Amid widespread concern about ash dieback, which could remove 80 million ash trees from the UK's landscape, little public attention has been paid so far to the internationally important species that will go with them.
Ash trees support a mini-ecosystem of lichens, mosses, fungi and liverworts – the colourful and mossy patches on and around old trees.
Scientists say their global environmental significance is comparable to Scotland's seabird population, and they are now under serious threat.
The British Lichen Society says the scale of the potential impact is clear from its database – it records 582 different lichens and fungi that grow on ash.
The society says: "The implications for lichens of the spread of ash dieback, a disease of ash trees caused by the fungus Chalara fraxinea, could be very serious.
"Of the different lichens that grow on ash, 220 are nationally rare or nationally scarce and 84 have a conservation status of near-threatened or above."
David Genney, Scottish Natural Heritage's expert on the subject, says the race is on to find an answer: "Attempts to propagate these fascinating organisms have been largely unsuccessful," he said. "It is very difficult to re-create the bark conditions and chemistry of a 100-year-old ash tree.
"These species in Scotland have an internationally important conservation value, particularly in the case of the west Highlands with our Atlantic rainforests, where we have some species which occur in very few other locations in the world."
He said many of the species do not have recognisable common names but are still important: "Fuscopannaria ignobilis is a lichen restricted almost entirely to ash in Scotland in the UK.
"The very rare liverwort, a type of bryophyte related to moss, called Lejeunea mandonii, is known from just two sites in Scotland, both of which are on old ash trees. It is a similar story for the lichen Leptogium saturninum and the west Highland specialist Leptogium cochleatum."
He said it is mainly the chemistry of the bark and its alkalinity that decides what species are associated with a tree.
"Several species that were already badly hit by loss of elm trees following Dutch Elm disease, found refuge on ash. Now they are threatened again in a double whammy."
He said unlike England, Scotland does not have suitable alternative native trees such as field maple, although aspen and hazel may help to some degree.
"Hazel might provide a home for some of the species, but not all of them. We are really just beginning to work out what to do and have started to talk to the universities about priority research. The obvious thing is to see what species of trees might replace the ash."
Sycamore and Norway maple have bark that several of the ash lichens can live on, but they are not native to the UK so scientists are cautious about using them.
Dr Genney said in other places where ash dieback had been rife, such as Denmark, there had been evidence of some ash trees being resistant to the disease. But establishing the science behind resistance had been difficult, because they had been felled and burned quickly.
"If we could isolate resistant trees we could plant saplings into the areas where the most important lichens under threat are located. But we would have to be very careful.
"Just because a tree doesn't die with a disease doesn't mean it doesn't carry it and we could end up spreading it. One thing we do have in our favour in Scotland is we have a lot of isolated populations of ash. We don't have the number of ash that they do in England, but as a result those we do have are more important."