Differing influences of natural and artificial disturbances on riparian cottonwoods from prairie to mountain ecoregions in Alberta, Canada
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Authors: G. M. Samuelson, and S. B. Rood
Date: 2004
Journal: Journal of Biogeography
Volume: 31
Number: 3
Pages: 435-450
Summary of Methods: To investigate plant adaptations along riparian areas across ecoregions in southwestern Alberta, the influences of natural disturbance (flooding) and artificial disturbance (cattle grazing) were measured on black cottonwood (Populus balsamifera trichocarpa). Age structure, percent cover, DBH, density of trees, saplings and seedlings were measured in July and August along two free-flowing, first order streams in three ecoregions (Montane, Parkland, Prairie). Transects were established in both moderately or heavily grazed and lightly or ungrazed areas along the entire elevational extent of the cottonwoods for both streams. Stream discharge was measured weekly during the field season and historical discharge records were used to determine cottonwood recruitment by stream flow.
Article Summary / Main Points: Montane and parkland cottonwood recruitment were strongly correlated with stream discharge; higher stream flow resulted in more recruitment. Prairie cottonwood recruitment was not correlated with discharge and overall recruitment was lower. Tree age increased while sapling density, tree density and cover decreased as elevation decreased – montane to prairie ecoregions. Ungrazed sites had higher tree and sapling density and greater tree cover than grazed sites and had a correlation between stream flow and tree recruitment. Tree cover and seedling density were similar between grazed and ungrazed sites. Flooding was important for cottonwood recruitment in the parkland region, less important in montane region and not important in the prairie region. Cattle grazing had the largest impact on the prairie region, which was thought to be the ecoregion impacted by grazing the longest.
Vegetation Types: Northern Mixed Prairie Riparian and Wetlands
MLRA Ecoregions: Not Applicable
Agrovoc Control Words: Riparian zones Rangelands Grazing Land
Article Review Type: Refereed
Article Type: Documented Case History
Keywords: age structure, cattle grazing, cottonwood, populus, flooding, mountain, prairie, reproduction, riparian, rocky mountains
Annotation: These results that suggest cottonwood tree age and density characteristics differ by stream flow, elevation and grazing pressure are important to keep in mind when trying to conserve or restore cottonwood riparian habitat. These findings are applicable to most northern cottonwood sites with livestock grazing. A more thorough characterization of the grazing pressure would have strengthened these findings by detailing how different intensities affect cottonwood recruitment.
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