Ask Mr. Smarty Plants

Rhus virens (Evergreen sumac) is, according to this USDA Plant Profile map, native in the area of Bexar County. It is not a true evergreen; like live oaks it drops its leaves in the Spring, which are quickly replaced by new leaves. The two small plants that you are seeing are actually shoots, still part of the original tree, and emerging from the same root. We are guessing, although this tree is drought tolerant, that our unusually hot and dry weather caused it to be drought-deciduous, dropping its leaves early to protect the roots.

You aready know, because of the shoots, that the roots are still alive. Use the thumbnail test to see if any of the stems have died. Beginnning fairly close to the top (or as high as you can reach) scrape off a very thin layer of the bark with your thumbnail. If there is a thin layer of green beneath the bark scraping, that trunk is still alive. You can check other areas or branches and, if you find no green layer, move farther down on the same branch.

We could find no projected age for this plant, but its mature height is usually about 8 to 10 feet. We did find information saying it could tolerate extra watering if it was in a well-drained soil but since, as you say, the care for it has not changed in 10 years, we doubt that could be the problem. At this point, we prescribe patience. If you found the green underskin, it is probably just waiting for better times. Leaves should begin to reappear in the Spring, per schedule.