Wednesday 6 February 2008

Water tank

Last summer we camped at a site called Yont the Cleugh a few miles south of Haltwhistle. A nice place run by nice people. I noticed this lichen-coated water tank there. It has a distinct line above which there are no lichens. The lichens are mostly Physcia tenella with some Melanelixia fuliginosa ssp glabratula and Parmelia sulcata.

It is quite obvious that this lichen line relates to the water level inside. During dewy weather, condensation must form on the tank up to the level of the cold water inside. Lichen propagules, mainly soredia and isidia, stick to the dew and when it dries out the propagules become stuck to the tank. Clearly, many of them have managed to establish a permanent foothold in this way. There is a plentiful supply of propagules since the tank is situated underneath trees.

The label on the tank adds another twist to the story. The lichens are confined to the dark lettering and diagrams while the white areas are almost completely bare. Touching the label I could tell that there is no difference in texture; both light and dark areas are equally smooth so it must be due to condensation forming preferentially on the dark areas. This takes me back to my Physics ‘A’ level at school and Black Body Radiation. Darker objects heat up more quickly in the sun, but they also lose heat faster at night. Consequently, the dark printed areas on the label get colder and are more likely to form dew, and catch lichen propagules, than the white areas.

The astonishing thing for me is that this effect works at such a small scale, even the thin, black border lines on the label have lichens growing on them.

On the left side of the tank is an area swept clean of lichens by wind-blown nettles, except just above and below a step in the tank which protects them.

On the far right of the tank there is a lichen-free column in the shelter of the blue pipe. Does the pipe keep that bit of tank warmer which prevents dew formation? Or is there a leak that washes that area clean?

Mosses are clearly not so good at attaching to smooth plastic. The only bit of moss is growing on a ledge formed by the peeling top edge of the label.

I wonder how dew formation might affect lichen distribution in natural situations.