Spatially significant effects of selective tropical forestry on water, nutrient and sediment flows: a modelling-supported review

INTRODUCTION Selective forestry is a set of commercial forestry practices that involves the selective removal of particular trees within an ‘annual logging coupe’ of forest (Conway, 1982). Selective harvesting within ‘natural forests’ (i.e. those forests that have not been clearfelled for non-forest uses or converted to plantation or agroforestry) covers a very wide range of practices, including highlead and tractor yarding, harvesting of only large, commercial trees, protection of riparian vegetation along rivers and protection of forest on very steep hills. As a consequence, the intensities of the impacts on the water environment (i.e. water, nutrient and sediment systems) are expected to be very varied. Some of these impacts can be profound. One of the most significant environmental impacts of all types of forestry operations within the humid tropics is accelerated soil erosion (Bruijnzeel, 1992). The resultant input of sediments into rivers leads to damage to fish populations (Martin-Smith, 1998), reduced quality of water supplies, reductions in channel capacity which affects flood risk and boat traffic (Sheffield et al. , 1995), and the inundation of offshore corals (MacDonald et al. , 2001). Development of selective harvesting techniques when applied to natural forests in the tropics are currently being focused on so-called ‘Reduced-Impact-Logging’ (RIL) or ‘closely supervised’ methods which aim to improve the ‘sustainability’ of timber production and reduce wider environmental damage.

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