H. John B. Birks 1,2,4 & Katherine J. Willis 1,3,4

1. ALPINES, TREES, AND REFUGIA H. John B. Birks1,2,4 & Katherine J. Willis1,3,4 1University of Bergen, 2University College London, 3University of Oxford, and 4Jesus College Oxford

2. Introduction Definitions Last Glacial Maximum Alpines in LGM Trees in LGM Southern and Mediterranean refugia Central, eastern, and northern refugia Current models based on available fossil evidence Is tree-growth in the LGM of central Europe possible? Holocene Cryptic refugia for alpines Conclusions

3. INTRODUCTION The Quaternary period is the past 2.8 million years (Myr) of Earth’s history. A time of very marked climatic and environmental changes Large terrestrial ice-caps started to form in the Northern Hemisphere about 2.75 Myr, resulting in multiple glacial-interglacial cycles driven by variations in orbital insolation on Milankovitch time-scales of 400, 100, 41, and 19-23 thousand year (kyr) intervals Glacial conditions account for up to 80% of the Quaternary Remaining 20% consist of shorter interglacial periods during which conditions were similar to, or warmer than, present day

4. Glacial conditions: • Large terrestrial ice-sheets • Widespread permafrost • Temperatures 10-25C lower than present at high-mid latitudes • High aridity and temperatures 2-5Clower than present at low latitudes • Global atmospheric CO2 concentrations as low as 180 ppmv rising to pre-industrial levels of 280 ppmv in intervening interglacials • Steep climatic gradient across Europe and Asia during the Last Glacial Maximum (LGM)

5. Major climate forcing for the last 450 kyr calculated at 60N. Global ice volume (f) plotted as sea-level, so low values reflect high ice volumes. Jackson & Overpeck 2000

6. Present day General circulation model (GCM) simulations of 21 kyr Last Glacial Maximum 21,000 cal. year BP Pollard & Thompson, 1997; Peltier, 1994

7. Ice-sheets Permafrost Relict soils Approximate extent of ice and of assumed continuous permafrost in Europe during LGM Willis 1996

8. Emphasis on Europe LGM Current interglacial – the Holocene ‘Alpines’ Trees Refugia Palaeobotanical evidence (macrofossils, microfossils) “…direct evidence can come only from fossils, indicating the existence, location, and duration of refugia, and their biotic composition in comparison with surrounding areas. Thus, palaeontology and genetics can operate synergistically, each suggesting fruitful geographical sampling areas for the other.” Stewart & Lister 2001 What do we know about the ranges of trees and alpines during the LGM? What do we know about ‘alpines’ during the current Holocene interglacial?

9. DEFINITIONS ‘Alpines’ - plants that today have their main occurrences above the altitudinal tree-line or beyond the latitudinal tree-line. Includes alpines sensu stricto and arctic plants Last Glacial Maximum (LGM) – about 18000-25000 years ago, coldest period of the last (Weichselian) glacial stage Holocene – last 11500 years (~10000 radiocarbon years) of Earth’s history, so-called ‘post-glacial’ period or current interglacial

10. Refugia – areas for the growth and survival of species during adverse or unfavourable environmental conditions. Sources for subsequent recolonisations when environmental conditions become more favourable - areas of survival for species during glacial episodes when temperate species survived in micro-environmentally favourable locations south of the continental ice-sheets and alpine species survived above or below the region of mountain glaciation and near the continental ice-sheets - refugia for other types of species also existed in areas far removed from glaciation (e.g. tropical rain-forest refugia) Cryptic refugia – restricted refugia in northern Europe; areas of sheltered topography with buffered, stable local microclimates (Stewart & Lister 2001). Possibly not detectable by pollen analysis

11. LAST GLACIAL MAXIMUM Vegetation 20 kyr ago Widespread ice, tundra, and steppe in north and east; park-tundra in south and east, and forest confined to Mediterranean basin Iversen 1973

12. Older Dryas (ca. 14 kyr) landscape in Denmark Abundant alpines along with species of steppe habitats (e.g. Helianthemum, Hippophae, Ephedra) Iversen 1973

