Download - Reduced ground disturbance during mechanized forest harvesting

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: [email protected]

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

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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 m–2) 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 m–2) made no differ-ence to increases in soil density at 0–5 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 at5–10 cm depths following five machine passes,increases in bulk density under 10 kg m–2 loggingresidue cover were 60 per cent greater than under20 kg m–2 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(150–200 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 3–6 (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

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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)

3 �

150

�12

(5)

3 �

150

�10

(5)

3 �

150

�15

(5)

Are

a tr

affic

ked

(%)5

9 (3

1)13

(31

)17

(42

)17

(42

)20

(50

)13

(33

)Te

rrai

n cl

ass6

4:2

:35

:2:2

4:1

:14

:3:1

4:2

:14

:1:1

Har

vest

erE

xcav

ator

Exc

avat

orT

imbe

rjac

k 12

70B

Tim

berj

ack

1270

BV

alm

et 9

11T

imbe

rjac

k 12

70B

Gro

ss w

eigh

t (k

g)15

000

1500

016

850

1685

015

000

1685

0Fo

rwar

der

Hem

ek T

D81

Val

met

860

Tim

berj

ack

1210

Hem

ek T

D81

Val

met

860

Val

met

890

Gro

ss w

eigh

t un

lade

n (k

g)13

000

1377

016

000

1300

013

770

1700

0C

apac

ity

(kg)

1200

012

000

1200

012

000

1200

018

000

Mac

hine

pas

ses7

4–1

69–

2810

–18

10–1

813

–18

9–20

1Py

att

(197

0).

21.

7m

spa

cing

at

tim

e of

pla

ntin

g w

ith

no s

ubse

quen

t th

inni

ng.

3B

ased

on

esti

mat

ed s

tand

ing

volu

me

(m3

ha–1

) of

sal

eabl

e m

ater

ial (

6 be

ing

the

low

est

and

24 b

eing

the

hig

hest

for

thi

s sp

ecie

s).

4N

umbe

r of

sec

onda

ry e

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

d, in

par

enth

eses

, 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

ning

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

emai

nder

by

the

forw

arde

r).

* A

utho

rs’ e

stim

ate.

†R

oute

s 1

and

2 w

ere

adja

cent

to

each

oth

er, r

oute

3 w

as lo

cate

d so

me

dist

ance

aw

ay in

the

sam

e st

and.

‡A

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 47

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 3–6) 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 (A–E)layers at sites 1 and 3–6 (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 sites4–6. 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 2–6, 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


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 10–14° (N–S), 360 mDrainage* Poor to moderateErosion NoneCoarse fragments NoneRock outcrops NoneGround cover Needle litter/mature Sitka spruce (planted 1951)L (0–2 cm) Sitka spruce needles, cones and twigs, abrupt change to next horizonF (2–7 cm) Wet, apedal, roots (fine, common, fibrous), abrupt change to next horizonH (7–22 cm) Dark reddish brown (5 YR 25/2), wet, apedal, roots (fine, few, amorphous), abrupt

change to next horizonAh (22–32 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 (32–52 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 (52–65 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 (65–82 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.

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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 2–6(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 4–6.

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 (A–E) 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.

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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 (<7 cm diameter)armoured by an upper layer of larger diametertree tops (stem diameter <7 cm) placed perpen-dicular to the direction of travel. In addition,non-saleable large diameter material from themain stem (>7 cm diameter and generally0.25–0.75 m length) including butt off-cuts (baseof the main stem where the diameter exceededthat required by the market or capacity of theharvesting head), forks (where the main stemsplit into two separate stems) and stops (portionof the main stem which exhibited a rapid changein diameter), were located randomly throughoutthe slash road. The thickness of the slash roadsalong primary (perimeter) extraction routes wasgenerally greater (unquantified) than that of slashroads along the secondary extraction routes dueto the larger amount of branch wood associatedwith trees along the forest edges.

Along primary routes (>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 3–6), 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 4a–f. The level of replication fell withdepth and, for the lower extent of the peaty (O)and mineral (A–E), 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 (A–E) layer. Despite traffic intensities ofbetween 16 (site 1) and 28 (site 2) passes, mean


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)

Large diameter material (>7 cm diameter and 0.25-0.75 m length)

Figure 3. Composition of the slash road.

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trafficked values for all sites (O layer n = between18 and 1, and A–E layer n = between 23 and 7)were either higher or lower than the mean untraf-ficked values (O layer n = between 8 and 1, andA–E 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


Dep

th (

cm)


Error bars represent ± 1 standard error.

Figure 4. (a–f) Mean soil dry bulk density at sites 1–6.

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

illustrated in Figure 5a–c. 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 0–21 cm, these differenceswere often significant (t-test assuming unequalvariance, P = 0.05). The pattern of mean soilresistance to penetration values at site 2 (trafficked


Figure 4. Continued.

c - site 3.

0

20

40

60

80

100

0.0 0.5 1.0 1.5 2.0


Dep

th (

cm)


d - site 4.

0

20

40

60

80

100

120

0.0 0.5 1.0 1.5 2.0


Dep

th (

cm)



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 6a–c; again the level of replication fellwith depth. Mean trafficked values beneath thewheel tracks (site 4, n = 24–10; 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


Dep

th (

cm)


f - site 6.

0

20

40

60

80

100

120

0.0 0.5 1.0 1.5 2.0


Dep

th (

cm)




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 (0–18 cm) and 5 (0–33 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


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)


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


Dep

th (

cm)



Figure 5. (a–c) Mean soil penetration resistance at sites 1–3.

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


Dep

th (

cm)



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. (a–c) Mean soil penetration resistance at sites 4–6.


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 A–E 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 � 10–2

mm s–1 (site 5, n = 8) and 1.6 � 10–2 mm s–1

(site 6, n = 9), and typical for a silty soil of lowpermeability (e.g. 10–2–10–4 mm s–1; Carter and


c - site 6.

0.0

0.5

1.0

1.5

2.0

2.5

3.0


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


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


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)


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.

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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 6a–c), 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. 30–50 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(A–E) 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. 50–100 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 (A–E) 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. 25 000–35 000 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


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 A–E 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 A–E layers (sites 1 and 3–6) 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 cm–3 (Froelich, undated), 1.2 gcm–3 (Canarache, 1990), 1.4 g cm–3 (Senyk andCraigdallie, 1997) and 1.5 g cm–3 (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 (A–E) 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 (A–E)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

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Received 26 April 2002


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