a ResearchNa&

1979

Soil Compaction Absent in Plantation Thinning

Tony King and Sharon Haines

SUMMARY

We examine the effects on soil bulk density by

using a TH-105 Thinner Harvester and two forwarders in a mechanically thinned slash pine

(Pinus elliottii Engelm.) plantation. Points in the

machine tracks were sampled before and after

harvesting at depths of 5 and 10 cm (2 and 4

in) for moisture and bulk density. Both the standard gravimetric method and a Troxler Model

3411 nuclear moisture-density gauge were used.

Sample points were trampled at least seven times

by wheels, each exerting a pressure in excess

of 120.7 kPa (17.5 lb/in*), but we detected no

compaction at depths sampled. Soil water content

was 13 percent during thinning.

Additional keywords: tree harvesters, thinning, soil compaction, soil density, plantation.

INTRODUCTION

Use of large logging equipment and the development of skidders permit year-round operations

in most southern forests. As a result, the potential

for site damage-compaction, puddling or displacement-has increased. But the effects of large

equipment on the properties of forest soils have

not been documented fully. In this study we evaluate the effects of the TH-105 Thinner Harvester’

on soil compaction in a plantation thinning operation.

Increases in density resulting from compaction

are greatest near the soil surface (Craul 1975):

little compaction occurs below about 20 cm. The

degree of compaction is greatest at the point with

the most passes. Weaver and Jamison (1951)

reported that the greatest increases in bulk density occurred during the first four passes.

Moehring and Rawls (1970) studying the effects of logging traffic in a 40-year-old even-aged

loblolly pine stand during wet and dry soil moisture conditions, concluded that in dry weather

neither tree growth nor physical soil properties

were altered significantly by logging traffic. However, in wet weather, deep ruts on skid trails,

compaction, and root damage were apparent.

Drainage was impeded, and tree growth was reduced greatly. Resulting root damage decreased

tree vigor and resistance to insect damage. Bulk

density of soil in the skid ruts increased 13 percent.

’ The use of trade, firm, or corporation names in this publication

is for the information and convenience of the reader. Such

use does not constitute an official endorsement or approval

by the U.S. Department of Agriculture of any product or service

to the exclusion of others which may be suitable.

Southern Forest Experiment Station/T-10210 U.S. Postal Services Bldg.. 701 Loyola Avenue, New Orleans, La. 70113

Forest Service, U.S. Department of Agriculture.

Serving Alabama, Arkansas, Louisiana, Mississippi, Eastern Oklahoma, Tennessee, Eastern Texas.
Soil recovery after logging operations takes

many years and depends on intensity of compaction. Hatchell (1969) detected no trend toward

recovery from compaction on eleven soil types

in the Atlantic Coastal Plain one year after logging. Under normal conditions, about 18 years

is required for bulk density on log decks to return

to that of undisturbed soil.

Omberg .(1969) reported that soil compaction

was minor when logging residues covered the

ground, providing a mat for vehicular movement.

This condition can occur when, for example, a

harvesting machine delimbs a tree and the

branches fall ahead of it.

Standard methods for monitoring soil bulk density are core sampling or radiation. Donnelly

(1976) compared bulk densities measured by volumetric core samples with measurements for a

nuclear surface-density probe. The latter slightly

overestimated soil moisture and slightly underestimated soil bulk density. Hassan (1977) working

with a light organic soil, found that a Troxler

Model 3401 surface moisture-density gage gave

much higher soil bulk density estimates than

those obtained by the core sample method. Both

methods are employed in our study.

PROCEbURE

The study site, provided by International Paper

Company, Baldwin Timberlands, Bay Minette,

Alabama, was in an 18-year-old slash pine plantation in the Coastal Plain of southern Alabama.

Soils of the Bowie-Tifton-Sunsweet association

predominated with a loamy sand topsoil about

10 cm (4 in) deep over a sandy clay loam subsoil.

The site is generally flat, and a recent prescribed

burn had left the mineral soil exposed in places.

Soil moisture in the O-10 cm layer at harvesting

averaged 13 percent by weight.’

