"植物区系组成" can be translated into English as "floristic composition" or "composition of the flora".
"Floristic composition" is a more technical and commonly used term in botany and ecology to describe the makeup or assemblage of plant species within a particular region or ecosystem.
"Composition of the flora" is another way to express the same idea, emphasizing the arrangement or structure of the plant species in a given area.
Both phrases are accurate and can be used interchangeably depending on the context and the level of formality required.