Changing Soil Hydrology Due to Rain Forest Logging: an Example from Sabah Malaysia

Journal of Environmental Management (1997) 49, 297–310

Changing Soil Hydrology Due to Rain Forest Logging: an Example from Sabah Malaysia S. M. Brooks and T. Spencer Department of Geography, University Road, Bristol BS8 1SS, U.K. and Department of Geography, Downing Place, Cambridge CB2 3EN, U.K. Received 20 June 1995; accepted 30 July 1996

Rain forest logging generally involves substantial changes in soil hydrology which lead to accelerated erosion. Many studies consider surface effects, focusing on increased runoff generation resulting from reduced soil permeability at saturation. Changes in soil profile hydrology are wide ranging and have a wider variety of consequences than simply a reduction in soil permeability. Soil moisture retention, saturated soil moisture content and hydraulic conductivity in the unsaturated zone influence moisture movement through the profile, pore water pressure distributions and moisture availability to plants between rain storms. Soil erosion at the surface is affected, but such factors also control the propensity for mass movement and the likely vegetation regeneration following abandonment of logged areas. This paper explores this range of hydrological properties and considers in detail some of the consequences of changing hydrological behaviour for an area of eastern Malaysia which is currently undergoing extensive logging. The results indicate that as well as a decline in saturated permeability, the soils undergo changes in moisture retention which affects unsaturated zone hydrology. The general model of Campbell is fitted to the data, the results of which indicate substantial changes in the relationship between moisture retention, hydraulic conductivity and soil suction following logging. These results form an essential requirement for the application of physically-based hydrology models which may elucidate in detail processes resulting from rain forest disturbance.  1997 Academic Press Limited

Keywords: rain forest, hydrology, permeability, moisture, retention, unsaturated zone, Malaysia.

1. Introduction Although reliable estimates of forest conversion are hard to determine due to problems of defining exactly what is meant by forest disturbance, it is clear that rain forest disturbance is taking place at an accelerating rate (Sayer and Whitmore, 1991). One of the main anthropogenic causes of this recent acceleration in disturbance rate is commercial harvesting of timber resources. In the Malaysian state of Sabah, particularly valuable Dipterocarpaceae tree species are present at relatively high densities and, coupled with 0301–4797/97/030297+14 $25.00/0/ev960091

 1997 Academic Press Limited

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a high export demand, this has led to some of the highest rates of forest loss in the tropical world. Marsh and Greer (1992) estimate that undisturbed rain forest in Sabah has been reduced from 60·1% in 1986 to 21·6% in 1992. One of the main timber extraction methods involves the use of tractors and chains. The employment of this technique is of considerable interest hydrologically, as it involves varying degrees of disturbance to the subsoil. This disturbance results in differential compaction and the development of a high level of spatial variability depending on the precise location from which individual trees are extracted. The main processes involved with this method include construction of log landing sites, where trees are collected together prior to transport from the area, and the use of skid roads radiating from these landings to the main clusters of extractable trees. Once the trees are felled they are then dragged by tractors, via the skid trails, to the log landings. Thus, the level of disturbance is conditioned by the number of tractor passes, involving areas of heavy soil compaction and other areas where soil is disturbed only to a minimal extent. Soil compaction has an important effect on soil hydrology in both the saturated and unsaturated zones (Van Genuchten, 1980; Hendrickx, 1990). At the surface, rates of infiltration are determined by soil permeability, while deeper in the soil pore pressure conditions determine rates of throughflow and the potential for large-scale slope failure. Soil erosion is a particular problem associated not only with the resulting higher volumes of surface runoff, but also with reduced pore pressure conditions which can lead to mass movement. However, very few studies have been carried out for ever-wet rain forest regions in the Tropics. Typically the erosion problem has been considered through measuring the sediment load of rivers. Available catchment data sets suggest increases in sediment concentration of over an order of magnitude for logged catchments compared with their pre-logged state (Malmer, 1990; Douglas et al., 1992). This methodology, however, does not permit elucidation of erosion mechanisms since it involves spatially averaged data for whole catchments; little idea is gained of catchment sub-areas from which sediment delivery is highest, nor of the processes responsible. Soil erosion has been evaluated for individual hillslope plots to a limited extent. Plotscale results suggest differences in erosion rates prior to and following logging of two to three orders of magnitude (Brooks et al., 1994). Plot-scale experiments have the advantage of permitting more detailed measurement, leading to closer evaluation of changing erosion mechanisms under different logging treatments carried out for different slope gradients. Processes of soil moisture redistribution can be measured and the main controls of soil erosion can be determined. However, even at this scale, the relative significance of mass movement and surface erosion have yet to be quantified. To implement successful erosion control strategies, it is important to have information about the relative importance of different processes, and precisely which hydrological changes lead to enhanced runoff generation. Control strategies are inhibited by the lack of data relating to the hydrology of tropical rain forest environments before and after logging, where changes are highly variable in space. Physically-based modelling offers an opportunity to apply a non-invasive technique to these problems but is hampered by lack of data for model calibration and validation. In particular, far more data are required for rain forest regions to quantify hydrological characteristics which relate specifically to differential degrees of disturbance. The few studies which consider soil hydrology tend to focus only on changes in soil permeability at saturation (Van der Weert, 1974; Malmer, 1990), as a reduction in this parameter can lead to substantial soil loss via generation of surface wash. Unsaturated

