Southwestern white pine (Pinus strobiformis, Pinaceae) seeds are dispersed by the Clark's nutcracker (Nucifraga columbiana, Corvidae) where the two species are sympatric. Nutcrackers store widely dispersed caches of 1-15 pine seeds for subsequent use in the winter and spring. A cache of unutilized seeds may germinate and eventually form a multi-genet tree cluster. Ecological damage may cause individual trees to resemble tree clusters but, in these cases, the trunks are of one genotype. The objective of this research was to examine southwestern white pine stand and population structure both within and outside of the range of Clark's nutcracker. It was hypothesized that sympatric populations would exhibit a higher frequency of multi-genet clusters in response to the foraging and caching behavior of the nutcracker. Four populations of southwestern white pine were sampled - two within and two outside the nutcracker's range - for the presence of tree clumps (multi-trunked trees of undetermined genetic structure). Foliage was collected from individual stems and subjected to allozyme analysis to elucidate the genotypic composition of the tree clumps. Although the northern sites located within the range of Clark's nutcracker contained a higher density of clumps, this did not differ significantly from southern sites, where the two species do not co-occur. Of the 35 clumps sampled at the four sites, allozyme analysis indicated 24 to be multi-genet clusters. Within the range of Clark's nutcracker, 19 of the 26 clumps sampled (73.1%) were revealed to be multi-genet clusters. Of the nine clumps sampled from outside the range of Clark's nutcracker, five (55.6%) were multi-genet. Whereas these data suggest that the Clark's nutcracker imposes a genetic structure on southwestern white pine populations where their ranges overlap, dispersal by other vertebrates, e.g., Steller's jays, undoubtedly plays a role in effecting structure, as well.

Key words: Clark's nutcracker, growth form, Pinus strobiformis, population structure, seed dispersal, southwestern white pine