Soil chemical changes after tropical forest disturbance and conversion: The hydrological perspective

The soil chemical and hydrochemical changes accompanying tropical forest disturbances of varying intensity are reviewed. Various methodological problems are discussed with respect to the establishment of ecosystem nutrient balances and the determination of changes therein due to forest exploitation or conversion to other land uses. It is shown that amounts of nutrients removed from the forest ecosystem in harvested timber generally exceed those associated with enhanced leaching, both after selective logging and clearcutting operations. Losses of base cations through volatilization and ash dispersal upon burning residual biomass are substantial, contrary to common belief. Approximate periods required for the replenishment of lost nutrients at the ecosystem level are estimated by comparing the losses with inputs via bulk precipitation and/or mineral weathering. The results suggest typical recovery periods of 30–60 years, depending on harvesting intensity, rainfall regime and soil nutrient retention capacity. Issues requiring further research include: (i) the evapotranspiration from young regenerating and planted vegetation; (ii) nutrient losses via rapid flow through macro-pores during rainfall (as opposed to slow matrix flow); (iii) nutrient losses via erosion after mechanized tree harvesting and burning of slash (particularly in the context of short-rotation plantation forestry); and (iv) the possible contributions to the overall ecosystem nutrient budget by mineral weathering. Particular emphasis is placed on the integration of hydrological, pedological and plant ecological process research at various levels of scale within an overall catchment ecosystem context in order to facilitate the integration of different disciplines. A plea is made to concentrate future research efforts at a relatively small number of carefully selected and well-researched locations, possibly joined together in a network capturing the chief environmental variability encountered in the humid tropics.