Human impacted landscapes facilitate hybridization between a native and an introduced tree

Spatial and temporal dynamics of hybridization, in particular the influence of local environmental conditions, are well studied for sympatric species but less is known for native-introduced systems, especially for long-lived species. We used microsatellite and chloroplast DNA markers to characterize the influence of anthropogenic landscapes on the extent, direction, and spatial distribution of hybridization between a native North American tree Juglans cinerea (butternut) and an introduced tree Juglans ailantifolia (Japanese walnut) for 1363 trees at 48 locations across the native range of butternut. Remarkably, admixture in anthropogenic sites reached nearly 70%, while fragmented and continuous forests showed minimal admixture (7.5% and 2.5%, respectively). Furthermore, more hybrids in anthropogenic sites had J. ailantifolia seed parents (95%) than hybrids in fragmented and continuous forests (69% and 59%, respectively). Our results show a strong influence of landscape on rate and direction of realized gene flow. While hybridization is clearly rampant in anthropogenic landscapes, our results suggest that even small forested landscapes serve as substantial barriers to hybridization, a key consideration for conservation planning for butternut, a species already exhibiting severe decline, and for other North American forest trees that hybridize with introduced congeners.