Hydrological effects and nutrient losses of forest plantation establishment on tropical rainforest land in Sabah, Malaysia

Hydrological effects and nutrient losses of forest plantation establishment on tropical rainforest land in Sabah, Malaysia

Journal of Hydrology ELSEVIER Journal of Hydrology 174 (1996) 129-148 Hydrological effects and nutrient losses of forest plantation establishment o...

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Journal of

Hydrology ELSEVIER

Journal of Hydrology 174 (1996) 129-148

Hydrological effects and nutrient losses of forest plantation establishment on tropical rainforest land in Sabah, Malaysia A. Maimer Swedish University of Agricultural Sciences, Faculty of Forestry, Department of Forest Ecology, S-901 83 Umed, Sweden Received 5 June 1994; revision accepted 7 February 1995

Abstract

New measurements of field saturated hydraulic conductivity, soil moisture, rain intensity, surface runoff and erosion are presented with earlier published data on soil physical properties, stream water quality and runoff and dissolved nutrient losses from a paired catchment experiment in Sabah, Malaysia. The catchment experiment compared crawler tractor extraction and burning (normal practice, catchment W5) versus manual extraction and no burning (W4). Another catchment where secondary vegetation after forest fire was cleared and burned before planting was also studied (W1 + 2). Two separate control catchments were also included; one was for rainforest (W6) and the other comprised secondary vegetation (W3). Although hydraulic conductivity was reduced in clay soils at 20 and 40 cm depths under tractor tracks, the decrease was less drastic than for steady-state infiltrability on both clay and sand topsoils, since control topsoils (0-20 cm) were well aggregated in contrast to more compact subsoils with higher clay content. Topsoils were close to saturation at all times, also after treatments, but control forest surface runoffin slopes (excluding valley bottom) comprised only 2.9% of rainfall in a wet year. Except for just after burning, clear-felling did not increase slope surface runoff on undisturbed clay topsoils, and surface erosion was not increased, except for the erosion of ash after burning. On tractor tracks, extensive surface runoff caused surface and gully erosion. Elevated concentrations of suspended sediment in stormflow in W5 were back to pretreatment levels within 2 years. Under manual extraction suspended particulate loss was approximately half, most of which emanated from sediments activated by increasing stream runoff. Dissolved nutrient losses from catchment W5 in kilograms per hectare were 39.9 (N), 1.3 (P), 189 (K), 27 (Ca) and 16 (Mg) in the 33 months during and after treatments; these amounts were equivalent to between 10 and 185% of the removal in the harvest. For the 'minimum disturbance' of W4, the increased nutrient loss was halved, except for Ca. Burning of residues in Wl + 2 and W5 increased dissolved losses dramatically over a short period.

0022-1694/96/$15.00 © 1996 - Elsevier Science B.V. All rights reserved SSDI 0022-1694(95)02757-2

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A. Maimer / Journal of Hydrology 174 (1996) 129-148

The bulk of soil water was concluded to move quickly as troughflow in topsoil. Hence, 'bypassing' of retention possibilities in the largest pores with small specific surfaces led to large dissolved nutrient stream losses in this humid tropical environment, even without soil disturbance and extensive surface runoff.

I. Introduction

Managed natural forest and extractive tree plantations are becoming increasingly important owing to the depletion of forest resources in the tropics (Whitmore, 1990). Consequently, forestry was one of the main issues at the United Nations Conference of Environment and Development in Rio de Janeiro 1992 (e.g. Fries, 1992). Logging and clear-felling of tropical rainforest disturb soil and water budgets. This disturbance varies in amount and duration according to the intensity of the operation and the methods used (Hamilton and King, 1983). To secure sustainability of forest production in the future, it is important to optimize silvicultural systems, so that selective logging and land clearing for forest plantations minimize negative impacts on soil and water (e.g. Dykstra and Heinrich, 1992; Mok, 1992). In Ghana, Nye and Greenland (1964) found large differences between total topsoil nutrient loss and a much smaller removal in harvest after 2 years of cropping; they attributed this to loss by leaching. Erosion processes and changes in hydrology have been studied on the plot scale as well as the catchment scale. However, to understand the dynamics of combined disturbances (e.g. clear-felling, soil disturbance and burning) in different parts of a catchment and the effect on nutrient dynamics and site sustainable production, different approaches and scales must be combined (Anderson and Spencer, 1991). Few such combined approach studies have been published for tropical forests (Bruijnzeel, 1990; Bonell with Balek, 1993; BoneU, 1993). The present paired catchment study, started in 1985, monitors the hydrological, hydrochemical, soil a n d biomass changes before, during, and after different conversions of selectively logged or fire-damaged tropical rainforest to forest plantation. The study began as an environmental impact assessment and a prerequisite for the location of a pulp and paper mill in Sipitang, Sabah. Quantification of effects on the catchment scale were combined with detailed studies of soil physical and chemical properties as well as with investigations of slope hydrological processes and biomass investigations. T h e research area, representative of forests being converted to plantation forest, was selected in intermediate elevation forest that had been lightly selectively logged in the early 1980s. It included areas affected by the extensive forest fires of 1982-1983 (Woods, 1989; Nykvist, 1995), as these were the areas selected for priority reforestation. Some of the results on which this discussion is based have been published elsewhere; new data on surface runoff, surface erosion, saturated hydraulic conductivity, soil moisture and rain intensity have been added to discuss the dynamics between hydrological processes, erosion and streamflow nutrient losses in forests subjected to different treatments.

