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The Palaeohistory of the Grey Alder (Alnus incana (L.) Moench.) and Black Alder (A. Glutinosa(L.) Gaertn.) in FennoscandiaAuthor(s): P. A. TallantireSource: New Phytologist, Vol. 73, No. 3 (May, 1974), pp. 529-546Published by: Wiley on behalf of the New Phytologist TrustStable URL: http://www.jstor.org/stable/2431124 .

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New Phytol. (i974) 73, 529-546.

THE PALAEOHISTORY OF THE GREY ALDER (ALNUS INCANA (L.) MOENCH.) AND BLACK

ALDER (A. GLUTINOSA (L.) GAERTN.) IN FENNOSCANDIA

BY P. A. TALLANTIRE

Botanical Institute, Trondheim University, N-7ooo Trondheim, Norway

(Received I3 November I973)

SUMMARY

The available '4C-dated and broadly context-dated pollen and macrofossil data for the immigration and spread of the two species of alder within Fennoscandia and certain adjacent countries is presented. The phases of immigration and regional spread take place during certain periods of time (e.g. 65oo, 6ooo, 5000 and 4500 14C years B.C.) and over certain restricted geographical areas. The latter yield a picture of climatic regionalism, with limits akin to that of the present day. Along its present, diffuse northern boundary Alnus glutinosa, on the evidence from Tr0n- delag in Norway, appears to have become established only in a late phase of the broad climatic optimum, after 2000 B.C., and to have lost ground again after c. 700 B.C., though the course of later events is confused by human activity. A tabulation of the postulated ecological requirements of the two species of alder at various stages of their life histories is attempted. Details of the sites mapped are included in an appendix.

INTRODUCTION

Sites with 14C-dated finds of macrofossils are still relatively sparse in Europe, yet they can often provide more reliable specific identification of plants whose pollen record remains at family or generic level. Such is the case for the two species of alder in northern Europe (Alnus incana and A. glutinosa); there is no evidence at present for the green alder (A. viridis (Chaix) DC) being found outside its present-day habitats in the Alps, Car- pathians and Arctic Russia earlier during the Postglacial period, with the possible exception of Poland during the Aller0d period (Szafer, I966). Recent finds of alder fruits from lake deposits in Asklundvatn in the Tr0ndelag area of Norway (site 5, Fig. 3, see Tallantire, I973) led to a comparison with other available evidence (14C-dated pollen curves on Fig. i and macrofossil finds on Fig. 2) from Fennoscandia, Britain and adjacent parts of mainland Europe, to determine whether there was evidence for climatic change or regional differentiation since c. 7000 B.C.

My aims are threefold: to bring up to date and extend in scope the previous two sur- veys for Sweden (Andersson, I893; Wenner, I968), and to compare them with the British and European ones (Firbas, I949; Godwin, I956; Kubitzki, I96I); to indicate the need for continued, dated, macrofossil studies in those areas for which we lack data at present; and to enter a plea for a more critical attitude in the choice of sites, as well as in the appraisal of existing data, paying more attention to the equal importance of all three of the main sources of evidence in Quaternary vegetational studies, stratigraphy, macrofossils and the pollen record, with selective 14C-dating of major changes in the first

529



530 P. A. TALLANTIRE

2' 0 2' 4 6 8' 10 12 14' 16' 182 20' 22 24' 26' 28' 30' 32' 34? 36'

4 563 A300 4570- 4 78 5 5000 5050 ----...

5 050 4998(30)

8': 6 30 (2 1^ 1? 14 61' 0 2 2 62

4810 2o~

FIG. I. A, '4C-dated pollen sites. *, Dating made at or just above the main rise of the alder pollen curve. The actual percentage value shown on the left, the dating on the right (T+ - 5568). *, Datings which fall below the main rise, or about which there are qualifications.



Alder in Fennoscandia 53I

24 ?W 24 4- 6. a' 10? 12' 14' 16. Is, 20" 22' 24? 26? 28? w 12, 34? 36

Fig. 2. Sites at which macroscopic remains of alder spp. have been encountered (for details see the appendix; site numbers on map C). 0, Finds of Alnus incana remains only; ?3, finds of both A. incana and A. glutinosa, together in same deposit; *, finds of A. glutinosa only; 0 finds of A. glutinosa together with remains of other mixed-oak-forest trees; ~, sites at which the basal sediments contained remains of A. incana alone, followed above by finds of A. glutinosa, alone or with some A. incana.




1, o 2 4' 6_ 8 0 12 1' 16 18 20 22 246 26 6 8 6 6 6 6 62 6 4' 3

68'/ / I &

2 6~/ 6

-1

6620 6130

64~ ~ ~~~~~~1 4~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~4

30

60' .7~~~~~~~~~~~~~~~~~~~~~~~1 1 8

66' / ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~~~~~~~~~~~~~~~~~~~~~6

Fig.3. ey o ste umbrs or al te steson igs an z, ogeherwit a ew npitte

sits.Th nubes efe t te lstd rde o stesinth apenix Siesontheinetma are listed at the end of the~ apenix



Alder in Fennoscandia 533 (Smith, I965) (on both sides of sharp boundaries), as well as the latter two. Regional pollen zones, or assemblages, provide too haphazard, and sometimes too metachronous (Hafsten, I970), a time-scale for studies of changes in any of the components of climate; studies which, of necessity, must compare contemporaneous events over wide geographic areas.

In the case of the alders the objection will be raised 'are they not as much or more dependent on local groundwater levels than on any temperature limitations, summer or winter?' My view is that although the climatic background to hydrological changes can be partially obscured, or augmented, by the eustatic-isostatic interplay controlling sea- level, especially in our area, these effects are relatively slow and long-term. The hydrology conditions the extent of land surface available for colonization, as well as the long- term thrift or survival of established trees, but seed production, dispersal, germination and seedling establishment are under more direct control by climatic factors. The fact that we are studying sites at which plant macro- and micro-fossils have been preserved implies that groundwater has been present all along, though river valley sites are not always as well represented as one could wish for drawing firm conclusions. Suspected gaps in the fossil record due to water-level changes, with resultant erosion or corrosion, are usually detectable in one form or another.

Alder pollen is now generally considered to be over-represented in the local pollen record (Andersen, 1970), due to high pollen production at a time when neither they nor other trees have come into leaf. However, the pollen may not be carried so very far afield in any noteworthy amount. Salmi (I962) reported pollen values of I I-34%, from A. incana trees growing on the margin of a raised bog in western Finland, in peat samples taken 200-300 m distant. Values had fallen to I-5 % at a distance of only i km across the bog. Regional transport of alder pollen, producing pollen values much> I %, seems unlikely even in unforested areas, so that the timing of the local appearance of alder in any abundance should be relatively easily detectable in a pollen diagram. The local course of events following immigration, either a steady spread, or at first restricted and only much later a general spread in the area, is complex, however, depending on vegetational suc- cessions, restriction of habitats by the spread of ombrogenous peats and, later, the direct and indirect effects of man and his animals.

