Water availability modifies tolerance to photo-oxidative pollutants in transplants of the lichen Flavoparmelia caperata

Abstract

The hypothesis that a daily water supply allows a lichen to endure the negative effects of environmental concentrations of NO(x) and O(3) was tested with a transplant experiment. Five groups (0, A-D) of Flavoparmelia caperata samples derived from the same thalli were used for destructive, pre-exposure measurements (0), or exposed for 5 weeks in the rural collection site (A), and in a urban site with high levels of NO(x) and O(3) (B-D). Two groups (C, D) were daily watered half an hour before the daily peak of NO(x) (C), and O(3) (D). The comparison between pre- and post-exposure measurements of stress biomarkers revealed that the different thallus hydration regime modified the pollution tolerance as well as the physiology of the exposed samples. The non-watered group B suffered an evident decrease in F(v)/F(m) and reduced glutathione, but increased ion leakage, whereas the watered groups C and D showed only decreased non-photosynthetic-quenching, possibly derived from NO(x) exposure. Ozone, which was higher in the rural than in the urban site, did not significantly affect the lichen metabolism. Our results re-open the discussion on the so-called "drought hypothesis", which suggests that the lichen desert observed in urban areas of central and eastern Europe is more a matter of dry microclimate than of air pollution.

Relative effects of vehicle pollution, moisture and colonization sources on urban lichens

“…This result is supported by a recent study suggesting that urban lichens may be less sensitive to pollution than to drought stress (Tretiach et al . ). However, the relatively weak effect of vehicle pollution on species richness was unexpected given that lichen species have been classified based on their tolerances to sulphur dioxide pollution (Hawksworth & Rose ) and to nitrogen enrichment (Gadsdon et al .…”

“…Many variables other than pollution have been observed to affect lichens in their natural habitats (Smith 1921;Barkman 1958;De Wit 1976). Within a city, lichen cover and richness at a site should also depend on the local site microclimate (primarily moisture level : Brodo 1966;Tretiach et al 2012) and on the rate of influx of lichen dispersal units (both vegetative and sexual) from nearby colonization sources (Ö ckinger, Niklasson & Nilsson 2005;Werth et al 2006a,b;Johansson, Ranius & Sna¨ll 2012). Gombert, Asta & Seaward (2004) found that patterns of the Index of Atmospheric Purity corresponded with general 'human impact' rather than major pollution sources, supporting the idea that urban lichen distributions may respond to factors other than, or in addition to, major pollution sources.…”

Functional over-redundancy and vulnerability of lichen communities decouple across spatial scales and environmental severity

“…Notably, in the study area, solar radiation on rock outcrops was, on average, twofold higher than that under tree canopies, which was quantitatively consistent with the maximum FOR difference between the corresponding lichen communities. Besides the obvious effects on photosynthesis, solar radiation was considered a reliable proxy for environmental severity at the sampling sites, as it is strictly related to water availability and temperature (Díaz-Barradas et al, 2018), which are fundamental for lichen physiology, especially in semi-arid ecosystems (Giordani et al, 2014;Matos et al, 2015;Tretiach et al, 2012). Consequently, our results indicate that, in lichen communities, species tend to cluster into a few over-represented FEs increases as environmental conditions become harsher and that, under these conditions, diversity-stability relationships (the insurance hypothesis; Yachi and Loreau, 1999) do not apply.…”

Lichen elemental content bioindicators for air quality in upper Midwest, USA: A model for large-scale monitoring

“…Evaluation in section 4.1 of multiple factors linked with strong spatial complementarity of Flacap and Phyaip in our study pointed to nearby forest cover as possibly a stronger driver of the pattern than pollution load. Modern European "urban drought" studies suggested a nonpollution mechanism for absence of Flacap from cities with low forest cover (e. g. Tretiach et al 2012). This might also apply somewhat to small and isolated rural forests, and dispersal limitation, interacting with response to pollution and humidity, is another possible mechanism linking lichens with landscape pattern (e. g. Ellis & Coppins 2010) that could be further investigated for bioindicator species.…”