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zone hydrology remains largely unexplored although it may be significant in several areas. For example, where tropical soils are deep and relatively permeable, slope stability may be governed by unsaturated rather than saturated zone hydrology (Anderson et al., 1988). However, the significance of unsaturated zone hydrology to surface and subsurface soil erosion on logged sites in the humid tropics remains unexplored. Another potentially significant consequence of mechanised logging is the effect that compaction might have on edaphic conditions for vegetation regeneration. Soil compaction results in changes in moisture retention at different tensions (Hall et al., 1977; Arya and Paris, 1981), as well as in the saturated soil moisture content. This has implications for water availability to plants. These changes also affect soil wetting and infiltration at the start of rainfall; drainage rates following cessation of rain; and throughflow rates throughout the storm. Rates of wetting and the relationship between soil moisture and tension determine pore water pressure distributions and the propensity for slope failure. The aim of this paper is to present data from recently logged areas, as well as from undisturbed rain forest, of the Malaysian State of Sabah to compare the full range of hydrological changes which result from different degrees of disturbance. The results enable a more complete assessment of hydrological behaviour, and fulfil a vital requirement for initial implementation of physically-based models which consider saturated and unsaturated zone hydrology. 2. Processes of rain forest logging in Sabah, Malaysia The field study area has been described in detail elsewhere (Brooks et al., 1993, 1994). The area near the Ulu Segama Forest Reserve has undergone logging in recent years, resulting in areas of varying land disturbance as well as retaining extensive areas of conserved primary rain forest. This region presents opportunities for the comparison of similar hillslopes under undisturbed forest and different logging treatments. The process of logging in Sabah, Malaysia results in four main categories of disturbance to the vegetation and subsoil (Nussbaum, 1991). The areas used as log landings are heavily compacted as they are frequently used by heavy lorries and machinery during the loading of large logs onto transport vehicles. Bulk density values for disused log landings taken shortly after abandonment are typically 1·8–2·0 g/cm3 (Nussbaum, 1991). These values represent the most extreme form of disturbance because the vegetation cover is completely removed and the soil is highly compacted. A second category of disturbance involves the use of skid trails, over which felled trees are dragged by tractor and chains. Vegetation is absent from such areas. Compaction tends to be somewhat lower than for the log landings but the precise degree to which the skids are compacted depends on the frequency of use. Generally, a central feeder road is used frequently, but leading away from this is a series of tracks running to individual or small groups of trees to be harvested. These subsidiary trails are used to a lesser extent. Compaction is heavy around the trails, with typical values for bulk density being 1·6 g/cm3, but as low as 1·3 g/cm3 where the trails are used less heavily. A third disturbance level involves regions from which the trees have been removed, and involves considerable disturbance to the vegetation but with a limited effect on the soil. In this category, soil bulk density is little higher than for the primary forest. Surrounding the regions from which trees have been taken are areas where the vegetation is only slightly disturbed, while the soil is virtually unchanged. Finally, the undisturbed primary rain forest represents a fifth category. In the latter three disturbance classes, soil bulk density is around 1·0–1·2 g/cm3.

