Dan sent me your message, which I have included.
He has been documenting vegetation loss in PA for a long time now. I shall leave those details to him.
As far as the reasons for this: it is our belief that human induced stress is causing death, not only to trees but also to other life.
Ozone is only part of the problem. Earthworms are changing the soil chemistry under conifer forests so as to render them unviable. You will find many other examples in the literature.
You are, of course, aware of vegetation zones migrating northward and upward. The same is happening to animals. We have coyotes in Chicago and soon they will be in Canada. Marmots and picas will be history as they run out of altitude in the mountains.
Animals can try and migrate, but sessile plants and trees cannot. They survive by spreading seeds, but that is stopped by inexorable human trespass into their habitats, including ozone and other pollution.
So as the world changes around them, we see more and more death in our familiar landscapes.
The dying trees are supplanted by invaders from the South, and from Eurasia, fast growing primary growth, such as Ailanthus. In time, if allowed, they in turn would be replaced by secondary and tertiary growth more representative of tropical climes. But we have stressed the land to the point that humankind consumes 40% of Net Primary Production, of all the green things that grow. And I do not know what can survive for our effluents kill a great deal of what remains.
I do recommend replacing dying plants by ones suited better to the changing climate. I have planted native American prairie plants such as coneflowers and grasses, as well as Colorado blue spruce, maples and birches (in soggy areas) with good results. All we can do is replace what we can, and teach the young children what is happening, for after all, they will inherit the Earth.
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