Net precipitation and soil water dynamics in clearings, old secondary and old-growth forests in the montane rain forest belt of Mount Kilimanjaro, Tanzania

摘要

The montane rain forest belt on Mt Kilimanjaro forms an important water source for northern Tanzania that is threatened bynboth logging and fire. The aim of this study was to investigate the consequences of forest fragmentation on various aspectsnof the water cycle. Soil properties, rainfall, throughfall and soil water suction were analysed for mature forest, secondarynforest patches and large clearings. A total of 10 plots located on the south-western slopes of the mountain between 2100nand 2300 m.a.s.l. were monitored from May 2000 to June 2002. Annual rainfall amounts ranged from 2000 to 2600 mmnwith high spatial and inter-annual variability. Rainfall interception ranged from 3 to 9% of incident rainfall in clearings, to anmaximum of 32% in forests. In general, soils under mature forest were wettest and showed only minor moisture fluctuationsnthroughout the year. Soils of secondary forest sites were driest and soil water suction exhibited the largest fluctuations.nAdditionally, the finer texture of soils under mature forest compared to secondary forest sites allowed higher amounts of soilnwater to be stored for similar soil water suction levels. Ventilation and radiation penetration, which influence the magnitude ofninterception, evaporation from the forest floor and transpiration, were probably higher in fragmented secondary than in closednmature forests. In clearings, higher throughfall and presumably lower transpiration rates led to moister conditions comparednto adjacent secondary forest sites. Thus, soil conditions under mature forest were most suitable for optimum water storage onnMt Kilimanjaro. The transformation of closed mature forest to a mosaic of forests in different stages of regeneration led tonincreased spatio-temporal variability in soil moisture.