Bryophytes on Aspens

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

Gordon Rothero
Strolonag, Glenmassen, Dunoon, Argyll, PA23 8RA. E-mail: GPRothero@aol.com

Introduction

Away from the oceanic woodlands of the west, where there is a rich and interesting bryoflora on all broadleaf species, interest in epiphytic bryophytes has tended to centre on those tree species known to have 'base-rich' bark, particularly Fraxinus excelsior, Acer pseudoplatanus, Ulmus spp, Salix spp and Sambucus nigra. These tree species often have a good assemblage of bryophytes, particularly when growing in relatively open sites, hence the value of wayside and parkland trees. To my knowledge, Aspen has mainly been celebrated as the host species for the single British record for Orthotrichum gymnostomum and it has certainly been undervalued.

Orthotrichum gymnostomum

Orthotrichum gymnostomum is a small, yellow-green, blunt-leaved moss which has a scattered distribution over much of northern and central Europe and also records from south-west Asia, Afghanistan and Newfoundland (Hill et al. 1994). Though it has been found on the bark of a number of different tree species throughout its range, most records come from species of Populus and through northern Europe most records are from old Populus tremula (Nyholm, 1979). In this sense, in Europe at least, it is probably more host-specific than other epiphytic bryophytes. Though sporophytes are apparently rare throughout its range, the plant does produce large numbers of gemmae, specialised means of vegetative reproduction, on its leaves, a feature it shares with the closely related Orthotrichum obtusifolium.

The solitary British record dates from 21st June 1966 when it was collected by J Dransfield in company with H.J.B. Birks and H.H. Lees near Loch an Eilein in the Rothiemurchus forest (Perry & Dransfield 1967). A small tuft consisting of about 15 stems was not recognised in the field and was collected. At first it was thought that the plant was Orthotrichum obtusifolium, but closer examination showed it to be Orthotrichum gymnostomum. After the discovery, other Aspens were searched but no further cushions were found. It would seem that the only cushion in that area had been collected, a sobering observation.

The site of the host Aspen for the 1966 record is not obvious from the description. The habitat description talks of "open pine-birch woodland with occasional Aspens on a north-facing slope at about 800ft in altitude" (Perry & Dransfield 1967), but unfortunately the six-figure map reference delineates a hectare on a south-facing slope near the loch margin. Perry and Dransfield opine that, given the frequency of Aspen in the Aviemore area, Orthotrichum gymnostomum should turn up elsewhere in the vicinity. Thirty five years on, that hope has yet to be realised and the plant is now classified as extinct in the Bryophyte Red Data Book (Church et al. 2001).

A number of competent bryologists have visited the Loch an Eilein area over the years and searched Aspens without success, but few have ventured further afield. Before giving up hope completely, it seemed a sensible idea to spend a small amount of time visiting some areas of Aspen in Strathspey and this suggestion was incorporated into a wider survey of 'Priority bryophytes' in Scotland, commissioned and funded by Scottish Natural Heritage. Other than the original locality, the sites to be visited were selected from a database of significant stands of Aspen compiled by the Malloch Society. It seemed sensible to look at as many Aspens as possible so only large stands were selected; at Invertromie, Insh, Torcroy, Creagan Breugach near Inverton, Speybank by Kincraig, Tomnagowan and Boat of Garten.

I had no more success at Loch an Eilean than other bryologists; the most likely site seemed to be Creag an Fhithich, the closest north-facing slope to the map reference with a few scattered Aspens, but a wider search was also made. Moving on to the other Aspen woodlands, two things were immediately apparent; Aspens had a much more diverse epiphytic bryophyte flora than I had realised, and searching all Aspens in a large woodland was not possible in the time available. Certainly more than 50% and probably more than 75% of the Aspens in the woods were checked, and it was the more bryophyte-rich trees that were targeted. On this basis I am reasonably convinced that Orthotrichum gymnostomum does not occur in the woodlands I visited. The general diversity of the flora is discussed below.

