Underground, it’s chemical warfare. Trees battle each other for root space with secretions that keep one another at bay, and keep intruders from gaining any foothold in their territory.
Oh, yes. Walnut trees and related species release juglone. I had wondered why my hickory leaf / nut (husk) mulch seemed to do in our tomatoes, until I found this out. Now I just use it (the mulch) for a worm-bed.