Population increases of mountain forest birds are linked to forest regeneration after reduction of sulphur dioxide concentration in ambient air

Abstract Mountain ecosystems are fragile and susceptible to various human pressures. At the same time, mountains are traditionally considered as clean and healthy environments, but this state can be compromised by long-distance transports of ambient air pollutants. Sulphur dioxide (SO 2 ) is one such pollutant with both direct (as a toxic gas) and indirect (via the atmospheric deposition) impacts on mountain ecosystems. These impacts were particularly strong in Central European mountains in the 1980s, where high SO 2 loads emitted by large thermal power plants concentrated and resulted in extensive tree dieback in mountain forests. However, the adoption of new stringent legislation resulted in a sharp decline in SO 2 emissions towards the 2000s. At the same time, consequences of these changes on populations of mountain species remain insufficiently known. To fix this knowledge gap, we analysed unique long-term datasets on SO 2 concentrations, forest cover and bird abundance collected under standardized protocol from 1988 to 2020 in the Giant Mountains, a mountain range in Czechia, Central Europe. SO 2 emissions decreased by nearly 90% over this period, while forest cover significantly increased as previously damaged areas underwent forest regeneration. Bird abundance data showed divergent population trajectories in respect to species forest dependence: whereas populations of species most dependent on forest cover increased, populations of open habitat species declined. These patterns were most likely driven by habitat changes linked to decrease in SO 2 concentration as open-habitat species formerly occupying damaged sites lost their habitat, whereas forest regeneration increased the amount of habitat available for forest-dependent species. The influence of external drivers is unlikely because our models took species ecological traits, as well as their national population trend, into account. Since we did not prove a direct effect of SO 2 on bird annual population growth rates, we suggest that SO 2 influences bird populations indirectly via ecosystem processes.