Mixed Northern Hepatic Mat, Or Hepatic Heath, Is One of the Most Beautiful and Conspicuous Bryophyte Communities Found in Britain and Ireland

Article Northern hepatic mat

The mixed northern hepatic mat, or hepatic heath, is one of the most beautiful and conspicuous bryophyte communities found in Britain and Ireland. It forms large, colourful mats of liverworts on mainly north-facing mountain slopes in western Scotland and Ireland, under heather and in rocky areas. As Rory Hodd and Micheline Sheehy Skeffington describe, it is extremely limited in distribution, as it requires very specific climatic and topographic conditions Mixed northern hepatic mat: which exist on only a handful of oceanic mountain ranges. For these reasons, this community is extremely a threatened and unique vulnerable to a number of threats, notably overgrazing and climate bryophyte community change. n the broadest sense of the term, a hepatic mat is term that will be used here when referring to m The eastern corrie of Mt Brandon, Co. Kerry, which and Bazzania pearsonii are relatively widespread, a continuous weft of leafy liverworts, and may mixed northern hepatic mat specifically. provides ideal conditions for the growth of hepatic mat but are more confined to areas where conditions species. R. Hodd be dominated by a range of relatively common Within Britain and Ireland, the most extensive are most suited to their growth. Scapania species, including Scapania gracilis, Plagiochila and species-rich stands of mixed northern hepa- The species composition of mixed northern nimbosa and Plagiochila carringtonii are widely spinulosa and Diplophyllum albicans. It also tic mat are found in the north-west Highlands of hepatic mat vegetation varies geographically, but distributed in western Scotland, but are very Irefers to a snowbed community that is composed Scotland. This community is also well represent- all stands have a similar community structure rare in Ireland, being restricted to two and entirely of liverworts (G. Rothero, pers. comm.). ed in other parts of the Scottish Highlands and and are dominated by species of a small group of one extant sites, respectively. Three species of The mixed northern hepatic mat is, therefore, a number of areas of western Ireland. Similar, large leafy liverworts. Herbertus aduncus subsp. Anastrophyllum – A. donnianum, A. joergensenii one of a number of types of hepatic mat, and less species-rich hepatic mats occur in the Lake hutchinsiae is a conspicuous species in many and A. alpinum – are not uncommon, and are may be more appropriately called ‘hepatic heath’, District of England and North Wales (Averis, areas, forming red to orange cushions on shady sometimes dominant, in Scotland, but are as it frequently grows in association with heath 1994). Species-poor hepatic mat vegetation has slopes and cliffs, with Pleurozia purpurea and unknown from Ireland. The rarest hepatic mat vegetation. However, as the majority of published also been found in Norway (Jordal & Hassel, Bazzania tricrenata also frequent throughout. species are Adelanthus lindenbergianus, which is literature uses the term ‘hepatic mat’, that is the 2010) and the Faeroe Islands (Averis, 1994). Scapania ornithopodioides, Mastigophora woodsii restricted to seven known sites in Ireland and

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one on Islay in Scotland, and Herbertus borealis, Campylopus setifolius, and the fern Hymenophyl- which has its worldwide headquarters on Beinn lum wilsonii (Hodd & Sheehy Skeffington, un- Eighe in Wester Ross. published). A number of common pleurocarpous A further number of species frequently grow and large acrocarpous moss species also often in hepatic mats, but cannot be said to be grow sparingly among the large hepatic species. characteristic of, or confined to, this community. These include the liverworts Scapania Worldwide distribution b Rocky hepatic heath, gracilis, Anastrepta orcadensis, Mylia taylorii, Many of the constituent species of hepatic mat Loch A’Mhadaidh, Plagiochila spinulosa and Lepidozia pearsonii, the are of disjunct distribution (Table 1), and are Fannich Hills, Wester mosses Sphagnum capillifolium, Racomitrium all, with the exception of Bazzania tricrenata, Ross. R. Hodd lanuginosum, Dicranodontium uncinatum and restricted within Europe to the hyper-oceanic

Table 1. The British, Irish and worldwide distribution of hepatic mat species, and altitude range at which they grow in Britain and Ireland (data from Paton, 1999)

