Riparian vegetation and processes: An analysis of elevational ecosystems in southwestern Alberta

Riparian (streamside) ecosystems are dynamic, disturbance-dependent systems that respond to a number of physical environmental factors. One aspect of the present study examined the environmental processes that dominated plant community development for riparian vegetation from alpine headwaters through the subalpine, montane, parkland and fescue prairie ecoregions along Yarrow and Drywood creeks in southwestern Alberta. Data analyses revealed that the dominance of flooding processes versus weather and climate patterns in structuring riparian plant community types varied both across ecoregion and with height above the stream. With regard to the ecoregion gradient, flooding processes were dominant for the lower elevation ecoregions, the fescue prairie and parkland ecoregions and the high elevation alpine ecoregion. Conversely, weather and climate processes were increasingly important in the montane and subalpine ecoregions where flooding processes are moderate in part due to bedrock control.; Riparian black cottonwoods, Populus balsamifera subsp. trichocarpa, were examined throughout their elevational range in the three lower elevation ecoregions along the same streams. The influences of a natural disturbance, flooding, and an artificial disturbance, cattle grazing were investigated. Flood events with appropriate hydrograph patterns were correlated with increased cottonwood recruitment in all three ecoregions but differences in dependency existed. Analyses of tree, sapling and seedling data indicated that clonal processes were more important than seedlings for cottonwood recruitment in the montane ecoregion. The correlation between flooding and cottonwood recruitment was strongest in the mid-elevation parkland ecoregion and there was little correlation with flooding and limited recruitment in the fescue prairie ecoregion probably as a result of cattle grazing. Comparisons between data of grazed and ungrazed transects revealed that cattle grazing has resulted in an even-aged, significantly older and significantly less dense cottonwood forest.; Comparisons of vegetation and environmental characteristics across two grazing intensities revealed that cattle grazing had negative impacts on the vegetation in all five elevational ecoregions of these two streams. There were variations between the ecoregions in terms of which vegetation type was most responsive to grazing. In the fescue prairie and montane ecoregions both woody and herbaceous vegetation indicated the impacts of cattle grazing. In the parkland ecoregion woody species were more sensitive and therefore better indicators of grazing impacts. In the subalpine and treeless alpine ecoregion, herbaceous vegetation characteristics were more sensitive grazing impact indicators. Similarity indices revealed that species composition was most impacted in the subalpine and alpine ecoregions in spite of their relatively short history of grazing compared to the lower elevation ecoregions.