Here’s a dictionary definition of a weed: “a wild plant growing where it is not wanted and in competition with cultivated plants.”
That definition leaves out a lot, doesn’t it? You hear the word “weed,” and somewhere in your mind you’re probably thinking “annoying” and “unsightly.”
People talk about “weed trees,” too. They spring up on their own, often in places where other trees may struggle to thrive or even survive. They’re exceedingly common. They may have characteristics that don’t fit the classic image of a tree, e.g., they may be bushy, misshapen, etc. And they’re scrappy: they can out-compete trees that we think of as conventionally beautiful, stately, grand, majestic, noble — all that good stuff.
Boxelder (Acer negundo) comes to mind. It’s a species of maple native to North America. Check out this guy. Talk about scrappy:

Here’s its compound leaf with leaflets (7 here but can be as few as 3), to help you identify the tree :

Weed tree? Yet entomologist Doug Tallamy says 285 species of Lepidoptera (moths and butterflies) rely upon boxelder to survive their caterpillar stage.
So many bugs! Here’s the secret: boxelder is terrible at doing what many other trees do incredibly well, which is to seal off decay. Which means that boxelder rots. And what loves rotting wood? Bugs!
And birds love bugs, of course. So boxelder indirectly feeds lotsa birds, too.
Can you build a beautiful, long-lasting dining room table out of boxelder wood? Nope. But can you sustain an ecosystem encompassing hundreds or perhaps even thousands of different insect and animal species with boxelder trees? ? Yep.

Yes, those are boxelder seedlings, bringing a stone staircase to life. Not bad for a weed.