Species British and Irish Worldwide Altitude range distribution distribution (m) in Britain and Ireland Adelanthus SW, W & NW Ireland, Islay Tropical and southern Africa, Central 380–800 lindenbergianus and South America, Antarctica Anastrophyllum NW Scotland Himalaya, W China, Alaska 700–950 alpinum Anastrophyllum Cairngorms, NW Scotland SW Norway, Tatra, Faeroes, 300–1,060 donnianum W Canada, Alaska, Himalaya Anastrophyllum Cairngorms, NW Scotland Norway, W China 700–950 b Bazzania pearsonii, joergensenii Herbertus aduncus Bazzania pearsonii SW, W & NW Ireland, Cairngorms, E & SE Asia, Himalaya, 300–1,000 subsp. hutchinsiae and NW Scotland NW North America Pleurozia purpurea in Bazzania tricrenata S, W & N Ireland, Scotland, Montane Europe, Faeroes, 0–1,220 Derrymore Glen, Slieve N England, N Wales North America, Guatemala, Asia Mish, Co. Kerry. Herbertus aduncus S, W & N Ireland, W Scotland, W Norway, Faeroes 0–1,040 R. Hodd hutchinsiae subsp. N England, N Wales regions of Ireland, Scotland, the Faeroe Islands Mountains of China (Long, 2008). However, Herbertus borealis NW Scotland SW Norway 380–550 and Norway. Outside north-western Europe, in the Sino-Himalaya, the species are associated Mastigophora woodsii SW & W Ireland, W Scotland Faeroes, NW North America, 100–1,000 most of these species occur in either north- with other similar liverworts which do not extend C and E Asia western North America or the Himalayas and to Europe, such as several other large Scapania Plagiochila carringtonii W Ireland, Cairngorms, W Scotland Faeroes, Nepal 300–1,070 western China. The one exception is Adelanthus species (D. Long, pers. comm.) Pleurozia purpurea SW, W, C & N Ireland, SW Norway, Faeroes, Jan Mayen, 0–915 lindenbergianus, which has also been found in Two hypotheses have been put forward for W & C Scotland Alaska, Himalaya, Guadeloupe tropical and southern Africa, South and Central the origin of the current disjunct distribution Scapania nimbosa SW, W & NW Ireland, Cairngorms, SW Norway, E Himalaya, W China 400–1,070 America and Antarctica (Paton, 1999). In these of these species: first, they may previously have W Scotland, N Wales areas, the British and Irish species, or closely had a continuous distribution between their cur- Scapania SW, W & NW Ireland, Cairngorms, Norway, Faeroes, Himalaya, Asia, 300–1,000 related species, often grow together, forming rent centres of population, but due to changes ornithopodioides W Scotland, N England, N Wales Hawaii hepatic mats, such as in the Gaoligong Shan in the climate have retracted to areas where

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Table 1. The British, Irish and worldwide distribution of hepatic mat species, and altitude range at which they grow in Britain and Ireland (data from Paton, 1999)

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Topography is also highly important for reach their full composition on north-east- to hepatic mat species, as the local topography north-facing slopes, where there is less sunlight determines the microclimate that many hepatic and lower evapotranspiration. They prefer steep, species depend on. Thus in most sites, hepatic well-drained slopes, but can also occasionally mats are confined to areas that have a specific occur on deep blanket peat. The degree of combination of altitude, aspect, slope and exposure also plays a role in determining where shelter. Hepatic mats generally do not grow hepatic mats will grow, meaning that deep below about 300 m, as at higher altitudes there corries, with high humidity and low sunlight is a combination of lower temperatures, less solar are the favoured habitat, along with areas shaded radiation, higher rainfall and more moisture due by boulders (Averis, 1994). All of these factors to the presence of more mist and cloud than at combine to greatly restrict the area of suitable lower altitudes (Hodd & Sheehy Skeffington, habitat for the growth of hepatic mat bryophytes 2011). Hepatic mats can grow at altitudes of in Britain and Ireland. up to 1,000 m in areas where snow cover pro- tects the species from winter frosts (D. Long, . The almost endemic Herbertus borealis, Beinn Eighe pers. comm.) Hepatic mats generally grow and NNR, Wester Ross. R. Hodd

