The vegetational history of Glen Diomhan, North Arran, site of endemic whitebeams, S or bus arranensis Hedl. and S, pseudofennica E.F. Warb.

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1 New Phytoi. (99), 7, 5-.S6 The vegetational history of Glen Diomhan, North Arran, site of endemic whitebeams, S or bus arranensis Hedl. and S, pseudofennica E.F. Warb. BY G. STEVEN AND J. H. DICKSON Department of Botany, University of Glasgow, Glasgow, G2 8QQ, UK {Received 6 August 99; accepted 2 November 99) SUMMARY A detailed pollen analysis was undertaken of sediment from a site in Glen Catacol on the Isle of Arran, Scotland, close to Glen Diomhan National Nature Reserve which has a large though possibly declining population of the endemic whitebeams, Sorbus arranensis Hedl. and S. pseudofennica E.F. Warb. Blanket peat formation began before 7 years BP. The results provide evidence that while parts of the glen saw the development and subsequent decline of birch, oak, hazel and alder woodland a large proportion of the surrounding vegetation has remained open throughout the Flandrian with grass-heath similar to that which occurs over much of the glen in the present day. Key words: Pollen analysis, Scotland, blanket peat, endemic Sorbits spp. INTRODUCTION Glen Diomhan, a steep-sided, rocky gully adjoining Glen Catacol in the north of the Isle of Arran oflf the west coast of Scotland has the largest concentration on the island of the local endemics Sorbus arranensis Hedl. and S. pseudofennica E. F. Warb. Part of the gully has been designated as a National Nature Reserve (NNR) by the Nature Conservancy Council (Fig. ). Little is known of the role of the endemics in the past vegetation of the island. The results presented here are part of a wider study of the local and regional vegetational history. Pollen analysis was also undertaken on sediments from a small loch to the northeast of Glen Diomhan, Loch a'mhuillin (Boyd & Dickson, 987a, Boyd et al., 988), and on basin mire deposits from Machrie Moor, a low-lying area near the west coast in the southern half of the island (Robinson, 98; Robinson & Dickson, 988). A major aim of the work was to provide guidance for management of the nature reserve in Glen Diomhan. THE ROLE OF SORIUIS IN THE PRESENT-DAY VEGETATION The endemics occur as small trees scattered alongside a few rivers and in scrub on steep inaccessible slopes, often with Betula pubescens Ehrh. and Sorbus aucuparia L., in an otherwise largely treeless landscape. They are known to occur at several localities in the north of the island and formerly occurred in a few other sites until historical times. All the localities, past and present, lie within an area of about km". In addition, Sorbus rupicola (Syme) Hedl. occurs on Holy Isle off south east Arran as well as on the mainland to the north. A study of the morphology of the endemics suggests that 5. arranensis is the result of the cross 5. aucuparia x S. rupicola and S. pseudofennica results from S. arranensis x >S. aucuparia (Hull & Smart, 984). The present disjunct distribution of the endemics has led to the suggestion that they were once more widespread, perhaps occurring through an extensive woodland of which the present stands are remnants (Bignal, 98). LOCATION AND SITE DESCRIPTION Geologically and physiographically the northern and southern parts of Arran differ greatly. The north consists predominantly of metamorphic and igneous rocks and is very mountainous, rising to 874 m O.D. at the summit of Goat Fell, with deep, glaciated valleys. The southern half of the island is made up of sedimentary rocks and has low, rolling hills. The north of the island has few indications of past human activity and these are predominantly coastal

