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Forest disturbance, conversion and recovery
D. F. Scott, FRBC Research Chair of Watershed Management, Okanagan University College, Kelowna, B.C., V1V 1V7, Canada,
L. A. Bruijnzeel, Faculty of Earth and Life Sciences, Vrije Universiteit, Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands,
J. Mackensen, Division of Policy Development and Law, United Nations Environmental Programme (UNEP), P.O. Box 30552, Nairobi, Kenya
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.).
Forest disturbance, conversion and recovery
D. Hölscher, Institute of Silviculture, University of Göttingen, Buesgenweg 1, D-37077 Göttingen, Germany,
J. Mackensen, Division of Policy Development and Law, United Nations Environmental Programme (UNEP), P.O. Box 30552, Nairobi, Kenya,
J. -M. Roberts, Centre for Ecology and Hydrology, Wallingford, OX10 8BB, UK
Many of the agricultural land use systems replacing tropical forest can be regarded as only semi-permanent. In shifting agriculture cultivated fields are abandoned within two to three years while pastures in Amazonia may have a productive period of only six to twelve years (Sanchez and Hailu, 1996; Uhl et al., 1988). Abandoned fields are usually left for secondary succession with trees and shrubs establishing in these areas initiating the forest recovery. In some tropical regions secondary forests already form the most important land cover and are likely to become more important as long as forest conversion proceeds and sustainable management systems for the deforested areas are lacking. Fearnside (1996) estimated that 30% of the deforested area of the Brazilian Amazon was covered by secondary vegetation in 1990. Similarly, in mainland South East Asia, a region with a long history of shifting cultivation, secondary forest comprises about one third in representative areas in northern Thailand (Fox et al., 1995), northern Vietnam (Fox et al., 2000) and southern China (Xu et al., 1999).
The conversion from forest to agricultural land use causes manifold changes in water and nutrient cycles and has been studied for different site conditions and degrees of disturbance (Malmer et al., Grip et al., this volume). This chapter reviews the changes in vegetation structure, dynamics of nutrient pools in soil and phytomass, and subsequent alterations of the hydrological cycle during the process of forest recovery on abandoned pastures and in shifting cultivation systems.
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