Mr Smarty Plants initial vote went for over-watering, you should really cut that down, but on consideration - transplant stress is another strong possibility since the Possumhaw seems to be reacting worse than the Anacacho Orchid! How can you tell if it's watered correctly? Test the soil near the tree. After watering it should be damp to a depth of 4-5 inches. Check out the Forest Service video referenced below. For information on the best growing conditions look at the plants record in the plant database.
Bauhinia lunarioides (Anacacho orchid tree) is found fully naturally in West Texas towards the Rio Grande Valley - It is acclimatized to quite dry conditions and the plant record [under Growing Conditions] indicates it does best in dry soil with low water conditions.
Ilex decidua (Possumhaw) is much more tolerant of water. It is native to Central to East Texas and the Plant Record indicates it likes moist soil and medium watering. It is a bit inconsistent with my call of overwatering that all of the possumhaw are affected and only one of two Anacacho Orchids is; this is a little more indicative of transplant shock.
Still 3X/week at 30 minutes per watering day is quite a bit of water. The Wildflower Center also has a page - Drought Resource Center - Watering information is there including The City of Austin Earthwise Guide to Tree Care during Drought and a Forest Service Video on Tree Watering Tips. Note especially on the video, the use of a screwdriver to test the soil to see if it's watered enough. A reasonable guideline for watering trees would be to set the bubblers to water once every 2 weeks for 2 hours instead of 15 minutes 2x a day. Once the trees are established, in about 2 years time, stop watering unless there is extreme drought.
Is something else going on - maybe. When I checked the other Mr Smarty Plants answers, they look fairly even that leaf browning may be watering [over OR under] and/or transplant shock.
- Anachacho Orchid leaves turning brown and curling in Driftwood.
- Leaves turning brown and dropping in Austin
- Possumhaw loosing leaves in Liberty Hill after transplant
- Brown Leave on Possumhaw in Grandview TX