Nitrogen loving (nitrophilous) lichens grow well in situations where there are relatively high levels of nitrogen compounds. Large quantities of nitrogen compounds enter our atmosphere from vehicle exhaust, through artificial agricultural fertiliser input (and subsequent decay into air bore pollutants) and from farm animal waste in areas where livestock (including birds) are raised in high numbers. Nitrophilous lichens are able to colonise and grow rapidly on surfaces affected by these man-made nitrogen sources. they are able to outcompete lichens that are nitrogen hating (nitrophobic) and those that can tolerate high nitrogen levels but continue to grow slowly. Three nitrophilous lichen species are now among the most abundant lichens in Britain. With all three easily seen on Whiteknights Campus. The tree trunks along Queens Drive (the Sports Park entrance to campus) are good places to start looking see images 1 and 2.
The three species:
This is a bright yellow species. It has broad yellow lobes see image 3 and a large number of fruiting bodies (apothecia) that have a yellow margin and an orange “disc” in the centre see image 4. Other yellow lichen species on campus either have narrow lobes, no lobes (the whole lichen is a powdery haze) or have lobes but no apothecia. When growing on twigs X. parietina can grow all around the circumference. When it is growing in shade beneath the twig, the lobes become grey, sometimes with a yellow edge see image 5 . This occurs because the sun screen effects of the yellow pigments are not required in continually shaded situations.
The other two nitrophilous species found on campus are difficult to tell apart. They are both species of Physcia, Physcia tenella and Physcia adscendens. Both have narrow lobes that sometimes remain flat against the substrate they are growing on, but often grow in a more bushy fashion see images 6 and 7 for P. adscendens and images 8 and 9 for P. tenella growth forms. Until the last ten years, these two species rarely fruited in Britain. However, fruits can now be found and although not unusual, this is still an uncommon occurence. Fruits on both species are apothecia with grey margins and a black disc, see image 10 for P. adscendens and 11 for P. tenella. The apothecia can have distinct stalks. These two species normally reproduce asexually. Each species produces soredia, small flour-like grains that each contain a few algal cells wrapped in fungal hyphae. These are usually slightly paler than the lichen thallus (main lichen body). The two species are distinguished by where the soredia have developed (without DNA testing young specimens of the two physcia could not be told apart).
P. tenella produces soredia on the underside of the leading edge of its lobes see image 12. These edges curl up slightly, so that the soredia are exposed to air currents, insect movements and other disturbance events that may transport the soredia to a new location.
By contrast, P. adscendens produces its soredia inside a swollen lobe tip see image 13. These swollen tips rupture to produce a shape commonly described as a “helmet”. Although its soredia are not as well exposed to the air as P. tenella, P. adscendens seems to spread just as rapidly as its sister species.
X. parietina. P. tenella and P. adscendens are not substrate specific (i.e. need a specific surface). Although, often commonly found on bark, they are equally easy to find on suitable rock surfaces. On campus, they are common on vertical concrete surfaces see image 14, but also grow on other undisturbed situations such as painted windowsills see image 15. Further to this, all 3 of the nitrophilous species can often be found growing together, see image 16.
I would like to acknowledge that this blog was written by Fay Newbery, who takes all credit for this excellent short publication, on lichen species identification.