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0.7

3

3

Saturated moisture content (cm /cm )

0.8

0.6

Category E

0.5 Category B 0.4

0.3

0.3 0.5

Category A

0.7

0.9

1.1 1.3 3 Bulk density (g/cm )

1.5

1.7

1.9

Figure 1. Relationship between soil bulk density and saturated soil moisture content for different disturbance categories (A=heavily compacted log landing areas; B=compacted skid trails; E=undisturbed rain forest).

To consider hydrological change on disturbance in detail, field plots were located in three of the disturbance categories. Several plots were located on the log landings (category A), on heavily compacted soils on slopes of low gradient. Second, a number of plots were located on disused skid trails (category B), having heavy compaction but being of higher gradient. Third, locations were selected within undisturbed forest on a range of slope gradients (category E). For each plot, laboratory measurements were taken of bulk density, saturated hydraulic conductivity (falling head permeameter) and the soil moisture retention curves (pressure plate extraction). Hydraulic properties of the unsaturated zone were determined from these measurements using the method of Campbell (1974), and compared for logged and undisturbed areas. 3. Changes in soil hydrology resulting from logging 3.1.     Soil compaction results in a decline in total pore space. This decline is important hydrologically, as soil having a high bulk density requires a lower moisture input to become saturated. Depending on the precise value for permeability at saturation, the attainment of saturation in the soil can result in large quantities of runoff being generated, especially under high intensity rainfall. The relationship between soil bulk density and saturated volumetric moisture content is shown in Figure 1; under the most extreme logging (category A), approximately half as much rainfall is required for soil saturation compared with soils of the primary rain forest (category E). This factor forms an important component in modelling the effect of logging on soil hydrology and runoff generation, and in assessing the nature and severity of erosion.

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Disturbance category

Saturated hydraulic conductivity (m/s)

1.0E-03

A

A

A

B

B

B

B

E

E

E

1.0E-04 1.0E-05 1.0E-06 1.0E-07 1.0E-08 1.0E-09 Figure 2. Saturated hydraulic conductivity variation for different disturbance categories.

3.2.    Values for saturated hydraulic conductivity have been measured and reported elsewhere for the disturbance classes described above (Brooks et al., 1994). In the earlier study, two methods were employed to quantify hydraulic conductivity. First, a series of runoff plots (50 cm×30 cm) was set up in the field and rainfall was applied at a constant intensity from a portable rainfall simulator. Runoff was collected at the plot outlet and once a steady state had been reached, the infiltration rate was calculated as the difference between the applied rainfall and the runoff volume. Second, two cores (5 cm in diameter) were taken from each plot following the runoff experiments and the permeability of the surface layer was found using a falling head permeameter in the laboratory. For recently logged areas there was very good agreement between these two methods despite the difference in scale of the measurements. Values for saturated hydraulic conductivity ranged from 2×10−5 m/s to 8×10−8 m/s. Although these values represent a range of over two orders of magnitude, the differences were found between plots rather than between the methods applied to an individual plot. Thus, values for saturated hydraulic conductivity seem to reflect the different treatment experienced by each plot during logging. Figure 2 presents (i) a larger set of field infiltration data found from the runoff plot experiments, conducted over a range of slope angles and landuse categories, and (ii) the laboratory data found from several additional falling-head permeameter measurements. Although there is scatter in the data, reflecting the methods used as well as spatial variability between the cores, there is a consistent difference between the primary rain forest permeabilities and those of landuse categories A and B (log landings and skid trails respectively) where soil compaction is highest. On average, saturated hydraulic conductivity is reduced by over two orders of magnitude following logging, and in the most extreme cases, four orders of magnitude is common. The effects of this reduction on runoff generation have been explored in the field and shown to be considerable (Brooks et al., 1994). However, further assessment of runoff generation mechanisms

