Some Observations on the Ecology of this Neglected Habitat from a Lichenological Perspective
by Alexandra (Sandy) and Brian Coppins
This paper concerns a neglected British plant, and a neglected habitat type that is almost unique to Scotland and of comparable, international importance to the much acclaimed machair. The plant is Corylus avellana - the Hazel. The habitat type is the Atlantic Hazelwood - found mainly along the coast of Western Scotland, from Knapdale in the South, to southern Sutherland in the North, but with a few examples in Western Ireland. Although they have been with us for 9000 years, the Atlantic Hazelwoods do not fit comfortably into any NVC community type, nor do they figure in the EU Habitats Directive.
Our interest and recognition of this habitat evolved from the study of the lichen flora of western Scotland. The story begins in the late 1970s, early 1980s when BJC began to look carefully at hazel as a host for lichens. He soon became aware that among his collections were many unidentifiable yet distinctive species. Subsequent study revealed that some of these species were new to Britain or Europe, and others were new to science. However, even in the Western Highlands and Islands, these species were not found everywhere on hazel - they were virtually absent from hazels under the canopy of oak and always absent from hazel stands that have developed from clear-cutting or had invaded open ground during the last 50-100 years or so.
The best hazels for lichens are found in woodlands where hazel forms the dominant canopy, on ridges and knolls or slopes close to the sea, or in steep ravines, also near the sea. Prime examples are found at Ballachuan (on Seil), south of Drimnin (Morvern), Struidh Wood (east coast of Eigg), and the Resipole ravine (Sunart).
A striking feature on entering a hazelwood is that thin, apparently 'bare' stems are white in colour - not hazel-brown. Closer inspection (especially with a hand lens), reveals that the white colouration is in fact a mosaic of small crustose lichens, some silvery white, some almost pure white, and others creamy-white. Some have black dots scattered on the surface, and others small lines like scribbles. It is this last group (the 'script' lichens) that lends the name to this group of crustose lichens - the Graphidion scriptae community (or Graphidion for short). This community occurs world-wide, but in the British Isles comprises about 65 species overall, of which about 20 can be considered 'specialist' species in that they are confined to Atlantic woodlands, especially hazelwoods. Some of these species are apparently endemic to the British Isles, and one of them, Graphis alboscripta, has so far not been found outside of Scotland. Several of these lichens are BAP (Biodiversity Action Plan) species, which means they are undergoing special study to understand their present status. Some species of the Graphidion also have their own host specific fungal parasites, which are even more restricted in distribution; e.g. Arthonia cohabitans on Arthothelium macounii, and Opegrapha brevis on Thelotrema petractoides.
A feature of these thin stems with their cover of smooth, crustose lichens is that rain-water drains fairly rapidly down the stems. When it stops raining, the stems dry very rapidly, and Graphidion lichens appear well-adapted to these alternating wet and dry conditions. On walking further into the hazelwood, where conditions are more shaded and humid, the hazel stems (or at least the older ones), appear darker and much more 'mossy'. These stems have become colonized by bryophytes, such as Frullania, allowing larger, leafy-lobed lichens to get a hold. It is these lichens that are more familiar to the casual observer. They include species of the genera Lobaria, Nephroma, Pseudocyphellaria, Sticta, Degelia, Pannaria and Leptogium.
All these lichens have blue-green algae (more correctly, cyanobacteria) as their photosynthetic partner. A physiological feature of the cyanobacteria is that they not only fix carbon from the air, but also nitrogen. Thus, when in abundance the lichens containing them play an important role in the nitrogen-cycle of the ecosystem.
What is also striking in the Atlantic Hazelwoods is the dearth of many larger lichens that have green algae as their photosynthetic partner and which are common and abundant in nearby woods dominated by oak or birch. These include the grey, leafy Parmelias, e.g. Parmelia laevigata, and the shrubby, coralloid Sphaerophorus globosus.
Without a detailed knowledge of lichens, how do you know if you are in a ‘good’ Atlantic hazelwood? One way is to attend one of the regular lichen courses run under the auspices of the NWDG and AGWA and get to recognize a few key species. Another way is to keep an eye out for the orange fungus, Hypocreopsis rhododendri ('Hazel gloves') (but absolutely nothing to do with Rhododendron ponticum!). This fungus somewhat resembles a lichen in that it forms thick, rubbery-orange rosettes, with fingers or radiating lobes that clasp round hazel stems (sometimes also nearby stems of blackthorn, rose and willow). It is also a BAP species, and in Western Scotland is an excellent indicator of a ‘good’ hazelwood, although it has so far not been recorded north of Eigg.
