"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 precise term used in botany and ecology to describe the makeup or structure of a plant flora in a particular region or ecosystem.
"Composition of the flora" is a more straightforward and easily understandable translation that conveys the same meaning.
Both expressions are acceptable, and the choice between them may depend on the specific context and the intended audience.