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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

    h si

    te

    Site

    (an

    d ha

    rves

    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

    spru

    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

    9 (3

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    (31

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    (33

    )Te

    rrai

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    ass6

    4:2

    :35

    :2:2

    4:1

    :14

    :3:1

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    :14

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    Har

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    70B

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    11T

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    70B

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    1500

    016

    850

    1685

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    000

    1685

    0Fo

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    D81

    Val

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    Tim

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    1210

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    met

    860

    Val

    met

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    n (k

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    000

    1377

    016

    000

    1300

    013

    770

    1700

    0C

    apac

    ity

    (kg)

    1200

    012

    000

    1200

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    000

    1200

    018

    000

    Mac

    hine

    pas

    ses7

    41

    69

    2810

    18

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    813

    18

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    ubse

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    ng.

    3B

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    on

    esti

    mat

    ed s

    tand

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    volu

    me

    (m3

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    ) of

    sal

    eabl

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    ater

    ial (

    6 be

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    the

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    and

    24 b

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    the

    hig

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    for

    thi

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    4N

    umbe

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    xtra

    ctio

    n ro

    utes

    , len

    gth

    and

    wid

    th (

    aver

    age

    wid

    th o

    f sl

    ash

    road

    was

    5m

    at

    each

    sit

    e).

    5B

    ased

    on

    the

    area

    of

    whe

    el t

    rack

    s an

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    par

    enth

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    , are

    a of

    the

    sla

    sh r

    oad

    as p

    erce

    ntag

    e.6

    Bas

    ed o

    n au

    thor

    s a

    sses

    smen

    t us

    ing

    FC t

    erra

    in c

    lass

    ifica

    tion

    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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    har

    vest

    ing

    and

    forw

    ardi

    ng m

    achi

    nery

    (at

    eac

    h si

    te t

    his

    com

    pris

    ed 2

    4 p

    asse

    s by

    the

    har

    vest

    er, t

    he r

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    by

    the

    forw

    arde

    r).

    * A

    utho

    rs e

    stim

    ate.

    R

    oute

    s 1

    and

    2 w

    ere

    adja

    cent

    to

    each

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    oute

    3 w

    as lo

    cate

    d so

    me

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    ance

    aw

    ay in

    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

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    ots

    pine

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    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...

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