Spring planted trees should be deeply soaked once about every seven to ten days. More established trees should be watered every two to three weeks depending on the species and age of the tree. Trees that are well watered and healthy are sent into winter dormancy in a stronger state better able to fend off the stresses of winter. Winter stress could include not only cold temperatures, but also dry conditions. Young trees should also be watered once or twice during dry winter conditions.
Stress is a compounding factor.
That is each stressful condition builds upon the next slowly weakening the tree. There is a point reached in which the plant cannot over come the problem and dies. Supplemental water is one factor to help reduce stress build up. In the case of native trees most of them will be fine. They are in a dormant state conserving as much of their resources as possible.