Relative Contributions of Avalanches and Chronic Tree Mortality as Wood Recruitment Processes in a Headwater Stream

Large wood (LW) is delivered to streams by a number of processes, ranging from steady inputs of riparian and hillslope trees to large, episodic inputs resulting from various natural disturbances. Although avalanches are often cited among input processes of LW to headwater streams, few data have documented their contribution. At the Fraser Experimental Forest, north-central Colorado, we measured channel and LW characteristics in twenty 50 meter reaches along Lexen Creek, a steep, first-order stream, approximately 2000 m in length. Mean bankfull width ranges from 1 m to 3.5 m. A total of 690 LW pieces were measured (length, end diameters), and evaluated for fall direction, distance from source, and most probable recruitment process. The Lexen watershed (124 ha; elevation 2900 to 3400 m) is dominated by subalpine fir, Engelmann spruce and lodgepole pine and about 15% of the basin area is above treeline. Multiple low-frequency avalanche events have originated in the alpine portion of the watershed and have uprooted, snapped off, and moved trees 100 to 500 m downslope and downstream, resulting in riparian and instream LW accumulations in which > 85% of the pieces were oriented parallel to the channel, and the number of instream pieces averaged 1.4 per meter channel length. For a distance of over 350 m, the stream is obscured beneath stacks of LW, including numerous pieces with root wads. By comparison, the mean number of pieces in reaches below the avalanche runout zone was 0.8 per meter channel length, and fall directions were mostly oriented towards the stream. Approximately 40% of the pieces in the lower reaches were traced to sources within 30 m of the channel and attributed to chronic tree mortality (35%) and bank erosion (5%). Although avalanches have contributed nearly 40% of the measured LW standing stock in Lexen Creek, this influence is highly localized, largely due to the configuration of the avalanche runout zone. The constraints of channel size in this small watershed severely limit transport of LW pieces and much of the wood has become incorporated into the structure of the channel and narrow valley bottom.