Dying Maples + Thriving Common Buckthorn and Gray Dogwood?

I've been on my property (zone 5a, northern catskills) for 2 years. The area is foothills--slopes and very swampy (most things I identify growing wild have "swamp" in their name if that's any indication). Trees growing include silver, red, and sugar maple, white ash, white pine, pitch pine, old apple trees (all are in decline/dying), northern red oak, bitternut hickory.

The maples are all struggling, even though they are about 30+ years old. An arborist came and drilled into a few of them for a stemx treatment last fall, but didn't give a diagnosis. One of the treated trees died over the winter, one silver maple is hanging on but had very tiny leaves this year, and the sugar maples are green but their leaves are shriveled and "crispy."

In the areas around the maples, thickets of gray dogwood are thriving, as well as some ~30ft buckthorn. The buckthorn isn't excessive--just a couple of them--so I could cut them down, but since all the other trees are struggling in the area, and they are the tallest/healthiest things I don't want to take them unless they are the problem. I really need the windbreak and the erosion control (and something sucking up the water during the spring thaw), so I figure something filling that role is better than nothing.

Any ideas on what could be troubling the maples? I see lots of standing/dead wood around this general area, so not sure if it's an overall decline, the gray dogwood or buckthorn is causing the issue, or if the buckthorn is just being opportunistic.

Any ideas or insights appreciated! The areas in question are on the north and west sides of the property and get absolutely pummeled by winter storms so I'm looking for something that will provide wind break and shelter that might compete or work with the common buckthorn.

Thanks!

I agree that seemingly few buckthorn could have a major effect across your property when considering the health of other trees like maples, but their growth and chemical aggressiveness may still be something you want to elminate, especially if you wish to plant new tree/plant species in the coming years.  See https://link.springer.com/article/10.1007/s11056-016-9551-7#:~:text=Our%20conclusion%20is%20that%20shading,control%20this%20invasive%20introduced%20shrub.  for some good information on why buckthorn is so successful at overtaking the lands it invades.  Other possibilities noted from Cornell University Extension can be found here:  https://cceoneida.com/home-garden/gardening/fact-sheets/lawn-landscape/tree-pests-diseases   Is there an extension office in your county that may have a specialist to come out and take a look at your property?.....That may be a resource that could give more site-specific information for your area.  Good luck!