The TH-105 harvester has 3 rubber-tired wheels

and a single steering wheel at the rear. During

thinning the machine straddles the row to be

harvested while the felling head grasps a tree,

shears it, pushes it forward, and lifts the butt end

through a delimbing ring, feeding the stem toward

the rear of the machine through a bucking shear.

The stem is severed into bolts 2.13 m (7 ft) long,

which fatI into a cradle. When the cradle is full,

about 1.2 m3 (% cord), it is lowered to the ground

and the bolts are dumped for pickup by a forwarder. As the TH-105 moves down a row, tops

and branches are deposited on the ground in

front of the harvester. Trees in adjacent rows are

selectively thinned.

The TH-105 weighs 9545 kg (21,000 lb) loaded.

Assuming that the weight is evenly distributed,

each tire supports a static load of 3100 kg (7,000

lb). The front tires of the harvester have a footprint

of 2,480 cm* (400 itY), indicating that the machine

exerts a pressure of about 120.7 kPa (17.5 Ibs/irV)

on the ground. The front tires track between tree

rows while the rear steering tire treads on top

of, or close to, the line of stumps. During harvest

the machine moves over the same soil area at

least three times, because it has to reverse with

the felled tree to prevent it from hanging up in

adjacent crowns. As a result, the TH-105 compressed each sample point with a pressure of at

least 120.7 kPa (17.5 Ib/irY) a minimum of three

times.

Pulpwood bolts were transported to the road

by two forwarders. Each machine has a capacity

of 7.2 m3 (2 cords) loaded, about 12,300 kg

(27,000 Ibs). The forwarders tracked the same

path as the TH-105 and passed over each sample

point with two wheels at least twice.

The plantation had 1722 trees/ha (697

trees/acre) before thinning and 1104 trees/ha

(447 trees/acre) after. Average height of 18 randomly selected trees was 10.1 m (33 ft); average

d.b.h. was 12.95 cm (5.1 in).

A rectangular area 31 tree-rows wide and 122

m (400 ft) long, encompassing 0.93 ha (2.3 acres),

was marked for the study. Fourteen randomly

located plots were established, each containing

three sample points: one in the left tire track, one

in the right track, and one in the undisturbed area

between adjacent thinned rows. At each point,

undisturbed 44 cc (2.7 in”) soil core samples were

taken before and after harvesting from 5 cm and

10 cm (2 and 4 in) depths and placed in moisture-tight containers. For comparison with the

gravimetric method, the Troxler surface moisture-density gage was used at or near the same

points to obtain direct readings of bulk density

and moisture at the same depths.

Samples were weighed at the Auburn laboratory

and cores oven dried at 105°C. Bulk density and

moisture percentages of each sample were calculated using the formulae:

Bulk density = oven dry weight of soil (g)

volume of soil (cc)

Moisture

Percentage =

wet weight (g) - oven dry weight (s> x ,00,

oven dry weight(g)

King is Research Engineer, Southern Forest Experiment Station, Forest Service-USDA, Auburn, Ala. Haines is Project Leader,

Soils Research, International Paper Co., Southlands Experiment Forest, Bainbridge. Ga.

2
Tab/e l.-Mean bulk density values (g/cm”) measured by soil cores’ and by Troxler= before and

after thinning at 5 cm and 10 cm depths

Before thinning

5 cm 10 cm

After thinning

5 cm 10 cm

Left wheel

track

Right wheel

track

. . . Grams per cubic centimeter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Core Troxler Core Troxler Core Troxler Core Troxler

1.51 1.23 1.67 1.31 1.52 1.22 1.65 1.29

(iO.11) (ltO.08) (iO.09) (+O.ll) (ItO.11) (kO.10) (kO.10) (kO.08)

1.54 1.21 1.62 1.29 1.50 1.27 1.66 1.34

(kO.09) (LtO.08) (kO.10) (kO.06) (+0.14) (fO.06) (+0.08) (+0.06)

Undisturbed 1.49 1.22 1.66 1.29

(10.12) (1.0.07) (f0.12) (kO.08)

’ Core bulk density values are mean of 14 observations.

2 Troxler bulk density values are mean of 8 observations.

Table 2.-Mean soil moisture percentage values’ measured by soil cores and by Troxler before

and after thinning at 5 cm and 10 cm

____-

Before thinning After thinning

5 cm 10 cm -~____-~5 cm 10 cm

Left wheel

track

Right wheel

track

Percent . . . . . . . . . .._..........................................