2 52 G. Steven and y. H. Dickson Figure. Location of Glen Diomhan and other sites mentioned in the text. unlike the southern half which has a large number of archaeological sites (Fairhurst, 98). In the area around Glen Catacol worked flints of Mesolithic age have been unearthed at Catacol Bay (Lacaille, 954) and the hill above the mouth of the glen has signs of settlement, as yet unexcavated. Glen Catacol is a large U-shaped valley running more or less south-north and ending at the northeast coast of Arran (Fig. ). Most of the glen is underlain by Tertiary biotite granite which gives rise to very acid soil conditions under the high annual rainfall of the area. The slopes above the coast at the mouth of the glen are composed of more nutrient-rich Dalradian schists and have birch rowan hazel woods. The broad valley floor has a superficial layer of glacial sands and gravels deepest near the mouth of the glen where they are several metres thick. The gentle slopes of the valley bottom have a thin layer of peat while the sides of the glen are generally very steep with thin, poor soil and areas of loose scree which occasionally have patches of scrub. The generally very acid soil conditions are ameliorated in a few places by seepage of base-enriched groundwater. The dominant vegetation of the glen is Molinia- Calluna-Erica tetralix blanket bog with carpets of Sphagnum, corresponding to the Molinia caerulea (L.) Moench - Myrica gale L. nodum of Adam, Birks & Huntley (977). Myrica is locally frequent in flush areas of the lower slopes with species such as Narthecium ossifragum (L.) Hudson, Car ex panicea L. and Selaginella selaginoides (L.) Link. Glen Diomhan is an offshoot of Glen Catacol about 2 km up the valley. It is a rocky V-shaped gorge with a small stream alongside which are scrubby patches of birch, rowan, willows, and holly. There are also occasional juniper and aspen among the rocks. The endemic whitebeams occur mainly on a very steep, inaccessible slope with loose scree. Less steep parts of the gorge have wet Calluna heath with Molinia, Erica tetralix L., Potentilla erecta (L.) Rauschel and Vaccinium myrtillus L. Huperzia selago (L.) Bernh. ex Shrank & Mart, is locally frequent. There is some localized base-enrichment but soils are mainly thin and poor. The coring site was a small depression on the lower slope of Glen Catacol at grid reference NR about 75 m from the mouth of Glen Diomhan at around m O.D. (Fig. 2). The site differs slightly from the general surroundings in being relatively well-drained and supports a localized stand of Pteridium. It is the nearest location of a significant depth of peat to the reserve. COLLECTION OF PEAT SAMPLES The sediments used in the pollen analysis were collected in 98. Two box samples, 45 x 5 x 5 cm, were taken from the surface layers after removing the growing vegetation and cutting away some of the peat to provide a section into which sharpened tins

3 Vegetational history of Glen Diomhan, Arran 53 Glen Diomhan National Nature Reserve X Coring site Boundary of NNR Exclosure metres 5 yards 5 Figure 2. Map of Glen Diomhan NNR showing coring site (x). 33.2

4 54 G. Steven and J. H. Dickson were driven and the section cut away. A Livingstone corer of 5 cm diameter was then used to take samples frotn 9 to 9 cm and 9 cm to the base of the peat at 235 cm. The peat was found to overlie granitederived sand and gravel. SEDIMENT STRATIGRAPHY A detailed description of the peat samples is given below (Table ). The stratigraphy is straightforward in that most of the sequence comprises highly humified peat containing remains of Molinia and Eriophorum, Only the uppermost and lowermost few centimetres difter; the base has a high content of sand while the top is fibrous and poorly consolidated. Charcoal occurs throughout the sequence but is not present as discrete layers obvious to the naked eye. Macrofossils encountered during the sediment characterization are also recorded. POLLEN ANALYSIS Samples for analysis were prepared by standard methods. Identification was aided by reference to the standard texts and the pollen reference collection held in Glasgow University Botany Department. The resulting pollen stratigraphy (Fig. 3) was subdivided by inspection into local assemblage zones. Individual pollen curves are presented in order of occurrence within each category and are to the same scale. Percentages of % and over are represented as a line, less than % by a cross and taxa encountered during scanning of slides by an asterisk. Taxa in four or less samples are recorded as lettered symbols; all of these are present as less than % of the total. The estimation of the time scale is based upon correlations with other pollen diagrams, in particular those from Loch a'mhuilinn (Boyd & Dickson, 987a) and Machrie Moor (Robinson & Dickson, 988). The sequence may have begun at or just before 