Only time will tell.

Think this is one of the live-and-learn lessons of gardening.

Update: Don't use Weed Be Gone Max around any plants, bushes or ornamental trees. My Red Maple survived, tho' some of the leaves on the bottom died, the tree is still alive. 2yr. old boxwoods - dead, 1 yr. old azaleas - dead, flowers that were planted ~ the timeframe of the Ortho. Max usage - dead. I live & I learn!

Maples are shallow rooted. Wind drift can toast leaves that receive just a small amount of product. Use mulch, not product. SPOT treat weeds if you must for control. Less is more.
hortster

I may have made the same mistake, but worse.

I just sprayed WBG on what I thought was a rather large weed coming out of the base of the trunk of our tree, only to discover that it is a a young branch with fresh leaves.

The tree is well-established and stands about 10ft. I can't tell you what kind it is, only that it's at least 20yr.s old and stands strong.

Have I started a slow death for this tree, or is there something I can do to prevent any damage?

Thank you...

I would cut the branch off as low as possible or as flush to the branch collar as possible, ASAP, to (again, as much as possible) prevent the product from entering the rest of the tree.
hortster