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The Late-Quaternary vegetation of Sweden 279Skane by T. Nn..ssoN (1964a) and others. However,it is still an unsolved problem whether or not thesetrees, which seem to have accompanied each otherfairly well, ever reached as far north as the provincesaround Lake Malaren (W of Stockholm). Low butnot extremely low amounts of Fagus and Carpinuspollen in diagrams from this area may indicatethat this was the case (voN PosT 1924, pp. llO,125; M. FRIES 1963a, p. 14). Scattered pollen grainshave been found even farther north. Anyhow, it isquite clear that the distribution of beech andhornbeam has been diminished during the last tenor fifteen hundred years, a process which to somedegree may be a result of climatic change but islargely caused by human interference (B. LINDQUIST1931, pp. 388-411).According to the above-mentioned circumstancesthe last Post-glacial period, the Sub-atlantic time(pollen zone IX in the Danish system, SA 1-2 inthe Scanian system) is characterized pollen-analyticallyby the occurrence of spruce, beech, andhornbeam and other events, caused by a series ofcomplicated climatic changes in a cooler and moisterdirection. Obviously these changes that ended thePost-glacial warm period interfered more seriouslywith the thermophilous flora in the north than inthe south. Therefore we may assume that the zoneborder VIII/IX transgresses in time from northto south. To what extent this is the case may beellucidated by future radiocarbon-age determinations.Beside these changes of the forest geography, aconsiderable formation of peat land took place.Lakes were gradually filled in and poorly drainedforest were paludified. Mainly in South Sweden and.parts of Central Sweden the peat accumulation evenled to a development of raised bogs, the furthergrowth of which was dependent on the excess ofprecipitation over evapotranspiration. The differencesin degree of decomposition betweenvarious peat strata, and the interbedding of fossilstumps and logs, mostly of pine, were postulatedby BLYTT (1876a, b, 1881) to indicate an alternationof wet and dry periods. This idea was furtherdeveloped by SERNANDER (e.g. 1910), VON PosT(1909, 1913, 1946), G. LuNDQVIST (1928) and, aboveall, by GRANLUND (1932), who considered the alternationbetween the highly humified and less humifiedpeat to be caused by a recurrent climatic cycle.Each change from a dry to a wet climate produceda "recurrence surface", in Swedish rekurrensyta(RY). Further studies of the "recurrence surfaces"through pollen analyses and radiocarbon datingsshow that the problem of their origin and chronologicalposition is complicated and far from beingsolved (GODWIN 1954; VAN ZEIST 1955; JAN LUND­QVIST 1957; 0LAUSSON 1957, pp. 28-30; OVERBECKet al. 1957; OvERBECK 1961; G. LuNDQVIST 1962,pp. 8-10, 1963 b, pp. 95-97, 171; ScHNEEKLOTHet al. 1963, pp. 50-52, 126-127, 174-176).The influence of manThe human influence on the vegetation wasexceedingly small in pre-agricultural time. Thefollovving N eo lithic and Bronze Age cultures wererestricted to certain areas in South Sweden, separatedby wide tracts of untouched forests. Thissituation prevailed in fact far into the Iron Age,in some parts into Medieval and even Modern times.It is still a matter of discussion to what extentthe climatic deterioration that occurred about500 B.C., i.e. about the transition between theBronze and Iron Ages, influenced the settlementthat was dependent on agriculture and animalhusbandry (STENBERGER 1962, pp. ll5-118; 1964,pp. 319-322, 330-333). In this connection it maybe emphasized that there were other climaticchanges, both earlier and later, although probablynot so severe as this one. In certain areas, however,they may have had even more far-reaching influence.Several pollen-analytical studies in South Swedenshow that the landscape with vestiges of Neolithicand Bronze Age cultures was increasingly utilizedby man during the late Iron Age (in Sweden up to1050 A.D.) and still more during Medieval andModern times. In the districts with ancient agricultureand animal husbandry the frequency ofpollen from cereals, weeds, and pasture plants increasein the sediment or peat layers formed duringthese periods (M.-B. FLORIN 1957 b; M. FRIES 1958a,1962, 1963a, b; HELMFRID 1958; S. FLORIN 1961,1962; T. Nn..ssoN 1961, 1964a, b). Districts intowhich agriculture spread comparatively late haveActa Phytogeog1'. Suec. 50