Branch propagation, not cladoptosis, permits dispersive, clonal reproduction of riparian cottonwoods

In riparian (floodplain) zones, physical disturbances of floods and ice-scour undercut and topple shrubs and trees into the river where branches are sheared off by tumbling and turbulence. These branch fragments are deposited downstream along bars with sediments that retain moisture and promote adventitious rooting to produce vigorous clonal saplings. We observed such ‘branch propagation’ along rivers of western North America for Tacamahaca cottonwoods, being common for P. balsamifera (balsam poplar) and P. balsamifera subsp. trichocarpa (black cottonwood) but less common for Populus angustifolia (narrowleaf cottonwood). It was rare for the Aigeiros cottonwoods, P. deltoides (prairie cottonwood) or P. fremontii (Fremont cottonwood). Branch propagation was sparse downstream from dams due to the trapping of branch fragments and sediments by the reservoirs combined with unfavorable patterns of flow regulation. Conversely, branch propagation was abundant after major floods such as for black cottonwoods along the Elk River, British Columbia, following the flood-of-record in 1995. In that case, excavation of saplings from branch fragments in 1997 revealed that 91% originated from sheared branches, 7% from beaver browse and only 1% included the basal enlargement characteristic of physiological abscission. Consistent with this minimal representation, only 3% of abscised branches from narrowleaf, prairie and black cottonwoods successfully propagated under favorable greenhouse conditions in moist sand, in contrast to 96% success of winter shoot cuttings from these species. These studies indicate that branch propagation enables dispersive clonal reproduction for riparian cottonwoods. It provides vigorous saplings and is promoted by geomorphic disturbance and beaver browse. These observations reject the concept of ‘cladoptosis’ which proposed that physiologically abscised cottonwood branches commonly serve as clonal propagules.