Tree growth on a compacted Oxisol

Soil& Tillage Research, 19 (1991) 331-344 Elsevier Science Publishers B.V., Amsterdam

331

Tree growth on a compacted Oxisol R.J. Cheatle* Ministry of Agriculture and Lands, Honiara (Solomon Islands) (Accepted for publication20 December 1989)

ABSTRACT Cheatle, R.J., 1991. Tree growthon a compactedOxisol. Soil Tillage Res., 19:331-344. For a site at Barora, Solomon Islands, assessment was made of survival and growth of Gmelina arborea and Terminalia brassii planted on a heavy clay soil degraded by crawler tractor loggingof primary forest and other processes. Experimentalpaired plots were establishedin 1981 on six classes of degraded soil and in undisturbed forest as a control. Owing to tree fall on site, the data from the "Uncleared" plots was unsatisfactory.On trafficked land, tree survivalwas poor and basal areas were about halfthose on the "'LeastDisturbed" soil class. Gmelina arborea performed better than T. brassiL Field pedologicalinvestigationsand bulk density measurements were found to be of use in identifyingsites of poor and better growth. Physicaland chemical status were establishedfor a representative profile under primary forest and for the experimental plots. Comparisonof degradedareas with the baselineprofile suggestedthat f¢~';iityhad been reduced and that there were few signs of soil recoveryin the plots after 7 years. Problems related to control of soil compactionand the need for sustainablecroppingsystemsare discussed.

INTRODUCTION T h e S o l o m o n Islands h a v e a l a n d area o f 27 750 k m 2, m u c h o f which is hill c o u n t r y u n s u i t a b l e for c o n v e n t i o n a l f o r m s o f field crop agriculture. In 1972 HanseU a n d Wall ( 1 9 7 6 ) e s t i m a t e d 2229 k m 2 as c u l t i v a t e d l a n d a n d regrowth, a n d 2528 k m 2 as l o w l a n d forest. Logging o f p r i m a r y forest is a m a j o r i n d u s t r y in the country. A F o r e s t r y D i v i s i o n e s t i m a t e o f 10 000 ha logged a n n u a l l y leads to the c o n c l u s i o n t h a t a b o u t t h r e e q u a r t e r s o f the l o w l a n d forest r e c o r d e d in 1972 has since been logged. In all logging with crawler tractors a n d skid haulage there is soil disturbance, a n d d a m a g e is generally m o r e severe in clear felling t h a n in selective felling operations. C o m p a c t i o n a n d soil loss are two m a i n f o r m s o f damage. *Present address: Swedish Embassy, Development Cooperation Office, P.O. Box 44391, Nairobi Kenya.

0167-1987/91/$03.50

© 1991 -- Elsevier Science Publishers B.V.

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Compaction is caused by tractor tracking and tractor blading. Topsoil is also removed by blading and is eroded through slope wash after logging, when ground cover is poor or absent. In the Solomon Islands volunteer growth of creepers of Merrimia spp. usually results in establishment of 100% cover within 12 months after clearance. Subsequent to expressions of concern about soil damage, Webb (1973) studied the effects of compaction at selective felling sites on Gizo and Kolombangara. He concluded that there was severe compaction damage to 15% of the logged areas off the main roads, 89% of this damaged area being along extraction tracks. He reported the main effects as elimination of macropores resulting in decreased permeability, mechanical impedance to root growth, and loss of topsoil. In association with Leach (1973a,b) he reported poor growth of oil palm on compacted soils. Recovery of structure and porosity in compacted tracks was reported as negligible after 10 years. Marten ( 1973 ), in another study, concluded that serious soil damage could exceed 20% of the replantable area although the average was around 15%. As 10% of the damaged land was classed as access roads he regarded this as a redeeming feature. In 1980 a logging company began a clear felling operation at Barora in Northern New Georgia. Elsewhere there had been little reinvestment of logging revenue to land, but here there was interest in prospects for redevelopment after logging. Wall (1980) produced proposals for land development at Barora. In his report he estimated that 70% of cleared land was affected by skidder tracks, yards, heaps of bulldozed soil and scraped areas. He regarded the logging approach as "wasteful of resources" and "bad environmental management". The following year consultants were called in to evaluate a proposed reforestation project (Fraser and Larsen, 1981 ). They reported that the m6st severe damage was on skid trails and that the area affected was probably as high as 25%. They pointed out that Wall did not distinguish between severe disturbance and less severe disturbance. Fifty percent of the land area was placed in a less severe category, where some scraping and/or compaction had occurred. They also stated that "the degree to which first rotation crops will suffer from topsoil removal is unquantifiable" but that there is "every likelihood that tree growth could be diminished in second and third rotations due to a low level of nutrient reserves". Attention was drawn to the potential role of N-fixing species for rehabilitation of these degraded soils. Fraser and Larsen ( 1981 ) put forward proposals for a research programme but neither these, nor Wall's (1980), proposals were ever implemented. The logging company stopped operations after 5 years because of a dispute with the landowners. Hc,wever, after the consultants had reported, an experiment was set up to determine the effect of soil damage on tree survival and growth. Tree growth was reported after 11 months (Neumann, 1987). No fu~her work was car-

