Answer: Chinese Elm are semi-evergreen or even deciduous, depending on the variety. This means that they can or will drop leaves when temperatures get cold. If you have been watering appropriately (deeply and as infrequently as possible) they should leaf out again. I've added some info below on watering trees in the desert.
Desert soil and water both contain salts, which can accumulate in the root zone over time. This salt buildup forms where the water stops penetrating. Short periods of watering cause salts to build up in the top layers of soil and damage or kill your plant. Salt burn shows up as yellowing along leaf margins, browning and leaf drop. We see this happen alot with drip irrigation because it doesn't supply sufficient water for deep watering. For example, an emitter that puts out one gallon per hour would only put a quart of water on the ground in 15 minutes. Think about dumping a Big Gulp drink on a tree, and you can visualize how ineffective this would be. For trees, water should soak 3 feet deep, as this is where most of the roots are located. It's essential that you allow your drip system (or hose or bubblers) to run long enough for water to penetrate the appropriate depth. Depending on the size and number of emitters, soil type, etc. this might take several hours or 10 hours or many more. You can reduce the time you run the system by putting on extra emitters or changing to emitters with higher gallon/hour flow rates. Deep watering?or leaching?prevents salt burn by flushing the salts past the root zone. Always water slowly, deeply and as infrequently as possible.
Use a soil probe (any long, pointed piece of metal or wood to poke into the soil) to check how far water has penetrated. The probe moves easily through moist soil, but stops when it hits hard dry soil. As trees go, move emitters outward to keep pace with the expanding canopy. Feeder roots that absorb water are located towards the outer canopy edge, or drip line. I hope this info helps!
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