Landscape genetics and population structure in Valley Oak ( Quercus lobata Née ) 1

Gene fl ow in plants occurs through the dispersal of pollen or seeds. Pollen-mediated gene fl ow is oft en extensive in plants that are pollinated by wind. Although long-distance (landscape-level) pollen movement is diffi cult to measure directly, paternity studies have shown that average pollination distances within stands of windpollinated trees is ten to hundreds of meters and pollen immigrations rates into study sites are about 50% (reviewed in Ashley, 2010 ). Th ese levels of eff ective pollen movement suggest that windpollinated tree species may exhibit little population structure over large areas, especially if woodlands are relatively continuous and major landscape impediments to pollen movement do not occur. In tree species that have discontinuous ranges or those confi ned to certain habitat types, however, gene fl ow may be restricted through parts of a species range, and be refl ected by signifi cant population structure between some populations. We studied population structure in valley oak, Quercus lobata, a wind-pollinated oak endemic to California. A winter-deciduous species, valley oak is the largest western North American oak, with trees 10-25 m tall and 0.5-0.7 m diameter at breast height (dbh; Munz, 1973 ). Valley oak is an ecologically important species in savannas and oak woodlands of California but has experienced extensive habitat loss as land within its range has been converted to farmland, vineyards, and other development ( Allen-Diaz et al., 2007 ; Whipple et al., 2011 ). Ecological niche models for valley oak indicate that their current distribution has been relatively stable since the late Pleistocene (~120 kya ; Gugger et al., 2013 ). Th is distribution forms a horseshoe around the Central Valley, found in the lower elevations of the Central and North Coast ranges and the Sierra Nevada ( Fig. 1 ). Th e southernmost populations are close but 1 Manuscript received 22 April 2015; revision accepted 5 November 2015. 2 Department of Biological Sciences, University of Illinois at Chicago, 845 W. Taylor Street, Chicago, Illinois 60607 USA; and 3 Lab of Ornithology and Department of Neurobiology and Behavior, Cornell University, Ithaca, New York 14850 USA 4 Current address: Department of Internal Medicine, University of Michigan, Ann Arbor, Michigan 48109 USA 5 Author for correspondence (e-mail: [email protected]) doi:10.3732/ajb.1500182 Landscape genetics and population structure in Valley Oak ( Quercus lobata Née) 1