We could offer various theories:

--perhaps expansion of the highway system; there seems to be an abundance of ailanthus taking over I-81 from NY south to VA at least

--perhaps something in acid rain and pollution of water tables; ailanthus seems to thrive in conditions other living plants languish in

--but something dynamically different must have happened during the last 30 years because we saw very little ailanthus in the Richmond area in the early 70s; there is a great deal of ailanthus today all over the city and even out here in the countryside 30 miles from Richmond; we found our first ailanthus on the farm year before last and immediately destroyed the copse. We've been looking out for it ever since because it takes off like wildfire.

It's different from kudzu; kudzu covers everything and eventually kills what it covers. Ailanthus simply occupies any space available--any empty available space--and then grows very swiftly upward, thereby blocking out light of species that do not grow as tall, such as dogwoods and redbuds.