The hydrological and soil impacts of forestation in the tropics

INTRODUCTION In response to the continuing degradation and disappearance of the world's tropical forests (Drigo, this volume) the establishment of plantation forest on degraded and previously forested sites as well as into (sub)tropical grasslands is becoming increasingly common (Evans, 1999). The hydrological effects of this practice and the potential of forestation to improve or restore the hydrological behaviour of degraded catchments constitute the prime focus of this chapter, expanding and updating an earlier review of the subject by Bruijnzeel (1997). Three aspects are highlighted in particular, namely: (i) the effects of tree plantations on annual and seasonal streamflow totals; (ii) the associated impacts on stormflow and sediment production; and (iii) concurrent changes in soil chemical characteristics (fertility). Because the hydrological changes associated with forest clearing and the establishment of a new vegetation cover during the first few years are discussed at length in the chapter by Grip, Fritsch and Bruijnzeel, much of what follows pertains to the post-canopy closure phase of plantations. EXTENT, DEVELOPMENT AND IMPORTANCE OF TROPICAL TREE PLANTATIONS The establishment of timber plantations is a notable and accelerating land-use development of the last half-century. It has been estimated that there are now some 40 to 50 million ha of forest plantations in the tropics and warmer subtropics, trees being planted nowadays at a rate of c . 2 million ha yr -1 compared to c . 1 million ha yr -1 a decade ago (Evans, 1999). One of the most widely used types of tree is the eucalypt ( Eucalyptus spp.).