Hi, I'm finding that I have ornamental apple trees, ornamental weeping cherry trees, and my Himalayan birch tree, are all defoliating. The ornamental apples have done this the past 2 years during the spring. They leaf out and bloom beautifully, the leaves turn dark and fall off with the exception of the top 10% of the canopy, and then they leaf back out and look beautiful for the rest of the year. But with the addition of the weeping cherries and now my Himalayan birch, I'm not sure what is really going on. On the two ornamental fruit tree varieties, the leaves are there and then darken and fall off. On the Himalayan birch tree however, the leaves look like they're being stripped. There's just no evidence of a leaf being there anymore and the branches are starting to arch instead of being more vertical. I've looked over these trees with a fine tooth comb and can find no eggs, insects, and so I'm at a loss. In totality I have six trees in my yard that are being affected. The maples and dogwoods don't seem to be having any issues. Any idea what could be causing this?