Tree and scrub species of the Treeline Ecotone in the Cairngorms National Park, Scotland


  • Andy Amphlett



forest line; scrub line; montane scrub; mountain birch; birch belt; introgression


Expansion of native woodland, in places up to a natural treeline with montane scrub above, is a priority within the Cairngorms National Park (CNP). Thirty native and six non-native tree and scrub species (phanerophytes and nano-phanerophytes) have records within the CNP treeline ecotone, defined as between 500 and 1000 m altitude. The altitudinal distribution (using a 25 m altitude interval) of the native species is analysed at monad (1 km grid square) precision. Results are displayed as the raw monad counts, monad counts calibrated to take account of altitudinal recording bias, and calibrated monad counts as a percentage of monads within the CNP. Summing the calibrated species monad counts in each altitude (25 m) class provides a combined measure of spatial and altitudinal frequency.

The same ten species occupy the top ten rank frequency positions in each of the altitudinal zones (forest line, treeline woodland and montane scrub). Therefore, across the whole treeline ecotone, we can expect woodland and scrub to comprise the same mix of species, though their proportions will vary, both altitudinally and geographically. Juniperus communis and Sorbus aucuparia are the two most widespread species in all three zones, Pinus sylvestris is in 4th or 5th place, while Betula pubescens is in 7th or 8th place.

In Norway there is typically a 'birch belt', in which Betula pubescens var. pumila (L.) Govaerts forms the highest treeline, above the altitudinal limit of Pinus sylvestris. It has recently been argued that an equivalent birch belt used to be present in Scotland, and should be a target for conservation interventions to restore it. The present analysis does not provide support for this view. While the low rank position of B. pubescens may reflect lack of seed sources and browsing pressure, that P. sylvestris occurs to very high altitudes, and comprises a higher percentage of the combined CNP calibrated taxon monad totals above 700 m than it does from 500-700 m strongly suggests that B. pubescens will not be able to form an exclusive 'birch belt' at elevations above the P. sylvestris altitudinal limit.

While B. pubescens var. pumila has not been confirmed as occurring in Scotland, introgression from B. nana into B. pubescens is widespread here, though only at a low level. The most parsimonious scenario that explains this is that during the post glacial period any highly introgressed B. pubescens, derived from tetraploid hybrids crossing with B. pubescens, rapidly crossed with 'ordinary' B. pubescens, hence diluting and dispersing the genetic contribution of B. nana. Within the CNP, Mar Lodge NNR is the most likely location where future hybridisation and introgression of B. nana into B. pubescens will occur.


Amphlett, A. 2003. Contexts, developing ideas and emerging issues in the conservation management of the RSPB Abernethy Forest Reserve. Botanical Journal of Scotland 55:135-148.

Amphlett, A. 2021. Identification and taxonomy of Betula (Betulaceae) in Great Britain and Ireland. British & Irish Botany 3(2):99-135.

Amphlett, A. 2022. A review of the vascular plant flora of the Cairngorms Connect project area, Scotland, and some possible implications of forest expansion to the natural tree line. British & Irish Botany 4(3):166-226.

Ashburner, K. & McAllister, H.A. 2013. The Genus Betula. A Taxonomic Revision of Birches. Kew: Kew Publishing.

Birks, H.H. & Birks, H.J.B. 2000. Future Uses of Pollen Analysis Must Include Plant Macrofossils. Journal of Biogeography 27:31-35.

Birks, H.H. 2003. The importance of plant macrofossils in the reconstruction of Lateglacial vegetation and climate: examples from Scotland, western Norway, and Minnesota, USA. Quaternary Science Reviews 22:453-473.

Bryn, A. & Potthoff, K. 2018. Elevational treeline and forest line dynamics in Norwegian mountain areas – a review. Landscape Ecol. 33:1225–1245.

Cairngorms National Park Authority 2022. Cairngorms National Park Partnership Plan 2022-27.

Cairngorms Connect (undated). Looking to Norway to see the habitats of our future.,200%2Dyears%20into%20the%20future [Accessed 9 February 2024]

Clapham, A.R., Tutin, T.G. & Warburg, E.F. 1952. Flora of the British Isles. Cambridge: Cambridge University Press.

Dines, T.D. & Pearman, D.A. 2023. Alnus incana (L.) Moench. In: Stroh, P.A., Humphrey, T.A., Burkmar, R.J., Pescott, O.L., Roy, D.B. & Walker, K.J. eds. BSBI Online Plant Atlas 2020 [Accessed 11 February 2024]

Druce, G.C. 1905. Report for 1904, p.33. The Botanical Exchange Club and Society of the British Isles.

Elkington, T.T. 1968. Introgressive hybridization between Betula nana L. And B. pubescens Ehrh. in north-west Iceland. New Phytologist 67:109-118.

Elven, R. & Fremstad, E. 2018. Salix – vier, selje og pil i Norge. (The genus Salix (willows) in Norway.) Gunneria 82.

