemental trails were made where equipment had to leave widely spaced (130 ft) designated trails to fell intervening trees.

2. Extent and severity of compaction was greater on flat units, where all yarding was by forwarders or skidders and where three of four units had soils of silt loam or loam textures.

3. Consequence of this disturbance to tree performance is unknown. Will this be assessed in the future?

104 Miller and Anderson

POINT 2. • The linkage between soil disturbance and tree perfor-

THE CONSEQUENCES OF SOIL DISTURBANCE FOR SUBSEQUENT

mance (the variable needed for economic analysis) must be quantified to know the practical consequences of soil compaction.

TREE PERFORMANCE ARE SELDOM MEASURED

• East of the Cascades. All east-side studies are based on data collected 8–64 years after overstory removal or clearcutting (Table 8). Effects of trails in eastside com­mercial thinnings on residual tree growth have not been reported. Note that all investigations are retrospective rather than controlled-treatment.

Table 8.—Eastside: Investigations of tree growth on skid trails vs. off skid trails, by type of harvesta.

Area and species Locations Soil texture Tree age Results Source

No. years

Thinning: no reports 0 — — — None

Overstory removal:b

WA, ponderosa pine 3 loamy (ash) 9-18 -20 % stem volume Froehlich et al. (1986) -5 % tree height

WA, lodgepole pine 1 ashy 11 0 % volume, tree height Froehlich et al. (1986) OR, ponderosa pine 1 sandy loam 64 -6 % to -12 % tree BA growth Froehlich (1979)

Clear cutting: OR, ponderosa pine 1 loamy 8 - 38 % tree height Cochran and Brock (1985)

(at + 20 % increase in BD) N. ID, ponderosa pine 1 silt loam (ash) 20-25 -20 % d.b.h. (displacement) Clayton et al. (1987)

-10 % d.b.h. (compaction) N. ID, lodgepole pine 2 silt loam (ash) 15-19 -22 to-25 % d.b.h. (compaction) Clayton et al. (1987)

15 to -25 d.b.h. (displacement) BC, conifers 4 loam to silt loam 16-18 -14 to +4 % tree height Smith and Wass (1980) BC, conifers 5 loamy (calcareous) 9-22 -12 to-15 % height Smith and Wass (1979) BC, conifers 3 sandy (acid) 9-22 +18 to 22 % height Smith and Wass (1979) BC, Engleman spruce 3 sandy loam 9-10 tree volume least on track, Senyk (2001)

most on berm BC, lodgepole pine 3 sandy loam 9-10 tree volume least on track Senyk (2001)

at two locations and most at one location

a Skidroads, not trails, were investigated in BC. These roads are bladed into steep slopes. Growth usually differs with position on running surface and sidecost (cut, track, fill).

b The influence of residual overstory trees on growth of younger, measured trees complicates inferences about skid-trail effects.

Miller and Anderson 105

POINT 3. WHAT EVIDENCE IS RELIABLE

FOR JUDGING RISK TO LONG-TERM SITE PRODUCTIVITY?

• Conventional soil monitoring (“effectiveness” moni­toring of USFS) has an area and severity standard. Sev­eral types of soil disturbance are recognized in addition to compaction. Compaction severity that exceeds a specified threshold (e.g., 20% increase in bulk density) is considered “detrimental” compaction; this “counts” as a risk to soil capacity or quality (Fig. 8). The com­bined, estimated area of “detrimental” disturbances should not exceed 20% of the total (gross) harvested area, including the permanent roads which obviously remove land from production (USFS 1998).

Figure 8.—Percentage of harvested area in trails and with a 20% or more increase over average preharvest bulk density (0 to 7.5 cm depth) in specified harvest unit.

* At Fritz (8 units)

• 6 to 42 % of the thinned area was in trails and cor­ridors

• We detected compaction (increased soil resistance) on these trails, but was it really “detrimental” to soil capacity to grow trees?

* Soil monitoring:

• Provides numbers indicating the types, severity, and coverage of soil disturbance

• Provides indirect evidence: soil properties are changed. From this circumstantial evidence, many assume that tree performance will be reduced; But performance can also be increased on compacted soil in some situations (Powers and Fiddler 1997)

* Tree monitoring (termed “validation” monitoring by the USFS) would test this assumed linkage between tree performance and changed soil properties.

* For example, one would measure:

• Seedling survival and early growth; this is simplest to do and indicative of short-term effects

• Growth of residual trees after thinning or partial cutting; this is more difficult to accomplish

• Cubic volume yields per acre in mature stands; the most difficult to estimate, but the definitive mea­sure

* We assert that monitoring tree growth

• Provides the necessary direct evidence for judging risk to long-term site productivity

• Can indicate which type, severity, and pattern of soil disturbance really affects tree performance

SUMMARY

Point 1. Increased soil resistance after harvesting at the Fritz Tim­

ber Sale are consistent with results from other investiga­tions. Ground-based harvesting equipment used to thin these overstocked forests disturbed soil over much of the harvested area. Estimated combined area of severe com­paction (> 2000 kPa) varied greatly among the eight units (0–42%).

Point 2. Relative to the millions of acres of commercial forest in

the Inland West, the consequences of soil disturbance for subsequent tree performance have seldom been measured. Without local experience and longer periods of observa­tion, current claims about dire consequences of soil com­paction to long-term site productivity must be based largely on assumptions, circumstantial evidence, and speculation.

Point 3. Uncertainty about the consequences of soil compaction

and other forms of soil disturbance will remain until tree performance is reliably measured over a wide range of re­gional soils and climatic conditions, and over a long pe­riod of time.