TREEGROWTHINACOMPACTEDOXISOL

333

ried out until 7 years after the experiment was initiated. The results o f the latter investigation are reported in this paper. SITE, MATERI/~LSAND METHODS The volcanic centre of Mt. Mase occupies much of northern New Georgia. Emanating from, and around the breached crater at about 1000 m altitude a radial drainage pattern is incised into andesitic lavas and pyroclastic rocks (Stanton and Bell, 1969 ). At the upper elevations, steep slopes lead to sharp subparallel ridges that separate narrow valleys. Below 600 m, reliefis smoother with broader ridges and some moderate slopes but there is still considerable dissection. Low slopes on valley sides are often steep. The experimeatal site is in this latter terrain, the Vina Land System of Wall and Hansell (1975), at an elevation of about 100 m. Rainfall is about 3000-4000 m m year-~. In the early 1970s the area was described by WaU and Hansell (1975) as Tropical Lowland Rainforest (Whitmore, 1969) of m e d i u m aeight and closed canopy, with small areas of subsistence garden cultivation. In 1981 the site was virtually clear felled by a tractor logging operation and all merchantable timber removed. The site extends for 500 m along a mid-slope section of a valley. Within the site six categories o f disturbance and a control were identified as treatments (Table 1 ). A triplicate paired plot experiment with Gmelina arborea and Ter.. minalia brassii was laid out as follows. The land area within each treatment category was identified and then subdivided by a labelled grid, enabling three paired experimental plots to be randomly located. Each 25-m 2 plot was bordered by a l-m buffer zone, inside which four rows of trees were planted at 1 X 1 m spacing, 16 trees in all. Woody vegetation was destroyed in the vicinity of each pair of plots by felling and stumping or poisoning. Site conditions and tree growth were re-examined in 1987 by means of a free auger survey, pedological examination o f soils within the paired plots, TABLE 1 Categoriesof disturbance and area affected Category (treatment) (l)

(2) (3) (4) (5) (6) (7)

Undisturbed areas and patches of unmerchantabletrees Light disturbance - no trafficking Lightlyused tractor track Heavily used tractor track Road cuR~.ng- bladed area to each side of a road Soil heaps Log yards - areas where logsare pooled and loaded

Percent of area 76.0 3.4 3.8 12.9 1.6 1.8 0.6

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R.J. CHEATLE

tree measurements, and the collection of core samples for bulk density determination and bag samples for fertility assessment. For bulk density determination of soils with an observable dense layer, four samples were taken from above and four samples from within that layer; in other soils four samples were obtained from the 0-15-cm layer immediately below any forest mat. Four bag samples were collected from the 0-15-era soil layer of each plot and converted to a single composite sample. Samples for bulk density were oven dried at 60 ° C. Other samples were air dried and ground to pass a 2-mm sieve. A standard hydrometer approach was utilized for particle size analysis. Bases were extracted with ammonium acetate and determined by atomic absorption spectrometry, as was the aluminium extracted in potassium chloride. Dichromate oxidation was used to determine organic carbon, while total nitrogen was determined by the procedure of Keeney and Bremner ( 1967 ). For soils with pH 5 or lower, soluble phosphorous was determined by the method of Bray and Kurtz ( 1945 ). For soils of higher pH the method of Olsen et al. (1954) was adopted. RESULTS

