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

Lichen doesn't have any roots - all the nutrients it needs are obtained from the air.

One of the best indicators of air quality is Old Man's Beard (Usnea longissima) - a wispy lichen that hangs from trees and plants. It only grows in unpolluted air. When Old Man's Beard starts dying off it's often the first indicator of declining air quality.