THE PRESENT-DAY DISTRIBUTIONS AND ECOLOGICAL BACKGROUND

On a basis of the information in the available literature, I have attempted to summarize the ecological requirements, at various stages of their life histories, for the two species of alder (Table i); the references used in compiling the table have been appended there. Their climatic and edaphic requirements differ, Alnus glutinosa being the more demanding species, particularly as regards its mean summer tetraterm or pentaterm temperature requirement. The climatic data for the Trondheimfjord area (see Fig. 3 in Tallantire (1973)) yielded an optimum 30-year running mean, for the pentaterm May-September of i I.9? C for the period 1924-53, implying that only in individual years and in locally favourable habitats could this tree have thrived here in recent decades.

The two alders, like many other tree genera, are singularly ill-served by the usual type of distribution map. The recent modification by Straka (Walter, I970) of Meusel's Eurasian map does at least attempt to show frequency differences, enclaves and outposts near the distributional limits, though the very scale and reduction in print preclude any opportunity of showing diffuse boundaries (see also the general map for Fennoscandia in




Table i Factor Alnus incana Alnus glutinosa

Flowering time Feb.-Apr., C. Europe (I). Early-mid Feb.-Mar., Britain (A). Mid-Mar. to and pollination March, Poland (V). Mar.-Apr., Sweden late Apr., central Europe (H). Mid-

(Q). Earlier than A. glutinosa in areas Apr. to May, Finland (H, J). Apr.- where both occur (I). 2-3 weeks before May, Sweden (Q). Mid-Mar.-Apr., A. glutinosa (Q). Prematurely opened Poland (V). Pollen viability 30-5O days catkins (> IOo C day max. for I4 days) according to humidity (H). Stigmas un- in mid-April, badly damaged by subse- affected by air frost before pollination quent 4 days' cold spell (<50 day, (A). Stigma life and pollen tube initial <-IO night) a week later with snow growth probably susceptible to both and north-west winds (Tr0ndelag, own frosts and drying winds (A, H), or to observations). strong north and west winds in north

and west Britain (A). Foliage production End May to mid-June (Trondelag, own 2-5 weeks after flowering, about latter

observations). half Apr., C. Europe (H); mid-May in Finland (J).

Time of embryo No data found. Late July to early Aug., varies with place fertilization and year, Britain (A). Pollen tube growth

halted from end-Mar. to mid-June (I). Once fertilized, embryo development proceeds to maturity, slowed but not prevented by lack of summer warmth; this may be more important for the laying-down of catkin initials previous summer (A).

Seed maturation No data found. Ripens throughout Sept., Britain (A). Ripening can continue into Oct./Nov. Britain (B). Ripening Sept.-Oct. (C). Europe (H).

Leaf fall Oct.-Nov. after first frosts or protracted Oct.-Nov. (C). Europe (H). Oct.-Nov. snowfalls (Tr0ndelag, own observa- with first night frosts, Britain (G). Early tions). mid-Nov. Finland (J).

Seed shedding and Mostly in late autumn, some retained in Seed fertility usually higher from cross- fertility catkins until spring (Tr0ndelag, own ob- pollination (A). Wet weather at pollina-

servations). Seed quality less affected by tion time depresses viability irrespective low temperatures during the vegetative of any effect on cross-pollination (A). period than either A. glutinosa or hybrid, Marked decrease in fertility with alti- Latvia (R). tude, Scotland (A). Seed shedding

normally not before the springtime (H). Seed shedding mostly in the late autumn, Finland (J). Some shed in late autumn, mostly late winter and spring, Sweden (Q). Capable of setting seeds, with endosperm but no embryos, even in absence of pollination or of fertilization! (A; E, p. 325; H).

Seed dispersal Well adapted for wind dispersal, but in Water dispersal more effective than view of the Bavarian (I), Polish (W) and wind dispersal. Seeds can float and re- S. Swedish distribution patterns water main viable, even in salt-water, for transport is also effective?-perhaps via many months. Wind-drift over water ice floes in spring? also effective. Seeds eaten by birds but

thereby rendered non-viable. Water- fowl may be involved in long-distance transport (B).

Seed viability Not more than I year (I). Usually less than I2 months (B). Will germinate after 3 years' dry storage, but only after 6 months after falling into water (H).

Seed germination No data found. Damp storage (high air humidity) more important than low temperature storage (2-4' C), but cold treatment for 6 weeks improves germination energy and lowers the min. germination temp. from c. I8? to c. 70 C. For C. Europe (Detmer cited in F) gives min. 7-8?, optimum 260 and max. 340 C. Periodic



Alder in Fennoscandia 5.35

Table i (contd.)

Factor Alnus incana Alnus glutinosa

drying-out during germination is unfavourable (B). Insensitive to light or to pH of substrate. Deep burial prevents germination due to low 02-tension; seed buoyancy helps avoidance of this fate but renders seeds more liable to subsequent drying-out (B). Cotyledons unable to reach surface if seeds buried under more than i cm of topsoil (G).

Seedling estab- Flooding of site during first few years Germination from late-Feb. onwards in lishment and highly unfavourable (P). Rapid growth Britain, depending on degree of winter growth of roots and shoots during first and cold conditioning. Early seedlings have

second years (P). Seedlings develop high mortality rate due to subsequent mycorrhizal roots and nodules (H), no low temperatures, rain splashing or details found. drying-out, all of which adversely

affect radicle growth and satisfactory root anchorage (B). Timing of spring floods plays a role in preventing drying- out on free-draining soils, yet unfavourable, by smothering, during first two years if too much sediment and/or plant debris deposited by winter or spring floods (C). Prolonged and/or repeated desiccation very unfavourable during first year, and explains why alder unable to compete with birch on such sites (C, p. I99). Alder withstands up to 5 weeks' submergence of seedlings, birch killed by only 2-3 weeks (C, p. 202). Seedling establishment only on a soil surface that comes within the capillary fringe of the water table, so that surface layers remain continuously moist for 20-30 days in the period April-June. In regions of high summer rainfall the water table does not dominate so (G). Alder will not establish at water-level unless protected from splashing and flooding, but can do so a short distance above (C, p. 204). Mycorrhizal rootlets and nodules developed in all but totally anaerobic soils (C). Seedlings relatively shade tolerant within limits of small food store available, but sapling intolerance only exceeded by birch (C, pp. 2I3,

2i6). Light requirement may be only a reflection of need for root-room (H).

