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  • Introduction

    The impacts of mechanized forest harvesting onsoil physical properties have been widelyreported in countries such as Canada, the USAand Australia. Tree-length extraction (whole orstem-only), where stems are dragged from the siteby a skidder, can cause soil compaction, deeprutting and erosion and, with time, loss of siteproductivity. In contrast, the shortwood systemof harvesting is commonly used in the UK,

    whereby cutting and sorting is done on-site bypurpose-built harvesting machinery, beforecarriage by forwarder to roadside log-loadings.To reduce soil disturbance under this system,logging residues (largely branch wood) are placedon the ground to form a protective layer, or slashroad, over which all machinery travels.

    Previous studies have demonstrated that wheretimber is carried, slash roads can be highly effec-tive in limiting soil disturbance, though theirlongevity is limited where stems are dragged. For

    Reduced ground disturbance duringmechanized forest harvesting onsensitive forest soils in the UKM.J. WOOD1,3*, P.A. CARLING1 AND A.J. MOFFAT2

    1 Department of Geography, University of Southampton, Southampton SO17 1BJ, England2 Forest Research, Alice Holt Lodge, Wrecclesham, Farnham, Surrey GU10 4LH, England3 Forest Research, University of Canterbury, PO Box 29 237, Christchurch, New Zealand* Corresponding author. E-mail: matthew.wood@forestresearch.co.nz

    Summary

    Field trials were undertaken in north-east England and south-west Scotland to investigate the degreeand nature of disturbance on selected forest soils during mechanized harvesting, where extractionroutes were armoured with a layer of logging residues (slash roads). Dry soil bulk density, soilstrength (soil penetration resistance) and saturated hydraulic conductivity, measured directly beneaththe machine wheel tracks on gleyed mineral and deep peatland soils (peat >45 cm deep), exhibitedonly minor changes despite high levels of trafficking. This was ascribed to (1) the role of the slashroads in reducing machine ground pressures; (2) the inherent strength and elastic recovery of theoverlying fibrous peaty soils, retained in situ as a result of the slash roads; and (3) the slow rates ofdensification associated with the underlying saturated fine textured mineral soils. In addition, theslash roads were observed to improve vehicle traction and efficient carriage of timber to roadside loglandings. This study demonstrated that disturbance on peaty or fine-to-medium textured mineralsoils at high water contents can be largely avoided, allowing operations to continue during periodswhen wet ground conditions may otherwise limit harvesting.

    Institute of Chartered Foresters, 2003 Forestry, Vol. 76, No. 3, 2003

    06 cpg030 11/6/03 12:21 pm Page 345

  • example, changes in soil penetration resistance,hydraulic conductivity, dry bulk density and airporosity at depths between 0 and 45 cm weresignificantly lower along areas protected bylogging residues (FW 18 kg m2) compared withunprotected areas after up to seven machinecycles (Jakobsen and Moore, 1981), where onecycle combined a loaded and unloaded pass ofthe skidder on a dry kraznozem soil in Australia.However, logging residues were quickly mixedwith the surface soil, deflected by wheel action orlog dragging, and after 15 machine cycles, differ-ences in soil physical properties betweenprotected and unprotected areas were non-significant. On dry sandy soils in the USA,McDonald and Seixas (1997) found that loggingresidues (FW 10 or 20 kg m2) made no differ-ence to increases in soil density at 05 cm depthfollowing a single pass by a loaded forwarder(due to the speed with which initial air voids werecompressed), though after five passes increases insoil density were up to 40 per cent lower alongprotected areas compared with unprotectedareas. At increased moisture contents, the densityof logging residues became significant, and at510 cm depths following five machine passes,increases in bulk density under 10 kg m2 loggingresidue cover were 60 per cent greater than under20 kg m2 cover.

    However, only limited data exist regarding theefficacy of slash roads on some of the more sen-sitive soils encountered in the UK uplands suchas deep peatland (peat >45 cm deep) and peatygleys (Forestry Commission, 1998). Wall andSaunders (1998) and Hutchings et al. (2002)investigated the effect of up to 12 forwarderpasses (combining laden and unladen passes) ona surface-water gley (Kielder Forest, north-eastEngland). Increases in dry bulk density and soilpenetration resistance under slash roads derivedfrom four, six, eight and 10 rows of trees wereless than those for bare ground, though no signifi-cant differences were found between the treat-ment types. The effects of higher trafficintensities, such as those associated with theshortwood system of extraction, on soil physicalproperties and on the longevity of the slash roads,were not considered.

    This study describes the effects of mechanizedharvesting operations on the physical propertiesof sensitive deep peat and peaty gley soils at six

    sites in north-east England and south-westScotland. Key to this study is the fact that theobservations were made under normal opera-tional conditions employing the shortwoodsystem of extraction associated with high traf-ficking intensities, typically 50+ and 8+ machinepasses for primary and secondary extractionroutes respectively (Wood, 2001). At each site,extraction routes where armoured with a layer oflogging residues (slash roads) from up to ninerows of trees.

