The importance of Aspens for lichen

The Biodiversity and Management of Aspen woodlands: Proceedings of a one-day conference held in Kingussie, Scotland, on 25th May 2001

Les and Sheila Street
c/o Flaxfield, High Road, Havenstreet, Ryde, Isle of Wight, PO33 4DL. Email: Les.Street@espb.org.uk

Aspens differ greatly, both structurally and chemically, from the majority of Highland woodland trees and in particular from Birch and Scots pine. In Strathspey, especially, they provide a very important niche substrate for epiphytes that is otherwise virtually absent. One hundred and thirty species of lichen and 12 lichenicolous fungi have been recorded on Aspen in Strathspey (Table 1).

The bark of Aspens can be both fissured and smooth. The characteristic 'diamond-shaped' rough parts provide a coarse substrate favouring some species such as the 'strap-like' green Ramalina spp. some of them up to 25cm long. The smooth areas host a different suite of lichens such as Lecidella elaeochroma and Pertusaria or Arthonia spp. which are crustose. Older trees tend to be more rough and fissured, and bark on the oldest can even superficially resemble that of oak. Scots pine, for example, cannot host these species and birches and pines typically hold the genera Bryoria, Usnea and Hypogymnia, which, themselves, are less common upon Aspens.

Different parts of each Aspen can also provide a variety of microhabitats. On the lower parts of the trunk, usually overgrowing bryophytes, are lichens such as Peltigera membranacea, a 'dog-lichen', so-called because the Velcro-like rhizinae attaching it to its substrate are said to resemble dogs' teeth. At the other end of the tree, small lichens such as Rinodina sophodes subsist on small twigs.

The most significant feature of Aspen bark is its low acidity. Whereas Scots pine and Birch are typically around pH 3.2-3.5 (ranges respectively 3.4-3.8 and 3.2-5.0), Aspen bark is pH 5-6 (Mikko Kuusinen, Annales Botanici Fennici, 1994). Interestingly, some Strathspey Aspen clones host lichen epiphytes more typical of acidic substrates, while others hold those favouring more neutral or alkaline conditions. Further research may reveal the reason for this being natural variation in bark pH. There is also evidence that Aspens probably provide a naturally enriched substrate. Therefore, species that prefer nutrient-rich, basic habitats grow on them such as Xanthoria parietina (probably the best-known British lichen occurring as orange splodges on most old asbestos roofs) and Physconia distorta, a whitish-grey placodioid species with pruina that resemble caster sugar. Both of these have prominent jam-tart shaped apothecia (fruiting bodies). It can be noted that the Xanthoria plants show a preference for the western facing sides of Aspens and one method of quickly spotting Aspen stands from afar is to look for any tree sporting orange lichens.

The longest established stands, as with most woodlands, are those which have the richest lichen assemblages. At Invertromie (Strathspey), the Aspen seminar study area, there appear to have been a succession of pulses of vigorous Aspen regeneration. One is presently underway following some fencing to exclude deer and rabbits. Another occurred in the late 1980s when sheep grazing ceased and at least two others; one around 30-50 years ago and another, earlier one, are evident resulting in the varied age structure seen today.

Lecanora populicola © Les and Sheila Street This Red Data Book lichen had not been recorded in Britain for 150 years and was thought extinct, until it was discovered on Aspen at the RSPB's Insh Marshes Reserve in 2001.

The most species rich Aspen stands hold some very scarce and endangered lichens. The Aspen seminar field trip study area at Invertromie, for example, revealed one new UK lichen species Arthonia patellulata, another not seen for over 150 years with the apposite specific name Lecanora populicola and two more lichens, each confined to less than 20 UK sites. One is Sclerophora pallida, a vulnerable Red Data Book (RDB) lichen resembling minute ginger-coloured pinheads which is found on the dry under-hangs of boughs and trunks. The other a RDB (vulnerable) and 'Schedule 8' (legally protected) rarity called Pannaria ignobilis. This has an oddly disjunct distribution from Norway to the Mediterranean and is found locally in central Scotland.

Another interesting feature is the presence of several lichens normally considered more 'oceanic' or 'western' in their distribution. Examples include Pannaria conoplea, Degelia plumbea and the large, leafy lungworts Lobaria pulmonaria and L. scrobiculata which are relatively common along the western seaboard of Scotland from Argyll to Assynt, but scarcer well inland amid the central Highlands of Scotland (F Dobson, Lichens [distribution maps], 2000). These species and others demonstrate yet another important feature in that they are all closely associated with, and some confined to, ancient woodlands. Ecological indices have been developed defining lichen species linked to native pinewoods and western Scottish broadleaved woods and it may be possible, in future, for a similar version to be developed providing better evaluation of the ecological continuity of Aspen woods. It is clear already from the range of bryological, lichenological and entomological taxa present, that many Aspen woods show strong evidence of very long ecological continuity.

Each taxonomic group speaker used the Aspen seminar as a forum to make a plea for feedback from delegates and lichens are no exception: an extremely rare species resembling a small dark pinhead about a millimetre long called Phaeocalicium praecedens apparently exists only on Aspen twigs. If something resembling this is found, please collect and send a small sample specimen provided there is plenty locally present to Sheila Street.

Caloplaca flavorubescens © Les and Sheila Street The base-rich bark of Aspen provides a suitable medium for many rare and threatened mosses and lichens. Strathspey is now the best area in Britain for Caloplaca flavorubescens, a rare Red Data Book lichen.