Established saplings Roots capable of withstanding tem- Both surface and deep root systems well and mature trees: porary low 02-tensions, due to biotic developed, from first year onward. Can thrift and root activity in damp but not flooded ground respond to slowly rising water-levels by system extent and in summer (P). Surface root system well production of stilt-roots. Able to survive depth developed (H). Can stand appreciable temporary or slow changes in water

periods of drought. At higher latitudes table once saplings properly established. and altitudes is found on increasingly Duration and rate of change probably drier habitats, especially if soil clay/silt more important than extent (see D and rich. No thrift in standing-water habitats K). Deep roots are capable of growth in or on peats (H, and own observations). a reducing medium; the surface nutri- Root system shallow but extensive and tional rootlets, bearing the nodules, intensive; can stand drying-out, at least require a higher 02-tension (D). Rapid on clay/silt soils (K), better than periods vertical shoot growth of seedlings and of flooding of more than a week's dura- saplings, as of coppiced stump shoots tion (P). Light requirement less than (no root suckering), gives good com- that of willows or poplars (P) or A. petitive powers over ground vegetation glutinosa (H) and less sensitive to winter (H). Growth ceases end of August, frost than latter (H). In sandy soils in Lithuania (S). Warmth during May and




Table i (contd.)

Factor Alnus incana Alnus glutinosa

Poland grew better than A. glutinosa October, sufficient rainfall in June- with groundwater level at I 30 cm depth; July, and absence of late spring frosts, at 70 cm reverse held (T). Rapid shoot all favourable for trunkwood growth (C). growth and production of both stump Europe (N and U). Although seeds will shoots and root suckers (H). Growth germinate on tussocks in acidic peatbogs cessation in mid-August, Lithuania (S). the plants do not thrive and soon die off.

This is probably due to poverty in mineral salts (e.g. phosphate) rather than standing-water, since A. glutinosa can establish itself on flush bogs and fen peats (C).

Full-grown height 10-25 m (I). 20-35 m (H, I). 3-12 m in nature, i8-24 m in cultivation, Britain (G).

Longevity 20-50 years depending on the habitat 25-200 years depending on the habitat (H). In S. Bavaria trees about 30 years, conditions (H), and coppiced stump coppice stump shoots I5-30 years (P). shoots 50-70 years (H). Ioo-o20 years In the Kola peninsula rootstocks, with usual (I). In Finland 8o-ioo years (J). regenerating shoots, were considered to In Sweden seldom over I20 years (Q). be > 200 years old (H).

Age at start of 6-I5 years (H). I2-20 years sometimes, though usually seed production not before 40 years old (H). Temperature and Thermic demands similar to pine (Pinus Summer warmth requirement some- rainfall require- sylvestris) (Y), i.e. tetraterm 8.4-IO.6? C, what greater than Betula verrucosa, but ments (deduced depending on the total length of the can stand winter frost (H). Pentaterm from present-day vegetative season (L). (N.B. Faegri mean 120 C, tetraterm 12.40 (L). Short distribution limits); points out that the comparison of these (June-Aug.) vegetative season com- represent a sum- two genera, based on observed alti- pensated for by higher mean tempera- mation of the tudinal limits in Norway, is very em- tures, E. Alps (0). N. distribution rela- requirements of pirical, since both physiology and life ted to duration of winter sub-zero tem- the various life- history are markedly different.) For a peratures, a mean daily temperature of cycle stages mean temp. of coldest month of < - 6? C o0 C or less for more than 6 months of

the max. mean temp. of warmest month year (G). A near-linear relationship is is c. i8' with a mean rainfall (May-Sept.) shown, from values for the mean temp. of of > 350 mm, but reduces to . io0 or less the warmest month c. I2.80 C and a mean with a rainfall value of c. i 8o mm. For the temp. of the coldest month of c. + 20, coldest month mean temp. of + I-2' C, changing to c. I4.4' and c. - I20 C however, the respective values are c. 15' respectively. From Fig. gi (X). Because (rainfall > 400 mm) and c. I0? or less of the dependence of this species on (rainfall C. 300 mm.) from Fig. I2f (X). groundwater levels in E regions Hin-

tikka has not worked out a three- dimensional graph including the rainfall. Faegri (I960) points out that in western regions, with no seasonal rainfall shortage, this alder also grows on hillside sites.

References used for Table i.

A McVean, D. N. (I955). . Ecol., 43, 46. B McVean, D. N. 0955). J. Ecol., 43, 6i. C McVean, D. N. (I956). J. Ecol., 44, 195. D McVean, D. N. (I956). J. Ecol., 44, 2I9. E McVean, D. N. (1956). J. Ecol., 44, 32i. F McVean, D. N. (I956). J. Ecol., 44, 33I. G McVean, D. N. (1953). J. Ecol., 41, 447. H Kirchner, Loew & Schroter (i908). Lebensgeschichte der Blitenpflanzen Mitteleuropas, 2(i), i96. I Hegi, G. (i957). Illustriertes Flora von Mitteleuropa, 3(I), I69. J Kujala, V. (I924). Comm. Inst. Forest. Fenn., 7, I. K Linkola, K. (I938). Suomal. eldin- ja kasvit. Seur. van. Sulk., 9(7), 14. L Helland, A. (I9I2). Tidsskr. Skogbr., p. i. M Kobendza, R. (1956). Roczn. dendrol., II, II3. N Elling, W. (I 966). Flora, Jena, Abt. B, I56, I 55. O Grossman, A. & Melzer, H. (i933). Zbl. ges. Forstw., p. I47. P Goettling, H. (i968). Forstwiss. Forsch., No. 29. Q Lagerberg, T. (i947). Vilda Vaxter i Norden, Vol. 2.



Alder in Fennoscandia 537 R Kundzin, A. V. (I969). From No. 358I in For. Abst., 30. S Koiriukstis, L. (I969). From No. 2039 in For. Abst., 30. T Kralikowski, L. (I963). From No. 3349 in For. Abst., 24. U Holmsgaard, E. (I955). From No. 4404 in For. Abst., i6. V Fer, F. & gedivy, Z. (I963). Sbornik les. fak. vys. stkoly zemgd. v praze, 6, i9i. W Szafer, W. (I966). The Vegetation of Poland, Vol. i. X Hintikka, V. (I963). Suomal. eldin- ja kasvit. Seur. van. Sulk., 34, No. 5. Y Faegri, K. (I 945). Norsk geol. Tidsskr., 25, 99. Z Jurkevic, I. D. et al. (I965). Alnus incana Forests and their Economic Utilization (in Russian).

Published by Izdatel'stvo Akad. Nauk. BSSR, Minsk. (not yet seen)

Hulten (I971)). Outposts, too, are of two types, advance guards and rear-guards, both occupying microclimatically and edaphically favourable habitats for that species, dis- regarding of course the influence of man. Demanding species under gradually deteriorat- ing climatic trends represent the latter type, and may be capable of holding out vegetatively for centuries (e.g. hazel in many present-day sites along its northern boundary, where the setting of fertile seed is exceedingly rare).

