2 year old plant measured about 3 1/2 feet tall with it's first single flower.
USE CAUTION! There are several mining pits within this die-off area. Dangerous fumes are noted to emit from pits in other parts of the park, and they may be a sinkhole risk.
To the east and within sight of the intersection of Red Town Trail and Rainbow Town Trail, there is what appears to be sword fern - Polystichum munitum - die-off area, perhaps one hundred meters at its widest. I suspect that it is relatively recent, due to the uncharacteristically thin underbrush that drew me to it when I saw it off-trail. I would estimate that 75% of the ferns are dead in the area, though there are sections where the die-off is complete.
This area of forest is relatively young, having been decimated by a history of mining. It appears to have been colonize by birch, which have completely given over primarily to maple. There are several small pits in the area that are the legacy of mining, so use caution! At first I suspected that the dead plants could be a result of mining-related toxins, but the sword ferns were consistently thriving along the rest of the several miles of trails that I explored. I noticed one odd-looking cleavers plant - Galium aparine - growing out of the base of one dead fern; it had taken on a deep red hue that I can't recall ever having seen before.
To capture the scope of the die-off, I took two videos which I have uploaded to youtube. I have no idea why I filmed it in portrait; I beg your forgiveness.