Answer: Usually, this is a sign of moisture stress. The newly planted tree may have roots still primarily in the original potting soil which may dry out faster than the native soil in the ground, so you need to be aware of that when you water. These trees are also sensitive to heat when newly planted, and this can increase moisture stress as well. Make sure there is a layer of several inches of organic mulch over the root zone (not touching the trunk though) and that the soil is kept evenly moist but not sopping wet.
You might also check for damage to the trunk (rodent or weedwhacker for example) which could interfere with uptake of water. Finally, there is a possibility of accidental herbicide drift or overdose of fertilizer or similar "event" that could cause browning. Maybe this will help you trouble shoot the problem.
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