Core Troxler Core Troxler Core Troxler Core Troxler

11.3 15.1 11.9 14.0 15.0 21.4 14.8 20.1

(lt2.7) (+3.6) (k2.5) (f3.1) (k2.7) (13.4) (f1.7) (k3.7)

12.3 16.3 12.6 15.0 17.9 20.2 15.4 19.4

(k4.3) (lt5.1) (k3.0) (k4.8) (k5.2) (13.4) (*2.9) (k3.4)

Undisturbed 12.7 15.2 13.3 14.9

(lt2.6) (k3.3) (12.3) (k2.8)

’ Core and Troxler moisture values are mean of 8 observations.

RESULTS AND DISCUSSIONS

We detected no damage to the soil. Little dis- Bulk density at 5 cm (2 in) was significantly

turbance to the soil surface occurred, and no tire lower (r = 0.05) than at 10 cm (4 in) because bulk

ruts appeared. There was little tire-to-soil contact density increases with soil depth (Donnelly 1976).

anywhere since tree tops and branches formed However, the TH-105 had no detectable impact

a mat over the soil surface. Soil bulk density on bulk density in the subsoil at 10 cm, even

measurements confirmed that little soil distur- though the subsoil texture is finer than the surface

bance had occurred. texture.

Mean bulk density values of the soil measured

either gravimetrically or with the Troxler before

and after thinning did not differ significantly

(7 = 0.05) (table 1). However, standard deviation

Troxler bulk densities averaged 0.30 g/cc (18

Ibs/fV) lower than corresponding gravimetric determination. We assume the lower readings are

a characteristic of its calibration. Soil moisture

was greater by the former method (0.08 to 0.14)

than by the latter (0.06 to 0.11) (table 1).

3
estimates with the Troxler averaged 3 percent and

5 percent greater than gravimetric means before

and after harvesting (table 2).

Several factors probably contributed to the lack

of detectable differences after thinning. Of major

importance was the low soil moisture content (13

percent). Soil moisture and texture have considerable influence on the degree of compaction.

Another mitigating factor was the layer of tops

and branches deposited on the soil surface during

the delimbing cycle. Tires seldom came into contact with the mineral soil surface. This layer of

logging debris served to distribute the machine

weight over a larger area and decrease compaction.

A TH-105 Thinner Harvester used with two forwarders to thin a slash pine plantation caused

no detectable soil damage. Moderate ground

pressure together with the distribution of

branches on the soil in front of the machine contributed to the absence of soil compaction. The

Troxler 3411 nuclear moisture density meter consistently overestimated soil moisture by 3 to 5

percent and underestimated soil bulk density by

0.3 g/cc (18.7 Ibs/ft3) compared to the gravimetric

method.

Craul, P. J.

1975. Physical limitations of soils. In Proc. Logging road and skid trail constr. work, Misc.

Rep. 6, p. 21-24. Appl. For. Res. Inst., State

Univ. N.Y.

Donnelly, J. Ft.

1976. Effect of compaction on soil properties

and tree growth in forested recreation areas.

p.1. Proj. Abstr. Univ. Vt. Dep. For., Burlington, Vermont.

Hassan, A. E.

1977. Effect of mechanization on forest soils

and regeneration. I. Coastal plain organic

soil. Am. Sot. Agric. Eng. Pap. 77-1571, 24

Hatc!/ell. G. E.

1969. The effects of soil disturbance in logging

on soil characteristics and growth of loblolly

pine. 207 p. D. For. Diss., Duke Univ.

Moehring, D. M., and I. K. Rawls.

1970. Detrimental effects of wet weather logging. J. For. 68.3:166-167.

Omberg, H.

1969. The formation of tracks made by forwarders on forest soil. In Proc. Thinning and

Mechanization. p. 144-l 50. IUFRO Meet.,

Royal Coil. For., Stockholm, Sweden.

Weaver, H. A., and V. C. .Jamison.

1951. Effects of moisture on tractor tire compaction of soil. Soil Sci. 71 :15-23.

4 Sr U.S. GOVERNMENT PRINTING OFFICE. 1990-771-081