What role have changes in climatic conditions and atmospheric pollution played in the spatio-temporal variation of epiphytic bryophyte communities?

Due to their ecophysiological characteristics and their position at the interface between the atmosphere and vegetation, epiphytic bryophytes have long been used as indicators of air quality. The spectacular recolonization of these organisms over the last twenty years has paralleled the drastic reduction in SO2 concentrations. However, the concomitant variation in the concentration of other major pollutants (NOx, NHx, O3, fine particles) as well as climatic conditions complicates the interpretation of the factors responsible for the observed floristic changes. We carried out a diachronic analysis since 1980 to separate the impact of climatic conditions and concentrations of major atmospheric pollutants on the historical variation in floristic composition. This analysis was complemented by an assessment of the contribution of major pollutants and pesticides on the spatial variation of flora observed today. The results show that temporal changes in floristic composition within the same community are more than twice as great as spatial changes in composition between communities at present. This temporal variation in flora is largely explained by the decrease in SO2 and NO2 concentrations, but not by variations in climatic conditions. The absence of a relationship between historical concentrations of major pollutants and contemporary variation in flora at regional scale suggests that epiphytes have sufficient dispersal capacity to recolonize the areas that are favorable to them within a few decades. Our analyses have shown that the drastic reduction in major atmospheric pollutants has enabled the emergence of a contemporary segregation of floras based on regional variation in climatic conditions, even though other pollutants, particularly fine particles, are playing an increasing role.