Orthotrichum obtusifolium

Orthotrichum obtusifolium © Peter Cosgrove
The Blunt-leaved bristle moss, Orthotrichum obtusifolium, was formerly widespread in Britain in the 19th century but declined dramatically because of air pollution and Dutch-elm disease killing its host trees. With recent records from only one site, this UK BAP Priority species was recently discovered on Aspens at the RSPB's Insh Marshes Reserve, including new host trees on the afternoon fieldtrip! The small bright green moss cushions in the photograph are Orthotrichum obtusifolium.

Map 1. The Distribution of Orthotrichum obtusifolium in the British Isles

At Invertromie, one Aspen produced a small, blunt-leaved Orthotrichum species but this proved to be the closely related Orthotrichum obtusifolium. This species is very close to Orthotrichum gymnostomum and requires some familiarity with the group to distinguish it in the field. It has the same neat cushions with blunt leaves and differs mainly in the character of the leaf margins, plain or erect in Orthotrichum obtusifolium and curled in over the leaf surface in Orthotrichum gymnostomum. There are also critical differences in cell ornamentation that require a microscope but are diagnostic (Smith 1978).

The initial disappointment was hardly justified as Orthotrichum obtusifolium is listed on Schedule 8 of the Wildlife and Countryside Act and, prior to this survey of Aspens in Strathspey, had just one extant site in Britain, at Leith Hall near Huntly. The woodland at Insh produced a further tiny stand of Orthotrichum obtusifolium but disappointingly, no more populations were found at the other sites visited. A third locality was found on a group of Aspens at Inveruglas by David Chamberlain, walking back to Insh village from a visit to the site at Invertromie. During the 'Aspen day' afternoon field trip, I found a further small stand close to the original tree at Invertromie and a subsequent search by David Long has revealed further stands, including one large one, on three more trees.

Orthotrichum obtusifolium, as the distribution map shows, has a much longer history in Britain than Orthotrichum gymnostomum.

Orthotrichum obtusifolium was widespread in Britain in the 19th century, though it has always been rare. The reasonable presumption has been made that most of the English localities were casualties of increasing air pollution, though with a rare species, chance events will always play their part. Of the four relatively recent (post-1960) sites, three are in Scotland (Cortachy in Angus, Fochabers and Leith Hall). At Cortachy the plant grew on 'parkland' Elms which all seem to have succumbed to Dutch Elm disease, and this may also have been the fate of the Elm on which it was recorded at Fochabers. At Leith Hall there is a healthy population on some eight trees, both Elm and Sycamore, and this remains the best British population. At the English site in Norfolk, a single tuft was found on an elder twig in 1989, and shades of Orthotrichum gymnostomum was collected and has not been seen again.

The new populations near Insh have increased both the geographical spread of recorded localities and the number of host species. Historically, Ash has been the most favoured substrate in Britain followed by Elm and Sycamore, but in northern Europe and North America, Orthotrichum obtusifolium shows a marked preference for Aspen, so its occurrence on this tree in Scotland should be no real surprise. The sites at Invertromie and at Inveruglas are in relatively open woodland where light levels in summer remain quite high and all historic records come from similarly open sites. It may well be that further survey work on Orthotrichum obtusifolium, and possibly Orthotrichum gymnostomum as well, should target smaller stands of Aspen on more open sites rather than areas of woodland with a complete canopy.

Other bryophytes on Aspen

The survey of large numbers of Aspen revealed that a good proportion of mature trees have an excellent epiphytic flora. Table 1 gives a list of bryophytes recorded from Aspen in Strathspey in 2000 and no doubt more could be added.

