Wendy K. Gram

ASSOCIATION BETWEEN ENVIRONMENTAL AND GENETIC HETEROGENEITY IN FOREST TREE POPULATIONS

Within a plant species, environmental heterogeneity has the potential to influence the distribution of genetic variation among populations through several evolutionary processes, including natural selection, differential gene exchange, and chance associations caused by genetic drift or founder effects. We evaluated the relationship between environmental characteristics and the distribution of genotypes for three common woody species (Quercus alba, Carya tomentosa, and Sassafras albidum) in Missouri Ozark forests. We measured the correlation between multivariate genotypes and forest structure variables, and assessed the influence of physical landscape on multivariate genotypes. For all three species, we found significant relationships between genetic variation and environmental heterogeneity. We discovered that populations in local habitat patches with different forest structures also differed in combinations of multivariate genotypes. In contrast, we did not detect significant differences in multivariate genotypes among soil-type/aspect classes in any of the three study species, suggesting that genotypic differentiation is operating on a finer scale than soil or aspect differences. We conclude that natural selection, possibly interacting with founder events, has influenced the population differentiation of these three long-lived plant species. Such microgeographic variation in response to environmental heterogeneity is expected for a broad range of species, even when extensive gene flow is present.

Winter participation by neotropical migrant and resident birds in mixed-species flocks in Northeastern Mexico

Mixed-species foraging flocks in northeastern Mexico are large, diverse, and include Neotropical migrant and resident species. I characterized the species composition and social organization of mixed-species flocks in four habitats of the El Cielo Biosphere Reserve during the winters of 1993-1995. Thirty-seven percent of all species found in the Reserve participated in mixed-species flocks, including 16 migrant species. Flocks in the dry pine-oak forest had more migrant individuals than flocks in the humid oak-pine/cloud forest or tropical semi-deciduous forest, but flocks in the tropical semi-deciduous forest had more migrant species than resident species. Migrant species rarely have been reported as nuclear species that attract other species to the flock and maintain flock cohesion, yet both Ruby-crowned Kinglet (Regulus calendula) in the dry pine-oak forest, and Blue-gray Gnat-catcher (Polioptila caerulea) in the tropical semi-deciduous forest, are migrant species that behaved as nuclear flock species. Bridled Titmouse (Parus wollweberi) and Crescent-chested Warbler (Vermivora superciliosa), Mexican resident species, were nuclear species in the dry pine-oak forest and the humid oak-pine/cloud forest, respectively. I found that Ruby-crowned Kinglet and Crescent-chested Warbler were follower species (species that occurred regularly in flocks but did not behave as nuclear species) in habitats where they were not nuclear species, indicating that the social role of a species within a flock may depend on habitat, resource availability, species density, or the other species present in the mixed-species flock.

Bird Response to Clear Cutting in Missouri Ozark Forests

Abstract We evaluated changes in breeding bird density and shifts in territory distribution with respect to clear cutting and timber stand improvement (TSI) of even-aged stands on >300 ha experimental management units as part of the Missouri Ozark Forest Ecosystem Project. After one harvest entry, clear cutting had positive effects on density of indigo bunting (Passerina cyanea), prairie warbler (Dendroica discolor), and yellow-breasted chat (Icteria virens) and negative effects on density of Acadian flycatcher (Empidonax virescens), ovenbird (Seiurus aurocapilla), and worm-eating warbler (Helmitheros vermivorus). In buffer regions within 100 m of clearcuts, indigo bunting, hooded warbler (Wilsonia citrina), wood thrush (Hylocichila mustelina), and Kentucky warbler (Oporornis formosus) densities increased and ovenbird density decreased. Breeding bird densities did not change in interior regions >100 m from clearcuts except for a small increase for wood thrush. Breeding Acadian flycatcher and ovenbird showed greater use of stands not treated with TSI. We recommend combining adjoining stands to keep clearcut sizes between 8 ha and 13 ha to reduce negative effects on ovenbirds by cutting. We suggest a 7-year offset between the timing of clear cutting and TSI to reduce their combined effects on ovenbird.