Reduced-impact logging in Indonesian Borneo: some results confirming the need for new silvicultural prescriptions

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Abstract

Reduced-impact logging (RIL) and conventional techniques (CNV) were compared in a mixed dipterocarp hill forest in East Kalimantan in three blocks of about 100 ha each. Damage was evaluated using pre- and post-harvesting assessments in 24 one-hectare sample plots. RIL techniques nearly halved the number of trees destroyed (36 vs 60 trees/ha). RIL’s main benefit was in the reduction of skidding damage (9.5% of the original tree population in RIL vs 25% in CNV). Before logging, mean canopy openness in CNV (three plots only) and RIL (9 plots) was similar (3.6 and 3.1%) and not significantly different (χ2=2.73, P=0.254). After logging, the mean canopy openness was 19.2% in CNV (n=9 plots) and 13.3% in RIL (n=8 plots), and the distributions of the canopy class in RIL and CNV significantly different (χ2=43.56, P<0.001). CNV plots showed a higher proportion of measurements in the most open class ≥30% than in RIL. At a larger scale, the area of skidtrail per unit timber volume extracted was halved in the RIL compartment (15 m2 vs 27 m2 m−3 for CNV). However, under high felling intensity (>8 trees/ha), both stand damage and canopy disturbance in RIL approached those recorded in CNV under low or moderate felling regime. Over this felling intensity threshold the effectiveness of RIL in reducing tree damage is limited. In mixed dipterocarp forest where harvestable timber density generally exceeds 10 trees/ha, a minimum diameter felling limit is clearly insufficient to keep extraction rates below 8 trees/ha. Based on these new results and previous studies in Borneo, we suggest three silvicultural rules: (1) to keep a minimum distance between stumps of ca. 40 m, (2) to ensure only single tree gaps using directional felling, (3) to harvest only stems with 60–100 cm dbh. Foresters, policy makers and certifiers should consider these as criteria for sustainable forest management. We emphasise the need to expand harvesting studies to look at impacts and trade-offs across larger forest landscapes, to expand RIL beyond silvicultural concepts and to include the maintenance of other forest goods and services.

Introduction

In the Indonesian selective logging and planting system (TPTI), all dipterocarps (i.e. timber trees in the family Dipterocarpaceae) with a diameter at 1.3 m (height dbh) over 50 or 60 cm (depending on the type of forest) can be harvested with a polycyclic felling schedule of 35 years. One year prior to harvest, forest concessionaires must carry out an inventory to determine the annual allowable cutting granted by the Department of Forestry. However, in practice these inventories are generally too inaccurate to define a satisfactory harvesting plan. In the highly productive dipterocarp forests of Borneo, where harvesting intensities commonly exceed 100 m3 ha−1 and more than 10 trees/ha, conventional logging generally damages more than 50% of the original stand (Nicholson, 1958, Kartawinata, 1978, Tinal and Palinewen, 1978, Abdulhadi et al., 1981, Cannon et al., 1994, Pinard and Putz, 1996, Bertault and Sist, 1997, Sist et al., 1998a). Because over-harvesting and poor operational practices are now recognised as an important cause of deforestation, ITTO member countries, including Indonesia, are being encouraged to revise practices in order to achieve sustainable management of the estate forests. In this context, reduced-impact logging (RIL) techniques are regarded as vital to reduce damage to a level that will preserve forest regeneration and integrity (Dykstra and Heinrich, 1996, Sist, 2000). Several experiments in mixed dipterocarp forests have demonstrated that RIL techniques can reduce damage by at least 30–50% compared with normal operation, also called ‘conventional logging’ (Pinard and Putz, 1996, Bertault and Sist, 1997). Most of the studies comparing damage under RIL and conventional logging have neglected the variability in natural forest and the variation in damage that this implies. The current coverage of such studies is very limited. This is important as the proportion of stems damaged is generally correlated with extraction rates (Nicholson, 1958, Bertault and Sist, 1997, Sist et al., 1998a) and possibly with other factors. Thus, it remains difficult to determine how general or local any conclusion might be. The main objective of this study is to assess how far RIL can reduce damage under varying felling intensity.

Section snippets

Study site

The study area is located in the Indonesian Province of East Kalimantan (Borneo Island), in the district of Malinau (2°52′–3°14′N, 116°–116°40′E), within a 50,000 ha forest concession managed by Inhutani II, a state-owned timber company. The climate is equatorial with an annual rainfall measured at ca. 4000 mm. The monthly rainfall varies from ca. 200 to 400 mm (PT Inhutani II, 1996). The topography is deeply eroded with a dense network of steep ridges and drainage gullies. Elevations at the study

Results

The mean density and basal area of the four additional plots in the CNV blocks (243.6 trees/ha, SD=41; 30.4 m2/ha, SD=4.9) were not distinct from those of the 12 plots (230 trees/ha, SD=35.8 and 32.85 m2/ha, SD=4.7) set up before logging (t=0.57, df=14, P=0.58 for density, and t=0.87, df=14, P=0.40 for basal area). RIL (n=11) and CNV (n=12) plots showed similar tree densities and basal area (t=0.52, df=21, P=0.60 for density, and t=1.39, P=0.18, Table 3). The mean density and basal areas in each

Discussion

Reduced-impact techniques reduced the number of trees destroyed by 40% in comparison with conventional harvesting practices. However, the proportions of trees injured were similar in both techniques, affecting about 25% of the original stand. The main benefit of RIL was to reduce skidding damage from 25% of the original stand in CNV to only 9.5%. Because skidding operations are the major causes of mortality (Bertault and Sist, 1997), the low proportion of trees killed in RIL appear to result

Conclusions

In mixed dipterocarp forests of East Kalimantan, where density of harvestable trees often exceed 10 trees/ha, the minimum diameter rule results in high felling intensities with excessive damage to the remaining forest. RIL techniques, though they would appear to be a vital part of the solution, are useful only under a moderate extraction regime in which 8 trees/ha is an upper limit. Restricting felling intensity is essential in any case, both from the perspective of the growth and survival of

Acknowledgements

This study was carried out as a part of the project on Forest, Science and Sustainability: Bulungan Model Forest, PD12/97 Rev. 1(F) funded by the International Tropical Timber Organization (ITTO) and implemented by the Center for International Forestry Research (CIFOR), the Forest Research and Development Agency (FORDA) of the Ministry of Forestry of Indonesia, and Cirad-Forêt. The present RIL experiment would not have been possible without full support and cooperation of Inhutani II,

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