Reduced ground disturbance during mechanized forest harvesting ...
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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: firstname.lastname@example.org
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 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.
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
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GROUND DISTURBANCE DURING MECHANIZED FOREST HARVESTING 347
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uteIncreasing numbers of
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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.
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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 26(n = 10 penetrations) were collected from theright...