In Fennoscandia a more accurate impression of the distribution patterns of the two alders can be obtained from the following sources: for A. glutinosa-in Norway (Faegri, I960), in Finland (Kujala, I964), in Sweden (Andersson, I893); for A. incana-in Sweden (Andersson, I893), in Finland (Kujala, I964), together with Segerstad's map (in Kari, I929) for its outposts in South Sweden. A clue to the variant distribution of the two species in their areas of overlap in western Norway (for the east see Faegri, I960) can be gained from Ve's (I968) distribution maps for the inner part of Sognefjord (not far from the finds of site io). As Faegri (I960) states 'Its (A. glutinosa) ecological range is much narrower, and it is not able to take over from other trees, like A. incana.' A. incana, being more tolerant both as regards its pentaterm temperature requirement during the growing season (see Table I) and in its hydrological demands, can be encountered today well within the optimum area for A. glutinosa, but closer investigation will usually reveal signs of human interference, such as removal of A. glutinosa for its timber, or drainage work (e.g. in S. Finland (see Brenner, I904)); otherwise A. glutinosa can compete success- fully because of its greater longevity and height, and its better tolerance of flooding. The optimum groundwater conditions for the two species are, however, sufficiently different as often to preclude direct competition (see Sjors, I956, p. I56). It seems more likely that A. incana lost ground during the first half of the Postglacial period due to shading-out by elm, lime, oak and ash (and at a later period by spruce) and because of a general rise and increased stability of groundwater levels in some-areas, than to any direct competition from A. glutinosa, except perhaps around lakes and rivers. The seashore distribution of the two species in Fennoscandia, primarily the Baltic area, is complicated by the continued isostatic recovery of the landmass and the constant renewal of ecological succes- sions (see also Firbas, I949, pp. 202-3; Havas, I967).

McVean (ref. G, Table I) prefers to express the minimum tem-perature requirement for A. glutinosa, during the vegetative season, in terms of a maximum duration of below-zero temperatures in winter of 6 months, but Scandinavian botanists since Helland's (I9I2) time consider that there is evidence for a minimum mean temperature of the five warmest months of the year of just above I 20 C (about the same as for lime, Tilia cordata, but higher than that for hazel or elm, Ulmus glabra, of II.40 C). In fact a detailed study of the distribution of Alnus glutinosa and the mean monthly isotherms suggests that (e.g. on Aland) higher values for the tetraterm (June-September) can com- pensate for a shorter vegetative season, especially if the autumn temperatures are favourable. This would also explain the greater northward extent of A. glutinosa on the




Finnish side of the Bothnian Gulf, as compared to the Swedish side, and its extension up the Norwegian coast (though well inland of the skerry fringe), though on a basis of thriving and extensive stands the northern limit here might be drawn well south of Trondelag. Although we now have proof of its former presence during the later part of the Postglacial climatic optimum in an area in which very scattered specimens are known to exist at the present time, I fully support Faegri's (I960) contention that the latter occurrences are not relicts but 'more probably recent immigrants with little or no opportunity of establishing the species permanently', i.e. under the present climate. McVean's (ref. E, Table i) remarks on the higher-lying alder colonies in Scotland, which do not set fertile seed, are also of interest here, though use of the terms 'relict' and 'recent' is probably inadvisable without defining them more accurately in chronological terms.

A complex interplay of various climatic factors is suggested by two examples, as well as MlcVean's earlier comments on the same theme (ref. G, Table i). First, the effect of cold treatment of seeds of A. glutinosa in producing more rapid germination at lower temperatures subsequently (ref. B, Table i) would be disadvantageous in northern areas subject to late spring frosts, but advantageous in areas with a long but fairly constant winter followed by late but warm springs and early summers. Secondly, the frequency and duration of droughts and/or floods can tip the scales in favour of either of the two species, once both have become established, just as man's intervention by clearing, grazing and draining can alter the local pattern of the mosaic. The effect of variable hydrological conditions in influencing the mosaic in the short-term is well illustrated by the distribution of the two species in the Ladoga lake region and in Ostrobothnia (refs K and J, Table i). The small, circular, spring-fed lake of Fowlmere in the Breckland region of E. Anglia, England, provides an interesting parallel to the hydrological conditions at Asklundvatn, though the fluctuations of the water level over the course of a few years duration at Fowlmere are on a much greater scale. A. glutinosa nevertheless thrives around the shores of Fowlmere, but the stands were planted in the first instance and early mortality was high (see refs D and E, Table i). The so-called 'relict' occurrences of A. incana in the northern part of the west Ukraine (Slobodjan, I965), outside the main distribution area, may well be vegetatively maintained outposts in favourable localities which were colonized during a moister period.

THE PALAEOHISTORICAL DISTRIBUTION PATTERN

The published evidence for the immigration, establishment and general spread of the two alder species in Fennoscandia and the adjacent regions to southward and westward, is presented cartographically (Figs. I, 2 and 3).

I have attempted to draw separate conclusions for the temperature conditions throughout the year (the thermoclimate, separable again into winter and summer components) and the hydrological conditions, precipitation, evaporation and groundwater levels locally and seasonally (the hydroclimate), whilst fully aware that the position is really one of a complex of continually inter-relating factors, as Faegri (i963) pointed out. The time- lag involved in the local and seasonal groundwater levels is especially difficult to assess independently. Faced with the interplay between the supply of water from the final melting of the dead land-ice (perhaps not a major factor in the water budget of the Baltic area after c. 6500 B.C.); the isostatic recovery of the landmasses and seafloors (differential in northern and southern regions and not yet everywhere completed); and the gradual eustatic rise of ocean levels (within 6-9 m of completion by c. 5000 B.C. according to



Alder in Fennoscandia 539 Bloom (I97I)); little agreement is possible on such questions as the height of the water table at any one point in space and time (see Alhonen, 1971; Freden, 1967; Kessel and Punning, I969; Nilsson, I969). The amount of landmass available for alder colonization at any time must, however, be borne in mind when interpreting the evidence shown on the maps, and it probably affected speed of seed dispersal.

DISTRIBUTION CHANGES WITH TIME AND SPACE

The pattern of change is as follows. 7400-6800 B.C. Traces of alder pollen (up to I% values), not always sporadic, in

deposits in which secondary contamination is not suspected, are now known from southernmost Norway, southern, south-central and possibly north-central Sweden, and eastern Finland. These traces either occur earlier than, or contemporary with, similar traces of hazel. The earliest, and again always sparse, occurrences of alder fruits are all of Alnus incana. This is still a period during which large quantities of meltwater, from the substantial amounts of dead-ice still extant in Fennoscandia, would be present at certain times of year; both the thermo- and hydro-climatic conditions in general were suitable for the local establishment of A. incana, although one and/or the other were still too variable for wider success.