Site conditions Woodland, 3-4 m tall and dominated by Melochia umbellata and Commersonia bartramia, had established over 60% of the experimental site. Much of the remainder was under heavy growth of Merrimia spp. creepers, indicators of the first seral phase. Very few of the primary forest species remained and cleared forest land formed the greater part of the undisturbed category of Table 1. Former logging yards and major tractor roads remained unvegetated. The soils across the site exhibited a catenary sequence from the crest of a broad backed ridge towards the valley bottom. Deep, heavy, brown clays were characteristic of the ridge and those parts of the valley flanks with treatmen~ plots. Below the summit of the ridge in a small patch of undisturbed forest, a "representative profile" site was chosen for the purpose of establishing reference data.

Representaiive profile description In the field, three sections of the profile are evident. These are the fore~!t mat (0-7 cm), the topsoil (7-18 cm) and the subso~:l (18-134 cm) (Table 2). In the field the mat can be seen as soil protection with a pore space that will permit a high infiltration rate. The sponge effect in water storage is also observable because water can be wrung out of the mat when wet. Colour, organic content, roots, pore space, field permeability and structural aggregation distinguish the topsoil from the subsoil. Blocky structure with good pore space

TREEGROWTHINACOMPACTEDOXISOL

335

TABLE 2 The representative soil profile Tentative classification: typic haplohumox, clayey, kaolinitic, isohyper.,hcrmic Location: Barora, New Georgia Physiographic position: Vina Land System Topography: below the summit of a broad ridge Drainage: well drained but slow permeability in the subsoil Vegetation: small patch of primary forest Parent material: pyroclastic rocks Colour: colours are for moist soil Depth (cm) 00- 07

07- 18

18- 30 30- 66 66- 99 99-134 134-176

Profile description Black (7.5YR 2/0) spongy, fibrous mat of finely woven roots associated with litter, partially decayed and finely comminuted organic material and some mineral material; crumb structures associated with roots; highly permeable; small animals and fungal mycelia common; abrupt boundary Dark brown (2.5YR 3/6 ) clay; strong subangular blocky, aggregates 7-9 cm breaking to 1-2 cm, marked cracking; slightly sticky, slightly plastic; common macropores; good permeability; common coarse and medium roots, abundant fine roots; clay lined root channels; clear smooth boundary Rich brown (2.5YR 4/7) clay; massive appearance when moist though some very weak subangular blocky structures observable; few visible pores; low permeability; few roots; clear smooth boundary Rich brown (2.5YR 4/7 ) clay; massive appearance; very few visible pores; low permeability; very few roots; rare subangular iron stones As for 30-66; separation was for sampling purposes Transition zone to saprolite; few fossil rock structures; no root penetration observed ~aprolite dominant

in the topsoil, a n d o t h e r useful attributes o f b o t h topsoil a n d forest mat, suggest these materials as satisfactory m e d i a for plant growth. By d e p t h the subsoil is 87% o f the profile. R o o t p e n e t r a t i o n , particularly below 30 cm, is limited. T h e r e is w e a k structural d e v e l o p m e n t a n d field tests suggest low p e r m e a b i l i t y in this h e a v y clay. In its o b s e r v e d c o n d i t i o n u n d e r forest the subsoil appears as a r a t h e r p o o r m e d i u m for plant growth.

Physical and chemical characteristics of the representative profile This is a clay soil with a bulk density o f a b o u t 0.95 g c m -3 t h r o u g h o u t the m i n e r a l soil (Table 3). T h e forest m a t has a very low bu!k density (0.44 g cm-3). Forest m a t plus topsoil are in m a r k e d contrast to the subsoil. Whilst t h e f o r m e r constitutes just 13% o f t h e volume, it contains a relatively high pro-

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R.J.CHEATLE

TABLE 3 Physical and chemical characteristics of the representative profile Depth (cm) 00-07

07-18

18-30

30-66

66-99

99-134

Particle size (mm) distribution (%, w/w) Coarse sand (2.0-0.2) 15 Fine sand (0.2-0.02) 16 Silt (0.02-0.602) 14 Clay (<0.002) 55 Bulk density (g cm -3) 0.44

