Actually, we do have rowan trees in the US and Canada...we just don't call them rowans. The name used over hear is Mountain Ash, and I believe that my Audubon Book of North American Trees there are a couple of varieties in North America, as well as escaped versions of the European Mountain Ash.

For those of you who like Kipling, the Oak, Ash and Thorn in "Puck of Pook's Hill" and "Farewell Rewards and Fairies" refers to Oak, Rowan and Hawthorn, which were all semi-sacred trees in pre-Christian days.