During winters trees shed the leaves and become leafless . Once the spring season comes two kind of behavior are apparent.
If only leaves will come , the tree was deficient in stored food material and hence must produce photosynthates and cant spend its energy in reproduction. Others which have enough of storage material sense that summer is coming at least in tropical areas and for their survival they must produce seeds . Most of the tropical trees flower with regularity. March April are months for flowering in trees and seed setting. Year after year trees do it with such a great precision: you can know the month of the year by looking at floral pattern of trees in tropics or even in temperate climate during summer months. What precise mechanism regulates the development of vegetative shoot or floral shoot .
Man is still trying to learn from the period of "Takhtjan" to modern biotechnology molecular basis of such changes .

It is not easy to understand plant behavior. Natures reveals but conceals also.