4 6 14 76 0.95

6 5 14 75 0.94

1 6 14 79 0.93

2 4 16 78 0.91

0 16 34 50 0.94

E×change~ble bases (cmol kg-~ ) Ca Mg K Na Base saturation (%)

36.6 8.68 1.02 0.38 58

5.4 3.78 0.08 0.08 72

2.6 4.22 0.09 0.13 64

0.9 2.73 0.07 0.08 34

0.4 0.63 0.05 0.01 12

0 0.06 0.02 0.06 1

Aluminium (cmol kg-~ ) Aluminium sat. (%) CEC (cmol kg -I ) pH (CaCI2) C (%, w/w) Total N (%, w/w) SO4-S (/tgg - l ) P (/tg ml -I ) P-retention (%)

58 5.8 40.1 2.2 84 14 56

13 5.5 3.7 0.2 11 5 55

11 5.1 2.9 0.2 25 3 60

1.02 9 11 4.2 2.1 0.1 198 2 63

1.91 21 9 4.1 1.3 0 252 2 69

6.17 56 11 3.9 0.5 0 407 2 48

p o r t i o n o f n u t r i e n t s a n d has a b a s e s a t u r a t i o n > 50%. A l m o s t 50% o f t h e exc h a n g e a b l e c a l c i u m a n d o n e t h i r d o f the exchangeable p o t a s s i u m are associa t e d w i t h t h e forest m a t ( T a b l e 4 ) . T h e s e d a t a e m p h a s i s e that the forest m a t is an i m p o r t a n t n u t r i e n t store. In t h e topsoil, c a l c i u m a n d m a g n e s i u m levels are satisfactory for c r o p g r o w t h b u t p o t a s s i u m a n d p h o s p h a t e are low. T h e subsoil is strongly acid. A l u m i n i u m s a t u r a t i o n increases with d e p t h a n d n u t r i e n t status a n d b a s e s a t u r a t i o n are generally low a n d decrease with d e p t h . Ca, K, P a n d acidity levels are all c o n s i d e r e d limiting factors to p l a n t g r o w t h a n d t h e C a / M g ratio is indicative o f i m b a l a n c e . H i g h P - r e t e n t i o n is an overall characteristic o f t h e soil. Although relatively high levels o f organic c a r b o n w e r e d e t e r m i n e d to a d e p t h o f 1 m, the high C / N ratios suggest organic m a t e r i a l s o f l i m i t e d utility to p l a n t nutrition.

Survival and growth of trees Both Gmelina arborea a n d Terminalia brassii d e m o n s t r a t e d an ability to s u r v i v e in d i s t u r b e d c o n d i t i o n s ( T a b l e 5 ). Overall G. arborea s u r v i v e d b e t t e r

TREE GROWTHIN A COMPACTEDOXISOL

337

TABLE 4

Nutrient distribution in the representative profile (kg ha-J ) Depth

Ca

Mg

K

P

N

C

(cm)

Forest mat Topsoil

0 0 - 07 0 7 - 18

Total Subsoil

18- 30 3 0 - 66 6 6 - 99 99-134

228 102

33 43

12 3

4 5

6776 1882

123 200 34 817

334

78

15

9

8750

158 017

59 60 24 0

58 111 23 2

4 9 6 3

3 7 6 6

2256 3348 0 0

33 66 38 16

840 960 610 450

Subsoi! !otal

143

194

22

22

5604

159 208

Profile total

477

272

37

31

14 354

317 225

TABLE 5

Survival of Gmelina arborea and Terminalia brassii (plot mean, % ) Treatment

Gmelina

Terminalia

Uncleared area Light disturbance Lightly used track Heavily used track

52 ~ 81 83 71 84 98 77

62 E 81 71 54 72 69 69

Road cut Soil heap

Log yard

1Plots damaged by tree fall.

