Clonal growth in Salix exigua

Salix exigua (sandbar willow) is a widespread early colonizing riparian species in North America. It can colonize sites both by seed germination and by clonal growth. Whereas, a moderate amount of research has been reported on seed germination in this species, this is the first work to explore the role of clonal growth in sandbar willow stands. Amplified Fragment Length Polymorphism (AFLP) molecular genetic analyses were used to identify clones and allowed for the measurement of clone characteristics. In addition to a general understanding of the role of clonal growth in populations of sandbar willow, the effects of disturbance regime change on the relative importance of cloning were also explored.;The field study sites were on the Cosumnes River, which is a high disturbance undammed river, and on the Mokelumne River, which is a low disturbance damned river. Three sites along each river were sampled for clone characteristics. Clones were found to be much larger than expected, with 3 to 9 clones per site covering from 44 to 87 percent of the vegetated area. The largest clone measured was 325 square meters and occupied 40 percent of the entire field site sampled. Average proportion distinguishable (PD) values of 0.46 revealed relatively low levels of genet variability and high levels of clonal growth on the large-scale measured. Willow seeds have been found to be extremely transient in the environment and seedling mortality is very high. It appears that productivity in Salix exigua stands is maintained by extensive clonal growth.;The importance of clonal growth as measured by PD values was found to increase with a reduction in disturbance. Even though clone sizes were not found to be significantly different between rivers, data did suggest that clones on the Mokelumne River were larger. The reduced disturbance regime of the Mokelumne River was also found to result in greater aboveground biomass and productivity. These combined factors lead to greater clonal growth, a reduction in genotypic diversity, and the eventual replacement of Salix exigua by later successional species.;Almost all genets from within sites sampled were found to be siblings. This supports a single cohort recruitment model. Despite the predominantly single-family representation per site, molecular variance was still highest within sites. Molecular variance indicated significant barriers to gene flow at both the between watershed and site scale.;To provide an objective means of identifying clones, a threshold value of a similarity index based on distributions of between sibling pairwise similarities and between clone pairwise similarities was developed. This method was tested in replicate sampling and also tested on samples from a field site along the Cosumnes River in California. Results showed that this approach can be useful in the accurate identification of clones.