Ecological - Second Order
The principal native trees in the semiarid regions of southern Alberta are riparian cottonwoods. These include narrowleaf cottonwood, Populus angustifolia James, balsam poplar, Populus balsamifera ssp. balsamifera L., black cottonwood, Populus balsamifera ssp.trichocarpa Torr. & Gray, and prairie cottonwood, Populus deltoides Bartr., and interspecific hybrids. These phreatophytic poplars occur on the moist floodplains along streams and require flood disturbance for seedling recruitment. The present study investigated the responses of cottonwoods to another physical disturbance, fire. Two fires occurred in April 1992, in adjacent groves along the Oldman River at Lethbridge, Alberta. The fires occurred prior to bud flushing and the cottonwoods responded by vigorous sprouting, particularly in the first summer. By September of 1992 about 75% of the burned trees had produced coppice sprouts, new shoots from the remnant stumps. Root suckers, adventitious shoots from the roots of the burned trees, were also common, averaging 1 sucker/3 m2 in the burned zone. Five years after the burns, the number of clonal sprouts was reduced by about half and those had reached an average height of 3 m. Because of the difficulties of species identification for juvenile cottonwoods, sprouts were designated as Populus section Aigeiros (P. deltoides) or section Tacamahaca (the other species). After 5 months, 90% of the sprouting trunks and 80% of the suckers belonged to section Tacamahaca. After 5 years, this ratio had increased to 97% of the sprouting trunks and 98% of the suckers. This indicates that Tacamahaca cottonwoods produced more numerous sprouts with better survival than P. deltoides. This pattern across Populus sections was also observed at seven other cottonwood burn sites in western North America, from northern British Columbia to southern Utah. The Aigeiros cottonwoods, P. deltoides and P. fremontii S. Wats., produced few clonal shoots following fire, whereas Tacamahaca trees sprouted profusely. This study demonstrates that fire disturbance can stimulate clonal regeneration of riparian cottonwoods, particularly of section Tacamahaca.