Long-term dynamics of the iconic old-forest lichen Usnea longissima in a protected landscape

•Usnea longissima declined in all old sites, with an overall decline of 42% in 37 yr.•The total extinction rate was 0.81, with higher stochastic than deterministic extinction.•The lichen was strongly dispersal-limited, with a median effective dispersal of 3.8 m.•The decline was likely driven by air pollution, climate change and denser forests.•The long-term survival of Usnea longissima may be at risk even in protected forests.