I too watched the trees on my beloved mountains die off, leaving the ridgetops looking forlorn. That was in the 80's, from the gypsy moth infestation. In the 70's, my grandfather proudly took us all to the top of his mountain to see a Chestnut tree in bloom, the first in decades. He remembered when the forests were mostly chestnut; they were wiped out by the blight in the first half of the last century.
Change is a natural part of the environment. Species rise up, become strong, then are knocked down by something else trying to become strong. This isn't new; it didn't start in the last 100 years, and it's not all the fault of humans. Species (plants, animals, viruses, molds as well as people) are constantly trying to expand their foothold on their territory. They battle each other for precious air, water and land, and there are always winners and losers.
Don't buy into the environmentalists view that all the changes we see around us are unnatural and caused by humans. Their real agenda is to control us.