than T. brassii and survival was reduced for both species on the heavily used tracks and log yards. Unfortunately, comparisor~ with growth in the control plots is not possible because some of the nearby mature trees, which were poisoned or ring barked at the start of the experiment, had fallen across these plots in the intervening period, killing a number of planted individuals. Growth data are meaningful only in a relative sense, since the plant spacing of 1 × 1 m is very different to forestry practice. The difference between trafficked over and other categories is marked (Table 6), with growth in the lat1 ~ 1L_I ter p~ots about uOuuie that in the former. Tree roots of T. brassii in one road cut had spread across the soil surface into a nearby soil heap, so disrupting the data of the road cut category. Good growth on soil heaps was expected as these are mounds of topsoil. However, the trees were of the poorest quality

R.J. CHEATLE

338 TABLE 6 G~'owth of Gmelina arborea and Terminalia brassii (basal area; m 2 ha-R ) Treatment

Gmelina

Terminalia

Light disturbance Lightly used track Heavily usea track Road cut Soil heap Log yard

0.44 0.17 0.08 0.16 0.43 0.06

0.06 0.04 0.03 0.07 0.08 0.02

because movement in the loose material had led to stem warp and other unsatisfactory characteristics.

Pedological observations in experimental plots Soil colour and structure in control and lightly disturbed plots were similar and indicated 8-14 cm of forest topsoil in situ. The soil surface was covered by a partly decomposed litter layer of up to l cm. A slight degree of soil compaction was noticeable in the lightly disturbed plots. The soil heaps are formed of loose material, mainly topsoil, bulldozed into piles some 1-2 m high and several metres across. There was no discernible compacted layer and tree roots permeated the entire pile. Sites which had been subjected to repeated trafficking (tracks and road cuts) were all underlain by a distinct compacted soil layer, usually at a depth between 11 and 28 cm, but sometimes nearer the surface. In some cases, lateral spreading of roots had occurred around the upper interface between less dense and dense material. Roots explored the minor fissures that occurred within the dense layer but these were relatively infrequent. Two examples were found of roots breaching a dense layer in this manner. On 60% of plots the 2.5YR 4/7 colour close to the surface arid lack of blocky structure suggest that there had been comp!ete removal of topsoil. In the dense layer macropores were virtually absent and field permeability very low, but in the upper layer structure and macropores indicated modifications since establishment of the experiment. In log yards the dense layer occurred almost at the surf~ce~ beneath a veneer of loose material. These yards were virtually bare of vegetation, presumably owing to mechanical impedance to roots. Saprolitic material at the surface in one plot suggested deep truncation. Bulk density data (Table 7) support the distinctions illustrated by the pedological descriptions. This is an area with high and well-distributed rainfall and interpretations of bulk density are confounded by the unknown quantity

TREEGROWTHIN ACOMPACTEDOXISOL

339

TABLE 7 Mean bulk density and moisture content of soils Treatment

Uncleared area Light disturbance Lightly used track Heavily used track Road cut Soil heap Log yard

Near soil surface

Compacted layer

BD (gcm -3 )

Moisture (°/0, w/w)

BD (gcm -3 )

Moisture (%, w/w)

0.9 1.0 1.0 1.1 1.1 0.8 1.4

34 29 30 29 32 32 34

1.2 1.3 1.4 1.3 0.9 1.3

30 36 31 33 35 35

of the temporal and spatial moisture relationships. Nevertheless the bulk density data are supported by other evidence. Neumann ( 1987 ) suggested a critical threshold concept above which root impedance might occur on this soil type. These data suggest a value of around 1.2 gcm -3 for this threshold.

Pedological investigations outside the experimental plots It was observed that the area had been clear felled. Limited free auger sampiing and shallow pit excavations indicated that the area disturbed by trafficking may have been greater than that recorded in Table 1. The detection of smear tracks etched by logs drawn across soil in areas classed as undisturbed, confirmed trafficking there.