A. Maimer I Journal of Hydrology 174 (1996) 129-148

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A. Malmer / Journal of Hydrology 174 (1996) 129 148

2. Research area and basin treatments

The Mendolong research area is situated at 650-750 m.a.s.1. (115.5°E, 5.0°N) at Sipitang, Sabah, Malaysia (Fig. 1). The bedrock of the research area consists of steeply dipping, interbedded sandstones, siltstones and shales (Malmer, 1993; Grip et al., 1994). Soils are Haplic Acrisol with clay topsoil and Gleyic Podsol with sandy topsoil with various intermediate forms (Fig. 1). Topsoils are loose with welldeveloped structure and high porosity (Malmer and Grip, 1990). The two control catchments are relatively homogeneous in areal distribution of the two main soil types, while the other catchments have more of a mix of the two soil types as well as intermediate forms. Five year means of precipitation and runoffwere 3215 m m and 1962 m m versus 3490 m m and 1950 m m for control catchments W3 and W6, respectively (Malmer, 1992). Catchments W l - W 3 comprised secondary vegetation after the forest fire (Nykvist, 1995). Catchments W4 W6 comprised lowland hill dipterocarp forest lightly selectively logged in 1981 (Sim and Nykvist, 1991). The catchment treatments (Malmer and Grip, 1990), applied in N o v e m b e r 1987-May 1988, are given below. W1 + 2 Non-mechanized clearing of remaining trees and secondary vegetation, no wood extraction, burning of all biomass and planting. (These two catchments were treated as one owing to uncertainty in location of the c o m m o n phreatic water divide.) W3 Control catchment for forest fire area. No treatments. W4 Manual felling, manual wood extraction, clearing of planting rows in the slash and planting in these rows, without burning (minimum disturbance). W5 Manual felling, wood extraction using crawler tractors, burning of the remaining biomass and planting (normal practice). W6 Control catchment for selectively logged area. N o treatments. The tree species planted in the treated catchments was Acacia mangium. The dry matter accumulation in the aboveground vegetation in trees and in ground vegetation

Table 1 Above ground biomass dryweight accumulation (t ha l) in Acacia mangium plantation in different catchments 1.5 and 3.8 years after planting in Mendolong research area, Sabah, Malaysia (after Sim and Nykvist, 1991 and Nykvist et al., 1994) Catchment

Biomass accumulation (t ha- l) 1.5 years

Wl÷2 W4 W5

3.8 years

Trees

Undergrowth

Trees

Undergrowth

2.3 10.5 5.4

6.2 1.8 3.3

15.8 ~.6 23.0

6.4 5.4 7.4

W1 + 2 was manual clearing of scecondary vegetation after forest fire and burning before planting, W4 was clear-fellingof tropical rainforest, manual extraction and no burning before planting, W5 was clearfelling, tractor extraction and burning before planting.

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in the plantations after 1.5 and 3.8 years are presented in Table 1 (after Nykvist et al., 1994).

3. Methods of monitoring Continuous streamflow gauging in the six catchment streams started in mid-1985 together with measurements of precipitation (Maimer, 1992). Streamflow was sampled for suspended sediment load during high and low flows (Maimer, 1990), and for dissolved element chemicals at various flow rates (Grip et al., 1994). The effects of treatments on runoff, on streamflow suspended and dissolved concentrations and on losses were calculated by regressions between the control catchments and the treated catchments. Soil disturbance after manual and crawler tractor log extraction was assessed. This was combined with measurements of steady state infiltrability (double ring) and dry soil bulk density in control forest as well as in different types of disturbed soils on two soil types (Malmer and Grip, 1990). Furthermore, field measurements (inversed auger hole method, Kessler and Oosterbaan, 1974) of soil-saturated hydraulic conductivity in control forest and in disturbed soils were carried out in late 1989 (after Andersson, 1990). Seven unbounded runoff plots (50-200 m 2) were installed in control forest and in clear-felled forest on clay topsoil without soil disturbance in W4 and W5, immediately after the completion of the treatments in early 1988. Area, length, slope and treatment of plots are given in Table 2. The plots did not include the flattest area of about 10 m width close to the stream, so that saturation overland flows or return flows (Dunne, 1978) were not included in the slope runoff collected. Measurements of rain intensity (OTA tipping bucket, 1 h resolution), soil moisture (tensiometers in profiles at 20 and 50 cm depth along slopes) and gully erosion (repeated measurements of cross-sections

Table 2 Description of unbounded runoff plots in Mendolong research area in Sabah, Malaysia Plot

Area (mz)

Length (m)

Slope %

Treatment

mean

interval

CA CB CC

192 108 55

40 17 7

27.7 36.4 19.6

22-50 22-45 18-23

Control forest, main plot like CA, upper half of slope like CA, top of slope

MA MB MC

198 111 61

33 18 7

37.4 26.9 42.8

23-54 5-56 15-51

Manual extraction, no burning (W4) like MA, upper half of slope like MA, top of slope

BA

194

25

31.7

17 44

Burning after clear-felling (W5)

All plots were located on soils undisturbed by wood extraction. C-plots denote reference forest, M-plots manual extraction and no burning after clear-felling (W4) and B-plots burning after clear-felling (W5)

A. Malmer / Journal of Hydrology 174 (1996) 129-148

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a l o n g slopes o f t r a c t o r tracks) were also included in this slope s t u d y d u r i n g 1988 ( M a l m e r , 1993).

