Landsliding and Effects of Timber Harvest in a Natural History Context

To better understand the long term geomorphic and ecologic consequences of modified landslide erosion rates in managed forests would require detailed field studies encompassing entire river networks over multiple decades. Logistical, funding, and institutional constraints limit that approach. As an alternative, we show how landsliding can be considered within a natural history context that provides a surrogate measure for interpreting environmental impacts. In the Oregon Coast Range, the natural history of landsliding is represented as empirically-estimated probability distributions of annual rates based on historical basin sediment budgets, landslide studies, and radiocarbon dating. In managed forests, the corresponding altered probability distributions of landslide rates are estimated by integrating timber harvest over time and integrating forest lands spatially over multi-ownerships and forest practices that typify the watershed scale. Timber harvest shifts the natural probability distribution of annual landslide rates by decreasing the occurrence of the very lowest values, including zeros. There is a corresponding rising of the right tail of the distribution, expressed by increasing higher rates. These rate modifications result in hypothetical shifts in the probability distributions of ages of landslide-related valley-floor landforms and of sediment supply to fish-bearing streams. The direction and magnitude of shifts in probability distributions of landslide rates and their effects on environments provide a new approach for considering environmental changes resulting from land use activities.