Elm trees, once abundant and widely distributed in North Dakota and highly valued as an ornamental died by the thousands, mostly in the 1970s. Something similar may occur with ash trees and the emerald ash borer.
A single cottonwood tree can produce a staggering number of small seeds each year. Estimates range from around 25-48 million. But of course, the chances of a seed becoming established and living to maturity are extremely slim.
Have you been noticing a 2-3 foot tall wildflower sporting a terminal cluster of conspicuous purple flowers? If so, it is probably Dame’s Rocket. That is not a native plant. It might look pretty, but as they say, looks can be deceiving.