lichens on Lisbon's trees

Fungi cannot produce their own food, but sometimes they host algae or cyanobacteria that share carbohydrates from photosynthesis in exchange for water and nutrients. This mutually beneficial relationship is called a lichen.

In my neighbourhood there is an avenue of mature nettle trees in the neglected park of a former royal palace. Pollution is relatively low due to surrounding trees and distance from roads, there is ample sun, and the trees are undisturbed and healthy. Shield and sunburst lichens cover their trunks and upper branches.

Lichens are rare on the nettle trees that stand on either side of the busy road a short walk from the park. But one small colony has established itself at a crossroads and lichens live somewhere on almost every tree.

A few streets over there is a row of lime trees beside the fence of another park. Their bark is covered in shield lichens. The trees in the park itself, including the limes, are bare except for a few nettle and palm trees.

I saw no lichens on the pines. This may be because they have slightly acidic bark. None grew on the plane trees either, which regularly shed their bark.

Nettle trees have relatively smooth bark but time, moss and the lichens themselves create irregularities in the bark which trap water and dust. This may contain calcium from construction materials which further alkalinises the microhabitat favouring nitrophilous urban lichens.

Conifers in Lisbon seem to host mainly leprose lichens, which appear as a fine dust. Other common trees such as olive, nettle, and lime often carry patches of crustose lichens, which form a thin crust, and foliose lichens, which are crinkled and leaf-like. Lichens need sunlight and water, so they thrive near the crown, but they also cover trunks and lower limbs that are exposed and undisturbed.

Although lichens seem to favour certain trees, this is due to conditions rather than the species itself. Their attachment to a surface has little to do with whether it is living. They grow just as well on rocks or walls.

They are most likely to be found where there are other lichens, since many reproduce asexually, either by a fragment breaking off and growing independently or by dispersal of a small clump of cells.