4. Results

4.1. Infiltrability and soil permeability B o t h s a n d y a n d clay topsoils in c o n t r o l forest h a d high m e a n s t e a d y - s t a t e infiltrabilities (/ms, T a b l e 3): a w e l l - d e v e l o p e d structure, 1 5 - 2 0 c m thick on clay soil b u t only a b o u t 10 c m t h i c k on s a n d y topsoils was i n d i c a t e d b y m e a n d r y b u l k densities (Bdma) a n d p o r o s i t i e s ( M a l m e r a n d G r i p , 1990). Because o f the increasing clay c o n t e n t a n d decreasing structure with d e p t h , the m e a n field s a t u r a t e d h y d r a u l i c c o n d u c t i v i t y (Kmfs) o f the c o n t r o l forest soils d e c r e a s e d c o n s i d e r a b l y with d e p t h (Table 3). T h e s a n d y t o p s o i l h a d lower Kmfs t h a n the clay t o p s o i l at 20 c m because the structure o f the s h a l l o w e r t o p s o i l was less developed. Soil d i s t u r b a n c e b y t r a c t o r s in the clear-felled a n d b u r n e d c a t c h m e n t (W5) c h a n g e d soil h y d r o l o g i c a l variables c o n s i d e r a b l y on 2 4 % o f the c a t c h m e n t area. O n m o s t surfaces o f t r a c t o r tracks, the forest t o p s o i l was p u s h e d away, a n d the 'new t o p s o i l ' consisted o f u n c o v e r e d d e e p e r soil layers with n a t u r a l l y higher Bdmd a n d l o w e r / m s . T h e surfaces o f the heaps o f soil m a t e r i a l at the sides o f the t r a c t o r t r a c k s (included in the 2 4 % ) consisted m a i n l y o f ' b e l o w t o p s o i l m a t e r i a l ' t u r n e d over by the tractors. H o w e v e r , these h e a p s also c o n t a i n e d t o p s o i l m a t e r i a l a n d were n o t c o m p a c t e d by p a s s i n g t r a c t o r s . / m s was r e d u c e d to p r a c t i c a l l y zero on b o t h t o p s o i l types (Table 3). Table 3 Steady-state infiltrability (/ms m m h -l) and field saturated hydraulic conductivity (Kmf~mm h -z ) at 20 and 40 cm depth on clay and sandy top soils in control forest compared with after manual and tractor extraction and on 6-year-old tractor tracks at Mendolong research area, Sabah, Malaysia (after Malmer and Grip, 1990, and Andersson, 1990) Clay topsoil Forest control

Sandy topsoil Manual

Old tractor

/ms (mm h -l ) Mean 154 SD 89.8 n 10

36.7 31.2 10

0.63 0.45 8

Kmts 20 cm (mm h -1 ) Mean 0.682 SD 0.479 n 10

-

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-

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11.6 21.2 8

0.032 0.022 10

0.105 0.035 10

-

0.176 0.146 10

0.013 0.010

0.033 0.017

-

0.052 0.032

10

10

-

1.26 1.16 8

10

A. Malmer / Journal of Hydrology 174 (1996) 129-148

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Fig. 2. Weekly means of measured soil suction (kPa) in profiles at 20 and 50 cm depth in the highest and lowest points of runoff plots in 1988 in the Mendolong research area, Sabah, Malaysia. CA, control forest; MA, manual extraction and with burning; BA, burning after clear-felling.

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A. Malmer / Journal of Hydrology 174 (1996) 129-148

The very low value of/ms measured on 6-year-old tractor tracks on clay topsoil was consistent with the observations of Van der Plas and Briujnzeel (1993) on 12-year-old tracks. Measurements made 2 years after disturbance showed that values of Kmfs at 20 and 40 cm depth beneath disturbed soil surfaces on clay topsoils were reduced about 10 times (Table 3). For disturbed sandy topsoils the value of Kmfs did not differ statistically from that of the forest control. In the manually extracted catchment (W4) some 4% of the surface was disturbed by manual skidding along wooden rails, which prevented topsoil removal on all but very small parts of these surfaces. Some compaction was detected as increased values of Bdmd and values of/ms experienced a minor non-significant reduction on both soil types (Malmer and Grip, 1990). 4.2. Soil moisture In the forested control plots, measured soil moisture tension at 20 cm was seldom above 10 kPa, except after weeks that had low rainfall and clear days. The lowest part of the plot was wetter than the upper part of the slope (Fig. 2, CA), and soil horizons at 20 cm generally had lower suctions than drainage equilibrium compared with suctions at 50 cm (i.e. the soil suction at 20 cm was less than 3 kPa more than that 30 cm below at a depth of 50 cm - - indicating drainage (Malmer, 1993)). On the clear-felled plot without burning (Fig. 2, MA), the suctions at 20 cm were even lower than those at 50 cm. At the b o t t o m of the plot, the suction was, even in absolute readings, lower at 20 cm than at 50 cm. The top of the burned slope (Fig. 2, BA) differed from all the other measured sites; it was the only site to show higher suctions for longer periods at 20 cm as compared with drainage equilibrium at 50 cm. This probably was due to the naturally dryer upslope position in combination with the burning of the vegetation and residues, which made evaporation from the soil surface more effective. The large amounts of residues, particularly, had a considerable shading effect on unburned soil, observed in soil temperatures (unpublished). 4.3. Rain intensity and surface runoff 1988, the year of the slope measurements reported here, was the most humid of 5 years for the area, with 4352 m m of precipitation at the runoff plots. The number of days with rain was 190, but 55 of these experienced less than 8 m m of rain. More than 50 m m of rain during 24 h were recorded on 21 days, and the 24 h m a x i m u m was 111 mm. M a x i m u m 1 h rain was less than 30 mm, but higher intensities of 5-10 min duration were observed at the start of rainstorms. Over a year, the surface runoff in the control forest was only 2.9% of total rainfall. A m a x i m u m 24 h surface runoff of 17.5 m m was recorded at CA on the occasion of the m a x i m u m 24 h rainfall. The scatter between daily surface runoff versus 24 h rainfall in the control forest was high and had a loglinear adjusted R 2 of 0.61. The relationship of surface runoff to hourly rain intensity was poorer, probably because 1 h was too large a time interval to separate out the highest intensity of storms. The

A. Malmer / Journal of Hydrology 174 (1996) 129-148

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response to short high rain intensities in the early stages of an event was also dependent on antecedent soil moisture. The partial slope plots experienced few events where surface runoff exceeded the detection limit of 0.1 ram. Hence, the small sample size together with the high variability of surface runoff resulted in the relationship between the main plots at the partial slope plots being poor. However, the ratio of total surface runoff during 1988 for plot CA to partial slope plots CB and CC was 0.64 and 0.23, respectively (cf. Table 2). Clear-felling (on undisturbed soil surfaces) did not increase surface runoff considerably, as shown by accumulated surface runoff in Fig. 3. Differences were mainly a response to major rainstorms. In May, immediately after the second burning in W5, BA had the highest surface runoff but later, after new vegetation cover had become established, the response to rainstorms was more moderate. During the whole of this 9 month period after treatment, MA (clear-felling and no burning) had the smallest responses in surface runoff from major rain events. Accumulated sums of surface runoff for the same period gave the ratio of 0.69 and 0.53 of MA surface runoff to partial slope plots MB and MC, respectively. Surface runoff on disturbed soils (tractor tracks) was not measured in this study. However, at the low infiltrability shown here, extensive surface runoff is evident, and was observed during treatments (cf. Bruijnzeel, 1990, plate 6; Malmer, 1993, book cover) and continues still several years after the initial disturbance. 4.4. Erosion

