There are discussions among evolutionary biologists why flowering plants (angiosperms) so successfully propagated. In addition to what has been discussed at length: the better protection of their seeds and the help of insects and other animals to reproduce and spread. Newer discussions have also focused on their leafs: the better photosynthesis because of a sophisticated vein system, and better energy conservation during winter times because angiosperms don’t keep their leafs. There is, however, an other advantage. In times of heavy snow fall, the leaf-free angiosperm trees don’t carry as much snow and are therefore nor threatened to break by the heavy load. As a consequence gymnosperm tree are slim and tall while angiosperms can spread their canopy.
From the viewpoint of fauceir evolution theory these aspects are only secondary, the primary cause why angiosperms dominate vegetation on our planet is that they have better evolution control devices.