Some plants put in an awful lot of effort to look good, a case beautifully illustrated by this blue passion flower.
There is a tremendous amount of energy needed to make flowers, even if there is no need for them to look as impressive as this one, and flowers will thus be often limiting in extreme environments.
That is why flowering is one of the main measures of success in our study system of plant invasion in a harsh mountain climate (an environment were the passion flower of course does not stand much chance).
High in the mountains, the growing season is often too short for the invading species to gather enough energy to produce flowers. If that is the case, that might very well be the ultimate limit to their elevational distribution.