Temporal and spatial patterns of Populus angustifolia along the upper Yellowstone River and clonal recruitment: Extent and requirements

Riparian cottonwoods (Populus spp.) have been declining in the Pacific Northwest due to changes in land use and river regulation. Because the majority of research on Populus species reproduction has focused on seedling recruitment, management programs have been limited to the hydrological requirements for seedling establishment and survival. Although many cottonwoods reproduce clonally, their extent, requirements and recruitment to maturity is not known. In this thesis, I used hydrology, and temporal, and spatial distribution patterns to study Populus angustifolia stand dynamics in Montana along the upper Yellowstone River. Additionally, I used DNA microsatellite loci to investigate population genetics and clonality. Combining these two analyses allowed me to investigate clonal reproduction ecology. I divided the study reach into four strata based on channel morphology with three sites per stratum. Cottonwood ages were used to delineate the age of stands and flood plain surface ages. Stand structure was analyzed using 100 m2 fixed plots within all age-patches and sites. There was no significant association of flood events with P. angustifolia seedling recruitment, and this may indicate clonal recruitment. Stand structure analysis revealed a large range in tree ages within age-patches, also an indicator of clonal recruitment. Because analysis suggested that clonal recruitment might be present, I used two DNA microsatellite loci to investigate the extent of clonality. This allowed me to characterized the population genetics as well as identify clones. The cottonwood population was not significantly subdivided into smaller subpopulations (P = 0.809 for 5 subpopulations). Interestingly, the population was not in Hardy-Weinberg equilibrium, probably due to local mating between close relatives. Because two microsatellite primers chosen were highly polymorphic, clone identification was possible. Clonality was similar across all strata with more than 71% of all trees sampled originating from clonal recruitment. Clonal recruitment occurred repeatedly resulting in a wide range in age for ramets from new recruits to over 200 years within fixed plots. Clonal establishment increased significantly following large flood events (P = 0.001). These results suggest that clonal recruitment plays a major role in the P. angustifolia population along the upper Yellowstone River.