News article

Old trees more important than pollarding for the diversity of lichen, fungi and mosses

Publisert 14.12.2017

Pollarded trees have a higher diversity of lichen, fungi and mosses, but this may be due mainly to the high age of these trees

Elm from Mogstad in Surnadal. Pollarded trees are among the oldest living deciduous trees in Norway today. Because of their age they are an important habitat for many species of lichen, fungi and mosses. Photo: Björn Nordén, NINA
Elm from Mogstad in Surnadal. Pollarded trees are among the oldest living deciduous trees in Norway today. Because of their age they are an important habitat for many species of lichen, fungi and mosses. Photo: Björn Nordén, NINA

Pollarding of trees is the harvesting of leaves and small branches for animal fodder. Up until World War II pollarding was a crucial addition to the animal fodder on farms all over Europe and in Norway.

Old pollarded trees are often host to rich communities of lichen, fungi and mosses. Because of this connection landowners in both Norway and Sweden can apply for funding to continue the pollarding of trees as a way of conserving biodiversity even though there is no longer a need for additional animal fodder.

Ash and elm trees in western Norway

But is it worth the cost? According to a study by Björn Nordén, researched at NINA, old ash and elm trees in the west of Norway are of particular importance in the conservation of endagered species of lichen, fungi and mosses. He has compared pollarded and unpollarded trees.

-As the old pollarded trees die we must determine whether a continued, and costly, effort to pollard young trees is necessary to replace the older trees. If the connection to lichen, fungi and mosses is mainly based on age-related characteristics the conservation of old unpollarded trees should be prioritized instead, says Björn Nordén.

""Elm from Bømlo, Hordaland. Unpollarded trees that are allowed grow sufficiently old may, in time, be able to serve the same ecological function as old pollarded trees. Photo: Björn Nordén, NINA.-The biggest thread to the communities of organisms living on the surface of old pollarded trees is that the host tree will die without there being other suitable host trees nearby. It is mainly the fact that the trees are old, and have the age-related characteristics of thicker trunks and crevices in the surface, that makes them the ideal substrate for a range of species of lichen,fungi and mosses. Björn Nordén has found large numbers of red-listed species both on pollarded and unpollarded trees, something that indicates that old unpollarded trees can replace old pollarded trees as a habitat for these species.

Old trees more important

-The biggest thread to the communities of organisms living on the surface of old pollarded trees is that the host tree will die without there being other suitable host trees nearby. It is mainly the fact that the trees are old, and have the age-related characteristics of thicker trunks and crevices in the surface, that makes them the ideal substrate for a range of species of lichen,fungi and mosses. Björn Nordén has found large numbers of red-listed species both on pollarded and unpollarded trees, something that indicates that old unpollarded trees can replace old pollarded trees as a habitat for these species.

-One conclusion is that pollarding is not crucial to maintain biodiversity of epiphytic species as long as other old trees are available in the surrounding area. On the other hand, the continued pollarding of younger trees might be important to accelerate aging and the creation of suitable microhabitats in areas where few other old trees are available. However, we need to test this further with experiments, says Björn Nordén.

Since previously pollarded trees are still among the oldest in Norway and of significant importance as host plants to endangered species, it is crucial that they are managed correctly. -Even though continued pollarding may increase survival with regard to weather, we still know very little about how pollarding of old trees might also increase mortality. This must be investigated further to better our understanding and our management of old trees and the species that depend upon them, Björn Nordén adds.

Read the article:

Can large unmanaged trees replace ancient pollarded trees as habitats for lichenized fungi, non-lichenized fungi and bryophytes?

Contact: Björn Nordén

 

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