I like this term & concept of ecotones:

An ecotone is a transition area between two biological communities,[1] where two communities meet and integrate.[2] It may be narrow or wide, and it may be local (the zone between a field and forest) or regional (the transition between forest and grassland ecosystems).[3] An ecotone may appear on the ground as a gradual blending of the two communities across a broad area, or it may manifest itself as a sharp boundary line.