I have trees that did not come into leaf in recent years and i think they died during the winter. What do you think?
Trees that stand in water for periods of as little as a week or two can suffer damage to the roots by deprivation of oxygen. The roots can die and the tree can die and not come into leaf. Even if it does come into leaf in spring, it can die soon after. Partly damaged trees can struggle, and carry small leaves, and may survive. Make sure that trees do not stand in waterlogged or flooded soil. Improve drainage if necessary, or unblock existing drains.