The study of lichens associated with Aspens in Britain is in its infancy. Only recently, Brian and Sandy Coppins discovered the first UK occurrence of a species called Bacidia igniarii on Aspen. Further research in Strathspey since the Aspen seminar has revealed a yet more diverse lichen flora living upon Aspens including two more new species to the UK: Caloplaca ahtii and (once it is confirmed) Rinodina laevigata plus the best UK population of the attractive, and now rare, RDB critically endangered Schedule 8 lichen Caloplaca flavorubescens. This research is also beginning to provide insight into the processes involved with this special and complex symbiotic association. Judging by the diverse range of specialist epiphytes or invertebrates that these Aspen stands sustain, they are evidently ancient ecosystems requiring further study.

Further reading and key references

Coppins, B. and Coppins, A. M. 2000. Thoughts on aspen and its present and future role as a habitat for other organisms (unpublished).

Coppins, B. and Coppins, A. M. 1990. Forest of Glentanar - Lichens (unpublished).

Coppins, B., Street, S. and Street, L. 2001. Lichens of Aspen woods in Strathspey. Unpublished report to the British Lichen Society and Scottish Natural Heritage.

Dobson F. S. 2000. Lichens. Richmond Publishing.

Hedenås H. and Ericson L. 2000. Epiphytic macrolichens as conservation indicators: successional sequence in Populus tremula stands. Biological Conservation 93: 43-53.

Hereford and Worcester County Council incentive tree planting scheme leaflet (1987) The Black Poplar.

Humphries C. J., Press J. R., and Sutton, D. A. 1981. Trees of Britain and Europe Country Life Books.

Kuusinen M (1994) Epiphytic lichen flora and diversity on Populus tremula in old growth and managed forests of southern and middle boreal Finland, Annales Botanici Fennici 31:245-260.

Moberg R and Holmåsen I (1990) Lavar, Interpublishing AB, Sweden.

Nature Conservancy Council. 1988. Keys to woodland NVC vegetation communities

Ripple W and Larsen E. 2000. Historic aspen recruitment, elk and wolves in northern Yellowstone National Park USA Biological Conservation 95: 361-370

Steven H M and Carlisle A (1959) Native Pinewoods of Scotland. Hartnolls.

Trees for Life 2000. Caledonian Forest Species Profile - Aspen.

Lichenicolous fungi (all on lichens growing on Aspen in Strathspey)

Arthonia subfuscicola in apothecia of Lecanora carpinea, not previously recorded in UK since 19th century

Arthonia sp. in apothecia of Lecanora populicola. Similar to A. intexta in having 1-2-septate ascospores

Dactylospora parasitaster associated with Biatoridium delitescens

Laeviomyces pertusariicola on Pertusaria leioplaca

Lethariicola sp. on Pertusaria coronata. Possibly the same undescribed species as previously found in Scotland on Pertusaria hymenea

Lichenodiplis lecanorae on Caloplaca cerinella and Lecanora persimilis

Muellerella lichenicola on Tephromela atra

Phaeosphaerobolous alpinus on Lecanora carpinea

Phoma physciicola on Physcia stellaris

Stigmidium congestum in apothecia of Lecanora chlarotera

Stigmidium pumilum on Physcia aipolia

Vouauxiella lichenicola on Lecanora chlarotera

Other microfungi on Aspen

Amphisphaerella dispersella on Aspen bark

Dasyscyphius corticalis on Aspen bark

Hysterographium elongatum on Aspen lignum

Lahmia kunzei on Aspen bark

?Melaspilea cf. proximella on Aspen bark

Teichospora sp. on Aspen bark

Notes on Priority lichens and species new to the British Isles

Arthonia patellulata	Apparently the first correctly reported finds of this 'Aspen specialist' in the British Isles. Previous records have proven to be other species, although there is a 1968 record from Braemar that may be correct.
Bacidia vermifera	Status: RDB(CE). Previously recorded only from two 10km squares in Britain, both in Strathspey, one of which was Abernethy in 1980s. The finds during this survey increases the number of squares to four, and the first UK reports from Aspen.
Biatoridium delitescens	RDB(V) species, previously recorded from six 10km squares (four of which are Scottish). Not previously reported on Aspen from the UK.
Caloplaca ahtii	A recently described lichen from Fennoscandia and Alaska, where it is mainly found on Aspens. All the Scottish material is without apothecia.
Caloplaca flavovirescens	RDB(CE) and Schedule 8 lichen sparsely known from only a handful of isolated wayside trees, mostly Ash. Widely recorded in the 19th century, but now declined almost to extinction. Clais Eich and another site near Rothiemurchus are believed to be easily their best UK locations.
Catinaria neuschildii	RDB(V). The find at Kinchurdy is the fifth 10km square record in the UK, and the first from Aspen. Previous records are from Juniper and Oak.
Lecanora populicola	Until this survey RDB(EX). Not recorded in UK for over 150 years when it was last seen at Coltishall, East Norfolk. Seen at four sites.
Pannaria ignobilis	RDB(V) and Schedule 8. The discovery on a single old Aspen at Invertromie is the first and only record from Strathspey, and the first in the UK from Aspen. Its main populations are in the Great Glen and Strath Glass.
Rinodina laevigata *	If the identity is confirmed, this will be its first Scottish record and the first in Europe outwith Fennoscandia.
Schismatomma graphidiodes	This internationally rare RDB(V) and Schedule 8 species has its known world headquarters on the Oaks at Cawdor Wood near Nairn. Otherwise, it is known from only a handful of scattered, mostly Scottish localities.

Lichens on aspen - related pages

• The Lichen Ecology of Aspen Woods – A Preliminary Analysis

Return to The Biodiversity and Management of Aspen Woodlands

For further information about aspen, please go to the Aspen Information Resource