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can be achieved by considering changes in soil hydrology relevant to the unsaturated zone. Of particular significance is the soil moisture retention curve. 3.3.     Soil compaction can have a large effect on the volume of moisture retained at different tensions in the soil (Hall et al., 1977; Arya and Paris, 1981). This effect is important for investigating runoff generation as infiltration during soil wetting is determined partly by the suction at the soil surface. Initially, in dry soil, infiltration is high and the rate at which infiltration slows depends on the reduction in suction in the surface layer. For a given input of water, soil which is compacted tends to develop suctions closer to zero with the result that infiltration rates decline quickly and runoff is generated more readily. Soil with high porosity requires more water to become saturated and maintains higher suction for longer after the start of rainfall. Thus, on steeper gradients, for a given mechanical strength, the hydrological behaviour of soils with high bulk density is more likely to lead to mass failure. Soil moisture retention curves are also important components of hydrological models, especially significant for short duration events where the soil profile does not become entirely saturated, or for events where rainfall intensity is varying to a large extent. Figure 3 shows the results of the laboratory measurements of soil moisture retention curves taken using the pressure plate extraction method (Avery and Bascomb, 1974; Hall et al., 1977). Clear differences are apparent between the samples, most notably between the curves from the undisturbed rain forest under different geological units [Figure 3(a–c)] and logged plots [Figure 3(d)]. The latter have far steeper curves consistent with the higher bulk densities (1·553–1·681 g/cm3) of these samples. In all cases, the lower density of undisturbed forest soils compared with samples from logged areas results in more moisture being retained at lowest tensions commonly associated with field capacity. Compaction alters pore size distributions, reducing the proportion of large pores which retain moisture at lower tensions. This effect is clearly demonstrated in Figure 4, where all curves from undisturbed forest are plotted together, alongside a group from logged regions. The more compact logged soils have steeper curves, displaced into a region of lower moisture content. Although the geology varies between the locations from undisturbed rain forest, the sample bulk densities have a small variation (0·79–1·47 g/cm3) resulting in similar hydrological characteristics. The range is related to the depth at which the samples were taken, with deeper samples having a higher density. The range in bulk density is reflected in a range in moisture content at different tensions, but the curves from logged and undisturbed categories do not overlap (Figure 4). From the soil moisture retention curves, the available moisture for plant growth can be estimated. Generally, below a tension of about 0·3 bar (333 cm water), moisture is unavailable to plants as it drains freely through the large pores. Above a tension of 15 bars (15 000 cm water), plants are unable to extract moisture from the soil as the tension at which it is held is too great. Between these extremes, water is available. Table 1 provides data for “field capacity”, “wilting point” and available moisture for all samples. Clearly, there is more water available for plants in undisturbed soils, although the difference is not as great as might be expected given the level of soil disturbance during logging. Plotting the two values against bulk density shows the clear differences in both parameters for logged and undisturbed soils (Figure 5). Compaction of the soil results in greater reduction in “field capacity” moisture content than in the “wilting

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(b) Crystalline Basement

200

200

150

150 Suction (m)

Suction (m)

(a) Chert-Spillite

100

50

0

100

50

0.5

1 3

0 3

0.5 3

3

Volumetric moisture content (cm /cm )

(c) Kuamut Formation

(d) Logged Plots (1992)

200

200

150

150

100

50

0 0.5 1 3 3 Volumetric moisture content (cm /cm )

Suction (m)

Suction (m)

Volumetric moisture content (cm /cm )

