Answer: Although the undergrowth will compete with the tree roots for water, it can also help moderate soil temperatures and slow evaporation so leaving it there probably won't harm your trees. It would be worth your trouble to determine why your trees are dying. Lack of water? Insect or disease problems? If it is simply drought stress, watering deeply once each week should help the trees recover. Insect or disease problems need to be eliminated if your trees are going to have any chance of recovering.
Best wishes with your trees.
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