Finger, A., Rao, S., Cowie, N., MacDonell, T., Beck, A., & Denny, B. 2023. Conservation genetics of montane willow populations in Scotland—Limited natural recovery despite long-distance gene flow and high genetic diversity. Environmental Research: Ecology 2:015001.

Fitter, A. 1978. An Atlas of the Wild Flowers of Britain and Northern Europe. London: Collins.

Forbes, J.C. & Kenworthy, J.B. 1973. Distribution of two species of birch forming stands on Deeside, Aberdeenshire. Transactions of the Botanical Society of Edinburgh 42:1:101-110.

Gaillard, Marie-Jose & Birks, Hilary. 2013. Paleolimnological Applications.

Gilbert, D. 2010. Interactions between climate and land use which drive dynamics in treeeline ecotone scrub in Scotland. A thesis presented for the degree of Doctor of Philosophy at the University of Edinburgh.

Gimingham, G.H. 2002. Vegetation. In: Gimingham, C., The Ecology, Land Use and Conservation of the Cairngorms. Chichester: Packard Publishing Limited.

Gullett, P.R., Leslie, C., Mason, R., Ratcliffe, P., Sargent, I., Beck, A., Cameron, T., Cowie, N.R., Hetherington, D., MacDonell, T., Moat, T., Moore, P., Teuten, E., & Hancock, M.H. 2023. Woodland expansion in the presence of deer: 30 years of evidence from the Cairngorms Connect landscape restoration partnership. Journal of Applied Ecology 60:2298–2308.

Halley, D.A. 2011. Parallel Universe - Montane Scrub depletion and regeneration in coastal southwest Norway. Scrubbers’ Bulletin No. 9:13-21.

Halley, D. 2021. Bringing Back the Birch Belt - Scotland's Lost Mountain Woodland.

Halley, D. (Undated) (accessed 7 January 2024). Mountain birch.

Hill, M.O., Preston, C.D. & Roy, D.B. 2004. Plantatt. Attributes of British and Irish Plants: Status, Size, Life History, Geography and Habitats. Biological Records Centre. NERC Centre for Ecology & Hydrology.

Jonsell, B. 2000. Betula. In: Jonsell, B., ed. Flora Nordica, Volume 1. Stockholm: The Bergius Foundation. The Royal Swedish Academy of Sciences.

Jonsell, B. 2004. Forests, in Features of Nordic environment and vegetation. In: Jonsell, B., ed. Flora Nordica, General Volume. Stockholm: The Bergius Foundation. The Royal Swedish Academy of Sciences.

Kullman, L. and Öberg, L. 2009. Post-Little Ice Age tree line rise and climate warming in the Swedish Scandes: a landscape ecological perspective. Journal of Ecology 97:415-429.

Kullman, L. 2013. Ecological tree line history and palaeoclimate, Swedish Scandes. Boreas 42:555-567.

Linnaeus, C. 1745. Flora Suecica. Stockholm: Laurentius Salvius.

Löve, A. & Löve, D. 1956. Cytotaxonomical conspectus of the Icelandic Elora. Acta Horti Gotob. 20:65-291.

MacKenzie, N.A. 2000. Low Alpine, Subalpine & Coastal Scrub Communities in Scotland. Highland Birchwoods. Munlochy.

Marshall, E.S. 1901. Plants of North Scotland, 1900. Journal of Botany 39:270-271.

Marshall, E.S. 1914. Betula account. In: Moss, C.E., The Cambridge British Flora. Volume 2. Cambridge: Cambridge University Press.

Maskew, R. 2023. Rosa spinosissima L. In: Stroh, P.A., Humphrey, T.A., Burkmar, R.J., Pescott, O.L., Roy, D.B. & Walker, K.J. eds. BSBI Online Plant Atlas 2020. [Accessed 4 February 2024]

McCosh, D.J. & Aspey, N. 2023. Rubus idaeus L. In: Stroh, P.A., Humphrey, T.A., Burkmar, R.J., Pescott, O.L., Roy, D.B. & Walker, K.J. eds. BSBI Online Plant Atlas 2020. [Accessed 4 February 2024]

McVean, D.N. & Ratcliffe, D.A. 1962. Plant Communities of the Scottish Highlands. London: HMSO.

Meikle, R.D. 1984. Willows and Poplars of Great Britain and Ireland. BSBI Handbook no. 4. London: Botanical Society of the British Isles.

Nagy, J., Nagy, L., Legg, C. & Grace, J. 2013. The stability of the Pinus sylvestris treeline in the Cairngorms, Scotland over the last millennium. Plant Ecology & Diversity 6.

National Trust for Scotland. 2012. Mar Lodge Estate Forest Plan 2012-2032.

National Trust for Scotland. 2022. Mar Lodge Estate Operational Management Plan 2022.

