I've recently lost over 20 green ash trees due to the emerald ash borer, and now am concerned that my American elm tree is not healthy. It's already losing leaves and has small browned branches throughout the tree, including the top. What can I do to protect this tree if anything?

Call the nearest state agricultural extension. Every state has one, and they are always very well informed and helpful. (I have a feeling that you won't be able to protect your elms, but I could be wrong. You do need to do all you can do to try to eliminate the ash borer, for the sake of the community. Most have groups that will help with that.)

You can't eliminate an ash borer. Once it establishes itself in a neighbourhood, it is there for good. Sorry for the alarming news.