The influence of fluvial and upland processes on the dynamics of riparian trees along small gravel-bed streams in the Canadian Rocky Mountains.

摘要

The aim of this dissertation is to combine the importance of upland processes, typically ignored in the riparian literature, to fluvial processes in order to understand changes in tree dynamics that occur along longitudinal and lateral dimensions within a watershed. The focus is on patterns of small, mountainous gravel-bed streams. The first chapter focuses on the longitudinal dimension and examines the commonly accepted assumption that areas along streams do not burn in wildfires. Stand origin maps and field observations reveal that cumulative time-since-fire distributions are similar for the riparian area and for the study watershed. Thus, areas along streams are not less likely to burn than the uplands. In addition, field observations reveal that the relative importance of fires and floods is regulated by a change in channel form associated with the creation of bars. The results demonstrate that fires solely control tree ages along straight streams without bars, while the influence of floods is observed at the onset of lateral and point bar formation. This occurs because bars are at low elevations and require smaller discharges to be flooded, compared to terraces and steep hillslopes. Consequently, bars are the only surfaces flooded more frequently than they are burned. The second chapter focuses on the lateral dimension and more closely examines the establishment dynamics of trees on these lateral and point bars. It investigates the assumption that species and ages change linearly with distance from the river within the riparian area. The results indicate that, contrary to large alluvial rivers which are regulated by in-channel deposition, tree age and composition on small gravel-bed streams are controlled by overbank sedimentation. At the scale of the bar, the patterns in species and ages are heterogeneous because of the patchiness in sediment deposition. Finally, in the third chapter, a model is developed for longitudinal and lateral vegetation changes based on stream morphologies. A channel classification system derived for mountain streams is used to synthesize changes in fluvial and upland processes with changes in drainage area. Vegetation changes are then predicted using the classification.