Can trees be considered weeds?

{@alamba] no trees do serve a purpose timber they bear fruit for both human and animal consumption weed to me appear to serve no purpose except to be dug out as unwanted in your garden

Yes, Elm Trees and salt cedars.

Yep. If you take a tree from one country and plant it in another, and it starts to spread and dominate the native ecosystem, taking resources from other species of plant and tree, then yes that tree is considered a weed.

If they have 7 leaves .., maybe.