First of all, "withheld" means "held something back from," as in "she withheld the cookies from her child" or "he withheld the information from Congress." So it doesn't mean "hold up to" a big storm. The word you want is "survive."
Second, Alex almost has it right in his second example. You have a few options in the past tense:
The tree was so massive it could have survived the strongest storm. ("The strongest storm" has happened, but we don't know whether the tree was exposed to it. If it was, then clearly it survived.)
The tree was so massive it could survive the strongest storm. (Hypothetical: "The strongest storm" hasn't happened. We project that the tree could survive it.)
In present tense:
The tree is so massive it could survive the strongest storm. (Hypothetical again. Even if we are in the present, the strongest storm is in the future.)