Tuatara: Volume 30, Issue 1, December 1988
Soil differences between secondary and old growth Agathis Macrophylla forest at nadarivatu, FIJI
Soil differences between secondary and old growth Agathis Macrophylla forest at nadarivatu, FIJI
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.
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.
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.
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
|Classification:||Humic latosol (Twyford and Wright. 1965).|
|Location:||Mt Lomalagi, Nadarivatu|
|Landform:||N facing upper slope Mt Lomalagi|
|Relief:||Steep overall but 15-20° at pit site|
|Vegetation:||Mixed Agathis dominant mature forest Syzygium, Arytera dense understorey.|
|H1 0-1 cm:||moist; brownish black (7.5 YR 2/2) silt loam; weakly developed crumb structures; very friable; boulders 6-8%; fine root mat with mull humus; indistinct smooth boundary.|
|H2 1-94 cm:||moist; dark brown (7.5 YR 3/3) silt loam; weakly developed crumb structure; very friable; slightly sticky; weakly cemented in parts; mull humus; abundant roots; few boulders in horizon; generally massive horizon with little variation throughout; indistinct smooth boundary.|
|H3 94-123+cm:||moist; dark brown (7.5 YR 3/4) silt loam; weakly developed crumb structure; very friable; less roots, slightly more compact than H2; few stones or boulders|
Modal Profile: Secondary Stand
|Location:||Mt Lomalagi, Nadarivatu|
|Landform:||N facing mid-slope|
|Relief:||Steep - 20°|
|Vegetation:||Agathis, Syzygium mixed, secondary forest|
|H1 0-8cm:||moist; brown (5YR 4/4) silt loam; strongly developed nut structure; friable; fine root mat; coarse fragments 10-20%; smooth diffuse boundary|
|H2 8-19:||moist; reddish brown (5 YR 4/6) silt loam; moderately developed crumb structure; friable; roots abundant; small pieces of charcoal: <10% coarse fragments; smooth diffuse boundary.|
|H3 19-86+:||moist; reddish brown (5 YR 4/8) silty clay; moderately developed crumb structure; firm; slightly sticky and plastic; roots less abundant than above; very few coarse fragments; large concretions found at 74cm (7.5 YR 5/8); organic stained patches possibly decayed root remains (5 YR 3/3), diffuse boundary|
|General:||Soil very compact increasingly so with depth. Stones frequent in upper 50cm of profile. Lowest depth reached throughout was 1.5 M (no sign of basement rock).|
The mean values for soil chemical properties for both sites. A = mature stand values. B = secondary stand values.
|Mg||K||Na||Exchangeable acidity||Tamm's Extract. (%)||Tamm's Extract. (%)||15 Bar Water Retn.|
|m.e. %||m.e. %||m.e. %||m.e. %||Al||Fe||(%)|
|CEC||=||Cation exchange capacity|
|∼ Bases||=||Total exchangeable bases|
|m.e. %||=||Milliequivalent percent|
Soil chemical properties (see Table 1)
During leaching, water percolates down through the soil and mobilises soluble cations (base nutrients) to lower horizons, making them unavailable to plants. Calcium, sodium, magnesium and potassium occur as simple, highly mobile ions and are readily removed in this process. Once these cations are leached, their positions on the soil colloid particles are replaced by hydrogen ions. An increase in the number of hydrogen ions present, increases the acidity of the soil. Thus, acidity can be used as an indicator of the degree of leaching, with increasing acidity (lower pH values) tending to correlate with increased leaching.
Soil acidity was consistently higher in the upper horizons of the successional stand. There is a correspondingly low value for bases in these horizons (see table 1). It would appear that the secondary stand soils are slightly more leached in these upper horizons than in the mature forest.
Cation Exchange Capacity (CEC) measures the ability of the soil to absorb cations (i.e. bases) to the colloid particles within the soil, thus indicating the ability to hold nutrients. In the organic H1 horizon of the mature forest site the value of 86.7 m.e. % is substantially higher than the equivalent reading for the secondary stand (i.e. 37.6 m.e.%). However readings become similar in the lower horizons. page 59 Organic matter increases the CEC of a soil. The higher CEC observed in the topson of the mature forest is likely to relate to the higher values for organic matter. This is indicated by the higher reading for carbon content compared with the secondary stand.
Total Exchangeable Bases (TEB) measures the base nutrients present within a soil at the time of sampling. A similar pattern is seen with TEB ratings between the sites. Again the lower horizons have similar values, whereas the H1 horizon has a considerably higher value of 85 m.e.% in the mature forest, compared with 12.3 m.e.% in the secondary stand.
Percentage Base Saturation provides a valuable indicator of “base status’ which is often related to the amount of leaching. Lower Base Saturation values correspond with higher leaching. The Base Saturation values were only recorded for the mature stand. Characteristically the H1 horizon shows a high Base Saturation rating of >98 m.e.% as a result of the corresponding CEC values and the TEB, of which calcium and magnesium make up the largest proportions (>50 m.e.% and 11.2 m.e.% respectively). The lower horizons exhibit lower values with a mean Base Saturation of 26 m.e.%.
Both the calcium and magnesium components continue the trend observed in the CEC and TEB readings. Values for sodium are lower for horizons in the secondary stand. Potassium values however are higher in the secondary stand for each horizon with high values in the H1 and lower values in the H3 horizon.
Phosphate in New Zealand soils may be classified into 4 classes. These are Low (0-30%). Moderate (30-60%), High (60-90%) and Very High (90-100%). By these standards the soils in both sites studied here show very high phosphate retention. Phosphate rentention in acid soils is related to compounds of iron and aluminium (Saunders, 1968). Tamm's extractable iron and aluminium show high values and are possibly associated with the binding of phosphorus.
Water retention in a soil increases as the clay content increases. High clay content correlates with high CEC by increasing the number of colloid particles that can absorb cations. 15 bar water retention values obtained for the lower horizons of both sties range from 27.5% to 38.9%. These values would seem to represent moderate retention of water and relatively high clay content. A comparison of soil water retention values between soils in this study area and soils studied by Gangaiya et al (1982) suggest that Nadarivatu soils in general have a relatively high clay content and concurrently moderate water retention status for Fijian soils. Another factor that influences water retention is organic matter. In this study the topsoil readings for 15 bar water retention also correlate with the pattern of carbon content between these two soils, with higher values in the organic rich H1 horizon of the mature forest stand.
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.page 60
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.
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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