rich: Yes. With an analog signal, you can watch a snowy picture, a skipping picture, a picture with no color, or lots of other things that indicate that your signal is poor or noisy. With digital signals, it pretty much either works or it doesn't, and noise from tree leaves in the Spring vs. bare in Winter can make all the difference in the world. Solution? Get a bigger antenna, or cut down all of the trees.
rich67, How trees affect the new digital TV signal depends also on how big and thick the leaves are too. I live in Florida the southern pine is much easier to get a signal through than other trees way north of here. Your choices are, 1) Try to get the signal through the trees, 2) Get close to or above the tree line, 3) If you own the property cut down some trees so you can get a more horizon view to the desired antenna farm.