Old cemeteries (pre-1900) often have gravestones that often host many types of lichens. However, it is not impossible to find old cemeteries where lichens are entirely absent. Given what is known about lichens, an old cemetery without lichens probably gets a great deal of rain, which favors the growth of competing bacteria. is subject to extremely cold winter temperatures. is close to a source of air pollution. has an unusually dry climate.

Lichens are pollution indicator hence they do not grow at a place where air pollution is high.

Explanation:

Lichens are used to absorb sulfur dioxide dissolved in water and they work as bio-indicators. In polluted air where there is a high concentration of sulfur dioxide are found, in those places we don't find the growth of lichens. In the clean air, lichens become abundant. Lichen is an association of fungus & algae. In this association, both the fungi and algae are benefitted. Algae work as photosynthetic partners & provide food to fungi while fungi help in the absorption of water & minerals from the soil. The fungal part is called mycobiont while an algal part is called phycobiont.