Paleoecologist Margaret Davis' work has become legend among ecologists. She analyzed layer upon layer of pollen buried in lake sediments in the Appalachian Mountains to determine the natural history of trees in the area.
She found an interesting pattern:
Spruce (Picea) pollen dominated the area 12,000 years ago. Beech (specifically Fagus grandifolia) pollen appears 8,000 years ago.
The American Chestnut pollen did not show up in the sediment record until 2,000 years ago; it quickly vanished around 1920.
Cryphonectria parasitica, a type of blight (type of fungus), was transported to North America on the Chinese Chestnut, which had evolved defenses against the blight. The American Chestnut had no such defenses, and has been all but obliterated.
The blight usually only affects the trees when they reach a certain growth point, leaving the roots of the tree intact. Essentially, it attacks the sporophyte (spore bearing generation) of the organism when it reaches a certain height above ground.
This will be an interesting phenomenon for evolutionary biologists to observe over the centuries. How will the chestnut adapt and evolve to avoid the detriment of infection by Cryphonectria? Is there any way that ecologists/dendrologists can assist the population in a natural, evolutionary recovery?
There were once 3 - 4 billion indigenous chestnut trees in NA, 25% of which were concentrated in the Appalachians. Today that number is scant comparatively, though there are several organizations that are specifically supporting the chestnut, including The American Chestnut Foundation and The American Chestnut Cooperators' Foundation. Both organizations have great information at their websites (you can even buy seeds to plant with instructions on protecting the tree from infection).
Consequently, Davis continued her research across North America, and found evidence that the Appalachians were dominated by conifers during the last glaciation 18,000 years ago. It was a very different world not so long ago.
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Since the chestnut gets blighted before it can bear seeds, its chances to evolve a defense thru multiple generations of sexual recombination of genes are not good.
There is always the possiblity that a 1920 mutant had a resistance to the blight, but in a century, no thriving grove of chestnuts has been reported.
Theres the possibility that the Cryphonectria goes extinct first, having killed its host.
One thriving grove of chestnut trees escaped blight until 1988. That was not due to resistance, but to geological displacement from the natural range.
I suppose that should be "geographical", not "geological."