The "close cousins" thing is interesting for me. I live in a tree-lined streets neighborhood. Historically, they chose species on a street-by-street basis. My street was all linden trees for the curb trees (except for a last remaining elm, we've lost most of them to Dutch Elm disease). A few years back, a severe wind storm took out seven of the curb trees on my block, in a cluster (and smashed a few cars in the process).

Rather than planting more lindens, the city put in a diverse new collection - I think every one is different. Our new tree is a hackberry. Our next door neighbor got a japanese maple, and there's a buckeye across the street, etc.

So now I'm wondering what that means for the long-term health of our scrawny little hackberry. I don't know of any other hackberry trees in the neighborhood - the dominant species are lindens, maples, walnuts, ashes, catalpas, and a few ancient elms. Within our yard, there's a big old ash, a catalpa, a sumac, a magnolia, and a dogwood.

Oh, and I live in an urban forest, apparently.