8 years BP. DESCRIPTION OF LOCAL POLLEN ASSEMBLAGE ZONES AND INFERRED VEGETATIONAL HISTORY The following local pollen assemblage zones have been determined by inspection and named from the most abundant and/or ecologically important pollen and spores taxa. Calluna Coryloid Betula (Local pollen zone GC-, cm, c. 8 to c. 7 BP) This zone is characterized by relatively high values of Betula and Coryloid but very low values of Alnus. The top of the zone has a marked rise in Alnus, the rational limit, which has dates of to yrs BPat Loch a'mhuilinn (Boyd & Dickson, 987(2). The high Coryloid values indicate a post Corylus date for the base of the sequence. The Corylus rise on Arran, dated at yrs BP at Loch a'mhuilinn and yrs BP at Machrie Moor (Robinson & Dickson, 988), is slightly later than at mainland sites (Boyd & Dickson, 986). Ulmus and Quercus both have fairly high values in the zone although both have generally higher values in the following zone suggesting that this was a time of widespread development of deciduous woodland on Arran. Betula reaches its maximum in this zone with a value of over % in one sample so that it is probable that Betula was present around the coring site. Calluna has values reaching over 5%, almost as high as the maximum values reached later in the deposit and the values remain fairly stable throughout the sequence rarely falling below 2 5 %. This contrasts with the situation at Loch a'mhuilinn where Calluna values remain low until around 4 yrs BP when there is an expansion of Calluna and a corresponding decline of Betula. Grass values are also fairly high, mostly over % and there is a high representation of taxa typical of unshaded ground such as Succisa, Drosera, Lotus, Polygala, Selaginella, Lycopodium and Huperzia. Drosera rotundifolia L,./angliea Hudson reaches 2 % and occurs in no less than 7 other samples scattered through all five zones. D. rotundifolia ' is confined to communities with an open shrub layer or with only a layer of dwarf shrubs; it occurs in full sun and in some shade from shrubs such as Calluna, but does not survive shading if the canopy becomes dense' and D. anglica 'does not tolerate shading' (Crowder et al., 99, pp. 235 and 253). In 9 samples, 5 in zone and the rest scattered through zones 2, 3 and 4, pollen of Lotus corniculatus h./uliginosus Schkuhr further emphasizes the open nature of the site during the long period of peat deposition. This zone and the zone above have generally higher values for Filicales undift. than in later zones, including one sample with a very large number of spores, and several Osmunda spores were recovered. There is good evidence, therefore, that while there may have been limited cover of hazel and birch on the slopes of the glen, perhaps with willows in wetter places, much of the glen appears to have been occupied by open Calluna heath. Calluna-Alnus-Quercus-Ulmus (Local pollen zone GC-2, 25-6 cm, c. 7 to 5 np) This zone sees an increase in Ulmus and Quercus to their maximum values (both under %) and trees and shrubs as a whole also reach peak values here probably reflecting the maximum development of woodland cover on the island. Alnus values increase steadily through the zone and it is probable that alder colonized the low-lying parts of the glen. There are the first records of Fraxinus and Ilex pollen.

5 shrubs- -trees /, ^ ^ < * * -s? ;?> /.<f <\' K^A/',. ^^^^M/^^ u unidentified pteridophytes + sphagnum - herbs and graminids heaths- ^ ^ - ^ >!>^.^>t<cp^ a.c^^l,-^^' u GC-5 ao eo ' 2 7 highly humified + Eriophorum * -h * + h LV J LV * -f 4 -J 4 -Molinia GC-4 peat eo GC ISO 6 sooo 7 GC-2 B 6 9 ' 2 ' 2-7 GC (6 2 so Pr Prunui So SorbuB oucuparia Glen Catacol, North Arran. + less than % * observed during scanning LV LalhyruB/VlcIa lyp«fo FabacAW undtlf. M M.ntha typ«h H«racf»um aphondyltum Ag Aorimonlupatorl«/odor«U Co CaryophyllacMB undiff. OB Oamunda rsgalib Vo Valadanaoftlcinalls Pm Plantogo mafuima St Sttllaria holobtca Op OphlogloBBum vulgatum Th ThaUctrummlnuB/alplnum Ga Qallum Ch ChrwoBplanlum It.rnHollum/oppoBltltoiium Lc Lycopodium clavitum/ilplnum Lo LycopodlumannoUnum/complanatum Tr Trirotium r«p«nb Ci CIrBlum Co ConvolvuluB arvvnbib Figure 3. Percentage pollen diagram from Glen Catacol, close to Glen Diomhan NNR. Values are expressed as % total pollen.

