Landslides

Timber Harvesting and Landslides:
A Closer Look by Paul Adams, Oregon State University

"Countless studies have shown that timber harvesting causes landslides, so it's time to do something about it."

Sound familiar? Statements like this are now often seen and heard in letters to the editor and other public venues, especially after the destructive storms of 1996. Are the links between logging and landslides clearly understood? Moreover, is there a clear direction for policy makers to take in dealing with this issue? Some pertinent facts and perspective follow.

In the Pacific Northwest there have been quite a few studies of landslides on forest lands that include harvested areas. These studies show that landslides occur in both logged and unlogged areas, but in some cases landslide rates are higher in cutover areas during the first decade or so after harvest.

Although simple at first glance, interpretation of these findings and their use in policy making are considerably more complicated.

First, very few of the studies were validated with ground surveys (i.e., most relied on aerial photos where landslides in forests are hard to detect), and those that were show substantially smaller differences between logged and unlogged areas. Second, most studies were done years ago and thus reflect logging practices of the 1960s and '70s, which generally resulted in more ground disturbance. Third, when surveys included logged areas that were several decades old, some of these areas showed lower landslide rates than those seen in mature, unlogged forests.

Thus, while there is some evidence that timber harvest can have an effect on landslide rates, the actual degree and scope of this effect under current forest practice standards is much less clear. For the decision maker, this lack of clarity is not a trivial matter because it has direct bearing on the benefits and costs of policies that may be crafted to address this issue. Would the policies affect 50 or 500,000 acres? And what specific practices would be required?

In the one available study of a policy specifically designed to limit landslides from logging (i.e.,"headwall leave areas," which are steep, unstable locations that are left uncut), there were no significant differences in landslide rates observed between forested, clearcut and the leave areas.

The task of the policy maker will not be easy, especially when economic, public safety, water resource and property rights issues are involved. Fortunately, research continues and is expected to provide more useful insights about the occurrence and management of landslides on forest lands. While this growing technical knowledge will provide some direction, enough complexity and uncertainty are also likely to remain. Decision makers should expect further challenges in refining policies that provide a reasonable balance of risks, benefits and costs in addressing the public's diverse interests.

Paul Adams is a professor and Forest Watershed Extension specialist, College of Forestry, Oregon State University, Corvallis, Oregon.