In a global context, it is interesting to note that the ‘good’ Atlantic hazelwoods have an abundance of crustose lichens belonging to the Graphidaceae and Thelotremataceae families found on smooth bark, and a preponderance of larger lichens with cyanobacterial partners on the older, mossy stems and branches. Where else in the World can the same combination (if not necessarily the same species) be found? - the Lowland Tropical Rain Forests!
Indeed, there is one species, Parmentaria chilensis, found in Resipole Ravine at Sunart, and in a small ravine on Mull, that has sub-tropical distribution in Central and South America, with outliers in Macaronesia, a couple of ravines in the western Pyrenees and a few woods in SW Ireland.
Why are the Atlantic Hazelwoods so good for their lichens?
- Mild oceanic climate - benefits of the Gulf Stream
- Inherent characteristics of hazel bark
- Geographical location - still largely unaffected by atmospheric pollution
- Ideal levels of illumination and ventilation within the woodland
- The peculiar characteristics regarding the dynamics of hazel stools
- The management history of the woods
The last two aspects are further discussed below.
Part of the remit when preparing species dossiers of some of Scotland's rare lichens on hazel for the Biodiversity Action Plan process, was to make comments on the habitat and ecology of the species under scrutiny, and to recommend the appropriate management for the habitat. To understand the dynamics of the lichen, we first needed to know about the dynamics of the habitat, and more specifically - the dynamics of hazel itself. Because of difficulties in locating literature on the way hazel behaves, we have attempted to formulate possible hypotheses based on casual observations we (and Peter Quelch) have made over the years.
We know from the pollen evidence that hazel was one of the first woody species to invade western Scotland in the early Holocene (Birks 1989). The pollen evidence also suggests that pure hazel scrub covered significant areas for a substantial period, perhaps 1,000 years (Birks 1989, McVean 1964). Today, remnant stands of pure hazel (exclusive of a tall tree presence) are still a feature in some places, especially in coastal areas of the western Scottish Highlands, suggesting the continual presence of an ancient relict habitat of nearly some 10,000 years (Birks 1989).
Are all of today's significant stands of hazel former coppice?
Literature searches on the ecology of hazel revealed that there was very little written about hazel per se. If hazel is noticed, or mentioned, it is dismissed at best as ‘scrub’, or ‘understorey’, or (the universal favourite) ‘hazel coppice’, and at worst as ‘neglected hazel coppice’. There does appear to be a mind-set in describing Corylus avellana as ‘hazel coppice’. It is true that hazel lends itself admirably to coppicing, and the uses to which hazel has been utilized over the centuries are many and varied. The classic coppice-with-standards practice in lowland southern England is recognized as a form of management that gives rise to herb-rich ground floras, supports high diversity of insects and birds, and as such is elaborately described in glowing terms in all handbooks of ecology and woodland management. Hence, students and woodland managers are swayed into thinking that this is the way in which hazel always has been - and should be - managed, so that all stands of hazel, where-ever they occur, are automatically regarded as ‘hazel coppice’. Indeed, there are still some ecologists who believe that hazel will die out if it is not regularly coppiced. Well, this is patently not the case.
The coppice question did present us with some problems: on the one hand, Atlantic Hazelwoods represent an ancient, relict habitat but, if, as we are led to believe, all hazel present today was formerly coppiced in the past, then we are left with a dilemma. We know from the work of Francis Rose (Rose 1976, 1992) and others, that certain critical species of lichens require long periods of ecological continuity. When a hazel stool is coppiced, all the lichens which occur on the hazel go with the cut stems. The stool will regenerate, the ground flora will recover, the woodland birds and insects will return as the hazel matures, but not the ‘old woodland’ lichens. True, some lichens will colonize the stems, but these will be the common and widespread species, the ‘weeds’.
Hazel dynamics - a proposed scenario based on lichen evidence
A typical hazel stool has a cluster of thin, medium and thick stems. The smooth-barked young stems are colonized by a distinctive community of crustose lichens (the Graphidion). As these stems become older and thicker, the bark roughens, and crustose lichens give way to bryophytes and foliose lichens of the Lobarion community. The ageing stems tend to gradually lean outwards, probably from the weight of the canopy they support. This creates a gap in the overall canopy, which enables new, young stems to arise and fill the space. Damage to the canopy from winter storms will break off canopy twigs, and abrasion from stems rubbing together in windy weather allows fungal pathogens to attack, and gradually kill off individual stems. This all leads to a considerable turn-over of stems within a stool.
