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278 MAGNUS FRIESzone system the Sub-boreal is designated as zoneVIII, in the Scanian system as zone SB 1-2.The Post-glacial, later partThe climatic deterioration that followed thewarm period and influenced the vegetation wasregarded by SERNANDER ( 1908, 1910, pp. 219-246,1912a, b, c) and others as a single abrupt event,almost of catastrophic character, happening in themiddle of the first millenium B.C. (at least in thesouth). In opposition to SERNANDER, G. ANDERS­SON (1909, p. 70, 1910, pp. 289-293) assumed aprotracted, gradual deterioration after the climaticoptimum. It is most likely, however, that thechange to a cooler and moister climate happenedin stages during a period of two or three thousandyears, probably starting as early as about 3000 B.c.According to modern interpretation of detailedpollen diagrams the vegetational change at about3000 B.C. in South Scandinavia is a result of bothclimatic and cultural influences, which are difficultto separate. The discussion whether a special Ulmusdecline in South and West Scandinavian diagrams,at about 3000 B.c., was caused by climatic orcultural influence or both, may be quoted as typicalof problems of this kind (e.g. FlEGRI 1940, p. 122;T. NILSSON 1948, p. 48; TROELS-SMITH 1960;MAGNUSSON 1962, pp. 50, 61-62).The immigration of new forest trees, spruce(Picea abies) from the northeast and beech (Fagussilvatica) and hornbeam (Carpinus betulus) from thesouth, gave the forests of Sweden a new face, coincidingwith a southward displacement of the regionsas a result of the climatic deterioration (map e,Fig. 2). Although the immigration started earlier,these events were completed in the middle of thefirst millenium B.C. in most of Sweden. So the forest·regions of today were established, viz. from southto north (disregarding the subalpine birch belt):(l ) deciduous-tree forest region with beech, (2)conifer-forest region with thermophilous deciduoustrees (partly except beech), and (3) conifer-forestregion, mainly without thermophilous deciduoustrees (e.g. Du RIETZ 1925 c, 1964; SJORS, 'Forestregions' in this volume).The chronology oi the immigration of spruce is.still rather uncertain, although it has been asubject of many studies since SERNANDER's work"Die Einwanderung der Fichte in Skandinavien"( 1892). An absolute date, about 1000 B.c., for thespruce invasion into a central coastal part of NorthSweden was obtained through pollen-analyticalstudies of geochronologically dated varves (FROMM1938). Some radiocarbon datings of the markedincrease of the Picea pollen curve in North-Swedishdeposits verify this date, other do not (G. LuND­QVIST 1957, pp. 14, 23, 1963 b, p. 92, etc.). Evenwithin rather limited areas the dates differ considerably.However, this index level shows atendency to transgress in time towards youngerdates from South-Central Sweden southwards(voN PosT 1924, pp. lll-112; GRANLUND 1932,pp. 131-141; FRoMM 1938, p. 380; M. FRIES l958a,pp. 25-26; JAN LUNDQVIST 1957, pp. 6-7, 19; G.LUNDQVIST 1957, pp. 15, 23, l963 b, pp. 95, 170).Certainly one should observe great cautiousnesswhen using the increase of the Picea pollen curve asan index level. The immigration of the spruce andits chronology ought to be subject to further, morespecial studies by means of pollen analysis andradiocarbon-age determinations.In North Sweden the spruce, being a shadetoleranttree, most likely mainly invaded the birchforests of the warm period. Large areas of North­Swedish forests were fundamentally changed in arather short time.Some pollen diagrams from different parts ofSweden show remarkable occurrences of Piceapollen, even values of 20 per cent and more (G.LuNDQVIST 1929, p. 377; S. FLORIN 1944, p. 567;cf. voN PosT 1924, pp. Ill, 125) , as early as in lateAtlantic and in Sub-boreal time (zone VII andVIII, resp.). All of them can hardly be interpretedas results of long-distance transport. There are alsoa few archaeological evidences of spruce beforethe Iron Age (S. LINDQVIST 1916, pp. 169, 180).These early occurrences, probably infrequent andvery restricted, may have been important for therather rapid spreading over most of the country,when the climate became more suitable for thespruce.The immigration of the beech and hornbeam,covering a comparatively small area of southernSweden, is better known, thanks to studies inActa Phytogeogr. Suec. 50