    Methods

    Six operational clearfell sites employing theshortwood system of extraction on deep peat orpeaty gley soils were visited in successionbetween June 1998 and November 1999(Table 1). Primary extraction routes (>200 m inlength) were located along the edge of the forest,and fed by secondary extraction routes(150200 m in length) spaced regularly over theentire site. As harvesting progressed at each site,a suitable experimental plot, comprising threeadjacent secondary extraction routes, waslocated where species, age, planting regime andground features (slope, presence of drains, etc.)were uniform. The design of the experimentalplot at each site is presented in Figure 1.

    Forest and plot descriptions, machine specifi-cations and machine ground treatments (combin-ing multiple harvester and laden/unladenforwarder passes) are summarized in Table 1.Replication of ground treatments based on theexact number of passes at any point was difficultgiven the heterogeneity of the ground, and oper-ational nature of each site. As a result, sampleunits (see Figure 1) were located as best to repli-cate minimum, low, high and maximum trafficintensities along each extraction route during theremoval of timber. The commercial nature ofeach site in this study did not permit traffickingwithout a slash road. The soil profile descriptionfor site 1 (Table 2), based on the classificationsystem described by Pyatt (1970), was consideredapplicable to sites 36 (following observation ofthe intact soil cores collected at these sites seebelow).

    Given the relatively homogeneous nature ofthe deep peat profile at site 2 (following

    346 FORESTRY

    06 cpg030 11/6/03 12:21 pm Page 346

  • GROUND DISTURBANCE DURING MECHANIZED FOREST HARVESTING 347

    Tab

    le1:

    Site

    and

    exp

    erim

    enta

    l plo

    t ch

    arac

    teri

    stic

    s at

    eac

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    te

    Site

    (an

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    ting

    dat

    e of

    exp

    erim

    enta

    l plo

    t)

    Des

    crip

    tion

    1 (J

    une

    1998

    )2

    (Aug

    . 199

    8)3

    (Oct

    . 199

    8)4

    (Mar

    . 199

    9)5

    (May

    199

    9)6

    (Nov

    . 199

    9)

    Gri

    d re

    f.N

    Y 6

    4594

    5N

    X 3

    7588

    5N

    Y 6

    8590

    5N

    Y 7

    4590

    5N

    Y 6

    7593

    5N

    Y 7

    1585

    5So

    il1Pe

    aty

    gley

    Dee

    p pe

    atPe

    aty

    gley

    Peat

    y gl

    eyPe

    aty

    gley

    Peat

    y gl

    eyC

    ompa

    rtm

    ent

    area

    (ha

    )70

    25*

    25*

    842

    .512

    .5Sp

    ecie

    sSi

    tka

    spru

    ceSi

    tka

    spru

    ceSi

    tka

    spru

    ceSi

    tka

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    ceSi

    tka

    spru

    ceSi

    tka

    spru

    ce

    Plan

    ting

    dat

    e219

    5119

    5019

    4819

    4819

    5119

    51Y

    ield

    cla

    ss3

    12

    12

    1014

    Plot

    siz

    e (m

    )44

    15

    0

    16 (

    5)3

    15

    0

    16 (

    5)

    3

    150

    12

    (5)

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    (5)

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    (5)

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    (5)

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    (33

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    ass6

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    n (k

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    000

    1377

    016

    000

    1300

    013

    770

    1700

    0C

    apac

    ity

    (kg)

    1200

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    Mac

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    pas

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    41

    69

    2810

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    esti

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    sal

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    gth

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    aver

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    was

    5m

    at

    each

    sit

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    area

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    whe

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    rack

    s an

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    par

    enth

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    a of

    the

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    oad

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    erce

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    e.6

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    ing

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    sys

    tem

    (Fo

    rest

    ry C

    omm

    issi

    on, 1

    996)

    .7

    The

    ran

    ge a

    cros

    s en

    tire

    plo

    t co

    mbi

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    and

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    ng m

    achi

    nery

    (at

    eac

    h si

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    his

    com

    pris

    ed 2

    4 p

    asse

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    vest

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    he r

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    r).

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    utho

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    stim

    ate.

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    oute

    s 1

    and

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    adja

    cent

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    3 w

    as lo

    cate

    d so

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    the

    sam

    e st

    and.

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    lter

    nati

    ng b

    etw

    een

    two

    row

    s Si

    tka

    spru

    ce a

    nd t

    wo

    row

    s Sc

    ots

    pine

    (de

    ad, p

    lant

    ed o

    rigi

    nally

    as

    a nu

    rse

    spec

    ies)

    .

