INTRODUCTION

Tropical regions have undergone dramatic land use changes in the last few decades. The myriad of changes that have, and still are, taking place are the effect of an equally large number of local causes and factors, highlighting a complexity that tends to defy easy generalisations. Major background driving forces can be recognised, however, such as rapid demographic expansion, with consequent booming demands for agricultural land and woodfuels, plus a change from a subsistence-based to a market-orientated economy, with the associated heavy pressure on natural resources to fuel economic growth and development or, in some regions, political instability and incurable conflicts.

From this complexity of tropical land use dynamics, one single resulting element has been recognized with considerable alarm over the past decades. This is the progressive depletion of natural tropical forests. It is in fact to assess the remaining area of forest and its rate of change, rather than to study the complexity of tropical land use dynamics per se, that large-scale studies have been carried out over tropical regions. From a hydrological point of view, the removal of forest cover causes important changes in runoff and sediment yields (cf. Grip et al., this volume). Moreover, global concern over the fate of tropical forest has been coupled, especially during the last decade, with similar global concerns over the build-up of greenhouse gases in the atmosphere and the negative role that tropical deforestation plays in this regard (cf. Costa, this volume).