Recorded surface erosion under control forest was low (38 kg ha -I year -1, plot CA), as was surface erosion on clear-felled slopes without soil disturbance: plot MA

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A. Malmer / Journal of Hydrology 174 (1996) 129 148

in W4 yielded 82 kg ha 1 and plot BA in W5 142 kg ha -I over 9 months after treatment. In control forest and in W4, the major part of the erosion occurred during 14 major rain events, while the bulk of 'undisturbed surface' soil loss in W5 occurred soon after burning. Half of the sediments lost during 9 months were lost as ash washed away in 2 days of the first moderate rains after burning in W5 (Malmer, 1993). Soil losses from tractor tracks and disturbed surfaces in W5 were considerable. Gully erosion was still apparent on tractor tracks in 1990, and soil losses on the worst eroded tracks were equivalent to a rate of 500 t ha 1 year- 1 during the first 2 years. The bulk of the sediments from the tracks was trapped downslope within the catchment, and only a minor fraction was carried away as sediments suspended in streamflow. 4.5. Streamflow runoff" In the forested catchments, streamflow response to rainfall was rapid; maximum flows occurred within 30 min (Malmer, 1990). Only 26 (W6) to 30% (W3) of control catchment total runoff occurred as baseflows (Malmer, 1992). During the hydrological year 1987/1988, including the 8.5 months of treatments, the increase in stream runoff was only about half for the 'minimum disturbance' regime in W4 when compared with W l + 2 and W5 (Table 4). For W4 and W l + 2 the effect was rather small in the third year 1989/1990, whereas W5 in that year had a second increase in stream runoff. This prolonged effect in W5 was related to a shift from increases in baseflows to increases in mean stormftows and mean peakflows in that year compared with those for the control catchment W6 (Malmer, 1992). In catchment W l + 2 mean baseflows decreased and mean stormflows and peak flows increased for all these years, while catchment W4 experienced no significant changes in discharge dynamics compared with control catchment W6. 4.6. Streamflow water quality Suspended sediment stormflow concentrations were low (typically below 30 mg 1-1), and did not differ significantly from baseflow concentrations from control catchments and before treatments in other catchments. During treatments, there were fast responses to soil disturbance at all flow rates in W5 but they were significant only Table 4 Increases in runoff(mm) from differentcatchments owingto different forestrytreatments in the Mendolong research area, Sabah, Malaysia the 3 years during and after treatments (after Malmer, 1992) Catchment

1987/1988

1988/1989

1989/1990

Total 3 years

W1 + 2 W4 W5

397 197 460

522 170 262

89 80 468

1008 447 1190

W1 + 2 was manual clearing of secondary vegetation after forest fire and burning before planting, W4 was clear-felling of tropical rainforest, manual extraction and no burning before planting, W5 was clearfelling of tropical rainforest, tractor extraction and burning before planting.

139

A. Malmer / Journal of Hydrology 174 (1996) 129 148

Table 5 Treated stream mean suspended sediment stormflow concentrations (mg 1-1) during different periods before, during and after treatments in different catchment streams in the Mendolong research area, Sabah, Malaysia Catchment

Reference period 10 months

During felling 3 months

After burning W1 + 2 and W5 5 months

1988.1989 1 year

1989/1990 1 year

W1 + 2

7.9 4.5 14.5

24.4 23.3 78.0***

59.1"** 17.8 55.2***

14.1 30.5*** 41.4***

9.3 3.4 5.6

W4 W5

Wl + 2 was manual clearing of secondary vegetation after forest fire and burning before planting, W4 was clear-felling of tropical rainforest, manual extraction and no burning before planting, W5 was clearfelling of tropical rainforest, tractor extraction and burning before planting (after Maimer, 1990). Asterisks indicate stormflow concentrations significantly different from baseflow concentrations.

in stormflows (Table 5). The maximum recorded concentration was 552 mg 1-1 in W5 during soil disturbance. The significant effect of treatments lasted during treatments and in the subsequent year, but was not detectable in 1989/1990. W4 had a significant increase in stormflow concentrations in the year after treatments, and W1 + 2 had a short-lived rise in stormflow concentrations immediately after burning (Malmer, 1990). Less than 10 days after the start of felling in the upper parts of W4 and W5, 10-

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A. Maimer / Journal of Hydrology 174 (1996) 129-148

140

streamflow concentrations of dissolved major plant nutrients were much higher than the very low concentration levels in the pretreatment peri'od (Malmer and Grip, 1994). Fig. 4 shows stream baseflow concentrations of potassium, as an example. W1 and W2 had minor rises in baseflow concentrations as ground vegetation underbrushing started in late December (Fig. 5). Higher concentration levels were seen in W1 and W2 in mid-January when all small trees were felled. The high concentration levels in W 1 and W2 in mid-January when felling small trees with comparatively small biomass, may be explained by the large amount of leaf biomass cleared in a very short time (2 days). By comparison, the felling in W4 and W5 was carried out over 2 months, but a much larger total biomass was felled (cf. Sim and Nykvist, 1991; Nykvist, 1995). Stormflow nutrient concentrations did not differ significantly from baseflow concentrations before treatments and in the control streams but, during clear-felling and after burning, concentrations rose to 10-100 times than in the baseflow (Malmer and Grip, 1994). Stormflow nitrate (NO3-N) concentrations were never above the W H O recommended health limit (10.3 mg 1-1) but were generally above 1 mg l-I after burning in W1 + 2 and W5. Concentrations of nutrients like NO 3 related only to biomass returned relatively quickly to pretreatment and control levels during 1989. Concentrations of other elements like potassium (Fig. 4) related to weathering sources were still raised at the end of 1990.