No firm conclusions are possible about the ultimate source(s) of the first colonists. A comparison of Firbas's map (Firbas, I949, Fig. 27) showing the relative first appear- ances of alder and hazel pollen in the early Postglacial period, and the map in Straka (Walter, I970, Fig. 246) for the present-day distribution of A. incana, suggests the importance as refugia of the Carpathians and the eastern Alps and of the northward- running rivers from these regions, in parts of which A. incana may well have returned at an early stage during the late-glacial. The early finds of A. incana fruits in east Finland are very reasonable in view of the available dry-land areas for colonisation at this time (cf. the similar conclusion reached by Sauramo (I94I, Fig. 3) on a basis of pollen finds), and the source of seed-parents may well have been to south-eastward in Russia. Both the prevailing winds and the general flow of meltwaters would assist in seed transport from east to west within the Baltic area. The absence of such finds in north-west Germany and Denmark would suggest that dry conditions in spring and early summer and/or too fluctuating or permanently low ground water levels predominated and prevented success- ful colonization by A. incana in the vicinity of those lakes and bogs which we know existed in these areas at that time. The same conclusion would apply to much of south Sweden, too, since macrofossil finds are few and sparse and come from coastal clays, not lake sites. Was the summer thermoclimate good enough for A. glutinosa in the localities at which the hydroclimate might have permitted it? Or is its absence during this period solely due to delayed migration from the south? We lack enough dated evidence to answer this question properly, but in view of the known spread of hazel, and to some extent elm, by 6800 B.C. in parts of Sweden and Denmark, both taxa with fruits which are unlikely to spread faster than those of A. glutinosa, and the lower thermic requirement of those two trees, I think the answer will prove to be no; summers were either too cool and/or too short.

68oo-6000 B.C. We have good evidence for a major spread and establishment of both alder species during this period, and at least circumstantial evidence that A. glutinosa spread both north-westward and north-eastward, and A. incana north-westward and south- westward, from the regions assumed to be their main dispersal centres in the south-west




and south-east, and in the case of A. incana also from the already established local bridge- heads in Fennoscandia. In coastal south Norway and in the south and east of Sweden A. incana seems not to have been able to colonize en masse before A. glutinosa and the mixed-oak-forest trees managed to occupy the suitable areas. Here, presumably, the thermoclimate in summer was already up to A. glutinosa's requirements, certainly after 6500 B.C., since at almost all sites there are also finds of lime pollen at the same time. In central Sweden, Praglowski (in Wenner, I968) found pollen evidence for an initial period during which only A. incana was present, followed by a period with both species. The macrofossil evidence is probably not early enough to confirm this finding, and the finds we have (sites 38, 39 and 40) show an almost total dominance of A. glutinosa in deposits of 'Litorina' age, in an area in which A. incana is today near its southern boundary.

There is a suggestion from the dated pollen evidence that there may have been a temporary halt in the colonization process by either species about 6500-6300 B.C. There is not enough evidence yet to decide whether this may have been due to a period of less favourable summer thermoclimate, affecting A. glutinosa, or to a temporary decline in groundwater levels connected with changes in the Ancylus Lake level, affecting .both species, or is simply due to gaps in the record which when filled will reveal a steady immigration and expansion of both alders. Nevertheless both Jessen (see Smith, I965, p. 339) for A. glutinosa in Ireland, and Lundqvist (I969, p. I9I) for A. incana in Jemt- land, have postulated early, limited colonization followed later by regional establishment, as temperature and/or groundwater conditions became modified.

Around 6000 B.C. A. glutinosa becomes generally established at most sites in south Finland, Estonia, Denmark, north Germany, Holland and west Norway, whilst A. incana becomes generally established at most sites in north Finland, north Sweden and probably in most of north central Sweden, and in north and central Norway. The gap in the records for south central Sweden, west of Uppsala, is mainly due to the continued submergence of much of the landmass here at this time. A dated pollen diagram from the Siljan lake area would be interesting, since the macrofossil finds here (site 33) show at first a mixture of the two species, as nowadays, but with A. glutinosa dominant later on (probably post-3000 B.C.).

It appears as though around 6000 B.C. we have the first indications of a change towards more stable groundwater and/or precipitation conditions, at least during the summer period, over a large part of the area surveyed. This trend towards a more even state of the hydrological conditions becomes even more pronounced after 5100 B.C. (see next section). Inland sites are affected as well as coastal ones. Most authors seem agreed that the Ancylus Lake achieves a steady-state, though with increasing salinity from shortly after 6000 B.C. at latest (see Freden, i967, Fig. 4), culminating in the first of the Litorina transgressions about 5000 B.C. We also have local sites for both A. glutinosa (site 47) and A. incana (sites 5, 6 and 32) in Fennoscandia at which the local hydrology was so extreme as to preclude alder establishment, despite an obviously favourable thermoclimate (cf. some of Konigsson's (i968) Oland 'alvar' area lakes, also purely spring and/or seepage water fed, which also appear to show a long delay in the local spread, if not establishment, of alder; to judge from the beech, hornbeam and spruce pollen curves from these sites open-water conditions, constant enough for continuous deposition of lake muds again, were not re-established until after c. 800 B.C.). A. glutinosa, furthermore, remains absent or very local over more or less the whole of Great Britain. This change around 6000 B.C.

does not seem to have been as significant, hydroclimatically, as the shift in conditions



Alder in Fennoscandia 54I

around 5000 B.C. The difference may be one of degree, or of seasonal restriction. For Britain it cannot be ruled out that the restriction was thermoclimatic, perhaps low summer temperatures due to lower surface temperatures of the north Atlantic at that time, perhaps reinforced by early summer drought. The full rise in ocean temperatures was not achieved until c. 4000 B.C. according to Wiseman's curve for the tropical Atlantic (Dunbar, i968).

6000-4500 B.C. During the first five hundred years of this period there seems to have been a gradual and steady consolidation by both alders at those sites at which they had secured a foothold, but with little expansion of territory. This occurs around o000 B.C.

and marks the general colonization of A. glutinosa in England and Wales and of A. incana and A. glutinosa, respectively, in the drier areas of north and south Fennoscandia. This spread almost certainly took place from the earlier local colonies established around 6000 B.C. For the intervening period, c. 5600-5200 B.C., there is a virtual absence of alder expansion datings, which is in good agreement with the evidence for the virtual cessation of peat growth (fen peats) in east England (site i6) and Holland (site io) and probably contemporary with the lowered water-levels attested from certain types of lake sites in Britain, Denmark and Sweden (and two unpublished peatbog sites in south Tr0ndelag) for which we at present lack dates. This interruption could well form one of the periods of predominance of high-index circulation conditions postulated in connection with the spread of spruce at a later stage in the Postglacial period (Tallantire, I972b).

At certain sites in eastern Ireland, eastern England, and south central Sweden, all regions in which the hydrological conditions are liable to fluctuate nowadays from year to year, due to lowered local summer and/or winter precipitation often accentuated by the effect of local edaphic conditions on run-off, the main spread of alder, irrespective of species, occurred around 4500 B.C. rather than 5000 B.C. The same is true for the sites in north-west Scotland, though here the explanation is more probably lack of local seed parents and seed dispersal problems following the onset of suitable thermoclimatic conditions around 5000 B.C. These are all lake sites and at only one (site 2I) is there any sign of possible sporadic presence of trees before the main establishment phase and there only after 5000 B.C.

