Sexual and Asexual Reproduction of Salix sitchensis and the Influence of Beaver (Castor canadensis) Herbivory on Reproductive Success

The influence of beaver (Castor canadensis Kuhl) herbivory on Salix reproduction, specifically the stimulation of asexual reproduction via browsed stem fragments, is relatively unknown. This study aimed to determine if beaver herbivory stimulates asexual reproduction of riparian willows and results in mature populations dominated by clones. The survival of seedlings and asexual propagules produced by beaver browse in populations of the riparian willow Salix sitchensis (Sanson in Bongard) were quantified to determine overwinter survival at 6 experimental sites. Salix sitchensis clonal diversity, using five microsatellite markers and the polymerase chain reaction, was calculated to detect if asexual reproduction had been stimulated by beaver herbivory. No sexual propagules survived overwinter in any of our study sites. Numerous asexual propagules were observed and 0–41% survived overwinter. Each sampled individual possessed a unique multilocus genotype, and clonal diversity was 1.0. Beaver herbivory did not create current willow populations dominated by clones. Beaver herbivory and asexual reproduction appeared to have played a minor role in the reproductive strategies of S. sitchensis at our sites in central British Columbia, Canada. 1. Introduction Whether herbivory positively or negatively influences plant reproduction is a debated topic. Depending on the species, response variable, and the seasonal pattern of the foraging, herbivory can have a negative, [1–7], positive [1, 5, 8], or neutral [1, 4, 5] influence on plant reproduction. A relatively unexplored aspect of this interaction is the stimulation of asexual reproduction by the production and relocation of vegetative propagules via animal browsing. While some studies have reported the incidence or survival of such translocated propagules [9–15], it is still unclear if they significantly contribute to plant reproductive strategies. Riparian habitat in central British Columbia, Canada, presents an ideal system to investigate this relationship. The dominant members of the riparian vegetative community are species of the genus Salix. Willows (Salix spp.) reproduce either sexually through wind dispersed seeds, or asexually through underground runners and severed branches or stems [10–12, 15–18]. Severed branches may play an important role in Salix colonization [19–23], once colonization has occurred, these areas of patchy vegetation can shelter sexual propagules, as well as stabilize and retain sediment. This ecosystem modification creates more habitat conducive to sexual propagule survival [19–23].