4.7. Streamflow particulate and dissolved losses The total suspended sediment outputs from the treated catchments during the

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1.5 y e a r s o f effects w e r e W 1 + 2: 1.1, W 4 : 2 . 2 a n d W 5 : 3 . 9 t h a -1, c o m p a r e d w i t h 0.3 t h a -1 y e a r - I f r o m t h e c o n t r o l f o r e s t ( W 6 ) a n d 0.6 t h a -1 y e a r -1 f r o m s e c o n d a r y v e g e t a t i o n ( W 3 ) ( M a i m e r , 1990). T h e c o n t r o l f o r e s t d i s s o l v e d loss w a s 0 . 1 6 t h a - l y e a r -1 ( W 6 ) a n d f o r t h e s e c o n d a r y v e g e t a t i o n c o n t r o l 0.22 t h a -1 y e a r -1 ( W 3 ) ( G r i p e t al., 1994). T a b l e 6 s u m m a r i z e s dissolved net losses of some nutrients, calculated by regression, compared with that f r o m c o n t r o l c a t c h m e n t s d u r i n g a n d a f t e r t r e a t m e n t s ; t h e t o t a l s f o r 33 m o n t h s a r e c o m p a r e d w i t h t h e n u t r i e n t s r e m o v e d b y h a r v e s t ( N y k v i s t et al., 1994). F o r b o t h periods, total losses and rates for most major nutrients were highest from W1 + 2 and W5 after burning. Total dissolved losses for W4 were around half of those from Table 6 Dissolved nutrient losses during different conversions of tropical rainforest to forest plantation as effect of treatments (kg ha -l) (after Maimer, 1993), in the Mendolong research area, Sabah, Malaysia Form of loss and time period

Element

Streamflow during harvest 4 months

N-tot P-tot K Ca Mg N-tot P-tot K Ca Mg N-tot P-tot K Ca Mg

0.3 0.1 3 -2 a -2 a 13.0 1.1 131 21 1 3.9 0.7 -50 a 9 -4 a

0.9 0.1 10 2 1 7.2 0.2 33 8 3 18.8 0.5 63 14 3

2.3 0.3 18 4 3 10.2 0.6 84 8 5 26.8 0.4 87 15 6

N-tot P-tot K Ca Mg N-tot P-tot K Ca Mg

17.2 1.8 84 28 -5 a -

27.0 0.8 106 25 8 142 3.0 123 242 46

39.9 1.3 189 27 16 118 2.5 102 201 38

Streamflow during and after burning 5 months

Streamflow during plantation establishment 24 months

Streamflow total 33 months

Removal in the harvest

Net loss (kg ha 1) W1 + 2

W4

W5

a Negative number owing to smaller loss than would have been the case without treatment, as indicated by regression analysis with control catchment. W 1 + 2 was secondary vegetation, no extraction, burning and planting, W4 was lightly selectively logged forest, manual extraction and no burning before planting, W5 was lightly selectively logged forest, tractor extraction and burning before planting. (Some differences between tabulated sum and the sum of subperiods are due to round numbers in subperiods.)

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W5, except for Ca. Total dissolved losses owing to treatments for the elements shown in Table 6 were between 9 (P) and 86% (K) of contents in easily decomposed parts (including leaves, fine branches and fine roots) of the clear-felled rainforest (Malmer and Grip. 1994).

5. Discussion

5.1. Soil physical properties and soil water

In the control forest, high infiltrability and little surface runoff, combined with rapidly decreasing hydraulic conductivities with depth, demonstrate the importance of the highly permeable topsoil for downslope water transfer in the main sloping parts covered by the runoff plots. Data on hydraulic conductivity from forests of the humid tropics have until lately been scarce. Even though rates could be expected to vary with degree and depth of soil structural development, recently, several studies have confirmed decreases in conductivity with depth for sites in Peru (Elsenbeer et al., 1992), Peninsular Malaysia (Naguchi et al., 1994) and Sabah (Bidin et al., 1993). In comparison with the results of these and earlier studies (Bonell with Balek, 1993), the conductivity of the Mendolong soils decreases rather rapidly with depth, and the sandy topsoils are an extreme case. By extrapolation (Kmfs 0-40 cm depth) and modelling (from clay content) of saturated hydraulic conductivity from textural data, Malmer (1993) calculated that maximum soil water flow through a 2.5 m deep and 7 m wide clay topsoil slope profile was 7.6 1 h 1, of which 93% occurred in the uppermost 10 cm. The generally lower soil suction to drainage equilibrium at 20 cm compared with that at 50 cm indicates the possibility of shallow subsurface flow in the nearly saturated topsoil between rains. The main reason for this would be the lower conductivity of deeper soil layers. This slow vertical water transfer was demonstrated days after the first rains after dry periods, when the soil water tension at the 20 cm level was low (0-4 kPa), while, at the 50 cm level, it was still relatively dry with higher tensions (8-12 kPa). In the partial slope plots the control forest surface runoff was proportional to the length of each in relation to the full plot CA, even in the heaviest storm. This indicates infiltration excess overland flow rather than saturation overland flow in the main sloping part of the slope. However, close to the stream, saturated overland flow, or even return flow, might account for the maintenance of stream stormflow because, in the model calculation above, the hourly maximum subsurface flow was equivalent to only 0.04 mm over the full plot CA. On clear-felled slope surfaces without soil disturbance the soil water relations described above were little changed. These topsoils were dryer, but only in the upper part of the burned plot was the general pattern of drainage from 20 cm disturbed. The clear-felled but unburned plot, MA, experienced less surface runoff than the longer control plot CA, Table 2; the somewhat dryer topsoil may have experienced higher sorbtivity and initial infiltration rates at the most intensive start to heavy rains. In plot BA in the clear-felled and burned area, there was a short period

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right after burning with higher surface runoff than the control (Fig. 3), despite being less steep and shorter in length than plot CA. As soon as the vegetation recovered and the ashes were washed away, the effect ceased. Changes in hydraulic conductivity were not as drastic as the reduction in steadystate infiltrability on surfaces with soil disturbance in the W5 catchment. However, these surfaces experienced considerable surface runoff, of infiltration excess, most clearly demonstrated by the extensive surface and gully erosion recorded.