New stems (whips) appear to be produced every year. Initiation of new stem production may be triggered by short periods in the spring, when light conditions are favourable, but if the canopy above is closed, then these stems will abort by late summer, due to heavy shade created once the existing canopy is in full leaf. But, if a gap is present, then they are ready to grow rapidly and fill it. The new stems appear to act as a sort of ‘fail-safe’ strategy to ensure the viability and perpetuation of the stool. Therefore, it is clear, that each stool is a self-perpetuating ecological unit, with always some young, smooth-barked stems (supporting crustose lichen communities), and generally always some older stems, supporting the leafy-lobed lichen communities. Hence, the value of a hazel stool for lichens. There is always this continual cycle of replenishment, which is so critical for retaining the ecological continuity required by the more specialized ‘old woodland’ lichens, and why old Atlantic Hazelwoods in western Scotland support endemic species.
However, the plot thickens, as hazel stools in different situations tend to behave slightly differently; on deep soils in sheltered conditions, the stools tend to be large, well-grown and widely spaced, often with the canopies of adjacent stools meeting, giving rise to widespreading canopies (e.g. 6.5 m diameter). In these situations, the canopy can be up to 6.0 m high. Stools in these situations tend to support mostly shade-tolerant ‘old woodland’ lichens of the Lobarion, with the more light-demanding species of the Graphidion only poorly represented. There are a large number of stems per stool, ranging from thin, spindly ones, to very thick, woody ones, the latter often becoming almost horizontal. The canopy twigs of these old stems gradually adjust to vertical as the main stem assumes a horizontal position. Hence, when the old stems eventually collapse, the canopy twigs are often well above browsing height. In some cases, layering of the collapsed stem takes place, and new stools are formed, but this is only successful if there is a ‘space’, as competition for light is the prime factor controlling successful hazel establishment. Within a closed stand of hazel, the shade created by summer canopies in full leaf is extremely dark, which maybe explains why tree seedlings are unable to establish and are rarely encountered.
In more exposed situations on thin soils (the typical slope hazelwoods), stools tend to be closer together, much smaller, and composed of a few thin to medium-sized stems, with rarely the thick woody stems present. In these situations, the Lobarion is often species-poor, with the Graphidion dominant on the smooth-barked young stems. Depending on exposure, the height of the canopy is 1Ò3 m. There is a rigidity within the stand, with a tight, interlaced network of small twigs, which means internally it is very sheltered, although the outer twigs are often wind clipped and distorted. Turn-over of stems appears to be far more frequent, with the oldest stems being only 12Ò15 years old (we have found - as a very rough rule of thumb - that 1 cm girth approximately corresponds to 1 year of growth).
Do hazels eventually die, or do the stools get bigger?
We know that where oak has been coppiced over many centuries, the resulting stool enlarges, with individual coppiced trunks forming a sizeable ring around an open central space (Quelch 2001).From observations particularly at Ballacuan Hazelwood, but also on Mull, we have seen what we believe are hazel ‘rings’ - a bit like the fairy rings formed by fungi in old meadows. These are best developed on gentle slopes or flat ground in the intermediate zones, between the exposed slopes and the sheltered, deep soils on damp ground. Are these evidence of ancient hazel coppice?
At Ballachuan and on Mull, there are examples of circles of ‘satellite’ stools around an empty space, which can measure from 1.10 m in diameter to 2.30 m. (Coppins & Coppins 2000a,b). Intermediate stages of this open circle formation were also detected. About the same time that we were pondering over the implications of this, Peter Quelch was photographing similar hazel stool rings in Scandinavia, and tentatively coming to the same conclusion: that these satellite stools may have evolved through a gradual outward expansion of new stems at the edge of a ‘mother’ stool until a point is reached whereby the centre of the stool becomes too shaded, and central stems are not able to replenish this space, due to canopy shade from the outer, more vigorous stems. The root stock at the centre of the stool may also become exhausted.
The potential extrapolation of this method of vegetative reproduction has interesting implications if taken to logical conclusions; whole series of rings of satellite stools could perhaps be plotted, many overlapping with adjacent rings, and satellite stools themselves could in turn, eventually become ‘mother’ stools, and form further rings. Genetically-related groupings could be traced and plotted, which could lead to conclusions about the ecological history of the stand. Where hazel reproduces by seed, there will be genetic variability, but where it reproduces vegetatively (by spread of stool area or by layering), then is it possible that some hazel stands may represent genetic relics that originated thousands of years ago? In an extensive hazelwood such as Ballachuan (27 ha), the implications of such long-term stand integrity go some way to explaining why this site today is of international importance for its lichen flora.