Chemical fertility The soils have been subjected to tractor blading, trafficking and erosional processes, including the selective removal of nutrients in runoffand leaching. As well as the site variability brought about by these processes, there is a degree of inherent variability. Overall variability is reflected in the results as seen from Table 8, where the range and modal value for each category are shown. Significant differences between catesories cannot be demonstrated. However, in relation to the representative profile some interpretation is possible. In general the 0-15-cm layer of the experimental plots had a lower nutrient status than the representative profile in spite of mineralisation from the large quantity of trash and logs left after logging. With the exception of the uncleared plots the organic carbon levels were lower than those of both topsoil and subsoil of the representative profile, suggestive of a decrease in the or-

340

R.J. CHEATLE

TABLE 8

Chemical characteristics of treatment plot soils (0-15 cm ) Treatment

pH CaCI2

C (%,~/w)

P K Ca Mg (/tgg - I ) ( c m o l k t , - ' ) (cmolkg -I) (cmolkg -I)

Uncleared area

Range 4.1-5.3 2.2-3.9 Mode 4.9 3.3

1-3 2

0.03-0.08 0.05

3.5-10.1 7,9

0.0-1.9 0.73

Light disturbance

Range 4,6-5.0 1.1-2.4 Mode 4.7 1.6

1-2 2

0.02-0.06 0.03

4.4-6.1 5,2

0.7-1.3 0.9

Lightly used track Range 4.3-5.2 2.1-3.1 Mode 4,6 2,6

1-2 2

0.04-0.06 0.05

5.1-9.2 " 7,2

0.9-1.5 1.1

Heavily used track Range 4.3-5.5 1.3-2.6 Mode 4.9 1.7

1-2 !

0.02-0.02 0.2

4.1-17.4 10.3

0.7-0.9 0.8

Roadcut

Range 4.2-4.8 1.1-1.4 Mode 4.4 1.3

1-2 2

0.02-0.05 0.04

d.6-19.4 9.2

0.5-0.6 0.6

Soil heap

Range 4.1-6.7 1.1-2.1 Mode 5.7 i.7

1-4 3

0.02-0.04 0.03

4.2-17.;' 12.7

0.4-9.7 0.5

Logyard

Range 5.2-6.6 0 a - ! 6 Mode 6.8 1.4

I-2 2

0.01-1).04 0.02

3.1~8.3 5.9

0.3-0.5 0.4

ganic nutrient bank. The difference was less l~ronounced in the plots of least disturbance. Le,wer p H values indicate an intensified acidity ,;ta~us. While calcium levels seemed higher and magnesiuni levels lower than the representative profile, in terms of plant requirements the levels appear satisfactory. Both potassium and phosphate levels are very low by any standards. Overall the fertility status was lower than before logging and generally unsatisfactory. Seven years after the logging operation there were no clear signs of natural restoration (rehabilitation) of these soils. DISCUSSION

The results indicate two major types of soil disturbance as a consequence of crawler tractor logging at Barora. Firstly, trafficking has produced a compacted layer in the soil, the density and depth of which is probably related to the intensity of heavy plant movements. Secondly, the developr~ent of logging infiastructure, plus tree felling and removal activities, have disturbed the forest mat and topsoil through mechanical redistribution and the imposition of conditions conducive to subsequent soil erosion. Both types of disturbance imply overall degradation of the. site in terms of future plant production. Soil compaction is likely to hinder root penetration and development, thereby decreasing the availability of rmtrients and water to the plant and limiting root anchorage capabilities. Removal of the top soil layers r~d~::,es the soil nutrient store, thereby lowefinr~ overall fertility. Sub-

