Identify the complete adjective clause.

The tree that withstands winter winds is strengthened.

"that withstands winter winds" is the adjective clause. Not only is it describing the tree, but it is a dependent clause as well, with S-V relation. (that withstands)

since "The tree is strengthened." is an independent clause, it is easy to pull out the clause to make ^^^ happen

In this sentence the complete adjective clause would be [that withstands winter winds]. That is a relative pronoun which usually begins a clause.