    06 cpg030 11/6/03 12:21 pm Page 347

  • 348 FORESTRY

    Unt

    raffi

    cked

    area

    Sin

    gle

    soil

    core

    and

    aug

    er h

    ole

    mea

    sure

    men

    ts

    Soi

    l res

    ista

    nce

    to p

    enet

    ratio

    n

    Sla

    sh r

    oad

    clea

    red

    toal

    low

    acc

    ess

    togr

    ound

    /soi

    lsu

    rfac

    e

    1 m

    3 m

    10-

    16 m

    150

    m

    Fron

    t

    Bac

    k

    Prim

    ary

    extr

    actio

    nro

    uteIncreasing numbers of

    machine passes

    S

    econ

    dary

    ex

    trac

    tion

    rout

    e

    4 m

    Sam

    ple

    unit

    Sla

    sh r

    oad

    Gro

    und

    surf

    ace

    prof

    ile

    Plo

    t

    *rig

    ht a

    nd le

    ft w

    heel

    trac

    ks a

    s vi

    ewed

    from

    the

    back

    of e

    ach

    seco

    ndar

    y ex

    trac

    tion

    rout

    e.

    Rig

    ht*

    lef

    t*w

    heel

    trac

    ks

    Figu

    re1.

    Des

    ign

    of t

    he e

    xper

    imen

    tal

    plot

    at

    each

    sit

    e.

    06 cpg030 11/6/03 12:21 pm Page 348

  • observation of the intact soil cores collected atthis site see below), a pit profile description wasnot undertaken. Particle size distribution for themineral horizons (sites 1 and 36) remained con-sistent from site to site (Wood, 2001), where thetextural class was predominantly silty clay (withoccasional loamy texture). During each trial,daily observations of the water level in augerholes (n = 5) showed that the mineral (AE)layers at sites 1 and 36 (peaty gley soils) weresaturated. At site 2 (deep peat soil) the water-table remained, on average, 30 cm below theground surface.

    At each site, intact soil cores were collectedwithin 1 week of timber removal from untraf-ficked and trafficked areas of the experimentalplot (Figure 1) at up to 1 m depth using a cylinderauger (Eijkelkamp Agrisearch Equipment, VanWalt Ltd, Surrey, UK), inserted by a Pionjar-120hammer action percussion drill (Atlas Copco AbLtd, SE-105 23, Stockholm, Sweden). Thecylinder auger comprised a steel pipe of 120 cm

    (c. 11 cm inside diameter) with a bevelled (30)cutting edge (c. 10 cm inside diameter). Toprovide an undisturbed reference at site 1, coreswere collected from an untrafficked area adjacentto the experimental plot (n = 4). For sites 2 and3, a single core was taken from the untraffickedarea at each of six of the 12 sample units (chosenrandomly), and from all 12 sample units at sites46. At all sites, trafficked cores (left and rightwheel tracks) were collected from all sampleunits. Occasionally, the presence of large roots orstones meant that a useable core could not be col-lected. In the laboratory, soil cores (initially10 cm diameter) were divided into 5 cm sections.For sites 26, each section was sub-sampled usinga 5 cm diameter coring tin (to reduce edge effectsduring core collection observed at site 1). Coresections containing large roots or stones were dis-carded. Dry soil bulk density and gravimetricwater content were derived using standardlaboratory procedures.

    Soil strength (soil penetration resistance) data

    GROUND DISTURBANCE DURING MECHANIZED FOREST HARVESTING 349

    Table 2: Soil profile description for site 1

    Date and location 31 August 1998, Kielder Forest, England (NGR: NY 652942)Soil type/parent material Peaty gley/clayey glacial tillSlope, elevation, aspect 1014 (NS), 360 mDrainage* Poor to moderateErosion NoneCoarse fragments NoneRock outcrops NoneGround cover Needle litter/mature Sitka spruce (planted 1951)L (02 cm) Sitka spruce needles, cones and twigs, abrupt change to next horizonF (27 cm) Wet, apedal, roots (fine, common, fibrous), abrupt change to next horizonH (722 cm) Dark reddish brown (5 YR 25/2), wet, apedal, roots (fine, few, amorphous), abrupt

    change to next horizonAh (2232 cm) Black (5 YR 25/1), silty clay loam, wet, coarse sub-angular structure weakly

    developed, roots (very fine, few, fibrous), abrupt change to next horizonEg1 (3252 cm) Light yellowish brown (10 YR 6/4), sand, slightly stony (large, angular, pebbly),

    moist, very coarse angular structure moderately developed, root remains (fine, few,fibrous), abrupt change to next horizon

    Eg2 (5265 cm) Light brownish grey (25 Y 6/2), mottles (dark yellowish brown; 10 YR 4/6, many,very fine, prominent), loamy sand, moderately stony (medium, sub-angular,pebbly), moist, very coarse sub-angular structure moderately developed, rootremains (fine, few, fibrous), clear change to next horizon

    B1 (6582 cm) Greyish brown (25 Y 5/2), mottles (dark yellowish brown; 10 YR 4/6, many, fine,prominent), loamy sand, moist, apedal, root remains (fine, few, fibrous), clearchange to next horizon

    B2 (82+ cm) Dark grey (25 Y N/4), mottles (dark yellowish brown; 10 YR 4/6, many, fine,prominent), sandy clay, wet, apedal, clear change to next horizon

    * Authors assessment.