5.2. Streamflow dynamics and water quality The lack of slope surface runoff and very low surface erosion (cf. Wiersum, 1984) contributed to the lack of differences in concentration of suspended load between stream stormflow and baseflows from control catchments and from other catchments before treatments. The same situation obtained for dissolved loads, except for some elements which were correlated to streamflow within the range of baseflows (Grip et al., 1994). Ammonia was positively correlated (W3 and W6) and calcium negatively correlated (W3) with baseflow; this indicated the dependence of high baseflows on topsoil flowpaths with shorter transit times and vice versa. Preferential flow paths for soil water have been suggested as an important mechanism to prevent nutrient loss from the rainforest soil matrix (Nortcliff and Thornes, 1978; Sollins and Radulovich, 1988). In the present study, in the forested control catchment the rapid saturation of topsoil macropores, led to low dissolved concentrations in stream stormflows. However, if topsoil macropores maintain high saturation so that hydraulic conductivity is close to saturated hydraulic conductivity between rains, the same process could give an effective contact between leaching and mineralization in litter and topsoil and the stream baseflow. This could explain the positive relation between streamflow ammonia concentration and stream baseflow, and the opposite relation for calcium. With large amounts of logging residues under decomposition, rain entering the residues and the topsoil will to a larger extent leach nutrients in excess of uptake and soil retention. This water, which under undisturbed circumstances would be poor in nutrients, is relatively effectively drained through the topsoil macropores. Thus, the change to higher stormflow concentrations compared with baseflows of most elements during and after treatments. A clear effect on baseflow concentrations also illustrated this point in agreement with the above discussion of baseflows of short transit time (Figs. 4 and 5). However, soil nutrient retention played a part in reducing concentrations in soil water reaching the stream. Catchments W1 and W2 had the same treatment but in W1 sandy topsoils predominated while in W2 the dominant topsoils were clays (Fig. 1). Thus, the differences in baseflow concentrations between W1 and W2 in Fig. 5 reflected differences in retention capacity of the two soil types. The possible effect of this soil difference on the results of streamflow quality and nutrient loss is likely to be rather small owing to the following. (1) The difference above relates to close to maximum difference in effect since both W1 and W2 are almost homogeneous as regards soil types, while the area of sandy topsoils ranges from 34 (W4) and 38 (W1 + 2) to 62% (W5) for the treated catchments compared.

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(2) Soil disturbance, including total change and removal of topsoil structure, removal of vegetation and inactivation of root systems on 24% of the W5 catchment may have had a greater effect in reducing the retention in baseflows when compared with W4 than the 28% difference in area of soil types. (3) Stormflow concentrations from W1 and W2 did not show this pattern; rather W2 had higher stormflow concentrations of cations during and after treatments (Malmer and Grip, 1994). As dissolved concentrations were much higher in stormflow than baseflow and for all treated catchments the majority of runoff was as stormflow, this difference in the retention effect on baseflow concentration had little effect on stream water quality, and thus also on total nutrient losses. Hence, the 'bypass effect' in high stream baseflows was more effective in coarser topsoils. This implies that sandy soils, with lower nutrient stores in the first place, are even more sensitive to nutrient losses at clear-felling or burning. Burning optimized the enrichment of dissolved elements in stream water. All nutrient-rich parts of the remaining vegetation and logging residues were mineralized to easily leachable salts in the ash. The killing of the vegetation increased runoff and reduced nutrient retention in living vegetation and mycorrhiza. Burning also induced a short-lived increase in surface runoff. In W4, the nutrient release from residues was much slower and the remaining living vegetation promoted nutrient retention and a reduction of the runoff increase. The prolonged effect of raised baseflow concentrations of elements related to soil and weathering sources compared with those of organic origin has been reported elsewhere (Zulkifli Yusop and Abdul Rahim, 1991). Malmer and Grip (1994) discussed how contributions from deeper soil weathering could bypass uptake by roots, until sufficiently deep tree root systems have developed. The low stream water suspended load from catchment W4 during and after treatments reflected the low impact of the soil disturbance in that catchment. The increase in surface erosion was moderate and no extra surface runoff was induced, despite some compaction and cleared planting rows parallel to the slopes. The later increase in suspended load in 1988/1989 must be related to possible stream channel erosion and reactivation by increased streamflow of stream sediments in old or new debris dams (Spencer et al., 1990; Douglas et al., 1992). In catchment W5, erosion on tractor tracks maintained raised suspended load concentrations for 2.5 years, which is consistent with the prolonged gully erosion observed. This gully formation led to increased length of stream channels and was probably the reason for the shift to increased stormflows and the second increase in runoff in 1989/1990. However, this late change in stream channel dynamics in W5 was not accompanied by any change in stream suspended load. Both burned catchments, W1 + 2 and W5, had short-lived increases in surface erosion of ashes on undisturbed slopes. In W1 ÷ 2, ash was detectable in stream suspended load concentrations (Table 5), but in W5 it was masked by the sediments from tractor track erosion. 5.3. Streamflow nutrient losses

This study reports much larger total dissolved nutrient losses than previous studies

