Differing influences of natural and artificial disturbances on riparian cottonwoods from prairie to mountain ecoregions in Alberta, Canada

Abstract

Aim Ecoregions represent biophysical zones where environmental factors enable the development of particular plant communities. Ecoregions are generally large but abrupt transitions occur in areas with rapid physical change. A particularly abrupt transitional sequence occurs in the Rocky Mountain region of southwestern Alberta where fescue prairie, aspen parkland and mountain ecoregions occur within 15 km. To investigate plant adaptation across ecoregions, our study investigated the influences of a natural disturbance (flooding) and an artificial disturbance (cattle grazing) on reproductive and population processes of black cottonwood (Populus balsamifera subsp. trichocarpa, Torr. & Gray), the dominant riparian tree.Location We studied cottonwoods throughout their elevational range along two free-flowing, first-order streams, Yarrow and Drywood creeks. Cottonwood was the only prominent tree in the prairie ecoregion, the dominant riparian tree in the parkland and extended upward through the montane ecoregion where it was a pioneer species for the mixed coniferous-deciduous woodland. Cottonwoods did not occur in the higher elevation sub-alpine ecoregion.Methods Thirty-six cross-sectional sampling transects were located across the three ecoregions with cottonwoods, and in ungrazed and grazed areas of each ecoregion. Rectangular 100 m 2 tree and 2 m 2 seedling quadrats were positioned along the transects, and substrate and vegetation were assessed. Historic hydrological data were analysed relative to flood recurrences and seasonal flow patterns.Results Overall, the cottonwoods displayed a sawtooth shaped 'punctuated progressive age structure' with many young trees, progressively fewer older trees, and about four pulses of increased recruitment over the past century. This was considered to provide a healthy cottonwood population and recruitment pulses were apparently associated with flood events with appropriate peak timing and magnitude and a gradual post-flood stage recession. However, analyses of tree, sapling and seedling data indicated that flood-associated seedling recruitment was less important and clonal processes were more important for cottonwood recruitment in the montane ecoregion, the highest ecoregion with cottonwoods. The correlation between flood events and cottonwood recruitment was strongest in the mid-elevation parkland ecoregion suggesting greater reliance on floodassociated seedling recruitment. There was little correlation with flooding and limited recruitment in the fescue prairie ecoregion in recent decades and the disturbed age structure probably results from cattle impacts that have prevented recruitment and produced a decrepit cottonwood forest population.Main conclusions These analyses suggested that a healthy cottonwood population displayed a sawtooth shaped 'punctuated progressive age structure'

Instream flows and the decline of riparian cottonwoods along the Yakima River, Washington, USA

“…Consistent with the population structure of black cottonwoods along Drywood and Yarrow creeks in southwestern Alberta (Samuelson and Rood, 2004), the age distribution for mature cottonwoods along the Yakima River demonstrated apparent pulses of cottonwood colonization ( Figure 5). As with other plants, there is expected to be a 'lazy J' distribution across ages with many more young than old trees, and this was observed for the mature trees (>20 years old) with many established between 1960 and 1980 and fewer established in prior decades ( Figure 5).…”

Growth of riparian cottonwoods: a developmental pattern and the influence of geomorphic context

“…and A. Gray) Brayshaw), extends westward from the upstream reaches of the study area (Kalischuk et al 2001;Samuelson and Rood 2004). The Summerview Reach is near the upstream and western limit of the narrowleaf cottonwood, Fig.…”

“…This geomorphic context determines the site environment that has a number of influences on cottonwood growth. It constrains the riparian forest stand and influences stand density and consequent competition between trees (Samuelson and Rood 2004). It also influences river channel and bank form and dynamics and this influences substrate composition of the floodplain (Hughes 1997).…”

Managing River Flows to Restore Floodplain Forests

“…Suitable floods need to be appropriately timed relative to the organism's life history and followed by sufficient flows during the vulnerable juvenile life phase Lytle and Poff 2004;Samuelson and Rood 2004).…”