Initial cottonwood seedling recruitment following the flood of the century of the Oldman River, Alberta, Canada

Abstract

Following heavy rain in early June 1995, flows of the Oldman River in Alberta, Canada were the highest on record (since 1911). This ‘flood of the century’ preceded cottonwood seed release, and created suitable sites for seedling establishment. After the flood peak, the Oldman River Dam and tributary dams were operated to deliver a relatively natural and gradual river stage recession of about 2.5 to 5 cm per day. Nine research sites were established on lateral and point bars to study establishment, survival, and growth of seedlings ofPopulus angustifolia, P. balsamifera. andP. deltoides. In 1995, transects were established perpendicular to the river to the zone of mature cottonwoods; 131 quadrats were established at positions along the transects that intersected cottonwood seedling bands and revisited in 1996 and 1997. At all sites, extensive areas of seedlings occurred in 1995, and seedling bands ranged in elevation from 0.6 to 4 m above the late summer stream stage. Low elevation seedlings were removed in 1996 and 1997 by ice and water scouring; high elevation seedlings died primarily due to drought stress. Seedlings that survived through 1996 and 1997 occurred at elevations ranging from 1.7 to 3 m, but seedlings above 2.5 m grew slowly. Within the seedling bands, densities after the first season ranged from 80 to 4,000 seedlings m⁻², and densities fell to about 10% and then about 5% over the second and third years, respectively. The seedlings averaged only 2 cm in height after 1995 and increased to about 8 and 25 cm in 1996 and 1997;P. deltoides seedlings were larger than those of the other species, with some reaching 1 m in 1997. Thus, a major flood enabled a massive cottonwood seedling recruitment event that commenced in the flood year. The extensive recruitment occurred along a dammed river and was probably promoted deliberately by gradual stream stage decline after the flood peak.