Early cultivation and bioturbation cause high long-term soil erosion rates in tropical forests: OSL based evidence from Ghana

Abstract

Soil erosion in forests is often considered negligible or minimal. In some tropical forest ecosystems, however, the combination of continuous surface exposure of fine soil particles at the upper slope by soil macrofauna and episodic precipitation can cause considerable downslope soil transport, and the introduction of slash and burn agriculture is often found to speed up soil erosion even more. Here we, for the first time, use OSL dating to quantify the long-term soil erosion rate along a hillslope in the moist semi-deciduous forest zone of Ghana. During the last ~4–5000years about 100cm of sediment has accumulated at an approximately constant rate of ~0.13–0.21mmyear−1 (1.95–3.15tha−1year−1). This is about an order of magnitude higher than literature estimates from similar environments, primarily obtained using short-term experimental setups; this suggests that care should be taken when extrapolating short-term erosion estimates to centennial or millennial timescales. We further deduce that an apparent acceleration in erosion began ~4500years ago. This may be due to the introduction of cultivation in the area, slightly earlier than previously suggested (2800–3500years ago). In this case the erosion rate based on deposition below 100cm (~0.05mmyear−1) can be taken to represent the (natural) erosion rate without human influence; this value is consistent with literature estimates. More retrospective studies of soil erosion rates are needed in order to confirm the validity of our methodology and results, as well as to test the reliability of extrapolated short-term erosion estimates.