(Does the lichen grow on the north side of the lamp post?)

Today’s blog is the result of John’s sharp eye and healthy exercise.  Riding his bike socially distanced around his neighborhood, he noticed something striking:   that multiple species of  lichens on the lamp posts are 100% confined to the north edge of the post.

North is on the left. Photos by John Bradford.

By contrast, when you look at the “moss” growing on a tree trunk…real moss, algae, lichens, liverworts, fern gametophytes…all the growth is scattered around the trunk irregularly and in response to such variables as sun and shade,  water flow patterns, lawn sprinklers,  bark texture, nooks and crannies in the tree, competitors, and who knows what else.   It is fun to try to figure these things out.   You often find lichens on the brighter exposed sunnier ridges, algae and mosses lower and shadier, and liverworts in “waterfalls” where branch crotches funnel water running down the trunk.

Lichens on a bald cypress, by JB.

But John’s experience resembles a well designed experiment with variables suppressed, a uniform surface, and under that circumstance, the lichens show their true predilection…north side, period.

Examining the poles in the “heat of the day” we noticed the sun lighting up the south, east, and west sides, leaving  a shaded vertical stripe inhabited by lichens of the north face. Like me under my beach umbrella. A quick look at a website showing shadow patterns year-round showed…during the hot hours…the north to be shaded all year.   Landscape architects know this.   We used to do a lesson in landscape design class showing that some points in the yard virtually never experience direct sun during the bright hours.

The poles are black fiberglass (or have fiberglass sheath covering something deeper within). How different were the two sides of the poles on a sunny afternoon with the air temperature 78 degrees F?   We checked several poles, on the sunny sides in degrees F: 115, 113, 113, in contrast with the  lichen-zones: 91, 93, 89.

Just for interest, we checked a palm trunk in the sun. The south side was an arctic 93, and the north side was 86,  which might help explain lichens surrounding the palm in contrast with the light posts.

The absolute absence of life on the poles with surfaces exceeding 100 degrees as opposed to plenty of lichens with temperatures in the 90s agrees with a broad general perception that at about 95 degrees life becomes supremely stressful in terms of cell membrane dysfunctions and protein denaturation (damage).   With obvious highly adapted specialized exceptions… cacti in Death Valley, bacteria in hot springs,…sustained unmitigated temperatures over 100 could be predicted to filter out most life,  even tough lichens.