After c. 5200 B.C., and throughout the period to c. 2000 B.C., a relatively high eustatic sea-level prevailed over the whole region under survey, with increased salinity in the Baltic and a progressively moderating rate of isostatic recovery, especially in the peri- pheral regions. In the southern half of our area A. glutinosa expands, and at those localities where it was already established the pollen curves for alder, lime, elm and oak usually rise. Increased convectional rainfall during the summer presumably compensated for the summer contribution to the c. 2-30 C rise in annual mean temperature generally postulated for northern Europe, since although there are indications of the limited onset of ombrogenous peat formation in parts of Britain, Norway, Sweden and Finland after c. 4500 B.C., the general evidence from topogenous and soligenous bogs and lakes is one of continued slow rates of growth or deposition. The effect was probably an evening-out of the hydroclimate throughout the vegetative period. Aario (I969) has some interesting calculations on water discharge from Lake Paiijanne in Finland during this period, whereby he concludes that dryness was probably a consequence of greater evaporation caused by a higher temperature, though in the absence of dates he cannot narrow this period further except within the broad limits of c. 58oo to c. 3800 B.C. Sundelin's (19I7) much earlier speculations, on the basis of Wallen's calculations, on the relation between annual mean temperatures and the shortfall of annual mean rainfall needed to produce




drought conditions in the area north-east of Viattern lake in south central Sweden, are also of interest in this connection.

By 4500 B.C., and for the next millennium, A. glutinosa is present and flourishing in all parts of its present range excepting Tr0ndelag and probably its other northerly localities in Norway, Sweden and Finland. The explanation for its absence from these areas, or relative scarcity compared to conditions after 2000-3000 B.C., would seem to be connected with the thermoclimate in summer rather than with the hydrological conditions by this time, since A. incana was already long established in these areas. Conceivably a slow- down in the rate of seed dispersal, due to these areas being cut-off from the main centres of distribution by regions in which the establishment of A. glutinosa was impossible (e.g. mountains, dense deciduous forest, edaphic dry-lands), could be the explanation in certain cases (e.g. Tr0ndelag, but scarcely in Sweden), but this makes its eventual appearance in these areas just as much of a mystery as before. We know that Bronze Age man used alder wood in constructing the framework for his skin-covered boats, the wood of both species being light but not rotting rapidly when kept in water, but he seems unlikely to have played any part in introducing A. glutinosa to these outlying areas. In view of the period of time available I do not think the seed dispersal argument is very convincing (see also Kubitzki, I96I). The thermoclimatic optimum was delayed here.

4500-700 B.C. The first millennium of this period has already been mentioned in the preceding section. For the remaining part the only '4C-dated evidence that I have is from Asklundvatn (site 5), though for the broad post-3000 B.C. period there are indications from the north Finnish and north central and north Swedish sites, with indirectly dated macrofossils, that A. glutinosa throve better there at that time than it does today, though not that it ever gained complete dominance over A. incana, nor that it ever extended its area further north than the present outposts at all (see Hintikka, ref. X in Table i, Fig. 24).

There is only slight evidence from Asklundvatn for any reflection of the climatic setback which slowly began to take effect after 4000 B.C. and which is locally registered as renewed glacier activity and lowered forest limits at higher-lying sites in Fennoscandia and the Alps, or as increased ombrogenous peat formation and the formation of recur- rence surfaces in raised bogs of lowland areas. Many of the effects can perhaps be explained in terms of the superimposition upon a curve of steadily declining annual mean temperature of a series of short (say 200-400 years) episodes of increased frequency of 'continental' conditions, summer and winter, over our area. The severity and degree of westward influence of these periods seems to have varied, and, in maritime districts more especially, to have produced renewed optima.

Pollen diagrams from some higher-lying peatbogs in central and west Norway (either undated or unpublished; e.g. sites 89 and 92) show a double-peaked alder curve, often paralleled by the birch curve, and at these sites almost certainly reflecting A. incana. At site 89 the peaks are dated to 3920 and I750 B.C. The period of diminishing values should therefore fall around 3600-3000 B.C., which is about the same time as the estimated start of Br0gger's 'Middle Tapes' and 0yen's 'Trivia' stages (Andersson, I909; Hafsten, I959) of the molluscan faunas in the Oslofjord, and indicating less favourable conditions then than during the preceding and subsequent stages of their 'Litorina/Tapes' period. The apparent contradiction in the conclusion drawn by the two geologists can be resolved if we read 'colder (winters) and warmer (summers)'. This period also covers the time of the 'elm decline' in Britain, on the continent of Europe, and in some parts of south Fenno- scandia (Ten Hove, I968), and of lowered, or continuing lowered, lake levels in parts of



Alder in Fennoscandia 543 the same regions (Troels-Smith, I956; Lamb, I969). This connection of the 'elm decline' with dry summers was first put forward by Tauber (I965). The provisional alder results would support this view, but need confirmation from further sites first. At Asklundvatn the alder pollen values (still A. incana) do show a slight decline at about this time, followed by the hazel values, but the samples are too widely spaced in time and only dated by interpolation.

After c. i6oo B.C. we have macrofossil evidence for the presence of A. glutinosa at Asklundvatn, and for a short period afterwards finds of A. incana seeds are in fact absent. A. glutinosa holds its own until about 700 B.C., thereafter giving way to A. incana again, though the presence of 'hybrid'-type seeds may indicate that a few trees held out, largely vegetatively. Since all macrofossils soon become exceedingly scarce there is no basis for further speculation. The present-day locality for solitary A. glutinosa on the margin of the lower-lying Logtumyra, just south of Asklundvatn, may represent a true 'relict' occurrence, which would have been reduced in vigour anyway by peat-cutting and drainage activities in historic time, or may represent a sporadic recolonization during periods of more favourable summers, for example during the tenth to twelfth centuries A.D., or even the 1920-40 period.

This piece of evidence that the most favourable 'all-round' climatic conditions in at least the western and northern parts of Fennoscandia occurred sometime during the two millennia after 3000 B.C., and not including the preceding two as in the more southern parts, was not unexpected. Scandinavian archaeologists had long since considered the Bronze Age (sensu tempora non artefactua) to have been a favourable period climatically.

Within this final period, as from 700 B.C. to the present day, the picture becomes increasingly complicated by changes due to agriculture and forestry, especially drainage work. More comprehensive material and more careful data analysis are required before any meaningful conclusions could be drawn. I shall not attempt this here. Further ecological studies of present-day stands of A. glutinosa in its northern and western outposts, and of A. incana on an altitudinal basis, would help to refine and test our postulated climatic requirements for both species.

