Excerpts from Jim Conrad's
from the February 23, 2014 Newsletter issued from the Frio Canyon Nature Education Center in the valley of the Dry Frio River in northern Uvalde County, southwestern Texas, on the southern border of the Edwards Plateau, USA
On a stem of a Mesquite tree standing alone on the valley floor several lichen species had found a home, and one of them looked new for us. On the dark stem it presented itself as a smooth, white patch only about the size of a thumbprint. The center of the white patch was cracked into irregular blocks and each block bore one or two black dots, as shown above.
Several lichen species appear in that picture. Clearly there's fierce competition for twig space. If you study the picture you can see where other lichen species are encroaching onto our subject's body, and where our subject appears to be expanding itself. A close-up of is black-dotted center blocks is shown below:
Among most easily visible, or "macroscopic," lichens the typical fruiting body producing sexually derived spores is a cuplike structure known as the apothecium. This lichen's black dots are apothecia, but they're unlike most apothecia in that they're less like cups or bowls than blisters with holes in their tops. This is unusual, and it's a good field mark for this species. Another good field mark is that the lichen body and its apothecia are so tiny. The apothecia holes are at most only 0.8mm across (1/32nd inch).
This species works out as PERTUSARIA PUSTULATA, sometimes known as the Pore Lichen, occurring on tree bark in Temperate Zones worldwide. Lichens living on tree bark are said to be "corticolous."
On the Mesquite tree, Pore Lichen occupied only twigs that were three or so years old. It was absent from branch tips, as well as branches thicker than approximately a pencil. Larger branches were occupied by bigger lichen species, especially of the foliose and fruticose types, which easily grew over our low, humble Pore Lichen.
Whenever confronted with a tree loaded with lichens, it's fun to notice how the various species specialize in particular niches.