TREE GROWTH IN A COMPACTED OXISOL

341

sequent plant growth is likely to lead to further, relatively rapid acidification of the topsoil and intensification of subsoil acidity. Basal area measurements of planted trees at Barora indicated that crawler tractor logging disturbance retards subsequent plant growth. In severe cases regrowth, whether planted or natural, is unable to re-establish for at least 7 years following logging operations and the exposure time of the soil to damaging elements of the environment is consequently extended. Most of the ~ 10 000 ha logged annually in the So!emon Islands is lowland primary forest on stable slopes below 400 m. Included in this area are some of the land facets most suited for agricultural development as well as reforestation. Assuming that activities at the Barora site were typical of other logging concerns and accepting the estimates made by Wall (1980) and Fraser and Larsen ( 1981 ) that at least 25% of the area has been severely disturbed then, since 1960~. the soil productivity potential of some 500 km 2 of land has been reduced. This represents a significant economic loss to the production potential of the country. Furthermore, other soils under lowland primary forest with merchantable timber, must be considered at risk. A small devel~ping country such as the Solomon Islands, with a high population growth rate and limited land resources, can ill afford such losses, especially when alternative logging practices, which could reduce logging damage, are feasible. C o n c e ~ over logging impacts in the Solomon Islands and e!sewher~ has shown that much could be done quickly and at low cost to impreve techniques to reduce damage. Where rainfall is temporally variable, compaction effects can be reduced if extraction is undertaken under relatively dry conditions. Suitable felling and extra_ction techn~u:~, using rubber-tyred skidders rather than crawler tractors, will minimise damage. In the U.S.A., for example, the use of the crawler on forest land is virtually defunct. More forethought in the location and construction of access tracks, log yards and other necessary infrastructure could considerably reduce the total land area subject to the most severe trafficking. Rehabilitation of compa~:ted soils is also possible, though costJy, ~nd may be needed bcfore plant growth in an area can recover adequately. More w~z-k is needed to ~dentfl}, cheap and efficient techniques for this work in d~veloping countries. The success of vegetation re-establishment on logged-over lands also depends on retainL~g or replacing the inherent fe~i_!ity of the systcm. One alternative is not =o clear torest land at ail but to crop it carefully, as traditional farmers would, using enrichment techniques to increase economic species that will h~ip increase productivity. Here the rattans (Calamus spp.), nut trees such as gnali (Canarium spp.) and the speciality rosewoods may have special merit, for they are crops of relatively high value. These arc crops of the forest that can be encouraged within the forest. Where some kind of cropping system is to follow logging, the optimum approach will be one which retains what is already there, maintains soil cover to avoid degradation and provides or-

342

R.J. CHEATLE

ganic or mineral inputs to replace the regular litter supply of the forest. Young ( 1986, 1987) has drawn attention to the potential of agroforestry for soil conservation, in respect to soil protection on slopes and the maintenance of fertility. On degraded soils, especially where acidification has been intense, rehabilitation will be costly. In commercial operations soil acidity can be ameliorated by inputs of lime and fertilizers, particularly phosphates. This approach is often out of the question in smaUholder systems. A temporal agroforestry approach, using block plantings of trees as an efficient fallow for rehabilita~ior:, cai~ improve soil, but in the se~'ere conditions identified ia this case, efficient short-term rehabilitation is likely to take seve~ai years of non-productive land use. The trce spe,zies chosen for rehabilitation will require to be ~zcid tolerant and possess the ability to root through dense sell layers. Though agroforestry approaches have been suggested for use in land dev~,lopment and soil rehabilitation (Fraser and Larsen, 1981 ) very little work has been done. At present the only form of po';t-logging nutrier~t maintainance practised in smallholder agricultural systems in the Solomon Islav~ds is the return to bush fallow after one or two years of cropping. Reforestation of customary land is rare after logging and the majority of the land is left to revert to bush. Any restoration of capital (as trees ) on much of the logged land will therefore be by natural regrowth, over a very ~_ongtime scale. Compaction and other ellects of tractor logging may be expected to restrict recovery there. However, this process should not be left to chance as a new growth of the speciality timber trees of these islands is a major requirement for logged over areas. Since reforestation cannot keep up witk extraction, financial resources and research energy could be usefully directed towards methodologies for improving natural regeneration. For all these approaches, technical and economic investigations should seek to assist decision making about soil conservation in an economic context. A financial framework, within which it is possible to describe loss due to the lack of respectable soil conservation measures, is likely to be more persuasive than technical description alone. Despite an awareness in the Solomon Is,a~us both of crawler tractor damage and of the existence of ways in which it could be reduced, there is at present virtually no practical control of any aspect of timber extraction and soil damage continues. CONCLUSIONS This study has demonstrated the vt;!nerability of heavy clays at Barora to crawler tractor operations. Truncation of the soil profile, loss of fertility and the development of a compacted layer were all consequences, their severity varying with the treatment received. Investigations of subsequent plant growth

