Most people link them to signs of a rotting or ageing tree. But lichens, mosses, fungi and liverworts represent one of Scotland’s ecological wonders, now threatened by disease, deer and alien plant species.
The global environmental significance of these colourful and mossy patches on and around old trees, is comparable to country’s seabird population, according to scientists.
It is estimated almost 600 different lichens and fungi that grow on ash trees alone.
Ecologists are deeply concerned over how they will be affected Chalara fraxinea, or ash dieback disease, that threatens to remove up to 80 million ash trees from the UK’s landscape.
Dr David Genney, Scottish Natural Heritage’s (SNH) Policy and Advice Officer on bryophytes, lichens and fungi, said ecologists are keeping a nervous vigil.
He added: “We haven’t observed any loss of rare or internationally important bryophytes (small plants that include mosses and liverworts), as a result of ash dieback in Scotland so far.
“This was to be expected though because most of the species we’re concerned about are associated with mature and veteran ash trees rather than young trees. While ash dieback can infect both young and old trees, young infected trees die faster than old trees.
“The concern for these species is longer-term because if the disease impacts trees as elsewhere in Europe, mortality of young trees is so great that it is unlikely that mature trees will be replaced once they die. So, it is the long-term loss of mature trees from our landscape that is a concern and it may take decades before the impacts on associated species are detectable.”
He said there were concerns following the recent discovery of ash dieback in Glen Nant, a Site of Special Scientific Interest and National Nature Reserve, south east of Oban, Argyll.
The area is thought to have been wooded for thousands of years.
Dr Genney added: “The species of bryophyte and lichen for which Scotland has greatest international responsibility tend to occur in west Highlands in our Atlantic temperate rainforests, where we have some species which occur in very few other locations in the world.
“Until recently these woodlands were thought to be in a disease-free zone, however a recent increase in surveillance has discovered the disease in a number of western localities, including some that are very important for bryophytes and lichens such as Glen Nant and on the Morvern Peninsula. We don’t know how widespread the disease is yet, or whether we will see the same high mortality of young trees, but the long-term outlook for species that depend on mature ash is worrying.”
He said there were other causes of concern for species dependent on mature ash trees.
“Ash is a fairly palatable species, favoured by deer. Some woodlands have therefore not seen successful ash regeneration for many years due to over-grazing. In addition, while great progress has been made in some areas, invasive Rhododendron continues to threaten bryophytes and lichens in many areas though the dense shade it can cast over these light-demanding species.”
A Forestry Commission Scotland spokesman said: “We are continuing to work hard at monitoring and managing ash woodlands in an effort to slow down the disease’s rate of spread.
“Since finding Chalara in the wider environment at Glen Nant, we are working with others to consider the best approach. We have started a process of reviewing the Chalara Action Plan for Scotland and this was a major part of the discussions at the recent meeting of the Scottish Tree Health Advisory Group.”