Take-away Message:

If you are concerned about tree performance, then collect direct evidence—measure trees.

ACKNOWLEDGMENTS We became involved with the Creating Opportunities

project (CROP) after Dr. Joan Landsberg retired from USFS. We summarized data (collected by Forest Service crews from Wenatchee Lab) and are preparing reports for publication. We thank the CROP and Dr. Landsberg for the opportu­nity to complete this work. We are also grateful to Jeffery Tepp for the orderly transfer of data files and assorted records, to John Senyk and Andrew Youngblood for tech­nical review, and to Sherry Dean for helping prepare our visual presentation.

106 Miller and Anderson

LITERATURE CITED Cochran, P.H. and T. Brock. 1985. Soil compaction and

initial height growth of planted ponderosa pine. Res. Note PNW-434. U.S. Department of Agriculture–For­est Service, Pacific Northwest Forest and Range Experi­ment Station, Portland, OR. 4 p.

Clayton, J.L., G. Kellogg, and N. Forrester. 1987. Soil dis­turbance-tree growth relations in central Idaho clearcuts. Res. Note INT-372. U.S. Department of Agri­culture–Forest Service, Intermountain Research Station, Ogden, UT. 6 pp.

Froehlich, H.A. 1979. Soil compaction from logging equip­ment: Effect on growth of young ponderosa pine. Jour­nal of Soil Water Conservation 34: 276-278.

Froehlich, H.A. and D.H. McNabb. 1984. Minimizing soil compaction in Pacific Northwest forests. In: Stone, E. L. (ed.). Forest soils and treatment impacts. Proceed­ings of the 6th North American Forest Soils Confer­ence, Department of Forestry, Wildlife, and Fisheries, University of Tennessee, Knoxville, TN. Pp. 159-192

Froehlich, H.A., D.W.R. Miles, and R.W. Robbins. 1986. Growth of young Pinus ponderosa and Pinus contorta on compacted soils in central Washington. Forest Ecol­ogy and Management 15: 285-294.

Geist, J.M., J.W. Hazard, and K.W. Seidel. 1989. Assessing physical conditions of some Pacific Northwest volca­nic ash soils after forest harvest. Soil Science Society of America Journal 53: 946-950.

Greacen, E.L. and R. Sands. 1980. Compaction of forest soils: A review. Australian Journal of Soil Research 18: 163-189.

Landsberg, J.D., R.E. Miller, H.W. Anderson, and J.S. Tepp. (Pending review). Soil resistance and bulk density as affected by commercial thinning on flat and steep ter­rain in northeastern Washington. U. S. Department of Agriculture–Forest Service, Pacific Northwest Research Station.

Page-Dumroese, D., A. Harvey, M. Jurgensen, and R. Gra­ham. 1991. Organic matter function in the western-montane forest soil system. In: Harvey, A.E. and Neuenschwander, L. F. (comps.). Proceedings-manage­ment and productivity of western-montane forest soils; 1990 April 10-12; Boise, ID. Gen. Tech. Rep. INT-280. U.S. Department of Agriculture–Forest Service, Inter­mountain Research Station, Ogden, UT. Pp. 95-100.

Powers, R.F. and G.O. Fiddler. 1997. The North American Long-Term Soil Productivity Study: progress through the first 5 years. In: Proceedings, Eighteenth Annual Forest Vegetation Management Conf., Jan 14-16, 1997, Sacramento, CA. Published by the Forest Vegetation Management Conference, Redding, CA.

Smith, R.B. and E.F. Wass. 1979. Tree growth on and adja­cent to contour skidroads in the subalpine zone, south­eastern British Columbia. Rep. BC-R-2. Canadian For­est Service, Pacific Forestry Research Centre, Victoria, BC. 26 pp.

Smith, R.B. and E.F. Wass. 1980. Tree growth on skidroads on steep slopes logged after wildfires in central and southeastern British Columbia. Inf. Rep. BC-R-6. Ca­nadian Forest Service, Pacific Forestry Research Cen­tre, Victoria, BC. 28 pp.

Senyk, J.P. 2001. Tree growth on displaced and compacted soils. Tech. Transfer Note No. 26. Canadian Forest Ser­vice, Pacific Forestry Research Centre, Victoria, BC.

Tepp, J.S. In review. Assessing visual soil disturbance on eight commercially thinned sites in northeastern Wash­ington. U.S. Department of Agriculture–Forest Service, Pacific Northwest Research Station.

USFS. 1998. USDA Forest Service Manual, FSM 2520 (Wa­tershed Protection and Management) R-6 Supplement No. 2500-98-1, Effective Aug 24, 1998.

Wronski, E.B. and G. Murphy. 1994. Responses of forest crops to soil compaction. In: Sloane, B.D., and van Ouwerkerk, C. (eds.). Soil compaction in crop produc­tion. Elsevier Science. Pp. 317-342.

Zulauf, A. and W.A. Starr. 1979. Soil survey of North Ferry area, Washington, parts of Ferry and Steven counties. U.S. Department of Agriculture–Soil Conservation Ser­vice and Forest Service and Washington Agricultural Experiment Station. 121 p. and 73 maps.

Authors Dick Miller, Retired USDA Forest Service-PNW Research Station Forestry Sciences Laboratory 3625 93rd Ave. SW Olympia, WA 98512-9193 360-956-2345 ext 669 millersoils@aol.com

Harry Anderson USDA Forest Service-PNW Research Station Forestry Sciences Laboratory 3625 93rd Ave. SW Olympia, WA 98512-9193

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

endorsement by the US Department of Agriculture of any product or service.