Tree lichens thrive better on dead wood

Wicklow People (West Edition) - - LIFESTYLE - JIM HURLEY’S

VERY slow progress was made the other day while I was cut­ting down the re­mains of a dead oak tree as I kept get­ting dis­tracted by the di­ver­sity of mosses and lichens grow­ing on the dead bark.

Lichens are amaz­ing plants. Mark Seaward’s cen­sus cat­a­logue lists 1,134 species recorded grow­ing in Ire­land, so they are a pretty sig­nif­i­cant as­pect of our nat­u­ral her­itage. While they are re­garded as in­di­vid­ual ‘plants’ and ‘species’, lichens are unique in that they are the only known life forms on the planet that are com­posed of two or three dif­fer­ent or­gan­isms liv­ing to­gether.

The two con­stituent or­gan­isms that form most lichens are ei­ther a fungus and an alga or a fungus and a bac­terium. Some­times all three live to­gether: fungus, alga and bac­terium. The two or three con­stituent or­gan­isms live to­gether in a close and in­ti­mate bi­o­log­i­cal re­la­tion­ship.

With so many species there are sev­eral dif­fer­ent do­mes­tic ar­range­ments, but the gen­eral rule is that the al­gal part­ner in the re­la­tion­ship makes food us­ing sun­light in the same way that grass and all other green plants grow. Pho­to­syn­the­sis is the chem­i­cal re­ac­tion that drives the con­tin­u­a­tion of so much of live on Earth.

What the fun­gal part­ner brings to the re­la­tion­ship is its abil­ity to ab­sorb nu­tri­ents from the en­vi­ron­ment. Some­times the fungus has to ab­sorb nu­tri­ents ei­ther di­rectly from the air or from rain­wa­ter when the lichen is grow­ing in such a challengin­g desert en­vi­ron­ment as the bare ver­ti­cal rock face of a head­stone in an old grave­yard.

Tree lichens thrive better on dead wood than on the wood of a live tree. The most abun­dant species on my dead tree was the very com­mon Ham­mered Shield Lichen, the one pic­tured above. It is an eas­ily iden­ti­fied species. Its body is made up of over­lap­ping leafy lobes each with a squar­ish end. These lobes are a very pale sil­very, bluish-grey colour above and a dark trea­cle-brown colour un­der­neath.

The dark colour un­der­neath borders the pale lobes giv­ing them dis­tinc­tive brown tips. Its fruit­ing bod­ies are shield-shaped. The leafy lobes have a net­work of sharp ridges and de­pres­sions giv­ing the lichen a ham­mered ap­pear­ance, hence its English name. Ham­mered Shield Lichen comes in a num­ber of forms. It is very com­mon and widespread and since it is one of the lichens that is very tol­er­ant of pol­lu­tion it is no stranger to tree trunks in parks and gar­dens in built-up ar­eas, vil­lages and towns.

Ham­mered Shield Lichen is a very com­mon lichen that grows on tree trunks.

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