Odland, A. 2015. Effect of latitude and mountain height on the timberline (Betula pubescens ssp. czerpanovii) elevation along the central Scandinavian mountain range. Fennia - International Journal of Geography 193(2):260–270. Available at:

Pears, N.V. 1968. The natural altitudinal limit of forest in the Scottish Grampians. Oikos 19:71-80.

Pears, N.V. 1970. Post glacial tree-lines of the Cairngorm mountains, Scotland: some modifications based on radiocarbon dating. Trans. Bot. Soc. Edinb. 40:536-544.

Peterken, G.F. 1981. Woodland Conservation and Management. Cambridge: Chapman and Hall.

Peterken, G.F. 1996. Natural Woodland. Ecology and Conservation in Northern Temperate Regions. Cambridge: Cambridge University Press.

Pratt, S. 2006a. Long-term landscape dynamics in a Caledonian pine forest. PhD thesis, University of Edinburgh.

Pratt, S. 2006b. Reconstructing past landscapes at Abernethy Forest: some new insights from palaeoecology and pollen modelling. Scottish Woodland History Discussion Group: Notes XI, 15-17.

Rao, S.J. 2017. Effect of reducing red deer Cervus elaphus density on browsing impact and growth of Scots pine Pinus sylvestris seedlings in semi-natural woodland in the Cairngorms, UK. Conservation Evidence 14:22–26.

Reforesting Scotland (undated). The Mountain Birch Project. [Accessed 18 February 2024]

Rodwell, J.S. (ed.) 1991. British Plant Communities volume 2. Mires and Heaths. Cambridge: Cambridge University Press.

Shaw, P. & Thompson, D. (eds.). 2006. The Nature of the Cairngorms. Edinburgh: Scottish Natural Heritage.

Smout, C. 1997. Highland Land-use before 1800: Misconceptions, Evidence and Realities. In: Smout, T.C. (ed.) Scottish Woodland History. Edinburgh: Scottish Cultural Press.

Smout, C. 2014. The History and the Myth of Scots Pine. Scottish Forestry 68.

Stace, C.A. 1991. New Flora of the British Isles. 1st ed. Cambridge: Cambridge University Press.

Stace, C.A. 2019. New Flora of the British Isles. 4th ed. Middlewood Green, Suffolk: C & M Floristics.

The Scottish Rewilding Alliance 2024. Rewilding Stories - The Mountain Birch Project. [Accessed 10 February 2024]

Towers, W., Hall, J., Hester, A., Malcolm, A. & Stone, D. 2004. The potential for native woodland in Scotland: the native woodland model. Battleby: Scottish Natural Heritage. The GIS layer is at

Walker, K. & Harding. M. 2024. The spread of Sitka spruce Picea sitchensis in Britain and Ireland. Evidence from botanical surveys and implications for biodiversity and forestry. Scottish Forestry 78:34-39.

Wang, N., Borrell, J.S. & Buggs, R.J.A. 2014. Is the Atkinson discriminant function a reliable method for distinguishing between Betula pendula and B. pubescens (Betulaceae)? New Journal of Botany 4(2):90-94.

Watts, S.H. & Dines, T.D. 2023. Betula nana L. In: Stroh, P.A., Humphrey, T.A., Burkmar, R.J., Pescott, O.L., Roy, D.B. & Walker, K.J. eds. BSBI Online Plant Atlas 2020. [Accessed 6 February 2024]

Watts, S.H. & Hutchinson, G. 2023a. Salix arbuscula L. In: Stroh, P.A., Humphrey, T.A., Burkmar, R.J., Pescott, O.L., Roy, D.B. & Walker, K.J., eds. BSBI Online Plant Atlas 2020. [Accessed 4 February 2024]

Watts, S.H. & Hutchinson, G. 2023b. Salix myrsinifolia × phylicifolia = S. × ⁠tetrapla Walker. In: Stroh, P.A., Humphrey, T.A., Burkmar, R.J., Pescott, O.L., Roy, D.B. & Walker, K.J. eds. BSBI Online Plant Atlas 2020. [Accessed 20 February 2024]

Watts, S.H. & Hutchinson, G. 2023c. Salix myrsinites L. In: Stroh, P.A., Humphrey, T.A., Burkmar, R.J., Pescott, O.L., Roy, D.B. & Walker, K.J. eds. BSBI Online Plant Atlas 2020. [Accessed 7 February 2024]

Worrell, R. & Malcolm, D.C. 1998. Anomalies in the distribution of silver birch (Betula pendula Roth) populations in Scotland. Botanical Journal of Scotland 50: Issue 1:1-10.

Zohren, J., Wang, N., Kardailsky, I., Borrell, J.S., Joecker, A., Nichols, R.A. & Buggs, R.J.A. 2016. Unidirectional diploid–tetraploid introgression among British birch trees with shifting ranges shown by restriction site-associated markers. Molecular Ecology 25:2413–2426.