Population Dynamics of Fraxinus pennsylvanica

Ash (Fraxinus, Oleaceae) species occur on most continents, across a wide range of forest tree communities. The accidental introduction of the emerald ash borer beetle, Agrilus planipennis Fairmaire (EAB; Coleoptera: Buprestidae), now threatens the North American Fraxinus resource. Introduced into the U.S. from Asia in the late 20th century, EAB has caused widespread mortality primarily in green ash, Fraxinus pennsylvanica Marsh. (Section: Melioides) and now impacts other North American ash species. The development and successful reintroduction of resistant trees requires genetic tools for the evaluation of population dynamics in order to inform seed collection initiatives and restoration protocols.

Here, I report 16 novel EST-SSR markers developed for green ash, most of which amplify and are polymorphic across 18 other species of Fraxinus, including six species native to North America. I utilize these markers to identify the principal variables necessary for reproductive success in a pollen and seed dispersal analysis in eastern Montana. In addition, I ascertain fundamental characteristics of genetic variation and population differentiation across the range of green ash, including a broad and fine-scale analysis of genetic structure.

I inferred pollination neighborhood size (0.53 hectares) and found significant differentiation among pollen pools (ɸ_FT = 0.118, p = 0.001) with few effective pollen donors per mother tree (N_ep = 3.97). The distribution of pollen flow follows a bivariate, two-parameter exponential power function and the mean distance of pollen flow, δ, is high (δ = 182 m). I found three genetic clusters in the range with high genetic variation and highly variable population differentiation (0.003 to 0.330). Although spatial factors did not estimate differentiation well, mean temperature of the driest quarter was the best single predictor of population differentiation.

Finally, I found that individuals from the “central prairie” region maintained a growth advantage after 37 years and with advanced reproduction. Additionally, trees from more northern latitudes and higher elevations were smaller and burst bud later than those from more southern origins. Differential mortality was found across latitude with individuals from more southern latitudes surviving longer than those from northern latitudes.