Disentangling climate change from air pollution effects on epiphytic bryophytes

At the interface between atmosphere and vegetation, epiphytic floras have been
largely used as indicators of air quality. The recovery of epiphytes from high levels
of SO2 pollution has resulted in major range changes, whose interpretation has, however,
been challenged by concomitant variation in other pollutants as well as climate
change. Here, we combine historical and contemporary information on epiphytic
bryophyte species distributions, climatic conditions, and pollution loads since the
1980s in southern Belgium to disentangle the relative impact of climate change and
air pollution on temporal shifts in species composition. The relationship between the
temporal variation of species composition, climatic conditions, SO2, NO2, O3, and fine
particle concentrations, was analyzed by variation partitioning. The temporal shift in
species composition was such, that it was, on average, more than twice larger than the
change in species composition observed today among communities scattered across
the study area. The main driver, contributing to 38% of this temporal shift in species
composition, was the variation of air quality. Climate change alone did not contribute
to the substantial compositional shifts in epiphytic bryophyte communities in the
course of the last 40 years. As a consequence of the substantial drop of N and S loads
over the last decades, present-day
variations of epiphytic floras were, however, better
explained by the spatial variation of climatic conditions than by extant pollution loads.
The lack of any signature of recolonization delays of formerly polluted areas in the
composition of modern floras suggests that epiphytic bryophytes efficiently disperse
at the landscape scale. We suggest that a monitoring of epiphyte communities at 10-year
intervals would be desirable to assess the impact of raising pollution sources,
and especially pesticides, whose impact on bryophytes remains poorly documented.