Abstract

A comparison was made between soils beneath second growth and mature Fijian kauri (Agathis macro-phylla) forest. It was assumed that there were no significant soil differences between these sites prior to the selective logging of part of this area. The soils beneath the second growth stand showed a higher degree of leaching in the upper two horizons and a thicker, yet less well defined, organic topsoil horizon compared with the soils of the mature stand. The lower horizons in both sites were similar. The differences observed are likely to have resulted from the logging activities and subsequent modification of the vegetation in the second growth stand.

Key words:Agathis, Fiji, Soil, logging secondary succession.

Introduction

The two sites being compared were once continuous sub-tropical rain forest dominated by emergent Fijian kauri (Agathis macrophylla). During the logging exercises, commercial species including Fijian kauri, having a diameter at breast height (dbh) of 35cm or greater were removed. The logging is believed to have occurred between 1935 and 1960. The area that was logged now carries secondary re-growth containing regenerating Fijian kauri up to 30cm dbh. The soils were studied to determine whether differences exist between the soils of these two sites, and whether these differences were a result of logging.

The study area was located on the North Western slopes of Mt Lamalagi, near the Nadarivatu Government station in Northern Central Viti Levu (17° 35′ S, 177° 55′ E) between 900 and 990m altitude. The mean annual rainfall of this area is 2540mm-3300mm with a 3-5 month dry season. Mean monthly maximum temperatures range from 25°C in December to 22°C in July, with mean maximum temperatures range wing the same pattern with 17 and 14 respectively. Mean relative humidity remains higher than 80% year round (Berry and Howard 1972). A more detailed vegetation description is summarised in a previous paper in this issue (Weaver 1988).

The soils of the study area have been classified as humic latosols developing from basic parent materials including olivine basalt (Twyford and Wright 1965). These soils have developed on steep 25-40 degree slopes in a moist environment beneath sub-tropical lower montane rain forest. Humic latosols are commonly red or reddish soils having a clay texture but may be classified as loams in the field. They tend to exhibit a low base saturation, with moderate to high cation exchange capacity and are moderately to strongly acid. They are derived from parent materials ranging from olivine basalt lavas to acid-intermediate rocks such as acid agglomerate. Such soils are of widespread occurrence in Fiji and are found throughout the climatic range of the territory. Twyford and Wright (1965) have described the soils of the Nadarivatu area itself as being derived from olivine basalt.

Methods

Soil profiles were described from 10 soil pits, carried out in both the mature forest and in the second growth stand. The soil pit sites corresponded to vegetation analysis plots that were randomly located within the two forest types. Soil profile descriptions included colour, texture, structure and horizon depth. Soil samples were collected for each horizon from 6 of the 10 sites for chemical analysis. The chemical page 56 analysis of the soil samples was carried out by staff at the University of the South Pacific, Institute of Natural Resources, in Suva. The soil chemical properties that were measured are shown in Table 1.

Results
Physical soil properties

Soils beneath both vegetation types possessed similar structure. They were generally massive, with very friable, weakly developed, crumb structure. Boundaries were diffuse and colour change was slight down the profile except for the organic H1 horizon. Soil colour in the mature stand generally followed a brownish black 1-2cm humus, overlying a dark brown to brown 100-150cm sub-soil. Basement rock was not reached even at 150cm where the horizon at this depth corresponded to a B2 horizon (Gibbs 1980). In the soils beneath the secondary stand the organic horizon was thicker and of a lighter colour, generally following a dark brown to brownish black 1-14cm organic horizon, overlying a dark brown to reddish brown sub-soil.

The thicker but less darkly stained organic horizon in the secondary stand may have resulted from different species compositions between the sites or from modification of the soil caused by logging. Higher soil surface temperatures are likely beneath this vegetation type, due to a lower degree of shading compared with the mature stand. This also allows a higher proportion of surface rooting herbaceous plants to contribute to top soil formation. Similarly the thin (1-2cm) carbon rich humus blanket, recorded in the mature forest, suggests that a cooler and wetter top soil micro-environment exists in this heavily shaded situation.