conditions are suitable for their growth. The between the amount of moisture in the air and other possible origin of these species in north- how much moisture the air can hold when it is western Europe is through long-range dispersal of saturated; Crawford, 2000). There is a gradient spores from other populations in the Himalayas, of oceanicity from east to west, resulting in North America or Africa (Schofield & Crum, a high climatological contrast between the 1972). The latter hypothesis is gaining ground maritime western fringe of Britain and Ireland over the former, despite the fact that they are not and the relatively continental interior of these known to regularly reproduce sexually in Britain islands (Sweeney, 1997). or Ireland. Frequency, rather than the overall amount of rain appears to be the main controlling Ecological requirements factor in the distribution of hepatic mats, since The climate of western Ireland and western their distribution is closely associated with Scotland is extremely suitable for the growth areas where there are more than 220 rain days of hepatic mat bryophytes. These areas have a (days with more than 1 mm of rain) per year hyper-oceanic climate which results in, among (Ratcliffe, 1968). This high frequency of rain other things, a lack of extreme high or low results in a constantly humid atmosphere and temperatures, a high temperature lapse rate (the low evaporation and transpiration, particularly rate of temperature decrease with height), high in mountain areas. Hepatic mats also show better cloudiness, high and frequent rainfall and a low growth in areas with cool summers and mild vapour pressure saturation deficit (the difference winters (Averis, 1994).

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m Mastigophora woodsii among m Hepatic mat, dominated by Herbertus aduncus subsp. hutchinsiae, finding grassy vegetation, Brandon, refuge from heavy grazing on a cliff ledge, Slievemore, Achill Island, Co. Mayo. Co. Kerry. R. Hodd R. Hodd conditions are suitable for their growth. The between the amount of moisture in the air and other possible origin of these species in north- how much moisture the air can hold when it is western Europe is through long-range dispersal of saturated; Crawford, 2000). There is a gradient spores from other populations in the Himalayas, of oceanicity from east to west, resulting in North America or Africa (Schofield & Crum, a high climatological contrast between the 1972). The latter hypothesis is gaining ground maritime western fringe of Britain and Ireland over the former, despite the fact that they are not and the relatively continental interior of these known to regularly reproduce sexually in Britain islands (Sweeney, 1997). or Ireland. Frequency, rather than the overall amount of rain appears to be the main controlling Ecological requirements factor in the distribution of hepatic mats, since The climate of western Ireland and western their distribution is closely associated with Scotland is extremely suitable for the growth areas where there are more than 220 rain days of hepatic mat bryophytes. These areas have a (days with more than 1 mm of rain) per year hyper-oceanic climate which results in, among (Ratcliffe, 1968). This high frequency of rain other things, a lack of extreme high or low results in a constantly humid atmosphere and temperatures, a high temperature lapse rate (the low evaporation and transpiration, particularly rate of temperature decrease with height), high in mountain areas. Hepatic mats also show better cloudiness, high and frequent rainfall and a low growth in areas with cool summers and mild vapour pressure saturation deficit (the difference winters (Averis, 1994).

