Does air pollution still impact epiphytic bryophytes in the post acidic rain era? Insights from spatial variation of community composition in southern Belgium.

Since the dramatic air pollution peaks that prevailed in the course of the 20th century in Europe, effective environmental policies, along with major shifts in fuel usage, resulted in the substantial decrease of SOx and NOx pollution. At the interface between atmosphere and vegetation, epiphytic bryophyte floras responded by massive back-colonisation of formerly polluted areas. Whether extant concentrations of these pollutants are indeed too low to impact species distributions, and whether other pollutants today play a more important role remains, however, an open question. Taking advantage of an air quality monitoring network for a wide range of pollutants in southern Belgium, we implement here variation partitioning and embedded covariate selection to assess the contribution of current air pollutant loads to variations in epiphytic community composition relative to that of background environmental factors. Factors accounting for variation in species composition included, by decreasing order of importance, background environmental factors, major air pollutants (SOx, NOx, O3, fine particle matter), pollutants of agricultural origin (NH3, pesticides), and heavy metals. The substantially larger role played by background environmental factors over air pollution points to the efficiency of air pollution reduction policies, even for such sensitive organisms as epiphytic bryophytes. Ozone was the most important pollutant. Its ecophysiological impact on cryptogamic epiphytes remains poorly known, and the difference of its concentrations between urban and rural areas suggests that it may actually be interpreted as a land-use marker. Pesticides, whose impact on epiphyte floras was not previously assessed, marginally contributed to community composition and species distributions.