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on rainforest clear-felling and plantation establishment, and much larger parts of these losses are attributable to storm flows which dominated stream runoff. Also, the range of streamflow drainage in this study (Grip et al., 1994) may have been sampled with better coverage than in studies based on soil water lysimetry (Russell, 1983) or mainly stream baseflow sampling (Drainage and Irrigation Department, 1989; Zulkifli Yusop and Abdul Rahim, 1991). The combination of high nutrient concentrations in stormflows and the fact that the larger part of runoff from these catchments occurred as stormflows resulted in the very high dissolved nutrient losses during and after treatments (Table 6). For example in W1 + 2, 73-96% of the estimated effect of treatment was carried in stormflows. W1 + 2 also had the largest dissolved nutrient losses recorded after burning, even though the slash of that catchment had less biomass (Table 6). This reflected the harder burn in W1 + 2 along with the least disturbance to pretreatment runoff generation and the largest relative increase in stream stormflows (Malmer, 1992; Malmer and Grip, 1994). For all major nutrients but calcium (Table 6) W5 had larger total dissolved nutrient effects than W4. The calcium concentrations in W5 after burning were surprisingly low compared with those of W1 + 2, and showed no significant difference between baseflows and stormflows (Malmer and Grip, 1994). The harder burn in W1 + 2 probably converted more of the calcium-rich bark and wood to ashes. On the other hand, in W4, stormflow concentrations of calcium were significantly higher than in baseflows in the period after the burning of the other basins. The actual stormflow concentrations of nutrients in W5 were not very much lower than in W4 during that period, but in W5, the mean baseflow concentration of Ca was almost double that of W4, making the differences between baseflow and stormflow concentrations in W4 significant (in contrast to W5 in which the differences were not significant). The short-lived, intensive surface wash at the first rains after burning in W5 supplied particulate ash to the stream and may have caused a substantial extra loss of nutrients. Preliminary results on particulate concentrations of available phosphorous (Sibbesen, 1983) in stormflow suspended sediments show rises in W 1 + 2 and W5 streams after burning to levels above the concentration in humus.

6. Conclusions. The results of this study confirm the hypothesis that the humid tropical environment risks high nutrient losses to streamflow at sites of intensive disturbance and of extensive infiltration excess surface runoff. In addition, the high rate of mineralization of biomass residues in combination with 'biphasic' pore systems and short transit times for a major part of stream runoff may provide possibilities for large hydrological nutrient losses without soil disturbance. In many environments the burning of residual biomass fulfils many agricultural and silvicultural goals without major adverse effects. Here, however, it has triggered large hydrological nutrient losses, both with and without soil disturbance. Furthermore, burning before tree planting was effective neither for the suppression of weeds

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nor for the promotion of tree growth (Table 1, Sim and Nykvist, 1991; Nykvist et al., 1994). The hydrological losses of major plant nutrients both from 'normal practice' and ' m i n i m u m disturbance' in this study ranged from 10-185% of that removed in the harvest. Such losses have to be quantified and included in any calculation of nutrient balances and long-term sustainability of tropical rainforest conversions to plantation forestry on these lands. M i n i m u m disturbance (avoiding soil disturbance and burning) reduced by about 50% the expected increase in runoff, stream siltation and nutrient losses. The minimum disturbance treatment may be difficult to apply technically on a wide commercial scale, but it reduces the adverse impacts of loss of nutrients and of siltation and runoff from the site, as well as potentially increasing production on site. Nykvist et al. (1994) have elaborated on techniques for reducing nutrient losses from the site. To reduce hydrological nutrient losses, the present study explains the importance of developing techniques to reduce soil disturbance, to leave more living vegetation with active root systems (including trees with deeper root systems) and to avoid burning.

Acknowledgements The initial years, 1985-1986, of this research were funded by Sabah Forest Industries Sdn Bhd (SFI) through Angpannef6reningen-Industrins Processkonsult AB (AF-IPK). Later the funding was shared equally by the Swedish Agency for Research Cooperation with Developing Countries (SAREC) and SFI. F r o m 1990 the Sabah Forest Department has also taken part in the study. Thanks are due to the joint project leaders, Research and Development Manager Sim Boon Liang and Professor emeritus Nils Nykvist, as well as the heads of laboratories, Fui Khiong W o n g in Mendolong and Ove Emteryd in Ume~ with their staff. Special thanks to Dr. Harald Grip for active cooperation throughout the study and to Dr. Sampurno Bruijnzeel and reviewers for valuable contributions to this paper.

References Anderson, J.M. and Spencer, T., 1991. Carbon, nutrient and water balances of tropical rain forest ecosystems subject to disturbance: management implications and research proposals. Man and Biosphere (MAB) Digest 7, UNESCO, Paris. Andersson, ,~., 1990. The change of hydraulic conductivity after clear-felling tropical rainforest. Report from Department of Forest Site Research, Swedish University of Agricultural Sciences, Umegt,Sweden (in Swedish, unpublished). Bidin, K., Douglas, I. and Greer, T., 1993. Dynamic response of subsurfacewater levels in a zero-order tropical rainforest basin, Sabah, Malaysia. In: J.S. Gladwell (Editor), Hydrology of Warm Humid Regions. IAHS Publ. No. 216, IAHS, Wallingford, pp. 491-496. BoneU, M., 1993. Progress in the understanding of runoff generation dynamics in forests. J. Hydrol., 150: 217-275.