    06 cpg030 11/6/03 12:21 pm Page 349

  • were collected within 1 week of timber removalfrom untrafficked and trafficked areas of theexperimental plot (Figure 1) at 3 cm depth incre-ments up to 45 cm depth, using a hand-heldrecording penetrometer (Holtech Associates,Rough Rigg, Harwood-In-Teesdale, Co. Durham,UK). To provide an undisturbed reference at site1, mean soil penetration resistance data (n = 6penetrations) were collected at 10 randomlyselected untrafficked locations adjacent to theexperimental plot. For the remaining sites, meanundisturbed soil penetration resistance data (n =10 penetrations) were collected from an undis-turbed area within each of the 12 sample units.At site 1, mean trafficked soil penetration resist-ance data (n = 6 penetrations) and at sites 26(n = 10 penetrations) were collected from theright and left wheel tracks within each sampleunit. Mean soil penetration resistance data (n =10 penetrations) were also collected from both inbetween and adjacent to the left and right wheeltracks within each sample unit at sites 46.

    At year 2 sites a profile of the ground surfacetopography (Figure 1) was completed for each

    sample unit within 1 week of timber removal,based on measurements taken at 20 cm intervalsalong a horizontal reference (tree stumps werenot recorded). Based on these profiles, andunquantified observations at year 1 sites whichdemonstrated virtually no change in the positionof surface and sub-surface soil horizons, coresections were aligned for comparison as shown inFigure 2.

    Saturated hydraulic conductivity of the uppermineral layer at sites 5 and 6 was based onmeasurement of the rise and fall of water levelsin auger holes (n = 12 untrafficked and 24 traf-ficked) lined with plastic pipe (5 cm insidediameter) extending into the upper 5 cm of themineral (AE) layer (Youngs, 1991).

    Results

    Following removal of timber from the experi-mental plot and adjacent areas at each site, anassessment was made of the structure and com-position of the slash road (Figure 3). The main

    350 FORESTRY

    Litter

    Incr

    emen

    ts o

    f 5 c

    m

    Soil cores before realignment Soil cores after realignment

    Peaty (O) layer1

    Mineral (A-E) layer2

    Derivation of mean values for either untrafficked or trafficked areas at each site:

    1At each site for either the untrafficked or trafficked areas, mean values for each 5 cm depthincrement equal mean a, b, c etc, mean d, e, f etc.

    2At each site for either the untrafficked or trafficked areas, mean values for each 5 cm depthincrement equal mean 1, 2, 3 etc, mean 4, 5, 6 etc.

    1

    a

    g

    j

    l

    7

    a

    8

    i

    k

    9

    g

    j

    h i

    k

    l

    2

    3

    1 2 3

    c b c

    h

    b

    6

    5

    4 4 5 6

    d e f d e f

    7 8 9

    Figure 2. Alignment of soil core sections for comparison.

    06 cpg030 11/6/03 12:21 pm Page 350

  • body of the slash road, derived from 4 (site 6), 6(site 5), 7 (sites 3 and 4) and 9 (sites 1 and 2) rowsof trees (based on the width of the extractionroute divided by the spacing in metres at the timeof planting), comprised a lower layer of tightlyinterwoven branch wood (50 machine passes) ateach site, and along secondary routes within theexperimental plot at site 2 only (up to 28 machinepasses), mixing of logging residues with surface

    soil was observed along wetter areas, along withminor breakage and/or deflection over thebroader area. Along secondary extraction routeswithin the experimental plot at the remainingsites (1 and 36), and following up to 20 machinepasses, only minor breakage and/or deflection ofthe logging residues was observed, and the under-lying surface soil horizons (litter and peat)remained wholly intact. Occasionally, and onslopes of >11 (site 1), logging residues were dis-placed due to wheel action as the machinery trav-elled uphill, and the ground surface was exposedto wheel rutting. At site 4, mixing of loggingresidues with surface soil took place at draincrossings.

    Mean values of soil dry bulk density foreach 5 cm depth increment are illustrated inFigure 4af. The level of replication fell withdepth and, for the lower extent of the peaty (O)and mineral (AE), replication was insufficient tomake useful comparisons. However, all depthincrements are shown for illustration. At eachsite, except for the deep peat at site 2, the sharpincrease in bulk density at c. 30 cm along bothuntrafficked and trafficked areas, was indicativeof the transition from the peaty (O) layer to themineral (AE) layer. Despite traffic intensities ofbetween 16 (site 1) and 28 (site 2) passes, mean

    GROUND DISTURBANCE DURING MECHANIZED FOREST HARVESTING 351

    Lower layer

    Upper layer

    5 m

    Left wheel track* Right wheel track*

    *looking from the back of each secondary extraction route.

    Tree top

    Branch wood (7 cm diameter and 0.25-0.75 m length)

    Figure 3. Composition of the slash road.

    06 cpg030 11/6/03 12:21 pm Page 351

  • trafficked values for all sites (O layer n = between18 and 1, and AE layer n = between 23 and 7)were either higher or lower than the mean untraf-ficked values (O layer n = between 8 and 1, andAE layer n = between 12 and 1), though rarelywere the differences found to be statistically

    significant (t-test assuming unequal variance, P =0.05). No relationship was found between drybulk density and the number of machine passesat any of the sites visited.

