Fine-scale genetic structure of whitebark pine (Pinus albicaulis): Associations with watershed and growth form

摘要

The fine-scale genetic structure of a subalpine conifer, whitebark pine (Pinus albicaulis Engelm.), was studied at nested geographic levels from watershed to adjacent stems in the eastern Sierra Nevada Range of California. A combination of several characteristics contributed to unpredicted genetic structure in this species. This includes being one of only 20 pine species worldwide with wingless, bird-dispersed seeds; having the reputed capacity to reproduce vegetatively; and forming distinct growth morphologies at different elevations in this part of its natural range. Genetic differentiation, as measured with 21 allozyme loci, among the three studied watersheds is virtually negligible (F sub(ST) = 0.004). This is a surprising result because the upper-elevation sites vary somewhat in slope aspect; thus, aspect was confounded with watershed effect. Differentiation between the upper-elevation prostrate krommholz thickets and lower-elevation upright tree clump growth forms is modest (F sub(ST) = 0.051). Much stronger differentiation was measured among the individual thickets and clumps within their sample sites (F sub(ST) = 0.334). Within krummholz thickets, multiple individuals are present and genetic relationships often resemble haft- to full-sibling family structure (mean r = 0.320). Canonical trend surface analysis in two intensively sampled thickets indicates greatest genotypic variation in the direction of the prevailing wind. At lower elevations, most (72%) of the tree clumps contained more than one genotype; the remaining clumps are probably multistemmed trees. Within tree clumps, family relationships are closer than those for krummholz thickets - commonly full-sibling to selfed structure (mean r = 0.597). Genetic structure is apparently profoundly influenced by the seed-caching behavior of Clark's nutcracker (Nucifraga columbiana Wilson). Western pine species typically show little among-population differentiation and high levels of within-population genetic variation. In whitebark pine in the eastern Sierra Nevada of California, genetic variation is highly structured, especially within the natural groupings - krummholz thickets and upright tree clumps.