Resource managers have become increasingly concerned over the apparent decline of Quaking Aspen (Populus tremuloides Michx.) in the western United States. To aid in the development of conservation and restoration strategies for aspen, we investigated genetic variability, clonal diversity, levels of differentiation, and patterns of geographic variation as measured by allozymes among 663 aspen individuals located in 82 stands from 8 watersheds throughout the western slope of the central Sierra Nevada, California. As is usual for the species, these aspens are highly diverse genetically (82% polymorphic loci, 3.1 alleles/locus). Stands are much less variable, and are genetically different from each other, with 46.5% of all genetic variation measured being the differences among stands. Clonal diversity was 30% with 198 of the 663 individuals sampled containing unique clonal genotypes. Fifty-six percent of the stands studied contain two or more clones. Monoclonal stands are usually small in size, and vary from 0.1 to 5 acres, averaging 0.8 acres. All watersheds contain more than one clone. In ten cases, a single clone extended across what are now classified as two or more stands. Although there are some general trends of increased genetic similarity with geographic proximity, stands that are in close proximity are not necessarily genetically similar, and can be, in fact, quite genetically distinct. Genetic variation detected by the allozyme data can be used to delineate genetic restoration units and to prioritize conservation efforts.
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