    For year 1 sites, mean values of soil penetrationresistance for each 3 cm depth increment are

    352 FORESTRY

    a - site 1.

    0

    20

    40

    60

    80

    100

    0.0 0.5 1.0 1.5 2.0

    Soil dry bulk density (g cm -3)

    Dep

    th (

    cm)

    Untrafficked Trafficked

    b - site 2.

    0

    20

    40

    60

    80

    100

    0.00 0.04 0.08 0.12 0.16 0.20

    Soil dry bulk density (g cm -3)

    Dep

    th (

    cm)

    Untrafficked Trafficked

    Error bars represent 1 standard error.

    Figure 4. (af) Mean soil dry bulk density at sites 16.

    06 cpg030 11/6/03 12:21 pm Page 352

  • illustrated in Figure 5ac. As for bulk density, thelevel of replication fell with depth, though all depthincrements are shown for illustration. At site 1,mean trafficked values (n = between 20 and 2) wereconsistently lower than mean untrafficked values (n= between 10 and 2) at all depths, though the

    opposite was observed at site 3 (trafficked n =between 24 and 22 and untrafficked n = between12 and 6). For the upper 021 cm, these differenceswere often significant (t-test assuming unequalvariance, P = 0.05). The pattern of mean soilresistance to penetration values at site 2 (trafficked

    GROUND DISTURBANCE DURING MECHANIZED FOREST HARVESTING 353

    Figure 4. Continued.

    c - site 3.

    0

    20

    40

    60

    80

    100

    0.0 0.5 1.0 1.5 2.0

    Soil dry bulk density (g cm -3)

    Dep

    th (

    cm)

    Untrafficked Trafficked

    d - site 4.

    0

    20

    40

    60

    80

    100

    120

    0.0 0.5 1.0 1.5 2.0

    Soil dry bulk density (g cm -3)

    Dep

    th (

    cm)

    Untrafficked Trafficked

    Error bars represent 1 standard error.

    06 cpg030 11/6/03 12:21 pm Page 353

  • n = between 22 and 1 and untrafficked between n= 12 and 1) was less consistent and rarely signifi-cant (t-test assuming unequal variance, P = 0.05).No relationship was found between mean soil pen-etration resistance and the number of machinepasses at any of the sites visited.

    For year 2 sites, mean soil penetration resist-ance (MPa) data for selected 3 cm depth incre-ments (to maintain clarity) are illustrated inFigure 6ac; again the level of replication fellwith depth. Mean trafficked values beneath thewheel tracks (site 4, n = 2410; site 5, n = between

    354 FORESTRY

    e - site 5.

    0

    20

    40

    60

    80

    100

    120

    0.0 0.5 1.0 1.5 2.0

    Soil dry bulk density (g cm -3)

    Dep

    th (

    cm)

    Untrafficked Trafficked

    f - site 6.

    0

    20

    40

    60

    80

    100

    120

    0.0 0.5 1.0 1.5 2.0

    Soil dry bulk density (g cm -3)

    Dep

    th (

    cm)

    Untrafficked Trafficked

    Error bars represent 1 standard error.

    Figure 4. Continued.

    06 cpg030 11/6/03 12:21 pm Page 354

  • 24 and 2; and site 6, n = between 24 and 3) weregenerally higher than mean untrafficked values (n= between 12 and 4) at depths up to c. 30 cm,and at sites 4 (018 cm) and 5 (033 cm) wereoften found to be statistically significant (t-test

    assuming unequal variance, P = 0.05). At allyear 2 sites, changes in mean soil resistance topenetration between wheel tracks (n = between12 and 1) and adjacent to the wheel tracks (n =between 24 and 10) were rarely significant (t-test

    GROUND DISTURBANCE DURING MECHANIZED FOREST HARVESTING 355

    a - site 1.

    0

    9

    18

    27

    36

    45

    0.0 0.3 0.5 0.8 1.0 1.3 1.5 1.8 2.0

    Soil penetration resistance (MPa)

    Dep

    th (

    cm)

    Untrafficked Trafficked

    b - site 2.

    0

    9

    18

    27

    36

    45

    0.0 0.3 0.5 0.8 1.0 1.3 1.5 1.8 2.0

    Soil penetration resistance (MPa)

    Dep

    th (

    cm)

    Untrafficked Trafficked

    Error bars represent 1 standard error.

    Figure 5. (ac) Mean soil penetration resistance at sites 13.

    06 cpg030 11/6/03 12:21 pm Page 355

  • assuming unequal variance, P = 0.05). Thepattern of soil response across the profile at eachdepth increment did not suggest that there was a

    loading effect (e.g. a bias towards larger increasesunder the right wheel tracks) due to the positionof the tree crowns (see Figure 3), where the

    356 FORESTRY

    c - site 3.

    0

    9

    18

    27

    36

    45

    0.0 0.3 0.5 0.8 1.0 1.3 1.5 1.8 2.0

    Soil penetration resistance (MPa)

    Dep

    th (

    cm)

    Untrafficked Trafficked

    Error bars represent 1 standard error.

    a - site 4.