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Hepatic mats are often considered to occur been found on the rocky floor of corries, cliff mainly as an understory beneath an open canopy ledges and in montane heath. The range of prim- of heather (Calluna vulgaris) in dwarf shrub ary habitat in which hepatic mats occur displays heath. However, they also grow in a wide range a strong gradient from north-west to south-west of other habitats in the UK, including montane Ireland. The majority of stands in Donegal, in the heath, upland grassland, snowbeds, open rocky north-west, occur in dwarf shrub heath, while birch forest and on mountain ledges (Averis et the Kerry stands in the south-west rarely occur al., 2004). In Ireland, it has been found that in association with Calluna vulgaris, growing hepatic mats primarily occur in two distinct primarily on rocky, grassy slopes (Hodd & Sheehy habitats: under a canopy of heather in dwarf Skeffington, unpublished). There is no evidence shrub heath and in rocky grassland on steep for differences in land use or grazing intensity slopes in corries. Stands of hepatic mat have also between the areas and, therefore, it is likely that this gradient is mostly climatically driven. . One of only two Irish sites for Scapania nimbosa, Corrán Tuathail, Co. Kerry, under threat from damage caused by walkers climbing Ireland’s highest peak. Threats m A severely overgrazed slope, Binn Bhán, Twelve Bens, McKee et al., 1998). This has led to widespread Erosion around the path is visible in middle right of Hepatic mats are under threat from a number Connemara, Co. Galway, which was formerly one of loss of the cover of hepatic mat bryophytes in photo. R. Hodd of sources, which are mostly anthropogenically only two Irish sites for Plagiochila carringtonii. R. Hodd many sites where they were previously frequent (Holyoak, 2006). The most well-documented driven. Hepatic mats are vulnerable to disturb- losses have occurred in the Twelve Bens of ance and change for a number of reasons. These Connemara, where hepatic mats were once species are extremely restricted in distribution to abundant and rich under a tall canopy of heather areas of very specific climatic and topographic (Ratcliffe, 1962), but are now mainly restricted conditions. They also have very few means of to occasional, small, depauperate patches with long-distance dispersal, as only one hepatic mat little healthy heather in the vicinity (Holyoak, species (Anastrophyllum donnianum, on one 2006). Plagiochila carringtonii is presumed lost occasion; Averis, 1994) has ever been recorded here from one of only two known Irish sites, as producing sporophytes in Britain or Ireland. while Adelanthus lindenbergianus, which was Spread is primarily by vegetative fragmentation, once abundant and plentiful throughout the making the establishment of new populations Twelve Bens, is now restricted to one small patch slow and unpredictable (Rothero, 2003). Their on Binn Gabhar (Long, 2010). tendency to form large mats on steep slopes also However, grazing pressure is uneven across makes them particularly vulnerable to erosion. the range of hepatic mat distribution in Ireland, Currently, the most pressing and widely and moderately extensive stands, containing recognized threat to hepatic mats is overgrazing reasonably healthy populations of all of the Irish by sheep. Burning is also a threat in some species, including A. lindenbergianus, are still pres- areas, especially in Scotland (Rothero, 2003). ent in parts of Mayo, Kerry and Donegal (Hodd Overgrazing has had the largest negative impact & Sheehy Skeffington, unpublished). Stocking in parts of Ireland, particularly in parts of counties densities have also been reduced throughout Galway and Mayo, where overstocking of sheep western Ireland in the last decade, but it is has led to severe degradation of the vegetation unlikely that already damaged or lost stands will cover of the mountains (Bleasdale, 1998; recover in the near future due to the widespread

