Abstract:
Tropical montane forests are frequently located on steep slopes with pronounced differences in topographic exposure, related microclimatic conditions and hence in composition and structure of the vegetation over small distances. The objective of this work was to test the hypothesis that topographic position significantly influences soil fertility and water flow in these forests. Soil properties were determined at various topographic positions and water samples of selected ecosystem fluxes analyzed over a 1-year period for oxygen isotopes in three small, steep watersheds under lower montane forest in the Eastern Cordillera of the Andes in southern Ecuador. The soils are subject to lateral material movement (landsliding and solifluction). This, together with the pronounced variation in climatic conditions and vegetation over small distances, resulted in high heterogeneity of soil properties. The pH of the A-horizon ranged between 3.7 and 6.4; concentrations of base metals (calcium, magnesium), sulfur and phosphorus, and trace metals (manganese, zinc) showed enormous spatial variation (coefficient of variation: 358?680% over a surface area of <30 ha). The steepness of the study area and the large contrast in hydraulic conductivities of the organic
layer and the mineral soil resulted in a hillslope flow regime dominated by fast lateral flow. During baseflow conditions, d18O values were similar to that of the subsoil solution, but rapidly became similar to values in the top-soil solution during rain storms. The chemical composition of stormflows resembled that of the litter leachate. Stormflow had lower pH and higher organic carbon and metal concentrations than did baseflow. It is concluded that topographic position and lateral
transport of water and matter (as a consequence of the pronounced inclination) are important controls of the water and nutrient cycles of the study forest.
