Most of the US Southwest is naturally desert. There are only a few kinds of trees- such as the native palm that can live in these harsh conditions, and they naturally would only occur in arroyos or places where water accumulates and wind speeds are lower during storms. The soil is also just sand, and cannot support a large tree. The Los Angeles area is filled with non-native trees that are planted in sand and watered at great expense, but they fall and destroy houses and cars everytime it gets windy.