Apparently, Usnea are sensitive to SOx pollution, and that has dropped greatly (over 90%) in Finland since 1990’s (early 90’s would be about when I was taught this, so that’d explain). NOx is the second one, but not as bad. High smokestacks have also made the pollution much more diffuse.
To quote her on current limitations: “As a result the occasional Usnea thallus can now be found far into cities in many places again. The limiting factors will often be availability of suitable habitat, dispersal, and agricultural ammonia pollution especially in areas with intensive animal husbandry.”
It was really interesting to update what I know.