DECLINE IN ROOT STRENGTH FOLLOWING WILDFIRE IN THE OREGON COAST RANGE

Although several studies have proposed that fire plays an important role in the temporal pattern of erosion in the Oregon Coast Range (OCR), there is a paucity of knowledge regarding how forest fires affect surface processes in the OCR. Previous studies have suggested that Holocene fluctuations in sediment production may be linked to wildfires, but sparse evidence exists to connect climate variations and increased fire frequency with an increase in erosion and sedimentation. The primary mechanism for increased erosion is damage or destruction of vegetation. Intense fires can damage the crowns, boles, and root structures of trees, which often leads to the death of the trees and subsequent loss of root strength. Root strength decline increases the propensity for shallow soils slips or landslides. Whereas empirical studies have shown a well-defined decline in root tensile strength following timber harvesting, no studies have investigated the change in root strength following fires in the Oregon Coast Range.

We investigated root strength and root network density at three forested OCR sites that burned in 1999, 2002, and 2003. Our method entails excavating soil pits approximately 1 meter deep, measuring the depths and diameters of all roots greater than 1 mm, and using a field scale to quantify the tensile strength of the roots. When compared to a recent study of healthy Douglas fir forests (Schmidt et al., 2001), our data show that root strength declines approximately 50 percent within the first 10 months following an intense wildfire. This decline in root strength is comparable to the decline seen following timber harvesting. Root strength remains diminished for at least 4 years following fire, and over this time period, smaller roots decay more rapidly than larger diameter roots.

Our results suggest that fires may significantly increase the likelihood of shallow landsliding in the OCR. The period of decreased root strength following fire may be longer than that following timber harvest due to the destruction of secondary vegetation during fires. This implies that the coincidence of landslide-triggering rainfall events with periods of depressed root reinforcement may be more frequent during fire-prone periods, potentially leading to elevated rates of sediment production.