13. Possible LGM landscape in central Europe Open steppe with abundant Artemisia and Chenopodiaceae, and extensive loess deposition

14. Alpines In LGM Besides familiar arctic-alpines found commonly as fossils such as also find fossils of plants not growing in central European mountains, only in northern Europe today

15. Koenigia islandica Lang 1994

16. Other northern plants found as fossils in central Europe in LGM Salix polaris Silene uralensis Pedicularis hirsuta Ranunculus hyperboreus

17. Common alpines in LGM throughout northern and central Europe Dryas octopetala Silene acaulis Bistorta vivipara Betula nana Saxifraga oppositifolia Lychnis alpina

18. Trees in the LGM southern and Mediterranean refugia Interglacial LGM N S Traditional refugium model – narrow belt in southern mountains van der Hammen et al. 1971

20. Location of Ioannina basin in Pindus Mountains, NW Greece Tzedakis et al. 2002

21. LGM Tzedakis et al. 2002

22. Pollen evidence for traditional southern European LGM refugial model Pinus Quercus Fagus Ulmus Corylus Alnus Pistacia Tilia Betula Abies Bennett et al., 1991; Birks & Line, 1992

23. Taxa that have reliable macrofossil or pollen evidence for LGM presence in south European refugia

24. What about trees in central, eastern, and northern Europe during the LGM? • Detection difficult • Low pollen values – do these result from long-distance pollen transport or from small, scattered but nearby populations? • Classic problem in pollen analysis since Hesselman’s question to Lennart von Post in 1916. No satisfactory answer. • Few continuous sites of LGM age • Pollen productivity related to temperature and some trees cease producing pollen under cold conditions • Pollen productivity may also be reduced by low atmospheric CO2 concentrations • Other sources of fossil evidence critically important – macrofossils, macroscopic charcoal, and conifer stomata

25. Fossil evidence for trees and shrubs in LGM in northerly locations: pollen & macrofossil evidence • e.g. Palaeoecological results from Bulhary, South Moravia • Buried peat dated to 25,000 yr BP • Pollen record indicates existence of park-forest vegetation (Pinus sylvestris, Pinus cembra, Larix, Picea abies, Juniperus communis) • Excellent macrofossil assemblage including Betula pubescens and Salix sp. E. Rybnícová & K. Rybníček, 1991. In: Palaeoevegetational Developments in Europe, Proceedings of the Pan-European Palaeobotanical Conference, 1991, Vienna Museum of Natural History, pp 73-79.

26. Fossil evidence for LGM refugia in northerly locations: pollen evidence from six sites in Romania Pinus Picea Betula Salix Juniperus Feurdean et al. 2007

27. Greatest diversity during the LGM found in mid-altitude sites – 800-1300m asl – in Romania Feurdean et al. 2007

28. Fossil evidence for trees in central and eastern Europe during the LGM: macroscopic charcoal evidence Willis & van Andel 2004

29. Scanning electron microscope images of wood charcoal

30. Pol CzR Ukr Svk Aus Hun Rom Slo Cro Ser B&H CzR – Czech Republic; Aus – Austria; Slo – Slovenia; Cro – Croatia; Pol – Poland; Svk – Slovakia; Hun – Hungary; Ukr – Ukraine; Rom – Romania; Ser – Serbia; B&H – Bosnia & Herzegovina Willis & van Andel 2004

31. Willis & van Andel 2004

32. Tree taxa that have reliable macrofossil evidence for LGM presence in central, eastern, or northern European refugia

33. Iberian, Italian, and Balkan peninsula LGM refugia – ‘classical’ model Not complete Southern + central + northern European LGM refugia Current model Bhagwat & Willis 2007

34. Ice sheet Northerly LGM refugia Mediterranean LGM refugia Current model based on available fossil evidence Willis et al. 2007 (in press)

35. What was the LGM landscape like? • In the lowlands north of the Alps, a mosaic of: • open-ground habitats on well-drained soils and exposed sites supporting a mixture of alpines, steppe, and ‘weed’ taxa • willow scrub on damper soils • tree populations on sheltered localities along river banks, in valleys, and in depressions where there was moisture and some shelter • In the mountains south of the Alps, a mosaic of: • low-altitude steppe or shrub steppe • belt of trees at mid-altitudes where there was adequate moisture and temperatures were not too cold • high-altitude open habitats with alpines and cold-tolerant steppe plants