Table 1. Bryophytes recorded on Aspen in selected woodlands on Speyside in 2000

Liverworts
Frullania dilatata	Frullania fragilifolia Frullania tamarisci
Metzgeria furcata	Radula complanata
Mosses
Dicranum scoparium	Dicranum fuscescens
Homalothecium sericeum	Hypnum andoi
Hypnum cupressiforme	Leucodon sciuroides
Orthotrichum affine	Orthotrichum lyellii
Orthotrichum obtusifolium Nationally rare, Schedule 8
Orthotrichum speciosum Nationally rare
Orthotrichum stramineum	Orthotrichum striatum
Orthotrichum tenellum	Syntrichia laevipila
Ulota bruchii	Ulota crispa
Ulota drummondii	Ulota phyllantha
Zygodon conoideus	Zygodon rupestris
Zygodon viridissimus var viridissimus

Apart from the excellent diversity of species, there are three important conclusions to be drawn from the list. The first is a simple observation, that Aspens in Strathspey are the centre of distribution for the nationally rare moss Orthotrichum speciosum. Given the restricted distribution of recent records of this species (see Map 2), its abundance on many Aspens in the woods visited is quite remarkable. The robust cushions with clearly visible capsules are a feature of most of the better trees, sometimes forming large stands. The second is that three of the mosses which occur regularly on Aspen in the area, Ulota drummondii, Ulota phyllantha and Zygodon conoideus, are 'Atlantic bryophytes' (see Hodgetts 1997), an affinity which is also reflected in the lichen flora.

Map 2. The distribution of Orthotrichum speciosum in the British Isles

The third observation is more subtle, but could be of considerable importance. The loss of Elms to disease, and the general loss of wayside and parkland trees over the past 100 years, has deprived epiphytic bryophytes of favoured sites, so much so that some species, like Orthotrichum obtusifolium but also Orthotrichum pallens and Orthotrichum pumilum are now endangered in Britain (Church et al. 2001). It may be that Aspens on open sites in the east of Scotland could have populations of these species that have been overlooked. Even if this does not prove to be the case, the substrate that Aspens provide for a good assemblage of regionally important species, which might otherwise be in decline, is a worthwhile discovery.

One further observation of epiphytic populations on Aspens is puzzling. Though some Aspens are clothed in a variety of epiphytic mosses, species which are known to need bark of a reasonably high nutrient status, others are almost devoid of bryophytes, except those which can cope with nutrient-poor conditions. So we have Aspens, close together and experiencing similar light and nutrient regimes, some having a flora similar to Ash or Elm and others to Birch or Alder. A similar situation seems to exist with the epiphytic lichen flora. One obvious explanation, given the structure of Aspen woodland, is that this may be a clonal difference, some trees being genetically different to others. Another possible explanation may relate to the sort of fungal infestation that the Aspen is subject to. Aspens seem particularly prone to damage and most large trees show signs of fungal invasion; does this affect the nutrient status of the bark or the run-off?

Conclusion

It is clear from the survey of a limited number of Aspens in 2000 that the importance of the species for epiphytic bryophytes has been distinctly under-estimated. Though Orthotrichum gymnostomum was not refound, the discovery of three new populations of Orthotrichum obtusifolium, the abundance of Orthotrichum speciosum and the diversity of epiphtyic mosses on Aspen fully justify the survey. Any increase in Aspen woodland will benefit bryophytes, and this is as true of smaller groups of trees as of larger woodlands, so there is a different emphasis here compared with the entomological interest. The epiphytic interest of the larger Aspens should be borne in mind when considering the management of the woodland for insects which require dead wood but sensible precautions should prevent any possible conflict.

References

Church, J.M., Hodgetts, N.G., Preston, C.D. & Stewart N.F. 2001. British Red Data Books. Mosses and liverworts. Peterborough, JNCC.

Hill, M.O., Preston, C.D. & Smith A.J.E. 1991. Atlas of the Bryophytes of Britain and Ireland, Vol 3 Mosses (Diplolepidae). Harley Books.

Nyholm, E. 1979. Moss Flora of Fennoscandia II Musci; fascicle 4. Swedish Natural Science Research Council.

Hodgetts, N.G. 1997. Atlantic bryophytes in Scotland. Botanical Journal of Scotland. 49: 375-386.

Perry, A.R. & Dransfield, J. 1967. Orthotrichum gymnostomum in Scotland. J. Bryol. 5: 218-221.

Smith, A.J.E. 1978. The Moss Flora of Britain and Ireland. Cambridge University Press.

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