Motivation

Lichens and mosses have been used as bioindicators for environmental pollution because many species, especially those that live on trees (aka epiphytic , from epi- "upon" + -phyte "plant), obtain all their nutrients from the surrounding air, moisture, and rain. This means that they also absorb many of the chemicals and pollutants present in the air and serve as mini living air quality monitors. How cool is that?

But before we can use these organisms as bioindicators, we must first be able to identify them. Once you learn how to spot them, you might start seeing them everywhere!

Lichen

Did you know lichen are not plants? They're actually a symbiotic relationship between fungi and a photobiont (an organism that lives on sunlight), usually algae and/or cyanobacteria. You can find them on the bark of trees, on leaves, on wooden benches, and even on rocks, and while they're usually pale gray or light green, they can come in all colors. Because they can't store water, they usually prefer shadier spots where sunlight won't immediately evaporate all their moisture. They're also sensitive to air pollution, so if you're in a highly polluted area, chances are you won't see much lichen (though just because you don't see lichen doesn't necessarily mean the pollution is off the charts; there may be other factors).

There are three main types:

Crustose (Crusty)	Foliose (Leafy)	Fruticose (Shrubby or bushy)
Crustose lichens are pressed against their substrate and can come in bright, vibrant colors like sunny yellow, orange, and red, as well as grays and greens.	"Leaves" grow parallel to substrate, and the leaf-like lobes can be flat, lettuce-like, or bumpy and ridged.	Fruticose lichens grow outwards from their substrate and can be similar to wispy hair, small cups, or shrubs.
Least sensitive to pollution	Moderately sensitive to pollution	Most sensitive to pollution

Check out these links for more examples (warning: possible trigger for trypophobia):

• OPAL Air Survey Lichen Identification Guide
• Meet the Lichen
• Leif Stridvall
• Stephen Sharnoff
• Search for Lichens by Characteristic
• Video on Identifying Lichens in the Field and with Chemical Testing

Moss

Mosses are plants with small leaf-like structures and stems, and like lichen, they tend to grow on tree branches or trunks and like moist, dark areas. Check the side of the tree that receives the least sunlight. Some species can also grow on soil, rocks, brick, and even poop! They can look like small tufts of leaves, or in sufficient quantities, they can form dense green cushions.

Check out these links for more examples:

• Lab 12 - Bryophytes
• Pinewood Plants Gallery
• CalPhotos

Tips

• Magnifying glasses can be helpful, but not necessary
• Make sure to stay on marked paths
• Algae can be confusing to distinguish from moss and lichen since they are also found on tree trunks and on the leaves of evergreen trees and shrubs. They typically appear as greenish powder on trunks.

Go Out and Explore!

You can start looking for these organisms on your next neighborhood walk or hike! Post some pictures and and share any tips for fellow lichen and moss spotters ^_^