Gene flow from Fraxinus cultivars into natural stands of Fraxinus pennsylvanica occurs range-wide, is regionally extensive, and is associated with a loss of allele richness

In North America, the ubiquitous planting of a comparatively small number of Fraxinus (ash) cultivars in both urban and rural environments over the last 70-80 years may have permitted extensive gene flow into naturally regenerated stands. In the light of multiple biotic threats to the North American Fraxinus, an assessment of the extent of gene flow from ash cultivars and the current state of genetic diversity in F. pennsylvanica (green ash), one of the most widely distributed species, is needed to inform seed collection strategies for the preservation of genetic diversity range-wide. We used 16 EST-SSR markers to genotype 1291 trees from 48 naturally regenerated populations of green ash across the native range, 19 F. pennsylvanica cultivars and one F. americana (white ash) cultivar. We detected first generation cultivar parentage with high confidence in 172 individuals in 34 of the 48 populations and extensive cultivar parentage (23-50%) in eight populations. The incidence of cultivar parentage was negatively associated with allele richness (R2 = 0.151, p = 0.006). The high frequency of cultivar propagule dispersal in our study suggests that a significant proportion of the standing genetic variation in local populations may not be of local origin, a result that has serious implications for the study of adaptive variation and the conservation of the Fraxinus gene pool.