Nature Notes
by Bob Thomas

Lichens are a common life form, and they are diverse and abundant in coastal Louisiana.

The grayish-green thalli (the vegetative body of the lichen) of our most familiar forms grow on most species of trees. Live oaks always have a healthy population, and some, like the one shown in the photos to the left, may be profusely covered.

Unless one is a lichenologist, it is easy to overlook lichens' presence, especially when driving past stately oaks in urban areas.

Local biologist and artist Susan Norris-Davis recently made an observation that required the keen eyes that typically accompany her talents.

Palmer Park is located at the southwest corner of Claiborne Avenue and Carrollton Avenue in New Orleans (New Orleanians would say "Uptown river corner"). During Hurricane Katrina in 2005, Palmer Park was severely flooded. Carrollton Avenue has considerably subsided near the Claiborne intersection, and the water there was about four feet deep.

During the flooding, live oak trees along the Carrollton side of the park soaked in brackish water for days. One of the most shocking aftereffects of the massive flooding of the city was the destruction of vegetation - everything was brown for months until the weeds took over and people returned and began resodding their lawns. Live oaks did well, evidence that they have adapted well to the hurricane belt over the millennia.

Norris-Davis noticed the impacts of the brackish water on the live oak lichens. All lichens that were submerged died, while those above the flood waters survived. Lichens are obviously extremely sensitive to the salt content of the flood water. The Palmer Park oaks look like someone took a razorblade, cut around the trunk and removed all lichens below the cut. The water line is striking.

Lichens will grow back on the oaks over time. In fact, on some trees there is evidence that the barren bark is now being recolonized. The types of lichens that grow on oak trunks grow at a rate of between 0.5 -4 mm (there are 25 mm in an inch) per year. At this rate, you have time to visit the park and see this most interesting phenomenon.

Live oaks always have lichens on their bark,                               Base of the same tree on the left.
and some have massive colonies.                                                Photo by Bob Thomas.
Photo by Bob Thomas.

Live oak on the Carrollton Street side of                                     Live oak on the Claiborne Avenue side of
Palmer Park, New Orleans - June 3, 2009.                                   Palmer Park, New Orleans - June 3, 2009.
Photo by Bob Thomas.                                                            Photo by Bob Thomas.