Savanna is considered a transitional biome because it is intermediate between two

Savanna is considered  a transitional biome because it is intermediate between two BIOMES.
A Savannah is a grassland area, where one can found scattered shrubs and trees. A Savannah is typically located between a tropical rain forest and a desert biome. The low level of rain fall experienced in the Savannah region is responsible for the inability of the region to support trees and shrubs growth.