Ecohydrological controls on soil erosion and landscape evolution

Abstract

ABSTRACTThe ecohydrological controls on soil erosion and landscape evolution are difficult to quantify and poorly understood. In many parts of the world, cyclone‐induced tree throw is a major source of disturbance. Tree throw may increase sediment transport by exposing a mound of fresh soil as well as providing a pit which may act as a knickpoint triggering gully erosion. Alternatively, while tree throw provides characteristic pit–mound topography, the amount of soil disturbed or exposed in a mound is relatively small on the hillslope and catchment scale and the effects may be minimal. The April 2006 tropical cyclone Monica that impacted the coast of northern Australia with winds' speeds > 100 m s−1 uprooted approximately 50% of the trees in the study catchment. We use a landscape evolution model with repeated occurrence of the cyclone over a 1000‐year simulated period to quantify the effect of pit–mound topography distributions on both sediment transport and landscape evolution by including the fallen trees into the digital elevation model both as a pit–mound and also as a pit–mound and tree trunk. The results show that the inclusion of pit–mound topography substantially reduced erosion for the first 10–15 years of its introduction and adding pit–mound–trunk topography reduced erosion rates even further. The pit–mound and pit–mound–trunk acted as sediment traps, capturing sediment from upslope and storing it in debris dams reducing hillslope connectivity. Model simulations predict average denudation rates for the catchment approximating field measured data. These findings suggest that any tree throw is unlikely to result in landscape instability. Copyright © 2011 John Wiley & Sons, Ltd.