Disentangling climate change from air pollution effects on epiphytic bryophytes

Abstract

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.

Thermophilisation of communities differs between land plant lineages, land use types and elevation

“…Another factor that substantially affects bryophyte communities is the atmospheric sulphur dioxide load 60 which in Europe likewise climaxed in the 1980s 61 . It has been shown for southern Belgium, that changes in epiphytic communities are predominantly owed to improved air quality when considering the last four decades but that currently the regional climate better explains variation in species occurrences than air quality 62 . Therefore, and because the sulphur dioxide load in Switzerland was much less severe 63 it is unlikely that changes in the sulphur dioxide load per se significantly influenced our results.…”

Roles of Bryophytes in Forest Sustainability—Positive or Negative?

“…Bryophytes that live on tree trunks and branches (epiphytes) may, in some forests, have more roles than ground-level species, and they carry out many of the same functions [181][182][183]. They are more difficult to sustain because they receive more airborne pollutants [184,185]. In addition, when they are torn from trees, particularly for moss harvesting for horticultural uses, it is more difficult for them to recolonize [67]; many vegetative diaspores that can help a colony expand on the ground have difficulty becoming anchored on tree trunks.…”

Ecological indicator values of understorey plants perform poorly to infer forest microclimate temperature

“…Plant EIVs are widely used as proxies for environmental conditions, and to infer spatial and temporal vegetation dynamics (Diekmann, 2003; Pinto et al., 2016; Hutsemékers et al., 2023). Plant EIVs for temperature can be averaged across species within a given plant community to obtain a community mean of temperature EIV, or “floristic temperature” (also called the community temperature index; Richard et al., 2021).…”