Usually bald cypress trees (Taxodium distichum) turn rusty brown in autumn before their needles drop. A tree stressed by environmental conditions may show prematurely brown leaves and begin dropping them to conserve energy to survive.
If a bald cypress tree grows where summers are long and hot, often the green needles naturally begin to attain a more yellowy and eventually brownish look as early as late summer. If the summer was unusually hot and dry, leaf browning or bronzing occurs much earlier, especially during the midsummer.
Premature browning and drop of needles are signs the plant is trying to become dormant, it's potentially under attack by a serious disease or it is wounded. While bald cypress is normally not afflicted by pests and diseases, a trunk, root or branch wound could provide pests and diseases entrance into the tree. When plant tissues dry out or die, the leaves brown on affected branches.
Although drought tolerant when established, the bald cypress tree benefits from deep irrigation during extended growing season droughts. This slows or prevents premature leaf browning. Even with watering during extreme droughts, the tree's foliage still yellows and browns as a defense mechanism and enters an early dormancy.