I have noticed that two different plantings of Zelkova serrata trees (that I specified!) are showing signs of dieback of the upper branches. The trees were planted as 6cm and 3cm caliper size, 3 years ago, and all were irrigated their first year. The first year they showed some dieback, and this year some are almost totally without leaves except for new leaves along the lower trunk. The smaller trees had been in root bags, which were removed on planting. The larger trees were either B&B or in wire baskets. The larger trees are in a commercial planting on automatic irrigation. There is no obvious sign of insect damage. What physical stress might be causing this dieback? Literature seems to indicate that the Zelkova is a good tree for tough urban situations... Thanks for any advice!