Upland wildfires and fluvial processes: the response of tree populations along a gravel-bed watershed in the Canadian Rocky Mountains.

In order to better predict the influence of upland processes on watershed responses, we must first understand how forested systems change along drainage basins. Riparian vegetation studies tend to only consider fluvial disturbances to be of importance and ignore the impact of other disturbances, such as wildfires. This is partly due to the fact that riparian research typically only focus on high-order reaches, while little is known about larger, basin-scale processes. The objective of our research was to determine the relative importance of floods and fires on riparian forests along a drainage basin. The study was conducted along a gravel-bed river in a small mountainous watershed of the Canadian Rocky Mountains. Sites were chosen to represent first, second, and third order reaches. At each site, we constructed flood and fire frequency distributions as well as age structures of the vegetation. The results show that location within the watershed and reach type (straight or meandering) both dictate whether fires or floods dominated. First, fires dominate on all low-order reaches, creating even-aged forests. With an increase in drainage area and the onset of meandering, point bars are formed and the relative importance of floods is increased. Floods dominate on meandering reaches, where bi-modal age distributions are observed, with young trees found on fluvially-disturbed bars and old trees on older, fire-disturbed surfaces. Along straight reaches, even-aged distributions tend to occur since floods are typically not large enough to overflow the banks and fires therefore dominate. The shift in the relative importance of disturbances along the basin indicates that terrestrial processes differ along a basin, causing changes in forest structure and composition. Incorporating these changes in watershed models will lead to better predictions of large-scale hydrological processes.