My Corylus contorta has greenish yellow lichen on the branches. Should I be concerned and if so what should I do?
The lichen will not harm your Corylus contorta. Lichen is actually a sign that the air is relatively unpolluted.
You may find this page of discussion from University of British Columbia
Botanical Gardens of interest.
Here is an excerpt from a 2007 entry formerly available on the site of Treelink, now part of the Alliance for Community Trees:
“No need for a preemptive strike against the lowly lichen. Lichens are
composite, symbiotic organisms made up from members of as many as three
kingdoms.
The dominant partner is a fungus. Fungi are incapable of making their own
food. They usually provide for themselves as parasites or decomposers.
“Lichens are fungi that have discovered agriculture”– lichenologist
Trevor Goward.
“The lichen fungi (kingdom Fungi) cultivate partners that manufacture food
by photosynthesis. Sometimes the partners are algae (kingdom Protista),
other times cyanobacteria (kingdom Monera), formerly called blue-green
algae. Some enterprising fungi exploit both at once.
Most lichens grow very, very slowly, often less than a millimeter per
year, and some lichens are thought to be among the oldest living things
on Earth.
“Lichens are important in many ways in the habitat. Some make
the nitrogen in the air usable to plants, They are homes for spiders,
mites, lice, and other insects. All are important in the nutrient cycle
in the places where they grow. Many lichens are very sensitive to
pollution in the air. When there are too many harmful things in the air,
lichens die. If you live where there are many lichens it probably means
the air is clean. But, if there are only a few lichens in your
neighborhood, the air you are breathing is probably clogged with
automobile fumes or industrial wastes. Some trees and shrubs can develop
a layer of fungi, algae, lichens or moss on their bark. These are
non-parasitic organisms and do not injure the plants on which they grow.”