If storms had constantly battered a tree, its trunk, and its branches, they would not have become tall or strong. When a storm hits, it can sometimes even uproot trees completely, meaning that trees can not suddenly grow strong from something like that. Especially, not when hit over and over. This also means that storms do not help trees grow stronger or taller.
There is also nowhere where it would imply a tree would be able to become ugly or tangled, although this is a close answer. Frail generally means weak or weakening. When something is hit over and over, it is most likely to become weak. These are context clues you could use in the future if you do not know what a word such as this means.
The answer is ultimately: "delicate" or "fragile."