Could it be hedge apple, more commonly known as osage orange around here? (Maclura pomifera)The female trees get large, green aromatic orange shaped fruit in the fall. They have a sticky white fluid in them. The roots have orange bark. The branches have sharp spines and have yellow interiors. The tree was planted as a living fence in the past and did a good job of keeping animals penned. Several grew along the edge of a cemetery near me when I was a child.

Can you find any leaves that came from your trees, maybe a few stuck in the lower branches? Or compare the twigs with a winter tree ID guide.

Any twig pix?

BTW, Osage Orange have thorns that are visible in winter. Here's a link to a picture of how the twigs look:

FataMorgana

Here is a link that might be useful: Osage Orange Picture

Fairly smooth yellow bark doesn't really sound like osage. And if there are no thorns, it's something else. But if it turns out that these trees are indeed osage, you may not want all of them on your property, but please don't let yourself think of them as 'just weed trees'! They are highly prized by many woodworkers, and especially traditional bowyers. Someone would be thrilled to take some of them off your hands.

Linda S.