100

50

0 0.5 1 3 3 Volumetric moisture content (cm /cm )

Figure 3. Soil moisture characteristic curves for different geological units with undisturbed forest cover (a–c) and for logged plots (d).

point” moisture content. This reduction in available moisture renders it difficult for plants to reestablish, with regeneration favouring species tolerant of high light intensities and drier conditions. The relationship between tension and moisture content is more readily incorporated into hydrological models as a continuous curve rather than as a series of data points. Curve fitting also permits comparison between curves for suction ranges where direct measurements have not been taken, and can be carried out provided the scatter of data points around the fitted curve is not too great. There are several methods for establishing equations to describe the shape of the soil moisture characteristic curve (Campbell, 1974; Van Genuchten, 1980). The method of Campbell (1974) has the advantage of being relatively straightforward to derive from a limited number of points on the curve, but is weak in describing the area between air entry suction and saturation where there are two main inflexions on the curve. This method also enables calculation of the unsaturated permeability at different tensions, an important component of hydrological models, but one for which direct measurement is time-consuming. Following Campbell (1974), the soil moisture characteristic curve is described by:

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200

A

B

Suction (m)

150

100

50

0

0.2 0.4 0.6 Volumetric moisture content (cm3/cm3)

0.8

Figure 4. Range in soil moisture content with suction for undisturbed forest (curve B) and for logged areas (curve A).

A B

w=a

hsat h

−b

(1)

where, w=suction (m), h=volumetric moisture content (cm3/cm3), a and b are constants. Using a logarithmic transformation, the constants can be quantified using regression analysis. Values for each of the curves shown in Figure 3 are given in Table 2. In each case the least squares regression is statistically significant, and the curves are described well using the Campbell (1974) method with only a small scatter around the average curve for each sample. To establish a more general relationship for soil moisture retention curves in logged and undisturbed rain forest soils the parameters of the Campbell equation were plotted against bulk density (Figure 6). The relationships that emerge provide a basis for predicting general curves for moisture retention resulting from logging. For undisturbed forest, the Campbell parameters show a wide range in values, but are linearly related to bulk density. High bulk densities produce lower values for both parameters. The logged soils tend to have higher values for both parameters at a given bulk density,

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T 1. Water retention at “field capacity”, “wilting point” and available moisture for logged and undisturbed regions h333 (cm3/cm3)

h15 000 (cm3/cm3)

Available water (cm3/cm3)

Primary forest 1 2 3 4 5 6 7 8 9 Mean St. Dev

0·5898 0·5499 0·5470 0·4794 0·3339 0·4990 0·4606 0·4176 0·4678 0·4828 0·0726

0·2526 0·1931 0·1898 0·2051 0·2255 0·2632 0·3043 0·2861 0·1908 0·2395 0·0400

0·3372 0·3568 0·3572 0·2743 0·1084 0·2358 0·1563 0·1315 0·2770 0·2433 0·0912

Logged areas 1 2 3 4 5 6 Mean St. Dev

0·3119 0·3317 0·3596 0·3325 0·3557 0·4591 0·3584 0·0478

0·0571 0·0907 0·1405 0·1625 0·1827 0·2788 0·1521 0·0707

0·2548 0·2410 0·2191 0·1700 0·1730 0·1803 0·2063 0·0337

Sample location

and are more closely grouped. This result reflects the more uniform nature of these compacted soils. The Campbell parameters may be thus ascertained for undisturbed and logged soils in this region, allowing initial quantification of the effect that rain forest logging has on soil moisture retention curves. 3.4.    Vadose zone hydrology needs to be quantified to consider moisture flux in response to suction gradients; the potential for landsliding in soils where additional shear resistance is gained from negative pore water pressures; and for infiltration during the early part of storms before soil saturation is reached. In many cases, unsaturated zone behaviour has little influence on “worst-case” conditions, but during the early and latter part of storms, when the soil profile is unsaturated, pore water pressure distributions have significant effects on surface and subsurface soil erosion potential. Currently, only limited data are available to compare vadose zone hydrology for logged and unlogged regions, although significant differences might be expected. Using the above data it is possible to produce general relationships between soil suction and permeability. Several indirect methods are available for this, and the method of Campbell (1974) represents an appropriate initial way forward. Using the soil moisture characteristic curve and the saturated hydraulic conductivity, Campbell (1974) derived the following equation to describe the dependence of unsaturated hydraulic conductivity on suction:

