Causes of crown formation of Umbelliferae trees

Hello, what is the reason for the formation of the umbrella crown tree? The reason for the formation of the canopy tree is that the physiological structure evolved by the canopy tree adapts to the tropical grassland climate. Canopy trees are mainly distributed in savanna ecosystem, because the physiological structure evolved by Canopy trees adapts to the tropical grassland climate. These areas are warm all year round, but the rainfall distribution is uneven, with an annual average temperature of 18? 24 1, annual rainfall of 500? 1500mm. The xerophytic trees scattered in the grassland background are short and multi-branched, with very special large and flat canopy, hard leaves, often pinnately compound leaves, scaly bud protection, thick bark, and a large amount of water stored in the trunk to adapt to the poor soil and dry climate in the tropical grassland.