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f o u n d that after 7 years, the basal area o f p l a n t e d trees on plots previously subjected to trafficking was h a l f that o f land which h a d only been lightly disturbed. S o m e heavily trafficked sites r e m a i n e d bare o f all vegetation, indicating that very little recovery can h a v e t a k e n place in the intervening period. Physical c o m p a c t i o n w o u l d s e e m to be a limiting factor, b u t o t h e r aspects o f r e d u c e d fertility m a y be influential in constraining growth, notably the limited supply o f nutrients owing to removaY, o f topsoil, which exposed a relatively inert growth m e d i u m . T h e r e t a r d e d growth on these logged o v e r areas represerltS a r e d u c e d p r o d u c t i o n potential for this land. It is likely that consequences o f logging operations in o t h e r parts o f the S o l o m o n Islands, a n d elsewhere, are similar. In m o s t lesser d e v e l o p e d c o u n t r i e s facilities for sophisticated e n v i r o n m e n tal analysis are not present. Reliance for i n f o r m a t i o n has to be placed u p o n cheap a n d efficient t e c h n i q u e s tbr m e a s u r e m e n t , such as those utilised in the field for this ~:.~udy. Basal area, bulk density a n d o t h e r pedological field records all reveal t h e m s e l v e s as m u t u a l l y supportive a n d efficient methodologies for rapid appraisal techniques. REFERENCES Bray, R.H. and Kurtz, L.T., 1945. Determination of total, organic and available for,~s of phosphorous in soils. Soil Sci., 59: 39-45. Fraser, T. and Larsen, A., 198 I. Evaluation of Reforestation Project North New Georgia. Chandler, Fraser and Larse,~, Rotorua, New Zealand, 127 pp. Hanseil, J.R.F. and Wall, J.R.D., 1976. Land Resources of the Solomon Islands, Vol. 1., Introduction and Recommendaiions. Land Resource Study Number 18, Land Resources Development Centre, Overseas Development Administration, U.K., 148 pp. Keeney, D.R. and Bremner, J.M., 1967. A simple steam distillation method of estimating flhydroxy-ot-amino acids. Anal. Biochem., 18: 274-285. Leach, B.J., 1973a. Progress report on the oil Fal:~ field trial at R5 (Banyan) near Ringgi Cove, Kolombangara. Oil Palm Report 73/4, Department of Agriculture, Honiara, British Solomon Islands Protectorate. Leach, B.J., 1973b. Progress report on the oil palm field trial at All (Ringgi) on Kolombangara. Oil Palm Report 73/5, Department of Agriculture Honiara, British Solomon lsland~ Protectorate. Marten, K.D., 1973. Extraction damage. Research Report, Forestry Department, Honiara, British Solomon Islands Protectorate. Neumann, A.J., 1987. Effect of soil disturbance caused by logging on the growth of Gmel~tza arborea and Terminalia brassii. Forest Research Note No. 30-2/87, Forestry Division, Ministry of Natural Resources, Honiara, Solomon Islands. OIsen, S.R., Cole, C.V., Wanatabe, F.S. and Dean, L.A., 1954. Estimation ofavailable phosphorous in ~ails by extraction with sodium bicarbonate. U.S. Dep. Agric., Circ. 939. Stanton, R.L. and Bell, J.D., 1969. Volcanic and associated rocks of the New Georgia Group. British Solomon Islands Protectorate, Overseas Geol. Miner. Resour., ! 0: I 13-145. Wall, J.R.D., 1980. Proposals for land development after logging on North New Georgia, Solomon Islands. Project Report No. 99, Land Resources Development Division, ODA, London. Wall, J.R.D. and Hansell, J.R.F., 1975. Land Resources of the Solomon Islands, Vol. 4, New

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Georgia Group and the Russell Islands. Land Resources Study 18, Land Resources Development Centre, Overseas Development Administration, London. Webb, I.S., 1973. The influence of logging operations on the soils of Kolombangara. Report, Department of Agriculture, Honiara, British Solomon Islands Protectorate (unpublished). Whitmore, T.C., 1969. The vegetation of the Solomon Islands. Philos. Trans. R. Soc. London, Ser. B. 255: 259-270. Young, A., 1986. The potential of agroforec,try for soil conservation, Part I, Erosion control. ICRAF Working Paper 42, International C,~u~cil for Research in Agroforestry, Nairobi, 90 pp. Young, A., 1987. The potential of agroforestq, fo~ soil ¢otlservation, Part iI, Maintenance of soil fertility. ICRAF Working Paper 43, International Council for Research in Agroforestry, Nairobi, 135 pp