The evidence of fire (charcoal fragments) seen in the modal profile for the second growth stand is likely to have been a result of localised fire caused by lightning strike. Trees scarred by lightning were a relatively common feature throughout the study area. This is not surprising considering the frequency of thunder storms in the region (Revel 1981). There was no evidence of fire in any other soil profile.

Modal Profile: Mature Forest

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Modal Profile: Secondary Stand

Discussion

The soils studied at Nadarivatu show features of both basaltic and andesitic origin. Exchangeable potassium values in the mature stand are more closely related to the range of exchangeable potassium characteristic of basalt soils (0.2-0.57 m.e.%). The exchangeable potassium of soils on this site do not exceed 0.2 m.e.%. The values recorded from the secondary stand are higher in the upper two horizons but do not fit completely into the potassium range characteristic of andesitic tuff soils (0.75-1.5 m.e.%). As the soils of the study area are located on slopes of >25 degrees, the soil mantle is unstable causing a constant drift of soil particles down the slope. Evidence for unstable slopes was seen throughout the study area. Thus, the direct influences of parent materials are not easily discernable due to constant renewal and truncation of the soil mantle, at least in the zone near the soil surface.

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Some Agathis species including A. australis in New Zealand (Eckroyd, 1982), A. borneensis and A. alba in Borneo (Whitmore, 1966) are known to podsolise soils. The nature of the soil parent material tends to influence the degree of podsolisation. In Borneo base poor parent materials combined with high leaching allows the mor-forming Agathis leaf litter to podsolise the soil. However, no evidence of podsolisation was seen in the Nadarivatu soils. The steep slopes of the study area contribute to soil rejuvenation through slipping and soil creep, thus masking any physical effects of podsolisation. Also a large proportion of the forest community present may be non-podsol formers. Whitmore (1966) found no evidence of podsolisation beneath A. macrophylla forests on Vanikoro Island in the Solomon group, where a similar situation exists.

In general the soils of these two forests differed. However, differences were observed predominantly in the upper two horizon, particularly in the organic H1 horizon. This horizon was thicker in the second growth stand and had a lower carbon content than the equivalent horizon of the mature forest soil. There was also evidence of a higher degree of leaching in the upper horizons of the second growth stand with lower base values and higher acidity compared with the mature stand. Whitlock (1985) noticed similar patterns in a successional sequence of regenerating Agathis australis in New Zealand. Here the organic zone in the upper horizons increased in nutrients over successional time. The horizon boundary between organic H1 horizon and its underlying horizon tended to be more distinct in the soils of the mature forest. However, the lower soil horizons of the two sites showed no major differences.

The differences observed in these soils must either pre-date the logging or, result from the logging activities. If these differences existed prior to logging, it may have resulted from a soil parent material disparity between the two sites, or there may have been significant differences in the species composition. The latter is unlikely, as the local area comprising the north western slopes of Mt Lomalagi at these altitudes maintains forest of the type present in the mature stand. Other indigenous vegetation in this area is second growth forest that maintains many similar species to that of the surrounding old growth forest (Weaver, 1987; MacDonald, 1987). If differences existed between the soil parent materials, evidence in the lower horizons would be expected. This was not observed in the present study.

Logging is more likely to have been the determining factor of the soil differences seen in this study. The direct effects of logging would include soil scarification (mixing of humus and mineral soil) resulting from heavy machinery activity, and a large build up of litter from discarded branches, damaged trees and crown foliage of logged trees.

A deeper organic topsoil horizon in the second growth stand would result from this process combined with scarification of the soil surface. The lower carbon content and base values in this soil may result from subsequent leaching and run off which would have increased following the removal of a large proportion of the protecting canopy.

Acknowledgements

This paper is the result of work carried out for part of a B.Sc. Honours thesis in Botany at Victoria University, Wellington. I would like to thank Ian McDonald for assistance in the field and a substantial contribution to the results. Dr Ross McQueen supervised the thesis and Stephen Fuller provided valuable discussion and useful comments on the draft. I would also like to thank Professor J. Morrison and Kamlish Chand for assistance with the soil chemical analysis.

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