AB

k=ksat

we w

2 b

2+

(2)

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Primary Forest

Logged Areas

0.6

Volumetric moisture content (cm3/cm3)

0.5

Field capacity

0.4

0.3

0.2 Wilting point 0.1

0 0.6

0.8

1.0

1.2 1.4 3 Bulk density (g/cm )

1.6

1.8

Figure 5. Available water for plants (h15 000–h333) for soils from primary rain forest and logged regions.

T 2. Campbell parameters for the soil moisture retention curves hsat (cm3/cm3)

q (g/cm3)

a

b

r2

Primary forest 1 2 3 4 5 6 7 8 9

0·702 0·694 0·691 0·593 0·468 0·555 0·494 0·445 0·478

0·79 0·81 0·82 1·08 1·41 1·18 1·34 1·47 1·384

1·492 1·577 1·579 1·090 −2·096 0·724 −0·570 −1·299 1·121

−4·492 −3·639 −3·599 −4·485 −9·695 −5·955 −9·185 −10·076 −4·246

0·814 0·835 0·813 0·875 0·919 0·917 0·895 0·889 0·912

Logged region 1 2 3 4 5 6

0·374 0·379 0·414 0·373 0·356 0·407

1·658 1·646 1·553 1·662 1·681 1·572

1·386 1·115 0·722 −0·022 −0·044 −0·061

−2·245 −2·932 −4·053 −5·320 −5·715 −7·639

0·812 0·876 0·921 0·834 0·922 0·885

Sample location

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0 (a) –2

"b" parameter

–4

–6

–8

–10 Regression lines for primary forest data only –12 0.75

1

1.25

1.50

1.75

2 (b)

"a" parameter

1

0

–1

–2

–3 0.75

1

1.25 3 Bulk density (g/cm )

1.50

1.75

Figure 6. Relationship between soil bulk density and parameters required to calculate soil moisture retention curves using the method of Campbell (1974). (a) Campbell “b” value, (b) Campbell “a” value. (Β) Primary forest, (Χ) Logged areas.

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200

Logged areas

Primary rain forest

Suction (m)

150

100

50

0 1.0E-09

1.0E-07 1.0E-05 Hydraulic conductivity (m/s)

Figure 7. Unsaturated hydraulic conductivity functions found from the method of Campbell (1974) for logged and undisturbed forest.

where k=unsaturated hydraulic conductivity (m/s), we=air entry suction (m), w= suction (m), ksat=saturated hydraulic conductivity (m/s), b is the constant found from the soil moisture characteristic curve (as described above). The relationships between suction and hydraulic conductivity are plotted in Figure 7 for each of the samples. As well as having higher saturated hydraulic conductivities, samples from undisturbed rain forest have higher hydraulic conductivity throughout the range of suction considered here. The difference between the two groups of soil is over two orders of magnitude. Under unsaturated conditions, infiltration depends on the solution to D’arcy’s Law, modified for inclusion of variable hydraulic conductivity and negative pore water pressures at the ground surface as shown by the following equation: q=−k(h)

∂w ∂l

(3)

where q=flux per unit area (m/s), k(h)=unsaturated hydraulic conductivity (m/s) and ∂w/∂l=suction gradient. For a given suction (w) attained in the surface soil layer, hydraulic conductivity is up to two orders of magnitude lower for logged plots. This reduction has several implications. First, runoff will be generated readily for the logged areas whereas for the rain forest runoff may not be generated at all. However, this condition ignores the