A. Malmer / Journal of Hydrology 174 (1996) 129-148

147

Bonell, M. with Balek, J., 1993. Recent scientific developments and research needs in hydrological processes of the humid tropics. In: M.M. Hufsehmidt and J.S. Gladwell (Editors), Hydrology and Water Management in the Humid Tropics - - Hydrological Research Issues and Strategies for Water Management. UNESCO-Cambridge University, Cambridge, pp. 167-260. Bruijnzeel, L.A., 1990. Hydrology of Moist Tropical Forests and Effects of Conversion: A State of Knowledge Review. UNESCO-IHP, Humid Tropics Programme, Paris. Douglas, I., Spencer, T., Greer, T., Bidin, K., Sinun, W. and Meng, W.W., 1992. The impact of selective commercial logging on stream hydrology, chemistry and sediment loads in the Ulu Segama rain forest, Sabah, Malaysia. Phil. Trans. R. Soc. London, Ser. B, 335: 397-406. Drainage and Irrigation Department, 1989. Sungai Tekam experimental basin, Final Rep. July 1977 to June 1986. Water Resources Publ. No. 7, Drainage and Irrigation Department, Kuala Lumpur, Malaysia. Dunne, T., 1978. Field studies of hillslope flow processes. In: M.J. Kirkby (Editor), Hillslope Hydrology. Wiley, New York, pp. 227-293. Dykstra, D.P. and Heinrich, R., 1992. Sustaining tropical forests through environmentally sound harvesting practices. Unasylva, 169(43): 9-15. Elsenbeer, H., Cassel, K. and Castro, J., 1992. Spatial analysis of soil hydraulic conductivity in a tropical rain forest catchment. Water Resour. Res., 28 (12): 3201-3214. Fries, J., 1992. UNCD - - A success or failure? IRDC Currents, 4:4 8. Grip, H., Maimer, A. and Wong, F.K., 1994. Converting tropical rainforest to forest plantation in Sabah, Malaysia. I. Dynamics and net losses of nutrients in control catchment streams. Hydrol. Process., 8: 179-194. Hamilton, L.S. and King, P.N., 1983. Tropical Forested Watersheds. Hydrologic and Soil Response to Major Uses or Conversions. Westview, Boulder, CO. Kessler, J. and Oosterbaan, R.J., 1974. Determining hydraulic conductivity in soils. In: Drainage Principles and Applications, Vol. 16, No. 3, International Institute for Land Reclamation, Wageningen, The Netherlands, pp. 253-296. Malmer, A., 1990. Stream suspended sediment load after clear-felling and different forestry treatments in tropical rainforest, Sabah, Malaysia. In: R.R. Ziemer, C.L. O'Loughlin and L.S. Hamilton (Editors), Research Needs and Applications to Reduce Erosion and Sedimentation in Tropical Steeplands. IAHS Publ. No. 192, IAHS, Wallingford, pp. 62-71. Malmer, A., 1992. Water yield changes after clear-felling tropical rainforest and establishment of forest plantation in Sabah, Malaysia. J. Hydro1., 134: 77-94. Maimer, A., 1993. Dynamics of hydrology and nutrient losses as response to establishment of forest plantation. A case study on tropical rainforest land in Sabah, Malaysia. Ph.D. Thesis, Swedish University of Agricultural Sciences, Department of Forest Ecology, Ume~, 182 pp. Maimer, A. and Grip, H., 1990. Soil disturbance and loss of infiltrability caused by mechanized and manual extraction of tropical rainforest in Sabah, Malaysia. For. Ecol. Manage., 38: 1-12. Maimer, A. and Grip, H., 1994. Converting tropical rainforest to forest plantation in Sabah, Malaysia. II. Changes of nutrient dynamics and net losses in streams due to treatments. Hydrol. Process., 8: 195-209. Mok, S.T., 1992. Potential for sustainable tropical forest management in Malaysia. Unasylva, 169(43): 28-33. Nagochi,S., Abdul Rahim Nik, Saifuddin Sulaiman, Sammori, T. and Tani, M., 1994. Hydrological characteristics of tropical rain forest in Peninsular Malaysia (I) - - General hydrological observations on a hillslope. In: T. Ohta, Y. Fukushima, M. Suzuki (Editors), Proc. Int. Symp. of Forest Hydrology, Tokyo, Japan, October 1994. IUFRO, pp. 275-282. Nortcliff, S. and Thornes, J.B., 1978. Water and cation movements in a tropical rainforest environment. I. Objectives, experimental design and preliminary results. Acta Amazonica, 8: 245-258. Nye, P.H. and Greenland, D.J., 1964. Changes in the soil after clearing tropical forest. Plant Soil, 21(1): 101 112. Nykvist, N., 1995. The above-ground biomass growth of secondary vegetation after the great 'Borneo fire' of 1983. J. Trop. Ecol., in press. Nykvist, N., Grip, H., Sim, B.L., Malmer, A. and Wong, F.K., 1994. Nutrient losses in forest plantations in Sabah, Malaysia. Ambio, 23(3): 210-215.

148

A. Malmer / Journal of Hydrology 174 (1996) 129-148

Russell, C.E., 1983. Nutrient cycling and productivity in native and plantation forests in Jari Florestal, Para, Brazil. Ph.D. Thesis, University of Georgia, Athens, GA. Sibbesen, E., 1983. Phosphate soil tests and their suitability to assess the Phosphate status of soil. J. Sci. Food Agric., 34: 1368-1374. Sim, B.L. and Nykvist, N., 1991. Impact of forest harvesting and replanting. J. Trop. For. Sci., 3(3): 251-284. Sollins, P. and Radulovich, R., 1988. Effects of soil physical structure on solute transport in a weathered tropical soil. Soil Sci. Soc. Am. J., 52:1168-1173. Spencer, T., Douglas, I., Greer, T. and Sinun, W., 1990. Vegetation and fluvial geomorphic processes in South-east Asian tropical rainforests. In: J.B. Thornes (Editor), Vegetation and Erosion. Wiley, Chichester, pp. 451-469. Van der Plas, M.C. and Bruijnzeel, L.A., 1993. Impact of mechanized selective logging on top soil infiltrability in the Upper Segama area, Sabah, Malaysia. IAHS Publ. No. 216, IAHS, Wallingford, pp. 203-211. Whitmore, T.C., 1990. An Introduction to Tropical Rainforests. Oxford University, Oxford. Wiersum, K.F., 1984. Surface erosion under various tropical agroforestry systems. In: C.L. O'Loughlin and A.J. Pearce (Editors), Proc. Symp. on Effects of Forest Land Use on Erosion and Slope Stability. IUFRO, Wienna and Est-West Centre, Honolulu, Hawaii, pp. 231-239. Woods, P., 1989. Effects of logging, drought and fire on structure and composition of tropical forests in Sabah, Malaysia. Biotropica, 21(4): 290-298. Zulkifli Yusop, 1991. Hydrologic nutrient losses following selective logging methods in a tropical rainforest. In: Soil Science Conf. of Malaysia, Genting Highland, Malaysia, 4-5 March 1991. Zulkifli Yusop and Abdul Rahim, N., 1991. Logging and forest conversion: Can we minimize their impacts on water resources? In: ASEAN Seminar on 'Land Use Decisions and Polices: Will Tropical Forest Survive Their Impact?', Penang, Malaysia, 28-30 October 1991.