CLIMATIC REGIONALISM

The broad regional distribution patterns of the two alders at unit points in time provide a provisional picture of the climatic regions in northern Europe during the Postglacial. The combined distributions yield clues on the development of the hydroclimate; the distribution of Alnus glutinosa taken alone yields clues to that of the thermoclimate, in so far as the two concepts can be considered in isolation at all. After allowing for the effects of the altered land/sea distribution during the first half of the Postglacial, and the cooling effects of a Baltic basin receiving large amounts of meltwater during early summer, of the restricted exchange of saline and freshwater in the Baltic and North Sea before 5000 B.C., and of the slow amelioration of sea surface temperatures in the north Atlantic, what regionalism do we find? One, I think, that agrees surprisingly well with the climatic regions recognized by Koppen and others (see the references in Bliithgen, I970) for the present-day. The climatic data base has changed both positively and negatively at various phases of the Postglacial, but the regions affected seem to retain their geographical identity throughout. This is of importance for Quaternary botanists, and others, considering the kind of areas within which relatively synchronous vegetational changes may be assumed. There is some hope, however, that local deviations from the regional




pattern may be predictable on a basis of present-day conditions, provided the variation within a 50-Ioo-year period at least is allowed for.

'4C-dated studies of ombrogenous peat formation within these various climatic regions could supply independent and more precise evidence about changes in the hydroclimate (P/E ratios), though evidence for periods of increased atmospheric turbulence (wind strength), affecting the advective component of rainfall in near-coastal and upland areas, must be sought elsewhere.

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APPENDIX*

Source list for the maps: Figs. I, 2 and 3

I Kaland, P.-E. (1970). Unpublished degree thesis, Bergen. 2 Hafsten U. & Tallantire P. A. (unpublished) Torgarden-Harams0y. 3 Persson, C. (I97 i). Geol. Foren. Stockh. Forh., 89, I89. Kristiansund. 4 Mack, G. (0972). Unpublished degree thesis, Trondheim. Stormyra-Dragvoll. 5 Lillealter, J. (I972). Unpublished degree thesis, Trondheim. Asklundvatn-Frosta. (See also

Tallantire, I 973). 6 Ibid. Brekkmyra-Frosta. 7 Hamberg, A. (i893). Geol. F6ren. Stockh. Forh., I5, 5II. Vaerdal. 8 Vorren, B. (i969). Unpublished thesis, Trondheim. Lite Kultj0nn. 9 Andersson, G. (i892). Geol. F6ren. Stockh. Forh., 14, 5i6-I7. Turtagr0.

Io Vorren, T. (1971). Unpublished thesis, Bergen. Kroken in Lusterfjord. ii Larssen, K.-E. In: Gabrielsen, G. (1959). Nature, Lond., 183, i6i6. 12 Holmboe, J. (i903). Vidensk. Selsk. Kristiania Skr. I. Math.-Naturv. Kl. No. 2. L0netjern. 13 Larssen, K.-E. In: Nydal, R. (1959). Radiocarbon, I. 'Tjonna', Salunn. 14 Ibid. Maler0d tj0nn, Brunlanes. 15 Sonesson, M. (I968). Bot. Notiser, 121, 49I. Vuolep Njakajaure. i6 Robertsson, A.-M. (I97I). Sver. geol. Unders., Ser. C, No. 658. Leveiiniemi. I7 Lundqvist, G. ( 956). Ymer, 76, 23 i. Adak. i8 Post, L. von. (i906). Geol. Foren. Stockh. Forh., 28, 20I. Langmyren. I9 Lundqvist, J. (i969). Sver. geol. Unders., Ser. Ca, No. 45. Hallviksmyren. 20 Andersson, G. (i894). Geol. Foren. Stockh. Forh., I6, 53I. Vainniis. 2i Eriksson, K. (I97I). Sver. geol. Unders., Ser. C, No. 66o. Hottojen. 22 Tolf, R. (i893). Kgl. Sv. Vet. Akad. Handl. Bihang, i9, Afd. III, No. i. Lit. 23 Andersson, G. (i894). Geol. F6ren. Stockh. Forh., i6, 53i. Granv'ag. 24 Nathorst, A. G. (i892). Ofversigt af Kgl. Vet. Akad. Forh. No. 9, 429. Askammen. 25 Lundqvist, J. (i969). Sver. geol. Unders., Ser. Ca., No. 45. Stockbergsmyren. 26 Post, L. von. (I906). Geol. Fdren. Stockh. Forh., 28, 20I. Kingstamyren. 27 Sandegren, R. (I924). Sver. geol. Unders., Ser. Ca., No. I2. Ragunda. 28 Wenner, C.-G. (i968). Stockh. Contr. Geol., I8 (3) 75. Bodsjo.

29-30 Fromm, E. (1938). Geol. Foren. Stockh. Forh., 6o, 363. Vignaisbrunnen and Sand. 3i Hedstr6m, H. (i893). Geol. F6ren. Stockh. Forh., 14, 29I. Timmermossen. 32 Lundqvist, J. (i969). Sver. geol. Unders., Ser. Ca., No. 45. T6nningsfloarna. 33 Hedstr6m, H. (1938). Geol. F6ren. Stockh. Forh., 14, 29I. Solleron. 34 Nathorst, A. (i895). Geol. F6ren. Stockh. Forh., 17, 69i. Skattmannso. 35 Andersson, G. (i896). Svenska vdxtvdrldens historia in korthet framstdlld. I36 pp. Stockholm. 36 Sunesson, S. & Sandegren, R. (1948). Sv. Bot. Tidskr., 42, 258. Lyckan. 37 Sernander, G. (i893). Geol. Foren. Stockh. Forh., 15, 345. Grava. 38 Sernander, G. & Kjellmark, H. (i896). Bull. geol. Inst. Univ. Uppsala, 2, 3I7. Gottersaitermossen. 39 Post, L. von. (I909). Geol. F6ren. Stockh. Forh., 31, 629. Vilsta. 40 Ibid. Tarnsj6moor, Skagershult. 4I Post, L. von. (I927). Sver. geol. Unders., Ser. C., No. 347. Hogebergsmossen.

42-44 Olsson, I. U. & Freden, C. (i969). Geol. Fo6ren. Stockh. Fdrh., 91, 20I. Solbergatjairn, Klippetorp- tjairn and Hakanbol.

45 Wenner, C.-G. (i968). Stockh. Contrib. Geol., I8 (3), 75. Grytsjon. * Author's note: due to space the editor has deleted the explanatory notes on the individual sites. Since

these formed an integral part of the argument (as in Tallantire I 972a) a copy will be sent on application.




46 Sernander, R. (I902). Geol. Foren. Stockh. Forh., 24, I25. Morklefmossen. 47 Magnusson, E. (I964). Sver. geol. Unders., Ser. C., No. 597. Dagsmosse. 48 Andersson, G. (10c. cit. site 35). 49 G6ransson, H. (I970). Radiocarbon, 12, 542. Striern. 50 Tolf, R. (I893). Kgl. Sv. Vet. Akad. Handl. Bihang, i9, Afd. III, No. i. Flahultmyr. 5I Andersson, G. (I893). Kgl. Sv. Vet. Akad. Handl. Bihang, i8, Afd. III, No. 8. Wallda Mosse. 52 Wenner, C.-G. (I968). Stockh. Contrib. Geol., I8 (3), 75. Langared. 53 Andersson, G. (10c. cit. site 5I). Munksgard Mosse. 54 Olausson, E. (I957). Lunds Univ. Arsskr. NF, Avd. 2, 53 (I2). Roshultsmyr. 55 Digerfeldt, G. (I 97i). The postglacial development of Lake Trummern. IV. Summary of developments.

