Abstract

Tropical mountain areas may undergo rapid land degradation as demographic growth and intensified agriculture cause more people to migrate to fragile ecosystems. To assess the extent of the resulting damage, an erosion rate benchmark against which changes in erosion can be evaluated is required. Benchmarks reflecting natural erosion rates are usually not provided by conventional sediment fluxes, which are often biased due to modern land use change, and also miss large, episodic events within the measuring period. To overcome this, we combined three independent assessment tools in the southern Ecuadorian Andes, an area that is severely affected by soil erosion. First, denudation rates from cosmogenic nuclides in river sediment average over time periods of 1–100 k.y. and establish a natural benchmark of only 150 ± 100 t km−2 yr−1. Second, we find that land use practices have increased modern sediment yields as derived from reservoir sedimentation rates, which average over periods of 10–100 yr to as much as 15 × 103 t km−2 yr−1. Third, our land cover analysis has shown us that vegetation cover exerts first-order control over present-day erosion rates at the catchment scale. Areas with high vegetation density erode at rates that are characteristically similar to those of the natural benchmark, regardless of whether the type of vegetation is native or anthropogenic. Therefore, our data suggest that even in steep mountain environments sediment fluxes can slow to near their natural benchmark levels with suitable revegetation programs. A set of techniques is now in place to evaluate the effectiveness of erosion mitigation strategies.

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