Shallow Landslide Erosion Rates on Industrially Managed Timberlands: Key Factors Affecting Historical and Contemporary Rates

Abstract

Timber harvesting and related management practices associated with industrial timberlands have changed dramatically in the last two decades. Industrial timberlands are now more carefully assessed and mitigated. Recent studies of mass wasting in northern California included a review of historical aerial photographs from the early 1940s through 2016 and field measurements of nearly 3,000 shallow landslides on industrially managed timberlands. Significant improvements have been seen in management practices over time that include but are not limited to reduced harvest unit sizes, increased streamside tree retention, reduced road density, and improved road-building practices. These improvements are a result of a variety of sources such as evolving state regulations, voluntary conservation plans, and increased professional oversight. Subsequently, significant decreases in management-related erosion are being observed across the area included in this study. Observations show that improvements in management practices have positively affected regional mass wasting. In this investigation, significant changes have been noted in both causal mechanisms and landslide erosion rates. The study data shows that before the year 2000, nearly 85 percent of landslide-related erosion was determined to be the result of historical logging, either by harvesting or from roads (generally poor design and/or location). Shallow landslide erosion rates have varied over the duration of time reviewed for this study, peaking in the 1970s. Since 2000, erosion rates across the study area have decreased to 20 m3/km2/yr, which is a 92 percent reduction compared with the historical rate.