Answer: If the very tops of the trees are dead, from wind or snow or frost, you can prune off the dead tops. Your trees will then begin to direct one of the top branches upward to form a new central leader. Or, it will send a new branch straight up which will become the central leader. It will take several years for this to occur, but eventually they will begin to grow upward again. You will notice a slight bend or curve in the top of the tree where the original central leader grew.
Best wishes with your trees!
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