Author(s)

H. SURYATMOJO, M. FUJIMOTO, K. KOSUGI & T. MIZUYAMA

Abstract

Tropical Indonesian rainforest is managed using an Intensive Forest Management System (IFMS). The main activity is selective logging for timber harvesting and intensive line planting to enrich the standing stock. These activities significantly alter the forest canopy cover and the hydrologic response of catchments, including peak discharge, direct runoff and soil erosion. Understanding the hydrologic effects in an IFMS is helpful to develop a forest management strategy. This study investigated the impact of vegetation changes on runoff and soil erosion characteristics in different periods of an IFMS resulting from their respective forestry treatments. This study was conducted in a natural tropical rainforest at Bukit Baka Experimental Catchment, Central Kalimantan, Indonesia. Runoff and soil erosion characteristics were investigated in three small catchments, a virgin forest catchment (C1), a 10-year-old line plantation catchment (C2) and a 1-year-old line plantation catchment (C3). The IFMS has increased in large scale of open forests and exposed the bare soil, especially in the early years after forest operation. This condition has changed drastically in the catchment hydrologic responses. The increase in discharge, direct runoff, and suspended sediment discharge in the C3 was higher than those in the C2 and C1, particularly for large rainfall events. The proportion of annual rainfall as water yield in the C1, C2, and C3 was 27, 30, and 41%, respectively. The annual suspended sediment yield in the C1, C2, and C3 was 0.15, 3.6, and 14.9 t ha−1 y−1, respec- tively. The results showed that the magnitude of runoff and soil erosion depends on the interaction among the rainfall, forest cover changes, forest treatment applied and catchment characteristics. Ten years after forest operation, forest cover has recovery close to natural condition, but still there are differences in hydrological response. Controlling soil erosion during logging activities by reducing the impact of logging and combining ecologically based vegetation structure design is an effective way to control runoff and soil erosion.

Keywords

Direct runoff, intensive line planting, logged-over forest, peak discharge, selective logging, soil erosion, virgin forest.