Mim. thesis, Lund. 56 Konigsson, L.-K. (I968). Acta phytogeogr. Suec., No. 55; see also Geol. F6ren. Stockh. Forh., 90, 5. 57 Sernander, R. (I893). Geol. Foren. Stockh. Forh., 15, 345. Lingomyr. 58 Andersson, G. (I892). Kgl. Sv. Vet. Akad. Handl. Bihang, i8, Afd. III, No. 2. Igelsjo Mosse-

Hallandsas. 59 Andersson, G. (loc. cit. above). Bjorkerods Mosse-Kullaberg. 6o Nilsson, T. (I964). Lunds Univ. Arsskr. N.F. Avd. 2, 59, No. 7. Agerods Mosse. 6i Digerfeldt, G. In: Hakonsson, H. (I969). Radiocarbon, II, 430. Ranvikan. 62 Digerfeldt, G. (I97i). Geol. Foren. Stockh. Forh., 93, 6oi. Torreberga. 63 Andersson, G. (loc. cit. site 5i). Mjellby Mosse. 64 Andersson, G. (loc. cit. site 5i). Ronneby. 65 Berglund, B. E. (I966). Opera Botanica, 12 (2). Hallarums Mosse. 66 Andersson, G. (10c. cit. site 35). The 'Blekinge' site. 67 Lappalainen, E. (I970). Bull. Comm. geol. Finl., No. 244. Virrttiovuoma. 68 Vasari, Y. (I967). Aquilo, Ser. Botanica, 6, 7I. Saiyna'ajajairvi. 69 Vasari, Y. (I963). Suomal. eldin- ja kasvit. Seur. van. Sulk., 33, No. I and appendix. Maijalampi. 70 Kanerva, K. (1956). Ann. Acad. Sci. Fenn., Ser. A, III, No. 46. Kainuu region. 7I Hyvvrinen, H. (I966). Comm. Biol., Soc. Sci. Fenn., No. 29. Alasenjiirvi I. 72 Valovirta, V. (I960). Bull. geol. Soc. Finl., No. i88, 4I. Valmossa. 73 Hyvarinen, H. (10c. cit. site 7I). Kaitalaminsuo. 74 Lindberg, H. In: Kujala, V. (I924). Comm. Inst. Forest. Fenn., 7, 295. Joroinen-Jarvikyla. 75 Alhonen, P. (I968). Bull. geol. Soc. Finl., 40, 65. Sarkkilanjiarvi.

76-78 Simola, L. K. (I963). Ann. Acad. Sci. Fenn., Ser. A, III, No. 70. Vanhaoja, Letensuo, Vanajavesi. 79 Donner, J. J. (I966). Comm. Biol., Soc. Sci. Fenn., 29, No. 9. Varassuo.

80-82 Andersson, G. (I898). Bull. Comm. geol. Finl., No. 8. Anta Mosse, Stubbangen, Humppila. 83 Virkkala, K. (I966). C.R. Soc. geol. Finl., 38, 237. Rabacka. 84 Valovirta, V. (I965). Bull. Comm. geol. Finl., No. 220. Heposaarensuo. 85 Valovirta, V. (loc. cit. above). Holmansuo. 86 Ilves, E., Sarv, A. & Valk, U. (I968). Pedobiologia, 7/8, 329. Teosaare (Jogeva). 87 Ilves, E. & Sarv, A. (I969). Izvest. Akad. Nauk. Eston, S.S.R., Ser. Chim.-Geol., i8 (4), 377.

Kalina. (N.B. wrongly plotted, lies further N at c. 59?0 1' N, 27? E.) 88 Ilves, E., Mannil, R. & Valk, U. (I967). Metsanduslikud Uurimused, 5, 235. Kuiksilla. 89 Hafsten, U. (unpublished) Vasst0l-Suldal. go Backman, A. L. (I919). Acta For. Fenn., :12, No. i. Rahkaneva. 9I Herlin, H. From Andersson, G. (loc. cit. site 8o). Kovelahti. 92 Faegri, K. (I945). Norsk Geogr. Tidsskr., 25, 99. Haugast0l. 93 Sorsa, P. (I965). Ann. Bot. Fenn., 2, 30I. Virtaniemi. 94 Neustadt, M. I. (I97i). Geol. F6ren. Stockh. Forh., 93, I05. Lake Bebrukas.

Sites used for the European inset maps in Figs. I and 3 (the Alder sp. presumed to be solely Alnus glutinosa)

I-5 Smith, A. G. et al. ( I97i). Radiocarbon, 13, I2I. 6 Pennington, W. et al. (1972). Phil. Trans. R. Soc., B, 264, I93. Loch Sionascaig. 7 Birks, H. H. (I970). J7. Ecol., 58, 827. Abernethy Forest. 8 Vasari, Y. & Vasari, A. (I968). Acta Bot. Fenn., No. 8o. Loch of Park. 9 Behre, K.-E. (I969). Beitr. Meeresk., 24/25, I22. Godwin, H. (1956). History of the British Flora,

Cambridge University Press. Dogger Bank area. I0 Zeist, W. van. (1957). Palaeohistoria, 4, II3. Emmen. II Baas, J. (1938). Abh. Senckenberg. Naturf. ges. Frankfurt, No. 440. Senckenberg Moor. I2 Iversen, J. In: Tauber, H. (I968). Radiocarbon, I0, 296. Draved Mose. I3 Kubitzki, K. & Miinnich, K. 0. (I960). Ber. dt. Bot. Ges., 73, I37. Siuderluigum.

I4-I5 Iversen, J. (I967). Danmarks Natur, I (Politikkens Forlag, Copenhagen). i6 Clark, J. G. D. & Godwin, H. (I962). Antiquity, 36, i0. Shippea Hill. I7 Hibbert, F. A., Switsur, V. R. & West, R. G. (I97I). Proc. R. Soc., B, I77, i6i. Red Moss. i8 Godwin, H., Walker, D. & Willis, E. H. (957). Proc. R. Soc., B, 147, 352. Scaleby Moss. I9 Krog, H. (I965). Baltica, 2, 47 and (I97I) Quaternaria, 14, 85. 20 Svitsur, V. R. & West, R.. G. (1972). Radiocarbon, 14, 239. Tregaron S.E. Bog. 2I Birks, H. H. (1972). New Phytol., 71, 73I. Loch Maree. 22 Pennington, W. et al. (loc. cit. site 6). Loch Clair.

