I don't think the trees cause a problem when they are alive. But lets say you leave them on there for 15-20 years and now the roots go all the way through the dam then for some reason one of the trees die from lightening or insects. As the roots of the tree rot water begins to flow through the dam following corse of the decaying roots. This happend with my 1.5 acre pond when cotton wood trees were left on it to long. Now the pond goes from 10' deep in the spring to almost dry in the summer months. It looses almost a 1" of water per day.