Many of you may have heard that lichens are indicators of clean air because they don't grow where it is polluted. There has been quite a lot of research to show how sensitive lichen species can be used to monitor air quality. What is less well known is that other species are capable of bio-accumulating significant pollutants and living in situations that are eutrophic (overly rich in nutrients).
Some lichens are indeed very sensitive to pollution, but others positively suck it up. Species belonging to the shield lichen family Parmeliaceae are very happy to grow where there is excess nitrogen, and rural roads like the one pictured are quite good habitat for them. The shield lichens are the pale blotches. The yellow patches are some other type of lichen I haven't identified.
At least one of the common shield lichen species is an ingredient in Chinese Traditional Medicine concoctions to treat erectile disfunction. Unfortunately, they've recently been discovered to contain a cytotoxic ester, which means that they cause cells to die. They can also grow so thickly on road surfaces that they cause slippery unsafe conditions, especially for motorcyclists.
The nitrogen cycle globally is changing, due to human activity, mainly the increased use of fossil fuel powered vehicles. Nitrogen oxides (NOx) are some of the most important roadside air pollutants and one of the ingredients of smog. In rural areas a significant contributor to the NOx problem is excess fertilizer applied to arable land over the last 50+ years. The shield lichens absorb far more than they need when they are in eutrophic environments and this can be measured to quantify the levels of pollution.