<b>Abstract</b><br/>Studies of genetic variation can clarify the role of geography and spatio-temporal variation of climate in shaping demography, particularly in temperate zone tree species with large latitudinal ranges. Here, we examined genetic variation in narrowleaf cottonwood, Populus angustifolia, a dominant riparian tree. Using multi-locus surveys of polymorphism in 363 individuals across the species’ 1800 km latitudinal range, we found that, first, P. angustifolia has stronger neutral genetic structure than many forest trees (simple sequence repeat (SSR) FST=0.21), with major genetic groups corresponding to large apparent geographical barriers to gene flow. Second, using SSRs and putatively neutral sequenced loci, coalescent simulations indicated that populations diverged before the last glacial maximum (LGM), suggesting the presence of population structure before the LGM. Third, the LGM and subsequent warming appear to have had different influences on each of these distinct populations, with effective population size reduction in the southern extent of the range but major expansion in the north. These results are consistent with the hypothesis that climate and geographic barriers have jointly affected the demographic history of P. angustifolia, and point the importance of both factors as being instrumental in shaping genetic variation and structure in widespread forest trees.

<b>Usage notes</b><br /><div class="o-metadata__file-usage-entry"><h4 class="o-heading__level3-file-title">Populus angustifolia locations and SSR data</h4><div class="o-metadata__file-description">This file contains all the SSR data collected for all unique P. angustifolia genotypes, formatted for the program STRUCTURE. It also contains the latitude, longitude, and elevation of the collections.</div><div class="o-metadata__file-name">PopulusAngustifoliaSSR.xlsx</br></div></div>

Dryad version number: 1 Version status: submitted Dryad curation status: Published Sharing link: http://datadryad.org/stash/dataset/doi:10.5061/dryad.82kv2 Storage size: 129478 Visibility: public