36. North of the Alps Sichuan, China South of the Alps Borah Peak, Idaho

37. Are there any ecological attributes characteristic of trees in southern LGM and northern LGM refugia? Bhagwat & Willis (2007) 23 trees - southern refugia: large, animal-dispersed seeds - northern refugia: wind-dispersed seeds

38. Could the trees identified in the macroscopic charcoal record have grown in the LGM environment of central, eastern, and northern Europe? Work in progress by Miguel Araújo, Shonil Bhagwat, and ourselves Basic approach is to model present-day tree distributions in relation to contemporary climate using seven different species-climate modelling algorithms (climate-envelopes, bagging trees, random forests, etc.) to develop an ‘ensemble forecasting framework’ for analysing species-climate relationships (Araújo & New 2006) Given modern tree-climate responses and LGM GCM model simulations from Paul Valdes, predict the LGM ranges for trees under LGM climates LGM GCM models – UGAMP (UK), ECHAM3 (Germany)

39. predicted today predicted LGM & refugia S,N Corylus avellana S,N Fagus sylvatica

40. predicted today predicted LGM & refugia S,N Alnus glutinosa S,N Betula pendula N Taxus baccata

41. predicted today predicted LGM & refugia S,N Pinus sylvestris N Juniperus communis S,N Picea abies

42. Combined probabilities of occurrence (potential quantity of suitable habitat) Some trees may have had more potential habitat in LGM than today

43. Early post-glacial migration rates in response to climate change based upon traditional refugial model Pinus Quercus Fagus Ulmus Corylus Alnus Pistacia Tilia Betula Abies Bennett et al. 1991

44. Tree-Spreading Rates • Currently assume spreading from southerly refugia only • These ‘rates’ of movement in early Holocene are then used in a number of climate envelope models to predict movement of plants in response to future climate change • But what if plants are not only in ‘southerly refugia’? • Similar study in USA found that some plants were much further north during the LGM than pollen evidence suggests (e.g. McLauchlan et al., 2005) • Spreading rates demonstrated to be vastly over-estimated • Significantly affects our predictions about how plants will respond to global warming

45. Genetic-diversity Hotspots • Currently assumed that most plants (and animals) were located in southern refugia and therefore this is where genetic diversity will be greatest. • Holocene migration from these refugial regions can be mapped through genetic patterns. • Increasing evidence for certain groups of plants (and animals) that do not fit this ‘southerly refugial model’. • In order to map and protect centres of genetic diversity properly, need to have proper understanding of where the plants (and animals) existed during the LGM. • Major challenge to palaeoecologists. More data needed from unambiguous sources like macrofossils and macroscopic charcoal. Need to critically reassess LGM pollen data.

46. Understanding ‘hotspots’ of genetic diversity very important to long-term conservation A. Hampe & R.J. Petit, 2005. Ecology Letters 8, 461-481; K.J. Willis & H.J.B. Birks, 2006. Science 314, 1261-1265

47. HOLOCENE Cryptic Refugia for Alpines Alpines widespread in LGM as shown by macrofossil evidence Became more restricted to alpine and arctic habitats above or beyond the tree-line in early Holocene with major climate warming and competition Some species also occur today in small, isolated ‘cryptic’ refugia within the potential forest zone Such cryptic refugia include sea-cliffs, other coastal habitats, inland cliffs and screes, open river-gravels, rocky gorges, and shallow soils on steep limestone slopes (Pigott & Walters 1954)

48. Ramasaig Cliff, Skye High Force, Teesdale Inchnadamph, W Sutherland Cronkley Fell, Teesdale

49. Mullaghmore, County Clare Yew Cogar Scar, Yorkshire Bettyhill, Sutherland Dryas octopetala, Sutherland

50. Cetry Bank, Teesdale Falcon Clints, Teesdale Scar Close, Yorkshire