    0.0

    0.5

    1.0

    1.5

    2.0

    2.5

    UntraffickedRight adjacentRight trackCentreLeft trackLeft adjacent

    Sample point

    Soi

    l pen

    etra

    tion

    resi

    stan

    ce (

    MP

    a)

    0-3 cm 6-9 12-15 18-21 24-27 30-33 36-39 42-45

    Figure 6. (ac) Mean soil penetration resistance at sites 46.

    Figure 5. Continued.

    06 cpg030 11/6/03 12:21 pm Page 356

  • increased diameter of the stem (to the right) may,in the absence of the lower layer of smalldiameter branch wood, serve to concentrate theweight of the machine over a smaller area.

    Mean untrafficked and trafficked saturatedhydraulic conductivities for the upper AE layer atsites 5 and 6 are illustrated in Figure 7. At each

    site, a small number of samples were omittedwhere the lined auger hole had become blockedwith soil and logging residues. Values for theuntrafficked areas were comparable: 1.4 102mm s1 (site 5, n = 8) and 1.6 102 mm s1(site 6, n = 9), and typical for a silty soil of lowpermeability (e.g. 102104 mm s1; Carter and

    GROUND DISTURBANCE DURING MECHANIZED FOREST HARVESTING 357

    c - site 6.

    0.0

    0.5

    1.0

    1.5

    2.0

    2.5

    3.0

    UntraffickedRight adjacentRight trackCentreLeft trackLeft adjacent

    Sample point

    Soi

    l pen

    etra

    tion

    resi

    stan

    ce (

    MP

    a)

    0-3 cm 6-9 12-15 18-21 24-27 30-33 36-39 42-45

    b - site 5.

    0.0

    0.5

    1.0

    1.5

    2.0

    2.5

    UntraffickedRight adjacentRight trackCentreLeft trackLeft adjacent

    Sample point.

    Soi

    l pen

    etra

    tion

    resi

    stan

    ce (

    MP

    a)

    0-3 cm 6-9 12-15 18-21 24-27 30-33 36-39

    Figure 6. Continued.

    06 cpg030 11/6/03 12:21 pm Page 357

  • Bentley, 1991). The trafficked areas at site 5showed an overall reduction in mean saturatedhydraulic conductivity of ~35 per cent (n = 17),and at site 6 a reduction of ~75 per cent (n = 24)

    was observed. However, as a result of the highlevels of variation, neither was found to be statisti-cally significant (t-test assuming unequal variance,P = 0.05).

    358 FORESTRY

    0.000

    0.005

    0.010

    0.015

    0.020

    0.025

    Untrafficked - site 5 Trafficked - site 5 Untrafficked - site 6 Trafficked - site 6

    Sat

    urat

    ed h

    ydra

    ulic

    con

    duct

    ivity

    (m

    m s

    -1)

    Error bars represent 1 standard error.

    Figure 7. Mean saturated hydraulic conductivity at sites 5 and 6.

    Peaty (O) layer

    (a) Machine ground pressuredistributed over larger area, slashroads remain largely intact

    (c) Loads (momentary)throughout saturated minerallayer are borne by increases inpore water pressure due to lowsoil permeability

    (b) Machine contact with groundsurface is avoided, and litter andpeat layers remain intact withrapid elastic recovery

    Litter

    Saturated mineral (A-E) layer

    Water table

    Wheel

    Slash road

    Machine

    Axle

    Pressure

    Figure 8. Reduced soil disturbance under slash roads.

    06 cpg030 11/6/03 12:21 pm Page 358

  • Discussion

    A model of the way in which slash roads reducesoil disturbance is illustrated in Figure 8. Slashroads spread the machine load over an area largerthan that of the machine footprint area (the pro-portion of each tyre or track in contact with theground surface), thus reducing the loads appliedto the ground surface (Figure 8a). However, treestumps often caused machinery to deviate fromits original course and cover an area greater thanthat defined by the machine footprint area.Consequently, increases in soil penetration resist-ance were measured in between and to the leftand right of each wheel track at year 2 sites. Asa result, and based on the area of the slash road,the site area at risk of traffic disturbance may beas high as 50 per cent (site 5, Table 1). Largediameter material within the slash road may alsoamplify machine loads by concentrating themover an area smaller than that of the machinefootprint area (Jakobsen and Moore, 1981; Mur-gatroyd, 1997). However, the orientation of thetree crowns (Figure 3) did not result in largeincreases in soil penetration resistance under theright wheel track (Figure 6ac), and instead, theyserved to protect the lower layer of the slash roadcomprising small diameter branch wood(Figure 3).

