Around the year 1920 is when the aspen trees in Yellowstone stopped regenerating.
Since the 1920's for around 70 years the aspen trees in Yellowstone have not done well and for awhile they have not been regenerating.
However as of 2020 I've heard that the aspen trees are making a comeback and so we may see more aspen trees regenerating again.
I haven't been to Yellowstone in a few years though so I cannot say from personal experience about the aspen trees.
But from what I've heard and read online the aspen trees are now regenerating and making a comeback after all those years.