A Survey of Soil Erosion Processes in Tropical Forest Ecosystems on Volcanic Ash Soils in the Central Andean Cordillera, Colombia

Erosion processes in relatively undisturbed tropical forest ecosystems were investigated along a transect across the Central Andean Cordillera between Pereira and Venadillo in the Departamento del Tolima Colombia. Of particular interest were relationships between the nature of soil erosion and the forest ecology. Twenty-three sites were selected at 200 m intervals of altitude on east and west facing slopes.Surveys were made of the geomorphology and soils, both of which were mapped, and field measurements made of various site and soil properties related to splash erosion (raindrop size distribution, area of exposed soil and lowest canopy height), erosion by overland flow (infiltration rates, occurring of runoff producing zones) and biogenic erosion. Soil samples from the A1 horizon were collected for laboratory analyses (bulk density, aggregation and organic matter content).The results of the investigation are reported with respect to forest type and altitude, and aggregate stability, permeability and infiltration rates are treated as dependent variables in multiple regression analyses.In an attempt to synthesise the total effect of site conditions on erosion, splash and overland flow indices are developed. These would suggest that splash erosion is most active in the Subandean forest zone due to the relatively large area of exposed soil and the greater height of the lowest canopy. However, conditions within the Subandean zone are quite variable. Runoff producing zones are precluded in some areas by a thick root mat and tend to be most frequently found in relatively open woodland in the zone of maximum rainfall in the Subandean forest. They also occur quite frequently in the Warm Tropical forest zone.The volcanic ash soils are in many cases less permeable than might be expected and it is concluded that under cultivation, when soils are able to dry out, the permeability is likely to increase. Lower water retention and higher rates of percolation might then encourage mass movements.The relationships described require verification of the assumptions inherent to the indices by field measurement. Further research should be conducted into the nature of the interrelationships between erosion processes and forest ecology within the broad ecological units considered here.