6 Vegetational history of Glen Diomhan, Arran 55 Table. Sediment description after {955) and macrofossils encountered. Troels-Smith O-OO-2-32 m ThH; highly humified Eriophorum- Molinia peat. Eriophorum remains from surface to -9 m; Molinia leaf fragments at '25 and -7 m; Potentilla fruits O'OS and 3 m; Ericaceae seeds -25 and -7 m; Sphagnum leaves -4 and -85 m; Calluna stem -35 m. Layer becomes gradually sandy below m Th'3, G min ; highly humified sandy peat. Considering the prevalence of poor, acid soils in the glen it is unlikely that ash or elm were ever present locally. Indeed, the low values for Quercus suggests that even oak may have been rare in the glen which contrasts with the situation at Loch a'mhuilinn where a maximum value of 48 % was recorded with many samples over 2%. Sorbus is first recorded at both Loch a'mhuilinn and Machrie Moor around this time. Open ground taxa continue to be important and Filicales undiff. still have relatively high values. Many spores of Polypodium were encountered through the deposit but especially in this zone which appeared to differ from typical P. vulgare L. in the appearance of the spore wall. These were placed in a separate taxon P. cf. interjection Shivas. The top of the zone is marked by a rapid fall in Ulmus values corresponding to the Elm Decline. This feature is dated at 536+ yrs BP at Loch a'mhuilinn and yrs BP at Machrie Moor. There is an associated decline in oak and the first appearance of indicators of disturbance e.g. Plantago lanceolata L., Artemisia providing the first signs of regional woodland clearances. Calluna-Alnus-Quercus (Local pollen zone GC-3, 6-2 cm, 5 to c. 35 BP) Defining this zone is a temporary recovery of Ulmus until it declines to very low values which continue to the top of the deposit. This secondary decline is seen in many pollen diagrams e.g. Machrie Moor (Robinson & Dickson, 988). Pinus reaches its peak value in this zone before falling to very low values. There is no conclusive evidence that pine was ever present on Arran and it is more likely that the pollen is longdistance transported. Quercus values begin to fall steadily in the zone, Alnus values fluctuate but remain fairly high while Coryloid shows something of a recovery. Cyperaceae values increase and fall. The immediate environment around the coring site appears to have remained unchanged through the zone with a continued prominence of open ground taxa. Calluna-Alnus-Pteridium (Local pollen zone GC-4, 2-25 cm, c. 35 to c. BP) Pteridium values increase significantly in this zone and there are many indicators of disturbance and possible signs of agriculture. There are the first records of Cerealia pollen (grass grains witb longest axis > 4/<), an increase in Plantago lanceolata and records for Trifolium repens L., Caryophyllaceae, Lathyrus/Vicia, Heracleum sphondylium L., Cirsium, Chenopodiaceae and Taraxacum type. Betula values remain steady while Quercus, Alnus and Coryloid all show a general decline. The single grain of Sorbus recovered occurs here and was assigned to S. aucuparia on tbe basis of its dimensions and surface texture (Boyd & Dickson, 9876). Calluna reaches its maximum, with values seldom below 4 %. Ericaceae values also increase somewhat. Poaceae values are generally higher tban in the previous zone. Cultivation may have been taking place in the glen but would probably be concentrated on the sandy soils near the coast. The decline of Alnus, Quercus and Coryloid may be an indication of gradual clearance of the woodland in the lower parts of the glen allowing an expansion of tbe Calluna heath and acid grassland. It was around this time that the main expansion of Calluna took place at Loch a'mhuilinn. Calluna-Poaceae Cyperaceae (Local pollen zone GC-5, 25- cm, c. BP to present) The pollen concentration falls away markedly in this the topmost zone due to the poorly consolidated nature of the sediment. It is characterized by large rises in Cyperaceae and grasses, both of wbicb attain their maximum values. There is an associated decline in Betula, Quercus, Alnus and Coryloid to very low values. The apparent recovery of Betula in the final sample is probably an anomaly caused by the very low pollen concentration. Calluna values generally remain high and there is a continued presence of agricultural indicators including a record of Convolvulus arvensis, very seldom encountered in pollen analysis. The zone thus appears to show the further clearance of any remaining woodland in the glen to the present level of cover. The decline of birch after relatively stable values throughout the rest of tbe sequence suggests tbat this was a time of retreat to isolated stands due to grazing/burning pressure. DISCUSSION Tbat no fossil pollen of tbe endemic Sorbus spp. was recovered in the analysis is neither surprising nor necessarily significant as these small trees have very

