Final answer:
The savanna is a transitional biome because it lies between the desert and tropical rainforest biomes, with attributes of both ecosystems.
Step-by-step explanation:
The savanna is considered a transitional biome because it is intermediate between two different biomes: the desert and the tropical rainforest. This unique ecosystem is found in regions like Africa, South America, and northern Australia. It features grasslands with scattered trees and experiences an extensive dry season with periodic fires, which has led to the evolution of plants with well-developed root systems capable of rapid regrowth.