Final answer:
The border between a forest and a grassland when they gradually merge is called an ecotone.
Step-by-step explanation:
The border between a forest and a grassland when they gradually merge into each other over a wide area is referred to as an ecotone. An ecotone is a transition zone between two different ecosystems, where characteristics of both ecosystems can be found. In this case, the forest and grassland characteristics blend together in the ecotone. For example, you may find a mix of trees and grasses in the ecotone area.