    At each site, secondary extraction routescovered the largest proportion of the ground areacompared with primary extraction routes. Yet,despite high trafficking intensities (up to 28machine passes), the slash roads remained largelyintact. This was ascribed to (1) the way in whichsmall diameter logging residues became increas-ingly woven together with each successivemachine pass and (2) the fact that timber wascarried rather than dragged, the latter resulting inrapid deterioration of the slash road (Jakobsenand Moore, 1981). Occasional failures of theslash road (e.g. displacement of logging residuesand exposure of the ground surface to machinetraffic) took place in relation to ground con-ditions (e.g. slope, drain channels), rather thanthe number of machine passes. Observations ofthe slash road outside of each experimental plotwhere the secondary extraction routesapproached, though rarely exceeded 200 m,showed no sign of imminent failure despite evenhigher levels of trafficking (c. 3050 machine

    passes). As a result, direct contact between themachinery and the ground surface was avoidedover much of the site and the integrity of thepeaty (O) and surface litter layers was maintained(Figure 8b). This is of particular importance asorganic matter (leaf litter, roots and peaty soilmaterial) can reduce the impacts of harvestingmachinery due to high levels of elasticity andrebound following load removal (Fries, 1974;Johnson et al., 1979; Wronski and Murphy,1994). The ground surface profiles at year 2 sitesdemonstrated minor depressions along traffickedareas relative to the surrounding ground level,though during the period of each trial, thesedepressions were observed (unquantified) tobecome smaller, suggesting that they representedan immediate compression of surface litter only,and were followed by rapid elastic recovery.

    Soil compaction results from the increase indensity of an unsaturated soil when soil air onlyis expelled during loading (Whitlow, 1995).However, the saturated nature of the mineral(AE) layers in this study, and their retention ina confined state, meant that increases in soildensity (and concomitant increases in soil pen-etration resistance) were possible only by theprocess of consolidation (Figure 8c). Consoli-dation is governed by the period of loading, soilpermeability and rate at which water drains andincreased pore water pressures dissipate, which,given the viscosity of water (c. 50100 times thatof air), generally occurs much slower than com-paction (Hillel, 1982). In this study, all the soilstructural requirements for consolidation weresatisfied in the mineral (AE) layers, but theperiod of loading was insufficient, and perme-ability of the predominantly fine textured claysoils so low, that consistent increases in soildensity did not occur.

    The relationship between machine load andsoil disturbance (or failure of the slash road) wasnot investigated directly in this study. However,soil response at each site remained similar despitethe range in forwarder fully laden gross weightsof c. 25 00035 000 kg, considered typical of for-warding machinery used on upland sites in theUK, whilst the longevity of the slash roadappeared more directly linked with ground con-ditions and terrain features.

    A further benefit of the slash road was the pro-vision of vehicle traction. In the absence of slash

    GROUND DISTURBANCE DURING MECHANIZED FOREST HARVESTING 359

    06 cpg030 11/6/03 12:21 pm Page 359

  • roads, and where machinery is in direct contactwith the ground surface, sustained off-roadvehicle mobility generally requires a degree of soilshear failure, compaction and rut formationbefore the underlying mineral horizons, ofgreater load bearing capacity, are able to supporttraffic (Karafiath and Nowatski, 1978; Jakobsenand Greacen, 1985). However, the incidence ofboth surface damage (deep rutting, erosive lossand root cutting) and adverse structural changeswithin the soil (compaction and strengthincreases) were greatly reduced under slash,whilst vehicle traction and uninterrupted haulageof timber to roadside log landings was main-tained.

    At each site, the rooting zone was confined tothe peaty (O) layer of the peaty gley soils (wherethe mineral AE layers were saturated), and theupper 30 cm of the deep peat soil (where themean depth of the water-table was 30 cm). Thisobservation was based upon the soil profiledescription for site 1 (Table 2) and, in addition,the absence of root material or root channels inthe AE layers (sites 1 and 36) and lower Olayers (site 2), noted while processing the intactsoil cores. For both soil types, this was ascribedto the position of the water-table (Table 1 andFigure 8), and subsequent anaerobic conditionsbelow the water-table, rather than mechanicalimpedance.

    Subject to species and soil type, soil dry bulkdensities of 1.0 g cm3 (Froelich, undated), 1.2 gcm3 (Canarache, 1990), 1.4 g cm3 (Senyk andCraigdallie, 1997) and 1.5 g cm3 (Wronski andMurphy, 1994), and mean soil strengths (pen-etration resistance) of 2.5 MPa (Gayel andVoronkov, 1965; Greacen et al., 1969), 3.0 MPa(Sands et al., 1979), and as much as 7.0 MPa(Greacen and Gerard, unpublished studyreported in Greacen and Sands, 1980), have allbeen cited as potentially limiting to root and treegrowth. Throughout the rooting zone at each sitein this study, mean soil dry bulk density and soilpenetration resistance remained well below thesepotentially limiting thresholds. Furthermore,differences in soil dry bulk density and pen-etration resistance between trafficked and untraf-ficked areas were rarely statistically significant(P = 0.05); those that were did not exhibit anyconsistent trends horizontally or vertically, andwere considered to result from the natural

    variability in the structural properties of the soilsrather than any treatment effect. Reductions insaturated hydraulic conductivity throughout thepeaty (O) layer, potentially of a similar order ofmagnitude to those measured in the uppermineral (AE) layer at sites 5 and 6, may affectsoil quality. For example, Hobbs (1986) notedthat the permeability of peat is highly sensitive tochanges in void ratio, where a reduction in thelatter of half an order of magnitude can result ina reduction in permeability of up to three ordersof magnitude.

