It's hard to miss the fruit of the Amur Cork Tree this time of year, the female trees are laden with the dark blue clusters. The compound leaves also turn a golden yellow, so the fruit stands out even more.
Native to eastern Asia; northern China, Manchuria, Korea, Ussuri, Amur, and Japan, the Amur cork tree is considered invasive in many parts of North America. (Wikipedia)
I first noticed these non-native trees a few years ago. The bark is very different in texture and color from any of our native trees, light gray with a raised wavy bark pattern. Both male and female trees are growing along the Maple-Basswood woodland edge in this park. A few are more upland and the other cluster is at the bottom of a east facing wooded hill.
This park is a few miles from the University of Minnesota Landscape Arboretum where a there is a Cork Tree Collection. It is quite possible that the seed was bird dispersed from the trees in their collection. A few years ago, the Arboretum removed all the female trees in this collection due to their potential to become invasive.
Based upon the size of these trees in the park, (they are 8-10" dbh) they've been here for several years.
Have you seen Amur Cork Tree naturalizing in your area?