The Lichen Ecology of Aspen Woods - A Preliminary Analysis

Project Outline

In April 2003 the Royal Botanic Garden Edinburgh implemented Phase II of its Scottish Rare Plants Survey. In recognition of the International importance of the British lichen flora, this initiative is to protect UK lichen species and their habitats under threat or in decline. Over the next one to two years the survey will focus on a hitherto neglected lichen habitat: Aspen (Populus tremula L.).

Stands of native Aspen have excited recent interest in conservation (Cosgrove & Amphlett 2002). The niches of certain lichen species are associated exclusively with the Aspen (e.g. Arthonia patellulata and Lecanora populicola) and populations of notable rare or threatened species occur in native Aspen woods (e.g. Caloplaca flavorubescens, Leptogium saturninum and Schismatomma graphidioides). The conservation status of Aspen as a habitat for lichens has been noted (Coppins et al. 2001) though requires further examination. The work at RBGE has two components:

• (i) a survey to inventory lichens, including an assessment of conservation status based on their rarity and commonness.
• (ii) Ecological research to describe the habitat requirements of lichen species.

Together these provide information towards the planning and implementation of conservation strategy.

A New Sampling Method

To examine how the abundance of lichen species might be controlled by environmental variables at contrasting spatial-scales, traditional ecological methodology has been adapted to suit lichens. To capture regional variation in the lichen flora, sites are being visited from biogeographic regions separated by major watersheds. At each site two to three Aspen trees of different ages are examined. The occurrence of lichen species on the trunk is quantified according to a standard methodology - the spiral-transect (Fig.1). A starting point (node) is marked, 5 cm above the base of the trunk, at a principal compass point. This point changes in turn for successive trees sampled, in the order: north, west, south and east. Subsequent nodes are marked on the same side of the trunk at heights above the ground of 100 cm and 200 cm, and on the opposite side of the trunk at 50 cm and 150 cm. A linear transect with sampling points at 20 cm intervals is then aligned in a westerly spiral around the tree, incorporating the shortest distance between the ascending nodes. To describe the lichen flora, a quadrat with sides 6 cm x 6 cm, divided into nine subunits (2 cm x 2 cm), is placed with its centre over consecutive sampling points on the spiral transect. Lichen species are recorded in the field as presence-absence in each of the nine subunits, providing a 9-point frequency of occurrence for each quadrat. Difficult taxa are noted and collected for examination in the herbarium.

The Lichen Habitat - A Preliminary Assessment

The abundance of lichens is being compared to environmental variation at different scales: i.e. between quadrats, between trees at a site, between sites within a region, or between regions. Preliminary analysis points to:

• (i) dramatic variation between biogeographic regions related to a steep east-west gradient in oceanicity.
• (ii) Variation in the lichen flora of stands in close proximity controlled principally by bark chemistry (e.g. pH and conductivity).

These points are exemplified by comparing the lichen flora of an Aspen stand in upper Speyside, Breakachy Bridge (NN 636929), first with a site ca 10 km westwards, across a watershed in the Laggan Valley (NN 514886), second with a site ca 10 km eastwards, Creagan Breugach (NN 742992), which lies downstream along the River Spey.

Of the combined species pool, Aspen from Breakachy Bridge and Laggan have ca 31% of species in common (Fig. 2). These species comprise the common elements of the Aspen flora e.g. Lecanora chlarotera, Lecidella elaeochroma and Ramalina farinacea. The Aspen specialist Lecanora populicola occurs at Breakachy Bridge and not at Laggan, though the most obvious difference between the sites is owing to a rich assemblage of oceanic epiphytes at Laggan, absent from Breakachy Bridge. These include the indicators of oceanic woodland continuity Collema fasciculare and Peltigera collina (Coppins & Coppins 2002).

The difference between lichen communities on the same tree species, over such a small distance (ca 10 kilometres), is the striking result of orographic effects on Scottish weather; Breakachy Bridge is in the rain-shadow of mountains surrounding Loch Laggan. However, comparing sites within the same valley, Breakachy Bridge and Creagan Breugach have still fewer species in common, i.e. 12% of the combined species pool (Fig. 3). The lichen flora of Breakachy Bridge is characterised by the Aspen specialists, Arthonia patellulata and Lecanora populicola, and species typical of more basic substrate, e.g. Caloplaca cerinella & C. holocarpa, Physcia aipolia & P. tenella and Xanothoria parietina. In contrast, the epiphyte flora at Creagan Breugach is similar to that found on relatively more acid-barked Birch and Pine, e.g. Bryoria fuscescens, Hypogymnia physodes and Platismatia glauca. A difference in the epiphyte flora of Aspen at closely occurring sites is tentatively explained by bark pH; that of the Aspen at Creagan Breugach (pH = 4.36) being relatively low compared to Aspen at Breakachy Bridge (pH = 5.17). Field observations and musings with Brian Coppins in the tap-room of the Suie Arms Hotel (NH 828057) have thrown light on a possible relationship between Aspen clonality and bark chemistry. Analyses on a wind-blown tree also suggest that an increase in the bark pH of Aspen upwards along the trunk is matched by a corresponding gradient in species composition.

Further Work

The study will continue to visit Aspen stands in Scotland, inventory lichens and refine the broad ecological analyses discussed in this paper. It will also include research to examine the lichen flora of the upper trunk and tree canopy and to explain the relative effect of clonality and environment (e.g. soil conditions) on bark pH.

Acknowledgements

The work in progress described here is carried out in collaboration with Brian Coppins, who I thank for his companionship in the field and patient training in lichen identification. The work is made possible by a grant to RBGE from the Esmée Fairbairn Foundation.

References

Cosgrove, P. & Amphlett, A. (2002) The Biodiversity and Management of Aspen Woodlands. Cairngorms Local Biodiversity Action Plan, Grantown-on-Spey.

Coppins, A.M. & Coppins B.J. (2002) Indices of Ecological Continuity for Woodland Epiphytic Lichen Habitats in the British Isles. British Lichen Society.

Coppins, B.J, Street, S. & Street, L. (2001) Lichens of Aspen Woods in Strathspey. Unpublished Report.

Figure 1. The spiral-transect method of sampling lichens on a tree trunk (see text for description).

Figure 2. Percent of the combined species pool occurring at Breakachy Bridge only, Loch Laggan only, or at both sites. Examples of species are given for each category, Aspen specialists are underlined and indicators of oceanic woodland continuity delimited by a 'w'.

Figure 3. Percent of the combined species pool occurring at Breakachy Bridge only, Creagan Breugach only, or at both sites. Examples of species are given for each category, Aspen specialists are underlined.

Lichens on aspen - related pages

• The importance of Aspens for lichen
• Epiphytic Lichens on Populus tremula: Implications for Conservation

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