The Role of Natural Vegetative Disturbance in Determining Stream Reach Characteristics in Central Idaho and Western Montana

We evaluated the relationship between natural vegetative disturbance and changes in stream habitat and macroinvertebrate metrics within 33 randomly selected minimally managed watersheds in central Idaho and western Montana. Changes in stream reach conditions were related to vegetative disturbance for the time periods from 1985 to 1993 and 1993 to 2000, respectively, at the following three spatial scales; within the stream buffer and less than 1 km from the evaluated reach, within the watershed and within 1 km of the stream reach, and within the watershed. Data for stream reaches were based on field surveys and vegetative disturbance was generated for the watershed above the sampled reach using remotely sensed data and geographical information systems. Large scale (>100 ha) vegetative disturbance was common within the study area. Even though natural vegetative disturbance rates were high, we found that few of the measured attributes were related to the magnitude of vegetative disturbance. The three physical habitat attributes that changed significantly were sinuosity, median particle size, and percentage of undercut bank; each was related to the disturbance in the earlier (1985-1993) time frame. There was a significant relationship between changes in two macroinvertebrate metrics, abundance and percent collectors/filterers, and the magnitude of disturbance during the more recent time period (1993-2000). We did not find a consistent relationship between the location of the disturbance within the watershed and changes in stream conditions. Our findings suggest that natural vegetative disturbance within the northern Rocky Mountains is complex but likely does not result in substantial short-term changes in the characteristics of most stream reaches. 1 Author to whom correspondence should be addresses. E-mail; [email protected] Introduction Current and historic geoclimatic conditions govern stream morphology (Montgomery and Buffington 1997, Ebersole et al. 1997). Physical components and processes within this setting, such as water and sediment movement through a watershed, form the basis for understanding how stream channels develop and are maintained (for reviews see Leopold et al. 1964 or Knighton 1998). These physical processes, however; are mediated by vegetation. The presence of vegetation can reduce peak stream flows (Hicks et al. 1991), decrease overland flow, and trap sediment (Gregory et al. 1991, Belt and O’Laughlin 1994), while the loss of vegetation can have the opposite effects (Minshall et al. 1989, Gresswell 1999, Benda et al. 2003). Since changes in vegetation alter physical processes, changes in vegetation can ultimately change channel form (Benda et al. 2003) as well as the composition of aquatic biota dependent upon that channel form (Rieman and Clayton 1997, Minshall et al. 2001, Dunham et al. 2003). Natural disturbance processes such as fire, wind throw, and insect activity alter vegetative structure and function (Agee 1993, Frelich 2002). Changes in vegetation that result from these natural processes vary in intensity (the amount of energy released), severity (vegetative mortality), and magnitude (spatial scale) (Frelich 2002). While each of these processes can lead to altered aquatic conditions, vegetative disturbances that are intense, severe, and proximate to streams have the greatest likelihood of causing changes in aquatic systems (Gresswell 1999). While there have been a number of studies evaluating the relationship between natural vegetative disturbance and stream channel conditions, much of the sampling effort has been opportunistic focusing in areas of high public interest (e.g., the