Okay for some reason this forum won't let me edit my post, but here are the pictures.

Tree full-length.

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Bark.

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Close-up of leaves.

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Ants and aphids on new growth (Eww)

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Callery Pear, this link explains it better than I can.

Here is a link that might be useful: Coming Plague of Pears

Awww, yep that's it. I've been looking for quite a while to identify it, and I couldn't because all the identifying tools must only include native species. They should include invasive species, because otherwise I might have let it grow. Although, as soon as it flowered, it would have been a tip-off that it wasn't the yellow birch I was hoping it was. Oh well. Thanks so much Sam md. That's exactly what I needed to know.

Here is a link that might be useful: Callery Pear Invasive Plant