CLONAL REPRODUCTION OF RIPARIAN COTTONWOODS IN SOUTHERN ALBERTA

From 1990 to 1993, 690 small cottonwood saplings were excavated along four rivers in the Oldman River Basin to determine their method of ori gin. Overall, 52% began as seedlings while 48% originated through clon al mechanisms, of which 30% (of the total) were root suckers, i.e., ad ventitious shoots from preexisting roots, and 18% were shoot suckers, i.e., new shoots originating from buried shoots. The shoot suckers aro se primarily through (i) flood training, i.e., new shoot emergence fro m young shoots that had been toppled and buried during flood events, a nd (ii) shoot regrowth following decapitation by ice scouring or anima l browsing. Only two saplings originated through cladoptosis, propagat ion from excised shoot fragments. The proportion of seedlings versus c lonally established shoots varied somewhat across years and sites. Wit hin a site, saplings near the river's edge were more commonly seedling s, whereas saplings further from the river were more commonly root suc kers. Three cottonwood species occurred in the forests studied and the ir reproductive strategies varied somewhat. Unlike the balsam poplar ( Populus balsamifera), narrowleaf cottonwood (Populus angustifolia), an d interspecific hybrids, the prairie cottonwood (Populus deltoides) di d not propagate through root suckers; all species produced seedlings a nd shoot suckers. This study demonstrates that clonal reproduction has the potential to contribute to the regeneration of riparian cottonwoo d forests in southern Alberta. The contribution through clonal propaga tion complicates the forest population structure and should be conside red in programs to conserve the remaining riparian cottonwood forests in southern Alberta and elsewhere in North America.