Effectiveness of needle cast at reducing erosion after forest fires

Needle cast from partially burnt conifer trees commonly occurs after forest fires. The effectiveness of needles in reducing soil erosion was investigated in this study. Two needle types, ponderosa pine and Douglas fir needles, were used at four different cover amounts (0, 15, 40, and 70 percent) on granitic and volcanic derived soils. Simulated rainfall was used to examine interrill erosion; added inflow was used to determine rill erosion in a laboratory setting. After a series of “runs,” data showed that sediment delivery was greater for the granitic soil compared with the volcanic soil. Douglas fir needles were more effective at reducing interrill erosion compared with the ponderosa pine needles. Ponderosa pine needles, because of their shape and being bundled together, often caused minidebris dams to form. The minidebris dams formed by ponderosa pine needles reduce flow within the rill, resulting in less rill erosion than the Douglas fir needles. A 50 percent cover of Douglas fir needles reduced interrill erosion by 80 percent and rill erosion 20 by percent. A 50 percent cover of ponderosa pine needles reduced interrill erosion by 60 percent and rill erosion by 40 percent. We also compared the effectiveness of using stream power, rather than shear stress, to model rill erosion. Stream power was a better predictor of sediment load than shear stress. Rill detachment rates based on stream power decreased with increasing cover for both needle types. These results challenge the use of shear stress detachment rates in current erosion models and provide insight into the use of stream power detachment rates.