Other questions then followed, such as ‘how old are the stools’. Again, Peter Quelch came up with a paper from an ecologist in Finland (H ggstr-m 2000), who had attempted to date hazel stools by applying systematic calculations relating to ages of stems and basal girths. He arrived at an amazing age of 990 years for his oldest stool. H ggstr-m also noted the tendency of hazel stools to form open rings.
But where does all this fit in with coppicing?
Of course hazel has been extensively utilized by man over the centuries, as is demonstrated from the archaeological Wetlands Project carried out on the Somerset Levels (Coles & Coles 2001). Several wooden trackways were laid across marshy ground in prehistoric times between the Polden Hills north to high ground at Westhay and Meare. One of these trackways is dated to 2,900 BC (around 5,000 years ago) and is late Neolithic. It is constructed of hurdles woven from hazel. Approximately 1,000 hurdles were used, and later repairs were added with further hurdles (Brunning 2001). Careful analysis of the stems that were used to construct the hurdles revealed that although they are all of a similar diameter, they are not of a similar age, and it was proposed that the individual hazel stems were cut on a selective basis (by drawing), from stools, rather than as a result of complete coppicing (Morgan 1982).
This practice has several advantages:
it reduces the effort of cutting a complete stool simply to obtain the few stems of the required size. Conceivably, though, the un-used cut wood could be used as fire-wood.
leaving the stool more-or-less intact, would stimulate new stems to grow quickly, and straight, up to the light to the gaps in the canopy left by the selective cut. This would mean that the stool could be selectively cut again at an earlier time, even the next year, when stems too small for the first year would have thickened up. Whereas, complete coppicing requires a gap of at least seven years before the complete new growth is sufficiently developed into straight-grown stems, suitable for hurdle making. The stems remaining in the stool would afford some protection from browsing animals.
Selective cutting (or Drawing) would reduce the distance involved in walking to new locations to find suitable stools for the next round of hurdles.
Selective cutting would ensure that the older stems were still present, and these are the nut-bearing stems, providing an important additional food source for late autumn.
Selective cutting would ensure that nearby stands of hazel were retained as shelter for flocks in the winter (not relevant perhaps in Neolithic times, but certainly an important feature and requirement for later farming communities).
Complete coppicing requires some substantial, labour-intensive method of protecting the coppiced stools from browsing animals to ensure continuing viability of the stool.
The recent experimental coppice plot at Abriachan Wood near Drumnadrochit, on the west side of Loch Ness demonstrates an example of modern-day conservation in action, following the belief that hazel needs to be coppiced (Coppins & Coppins 2001). Several stools have been completely coppiced and the site enclosed against grazing by erecting a palisade made from hazel stems. A great deal of effort has gone into making the protective palisade, and used an awful lot of the cut product to build it. On the evidence of this experiment, this method seems to be counterproductive, as the net result in numbers of useful stems obtained for other purposes would surely be too few to justify.
Evidence for selective cutting
The evidence of the lichen flora in many of the stands of western Scottish hazelwoods strongly suggests that they were not coppiced in the past, but have retained integrity over a very long period. These stands, where they are not over-topped by trees, retain a closed canopy, and appear to have a self-regulating system of continual replenishment, and support unique lichen assemblages found only in western Scottish and Irish coastal hazelwoods.
The selective cutting method is, we believe, the method that was widely used where extensive stands of pure hazel occur in western Scottish Atlantic Hazelwoods. This method retains the majority of stems within a stool, and as such it inadvertently retains the necessary ecological continuity that the associated lichen flora requires, which may be why these western Scottish hazelwoods are so important today.
It may not be the whole story, as there is documentary evidence (e.g. for the Portalloch Estate in Argyll) that large areas of hazelwood were intensively harvested to provide hoops for barrels: ‘February, 1836, 51 men did 700 man-days cutting wands and hoops’ (Gordon Gray-Stephens, pers. com., quoting from Estate Papers GD43.80.96). But even here, ‘cutting wands’ could imply selective cutting of stems suitable for splitting to make barrel hoops. Certainly, the value of hazel as shelter for flocks, as well as the needs of local people for this useful commodity of everyday life, would probably have precluded all hazel being intensively managed in this way in a given area.
