Answer: The sun has a wondrous effect on icy birch branches.
Explanation:
Robert Frost in this excerpt speaks of how birches are affected by ice and then by the environment around them from how the birches are bent by ice to how the icy branches are then affected by the wind, rain and sun.
At the end Robert describes the effect the sun has on the branches and notes how the sun enables the birches to shed to snow in such heaps that one might think a part of heaven has fallen. As heaven is considered so beautiful, it must mean that the effect the sun had was a beautiful and wonderful one.
