Limits of windthrow-driven hillslope sediment flux due to varying storm frequency and intensity

The uprooting and toppling of trees during storms transports soil, exhumes bedrock, and thus influences the evolution of hillslopes. Given predicted increased storminess due to future climate change, we adapt a forest-gap model (ForGEM) to explore windthrow-driven sediment transport in a Douglas fir forest under increasingly severe wind regimes in order to better assess potential future impacts on soil erosion. Larger trees are more sensitive to wind loading and are therefore preferentially toppled as storm frequency and intensity increase. Because larger trees have larger root-plate volumes and can move large volumes of sediment, increased wind velocities lead to an increase in sediment flux. With increasingly stormier conditions, however, the proportion of large trees dwindles. The net effect of these two countervailing trends is that sediment transport increases as average annual rates of windthrow approach eight trees per hectare, but begins to decline thereafter. Our results highlight the complex relationship between climate and sediment transport, particularly when it is mediated by the biota.

► The toppling of trees influences the evolution of hillslopes. ► Large trees are preferentially toppled as storm frequency and intensity increase. ► Increased storminess exhausts the supply of large trees. ► Sediment flux increases until rates of windthrow exceed eight trees per hectare. ► Diffusivity by windthrow peaks at nearly 3.5 square meters per thousand years.