Lichen as a air pollution indicator ?

Question

I wonder why lichen is used to determine the SO2 polluting level, according to their morphology, population.......
But not other plants, such as tree, grass...... which grow on the road side?

Answer 1

Because they are better at it:

"Lichens are sensitive to air pollution, especially sulphur dioxide produced by the burning of fossil fuels. The types of lichens present in any environment can indicate the purity of the air. Through their absorption of heavy metals and radioactive compounds, lichens may indicate environmental contamination."

Answer 2

They are slow growing and highly sensitive so can show how pollution has changed over several centuries. SO2 in rain water is acidic which results in flat lichens and kills off spores which need rain water for distribution (The spores have 2 sections algae and fungal which need to be deposited together for a lichen to grow, acid breaks that bond so fewer lichens grow) In cleaner conditions you get lots of fronds which increase surface area.