7 56 G. Steven and jf. H. Dickson Table 2. Results of surface pollen analysis for selected taxa carried out in Glen Diomhan. Site is c. m from the reserve; site 2 is from beneath Sorbus spp within the reserve. Figures are percentage of total pollen other than Sphagnum (% total /)o//em- -Sphagnum). From Boyd and Dickson {987a). Taxon Site Site 2 Betula Quercus Alnus Pinus Fraxinus Ulmus Sorbus Salix Coryloid CaUuna Erica Poaceae Cyperaceae Sphagnum occasional burning and by the grazing of large herbivores such as red deer, known to be indigenous to the island (Gibson, 985). CURRENT MANAGEMENT Part of the NNR is surrounded by a deer fence to decrease grazing by sheep and deer. Attempts have been made to establish young trees of the endemics within this enclosure mainly on the upper slopes of the gully but this has met with only limited success (Boyd et al., 988). Other trees and shrubs have also been planted in the enclosure with the aim of providing more sheltered conditions in the gorge. The endemics favour very steep slopes which are subject to soil movement and rock falls which limits the trees to a certain stature above which they become prone to windthrow. The reduction in grazing pressure may enable the endemics to spread to less precipitous parts of the gorge. poorly dispersed pollen (Boyd & Dickson, 987(2, b). Modern pollen deposition was examined by surface pollen analyses using mixed moss polsters (Table 2). The results clearly show the localized nature of Sorbus pollen deposition. Nevertheless, it is unfortunate that we do not know when the endemics first appeared in Glen Diomhan or whether they were ever rnore widespread in the area. Their appearance at Loch a'mhuilinn is tentatively correlated with an opening up of the surrounding woodland (Boyd & Dickson, 987a). Around Glen Catacol, however, it appears that the vegetation has been open throughout the Flandrian at least in the upper parts of the glen. Alder woodland may have extended far up the valley floor with hazel/oak woodland occupying parts of the slopes but CaUuna heath, perhaps with occasional patches of birch and rowan, seems to have dominated the upper reaches of the glen. There is no alder woodland in the glen today but substantial hazel/oak woodland occurs on the slopes above the coast. It seems unlikely that the endemic Sorbus spp were ever important members of this type of woodland from their absence in the present day community and it may be that they have always been restricted to places with little competition from other species. The fact that the sampling site, situated so close to the nature reserve, has remained free of closed tree cover throughout the last 8 years or so suggests that the woodland in the gorge has never been significantly more extensive. The open nature of the local vegetation could have been maintained by the poor thin soil in conjunction with the climatic conditions very conducive to peat growth, by ACKNOWLEDGEMENTS This research was funded by the Nature Conservancy Council (South West Scotland Region) and carried out in the Department of Botany, Glasgow University. REFERENCES ADAM, P., BIRKS, H. J. B. & HUNTI.EY, B. (977). Plant communities on the Island of Arran, Scotland. New Phytologist 79, BlGNAL, E. (98). The endemic Whitebeams of north Arran. Glasgow Naturalist 2, BOYD, W. E. & DICKSON, J. H. (986). Patterns in the geographical distribution of the early Flandrian Corylus rise in south-west Scotland. New Phytologist 2, BOYD, W. E. & DICKSON, J. H. (987a). A postglacial pollen sequence from Loch a'mhuilinn, north Arran: a record of vegetation history with special reference to the history of endemic Sorbus species. New Phytologist 7, Ill-IAA. BOYD, W. E. & DICKSON, J. H. (9876). The pollen morphology of four Sorbus species, with special reference to two Scottish local endemic species, 5. arranerisis Hedl. and 5. pseudofennica E.F. Warb. Pollen et Spores 24, BOYD, W. E., LAING, A. E., STEVEN, G. & DICKSON, J. H. (988). The history and present management of two rare endemic trees on the isle of Arran, Scotland. Environmental Conservation IS, CHOWDER, A. A., PEARSON, N. C, GRUISB, P. J. & LANGLOIS, P. H. (99). Drosera L. Journal of Ecology 78, FAIRHURST, H. C. (98). Exploring Arran's Past. Central Press, Glasgow. GIBSON, J. A. (985). The land and marine mammals of the Isle of Arran. Arran Naturalist 8, HULL, P. & SMART, G. J. B. (984). Variation in two Sorbus species endemic to the Isle of Arran, Scotland, Annals of Botany 53, LACAILLE, A. D. (954). The Stone Age in Scotland. Cambridge. RoniNSON, D. E. (98). The vegetational and land use history of the west of Arran, Scotland Ph.D. Thesis, University of Glasgow. RoniNSON, D. E. & DICKSON, J. H. (988). Vegetational history and land use : a radiocarbon-dated pollen diagram from Machrie Moor, Arran, Scotland. New Phytologist 9, TROEI.S-SMITM, J. (955). KaraUterisering af lose jordater. Danmarks Geologiske Undersogelse 3 (), -73.

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