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effect of litter in the forest which has been shown to be an important agent in runoff initiation (Coelho Netto, 1987), and compounds the excess runoff which results from a lower value of saturated hydraulic conductivity. Second, logged regions will generate runoff under lower rainfall intensities than for the undisturbed rain forest. Third, primary forest soils are likely to remain unsaturated during rainfall, given their requirement for more moisture to fill the greater void space. Thus, they are more likely to maintain large negative pore water pressures promoting higher infiltration rates. Fourth, as a consequence of lower suctions potentially being attained for logged regions, the propensity for slope failure here will be much greater than for undisturbed forest. The precise extent to which this effect is significant needs to be investigated as research to date has focused only on changes in surface erosion following logging. The data provided here demonstrate a clear need for further consideration of vadose zone hydrology and subsurface soil erosion, as changes resulting from logging may be considerable, and result from processes other than surface runoff generation and erosion. 4. Conclusions Opportunities to study in detail hydrological changes resulting from rain forest logging have been few and there is a paucity of data relating to the full range of hydrological parameters which are altered under rain forest logging practices. Traditionally, research has focused on the most obvious effects of enhanced surface erosion following a reduction in permeability due to subsoil compaction. The full extent of hydrological alteration is far more complex. This paper has demonstrated a number of ways in which hydrological change is significant and for which few data exist. First, soil moisture retention at different tensions has important implications for vegetation regeneration. This paper has demonstrated a clear reduction in available moisture resulting from logging, where subsoil compaction is considerable. Second, prevailing tensions in the unsaturated zone are significant to soil shear resistance. As well as determining the ease with which sediment can be detached at the surface (Luk and Hamilton, 1986), negative pore water pressures can enhance slope stability. Clearly, this is an area deserving closer attention in future, especially when the combined mechanical and hydrological effects of vegetation removal are concerned. Third, soil moisture movement in the vadose zone, especially in the region close to saturation, determines the timing of initial runoff generation and the dynamic changes in pore water conditions which occur at the start and end of storms. The rate of migration of the wetting front depends on subsoil pore water pressures, and this determines the total volume of water that enters the soil during storms, and hence the overall sediment delivery from the catchment. There is, therefore, an urgent need for reliable data to quantify rain forest soil hydrology and to provide estimates of the extent to which logging alters each hydrologically significant property. This paper has provided initial estimates of the extent to which soil hydrology is altered, and has shown it to be considerable, with potentially wide-ranging consequences. Hydrological modelling offers the potential to explore the likely significance of these changes, but in the absence of reliable data, model applications are hampered considerably. The results presented here provide the possibility for an initial application of plot-scale physically-based models to consider changing soil erosion rates and mechanisms under rain forest logging. Given the accelerating rate of forest disturbance, it is important that the full implications are realised and the hydrological mechanisms governing all forms of soil erosion properly understood. In particular, further measurements are required which take into account the spatial variability of

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Changing soil hydrology

soil disturbance under selective timber extraction procedures. Logging has been shown to cause enhanced runoff volumes, along with greater soil erosion, but most estimates are based on values averaged over whole catchments. In order to evaluate the true impact of soil hydrological change, comparative studies are required for logged and unlogged areas, within similar catchments. In this paper, four major hydrological changes have been quantified for recently logged rain forest and undisturbed forest from Sabah, Malaysia. All four parameters—saturated soil moisture content, saturated hydraulic conductivity, soil moisture retention curves and unsaturated hydraulic conductivity functions—show considerable differences between logged and unlogged areas, which suggests the need to focus on a greater variety of consequent soil erosion mechanisms than hitherto has been acknowledged. This research was conducted in collaboration with the Danum Valley Rainforest Research and Training Program, of the Forestry Division of Innoprise Corporation, and in association with research conducted by Professor I. Douglas of the University of Manchester and Dr J. Anderson of the University of Exeter. Funding was provided by the Overseas Development Administration (Project 4711).

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