    Conclusion

    This study has demonstrated that slash roadsprovided an effective means of limiting soildisturbance on sensitive deep peat and peatygley soils, despite high trafficking intensitiesunder the shortwood extraction system. Directcontact between the machinery and the groundsurface was avoided, and the surface litter andpeaty (O) layers remained undisturbed. Theunderlying saturated fine textured mineral (AE)layers remained largely unchanged due to theirlow rates of permeability and the short period ofmachine loading. This has important impli-cations for the timing of harvesting operationswhich may, assuming to use of slash roads,continue on these soil types during wet periodswithout a significant risk of soil disturbance andlonger term effects on site productivity.

    Acknowledgements

    We would like to thank the Scottish Forestry Trust andUK Forestry Commission for financial support, staff ofForest Enterprise (Newton Stewart and Kielder Forest)Districts for site access and Forest Research (NorthernResearch Station, Technical Support Unit North andAlice Holt Lodge) for support both in the field and withthe presentation of this work.

    References

    Canarache, A. 1990 PENETR a generalised semi-empirical model estimating soil resistance topenetration. Soil Till. Res. 16, 5170.

    Carter, L.P. and Bentley, S.P. 1991 Correlations of SoilProperties. Pentech Press, London.

    Forestry Commission 1996 Terrain classification.

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  • Forest Research Technical Development Branch,Technical Note 16/95, pp. 15.

    Forestry Commission 1998 Forests and Soil Conser-vation Guidelines. Forestry Commission, Edinburgh.The Forestry Authority, Edinburgh, 24 pp.

    Fries, J. 1974 Views on the choice of silviculturalmethods and logging technique in thinning. Aspectsof thinning. Forestry Commission Bulletin No. 55.HMSO, London.

    Froelich, H.A. (undated) The effect of soil compactionby logging on forest productivity. Forest EngineeringDepartment, Oregon State University, Corvallis.Final report to Bur Land Management, Portland,OR.

    Gayel, A.G. and Voronkov, N.A. 1965 Root system ofthe pine Pinus sylvestris L. on sandy soils of theKazakhstan and the Don. Bot. Zh. (Leningrad) 50.In Greacen, E.L. and Sands, R. 1980. Compaction offorest soils a review. Aust. J. Soil Res. 18, 163189.

    Greacen, E.L. and Sands, R. 1980 Compaction offorest soils a review. Aust. J. Soil Res. 18, 163189.

    Greacen, E.L. Barley, K.P. and Farrell, D.A. 1969 Themechanics of root growth in soils with particularreference to the implications for root distribution. InRoot Growth. J. Wittington (ed.). Butterworths,London, pp. 256269.

    Hillel, D. 1982 Introduction to Soil Physics. AcademicPress, London.

    Hobbs, N.B. 1986 Mire morphology and the proper-ties and behaviour of some British and foreign peats.Q. J. Engin. Geol. 19, 780.

    Hutchings, T.R., Moffat, A.J. and French, C.J. 2002Soil compaction under timber harvesting machinery:a preliminary report on the role of brash mats in itsprevention. Soil Use Manage. 18, 3438.

    Jakobsen, B.F. and Greacen, E.L. 1985 Compaction ofsandy soils by forwarder operations. Soil Till. Res. 5,5570.

    Jakobsen, B.F. and Moore, G.A. 1981 Effects of twotypes of skidders and of slash cover on soilcompaction by logging of mountain ash. Aust. For.Res. 11, 247255.

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    stands. Michigan Technological University, FordForestry Centre, Research Note 27, 13 pp.

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    Murgatroyd, I.R. 1997 Soft ground clearfell harvestingtechniques. Forestry Commission Technical Develop-ment Branch, Technical Note (unpublished),pp. 19.

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    Sands, R., Greacen, E.L. and Gerard, C.J. 1979Compaction of sandy soils in Radiata pine forests.1. A penetrometer study. Aust. J. Soil Res. 17,101113.

    Senyk, J.P. and Craigdallie, D. 1997 Effects ofskidroads on soil properties and forest productivityon steep slopes in Interior British Columbia. PacificForestry Centre, Forestry Research ApplicationsTechnical Transfer Note 8, pp. 14.

    Wall, M. and Saunders, C.J. 1998 Kielder harvestingslash road trial: an assessment of soil protection.Forestry Commission Technical DevelopmentBranch, Technical Note, pp. 110.

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    Wood, M.J. 2001 The effects of mechanised forestharvesting on soil physical properties. Ph.D. thesis,University of Southampton.

    Wronski, E.B. and Murphy, G. 1994 Response offorest crops to soil compaction. In Soil Compactionin Crop Production. B.D. Soane and C. vanOuwerkerk (eds). Elsevier Science, Amsterdam,pp. 317342.

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    Received 26 April 2002

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