Another factor in the life of hazel is the effect of long-term grazing on the stool. This is a fascinating field of study, but will have to be the subject of another talk at another time. Suffice it to say, that hazel deserves to be more widely appreciated and recognized. Apart from the lichen evidence, the comments and conclusions discussed in this paper are purely theoretical and untested, but are set out to provoke discussion and hopefully to promote more detailed research.
Hazel should not be used with the epithet ‘coppice’ unless there is strong evidence to support the fact that stools or stands have been managed as coppice in the past, or are currently being so managed.
Hazel should be recognized as a valuable component in its own right where it occurs within a woodland.
Where hazel stands occur at the edge of a woodland, or form stands in what is termed ‘scrub’ development, they should be recognized as having high ecological potential for a variety of associated wildlife, particularly lichens and bryophytes.
Stands of hazel without trees should not necessarily be regarded as areas of degraded woodland, or dismissed as secondary scrub development. There is the possibility (especially in coastal areas of Western Scotland and Ireland), that stands of pure hazel form an ancient relict habitat that dates back to post-glacial times, with important implications for genetic integrity and associated epiphytes.
Where stands of hazel occur without a significant presence of trees, it is not desirable to introduce trees within the stand, as this will lead to a degradation of the established hazel habitat, and an overall loss of diversity.
Precautionary advice is that hazel should not be coppiced as a general management practice, especially in the Scottish Highlands. We are currently working towards providing guidance for assessing the potential ecological importance of existing hazel stands, using one or two ‘indicator’ species, but until this is ready, we would advize that selective cropping of desirable stems from individual stools should be carried out rather than wholescale coppicing.
Birks, H.J.B. (1989) Holocene isochrone maps and patterns of tree-spreading in the British Isles. Journal of Biogeography 16: 503-540.
Brunning, R. (2001) The Somerset Levels. Current Archaeology [No. 172], 15(4): 139-145.
Coles, J. & Coles, B. (1982, 1990, 2001) Prehistory of the Somerset Levels. Cambridge and Exeter.
Coppins, A.M. & Coppins, B.J. (2000a) Ballachuan Hazelwood, SWT Reserve, Argyll Main (VC 98): Lichen Survey II. Unpublished report for Scottish Wildlife Trust.
Coppins, A.M. & Coppins, B.J. (2000b) Mull: investigation of selected hazelwoods for three BAP species: the lichens Arthothelium macounii and Graphis alboscripta and the fungus Hypocreopsis rhododendri. Unpublished report for Scottish Natural Heritage and Royal Botanic Garden, Edinburgh.
Coppins, A.M. & Coppins, B.J. (2001) Abriachan Wood, Easterness (VC 96) Lichen Survey. Unpublished report for Woodland Trust Scotland.
Hæggström, C.-A. (2000) The age and size of hazel (Corylus avellana L.) stools of Nåtö Island, Åland Islands, SW Finland. In: Agnoletti, M. and Anderson, S. (eds) Methods and Approaches in Forest History: 47-57. CAB International: Wallingford.
McVean, D.N. (1964) Pre-history and ecological history. In: Burnett, J.H. (ed.) The Vegetation of Scotland: 561-567. Oliver & Boyd, Edinburgh & London.
Morgan, R.A. (1982) The Eclipse Track: Tree-ring studies. Somerset Levels Papers 8: 36-38. Cambridge and Exeter.
Rose, F. (1976) Lichenological indicators of age and environmental continuity in woodlands. In Brown, D.H., Hawksworth, D.L. & Bailey, R.H. (eds), Lichenology: Progress and Problems: 278-307. Academic Press: London etc.
Rose, F. (1992) Temperate forest management: its effect on bryophyte and lichen floras and habitats. In Bates, J.W. & Farmer, A.M. (eds), Bryophytes and Lichens in a Changing Environment: 211-233. Oxford University Press.
[Our attention has been drawn to a forthcoming paper (Tallantire, P.A. (2002) The early-Holocene spread of hazel (Corylus avellana L.) in Europe north and west of the Alps: an ecological hypothesis. The Holocene 12(1): 91-106), as perhaps throwing more light on our own hypotheses. However, having obtained a proof copy, we find there is extremely useful data on how hazel spreads under various climatic regimes, etc., but again, no consideration given as to long-term behaviour of individual hazel stools.]
For further information about hazel, please go to our Hazel Information Resource.
Last updated: Wednesday, 25-Aug-2010 16:59:12 CEST