The first summer we were here we assumed that all the little trees had seeded in under the large one, but the first time we tried to dig one up we realized that wasn't the case. They were all suckers off the large tree, and while it's cool to be able to trace the path of the roots all over the yard, having about 1000 tiny Crataegus trees, knee high, covered with long thorns isn't cool.
Clearly this is why they have been so hard to kill. Check out the roots on these things. They are practically little bonsai trees from all of the years of being mowed down.