Larian Studios Forums Divinity: Original Sin 2 - The Modding Dungeon Divinity - Original Sin 2 - Modding General Trees that you walk through.

You can manually block certain areas from being walkable with the AI Grid Panel. Bring up the AI Grid Panel, select Paint Movement, and 'Block'. Choose a brush (round or square) and start painting. It will show up purple. Regenerate AI.

You could paint these blocked AI areas around your instance placed trees if you wanted.