fulltext - DiVA

274 MAGNUS FRIESseems to have lasted about 6500 years in the south(ea. 6800 to 300 B.C. in Skane, according to T.NILSSON 1964a), but less in the north (see below),and culminated about 4500 B.C. (op. cit.). It mayhave taken more than one thousand years beforethe whole assembly of thermophilous trees got apermanent foothold in South-Central Swedennorth of the Vanern-Ma1aren depression (whichto a great extent still was below sea level). Withaid of radiocarbon datings of pollen-analyzed lakesediment marking the isolation from the sea, S.FLoRIN (1961, p. 343; cf. 1944, p. 569) estimates theTilia pollen curve to begin at about 5500 B.c. inSodermanland (south of the present Lake Malaren).The gradual opening of the English Channel andthe submergence of the Norh Sea banks wereprobably of importance for giving the climate insouthern Scandinavia a more humid characterthan in the beginning of the Post-glacial time.Comparatively high temperature and humiditywere assumed to have been characteristic of theAtlantic time in early works by BLYTT (1876a, b,1881). The Atlantic period is designated as zoneVII in the Danish and AT 1-2 in the Scanianpollen-zone system.There are several observations indicating thatduring the warm period the thermophilous treesand their accompanying flora were more commonand occurred farther to the north than they dotoday. Some evidence may be mentioned.Fossils (nuts and pollen) of hazel (Corylus avellana)found considerably farther to the north andnorthwest than the area of general occurrencetoday (HEDSTROM 1893; G. ANDERSSON 1902; E.PETTERSSON 1956) made it possible to estimate thedifference in summer temperature between thePost-glacial warm period and our time. This differencewas given as 2.4°C by G. ANDERSSON(op. cit.).Trapa natans may be regarded almost as a symbolof the Post-glacial warm period. It spread obviouslyrather rapidly to a great many lakes asfar north as the Vanern-Malaren depression andeven somewhat north of this area (MALMSTROM1920; M. FRIES 1951, pp. 165-169, 206-207, mapp. 168). Its immigration to Southwest Swedenseems to have coincided in time with the invasionof the alder (M. FRIES, op. cit.). It disappearedfrom most of the shallow lakes, where the nuts arenow found in the bottom deposits, at the time of theclimatic deterioration in the first millenium B.c., ifnot earlier because of the filling-in of the lkes bysediments.Several finds of stems and stumps of pine (Pinussilvestris) above the present timberline in themountains indicate a more favourable climate alsoin this area, even when the influence of the laterland uplift is taken into consideration (GAVELIN1909; H. SMITH 1920, p. 120-127; G. LUNDQVIST1959 b). Radiocarbon datings of the stumps placenearly all the finds in the beginning and the middlepart of the warm period (G. L UNDQVIST 1959 b,1962). The lack of stumps dated to· the later thirdof this period (in the South-Swedish sense) showsthat the climatic deterioration had a negative effecton the comparatively thermophilous vegetationearlier here than farther south.On the basis of the pollen diagrams hithertopublished, it is hard to know more exactly to whatextent Sweden during the warm period was coveredby forests of the broadleaved thermophilous trees,and how far to the north they reached. Finally,the actual composition of the forests is difficult toimagine on the basis of pollen spectra.. In most ofSkane and a few other similar areas nearly all landwas occupied by these trees. In the rest of SouthSweden (up to about N 60°) they probably coveredover 50 per cent of the land. In the coastal areasof Norrland and locally in the inland they obviouslyformed scattered stands, as was also thecase with the hazel at the same time. The presentisolated occurrences of elm ( Ulmus glabra ssp.montana) not far from the mountains are certainlyrelics from the warm period and have originatedfrom west of the mountain chain in coastal Norway(ANDERSSON & BIRGER 1912, p. 186).As to the composition of the deciduous forestsit is customary in European pollen analysis to speakabout mixed oak forest Quercetum mixtum (QM inthe pollen diagrams), constituted by elm, oak,ash, lime, and maple. To a certain extent it may bereasonable to do so. Judging from studies ofdeciduous forests in modern time, for instanceDalby Soderskog in Skane (B. LINDQUIST 1938),Acta Phytogeogr. Suec. 50