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loss of vegetation cover and soil erosion, making Therefore, first, it is imperative to establish Bleasdale, A. (1998). Overgrazing in the west of Ireland – montane heath and associated plant communities in western restoration nearly impossible (Long, 2010). the current condition, composition and distri- assessing solutions. In Towards a Conservation Strategy for Irish mountains. In Climate Change, Ecology and Systematics. the Bogs of Ireland, pp. 67–76. Edited by G. O’Leary, & F. Edited by T. Hodkinson, M. Jones, S. Waldren & J. Parnell. bution of this community in these islands, and Gormley. Dublin: Irish Peatland Conservation Council. Cambridge: Cambridge University Press. Response to climate change to ensure that further damage due to grazing and It is thought that future changes in the climate other preventable factors does not occur. Second, Crawford, R.M.M. (2000). Tansley Review no. 114. Ecological Holyoak, D.T. (2006). Progress towards a species inventory hazards of oceanic environments. New Phytologist 147(2), for conservation of bryophytes in Ireland. Proceedings of the are likely to have an impact on northern hepatic long-term monitoring is necessary to establish 257–281. Royal Irish Academy 106B, 225–236. mat species (Porley & Hodgetts, 2005). The what impact climate change may have on this main aspects of climate change that may affect community, and to enable timely conservation Hodd, R.L. & Sheehy Skeffington, M.J. (2011). Climate Jordal, J.B. & Hassel, K. (2010). The rare liverwort Scapania change and oceanic montane vegetation: a case study of the nimbosa – new knowledge about distribution and ecology in these species are potentially decreased summer efforts. Research is currently underway into this Norway. Lindbergia 33, 81–91. rainfall and increased year-round temperatures, community in both Ireland (Hodd & Sheehy as well as an increase in extreme weather events. Skeffington, 2011) and Scotland which will Long, D.G. (2008). The Gaoligong Shan mountains of the Sino-Burmese border. Field Bryology 96, 28–38. Work on modelling the future distribution increase the understanding of this community. of a number of hepatic mat species in Ireland But there is a need for continued monitoring, Long, D.G. (2010). The tragedy of the Twelve Bens of (Hodd, Bourke & Sheehy Skeffington et al., as well as addressing the technically difficult Connemara: is there a future for Adelanthus lindenbergianus? Field Bryology 100, 2–8. unpublished) suggests that suitable climate space and ecologically problematic issue of potentially for these species will be lost in the south of their transplanting or propagating the component McKee, A-M., Bleasdale, A. & Sheehy Skeffington, M. (1998). range and gained in northern areas of Ireland. species. However, despite continued research it The effects of different grazing pressures on the above-ground biomass of vegetation in the Connemara However, considering the limited dispersal may be that the luxuriant, multicoloured mats uplands. In Towards a Conservation Strategy for the Bogs of capacity of these species, they are unlikely to of these liverworts will no longer be a prominent Ireland, pp. 177–188. Edited by G. O’Leary & F. Gormley. be able to move their range northwards in time feature of the vegetation of the Scottish and Irish Dublin: Irish Peatland Conservation Council. with the climate. Populations may therefore mountains in the future. Paton, J.A. (1999). The Liverwort flora of Britain and Ireland. be lost in southern areas, but no new ones are Colchester: Harley Books. Acknowledgements likely to be established in the north. Increased We would like to thank David Long, Gordon Rothero and Porley, R. & Hodgetts, N. (2005). Mosses and Liverworts. The competition from other species able to expand Maren Flagmeier for helpful comments, particularly in New Naturalist Library. London: Harper Collins. into the hepatics’ niche, as a result of changes providing a Scottish perspective to the article. Thanks also to Ratcliffe, D.A. (1962). The habitat of Adelanthus unciformis in the climate, may also have a major negative Neil Lockhart for useful comments on the manuscript. Work (Tayl.) Mitt., and Jamsoniella carringtonii (Balf.) Spr. in on these hepatic mats is part of a PhD being undertaken by the impact on existing populations of hepatic mat Ireland. Irish Naturalists Journal 14, 38–40. species. Therefore, climate change could provide first author, funded by the Irish Research Council for Science, Engineering & Technology and the National Parks and Wildlife Ratcliffe, D.A. (1968). An ecological account of Atlantic a significant future challenge to the survival of Service, Ireland. bryophytes in the British Isles. New Phytologist 67, 365–439. these species in Britain and Ireland. Rothero, G. (2003). Bryophyte conservation in Scotland. Rory Hodd & Micheline Sheehy Skeffington Botanical Journal of Scotland 55(1), 17–26. What does the future hold for hepatic mats? Botany and Plant Science, National University of Schofield, W.B. & Crum, H.A. (1972). The future of the mixed northern hepatic mat Ireland Galway, Ireland (e [email protected]) Disjunction in 59(2) is currently unclear. Much damage has been bryophytes. Annals of the Missouri Botanical Garden , 174–202. caused by overgrazing in certain parts of its References range, but extensive, thriving stands still occur in Averis, A.M. (1994). The ecology of an Atlantic liverwort Sweeney, J. (1997). Ireland. In Regional Climates of the British Isles, pp. 254–275. Edited by D. Wheeler & J. Mayes. some areas, particularly in Scotland. However, if community. Unpublished PhD Thesis, University of Edinburgh. London: Routledge. climate change does eventually have a widespread Averis, A.M., Averis, A.B.G., Birks, H.J.B., Horsfield, D., and far-reaching impact on the distribution and Thompson, D.B.A. & Yeo, M.J.M. (2004). An Illustrated b The very rare Adelanthus lindenbergianus, in one abundance of hepatic mats, they may become Guide to British Upland Vegetation. Peterborough: Joint of only eight European sites, Errigal, Co. Donegal. extinct in several regions. Nature Conservation Committee. R. Hodd

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