Why is my Chinese Pistache losing foliage on entire limbs?

More information would be helpful in determining what to do. Is there any damage at the base of the trunk? Are buttress roots visible? How wet (or dry) is the soil on average during the summer? It sounds like a portion of the crown has been dead for more than one season. What percentage of the entire crown would you estimate is currently healthy? It will be best to set up an appointment to have the situation assessed.