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The Late-Quaternary vegetation of Sweden273cates a remarkable climatic amelioration. On thebasis of this climatic change the forest is consideredto have followed close to the retreating ice margin,preventing the arctic-subarctic plants of the Lateglacialfrom moving northwards. However, thisassumption, although plausible in itself, is notsupported by any valid pollen-analytical or otherpaleontological evidence. Detailed studies of earlylake deposits in the area between the Central­Swedish Moraines and the mountains are badlywanted.In the pioneer forest of the Preboreal the birchwas the predominant tree. In fact many pollendiagrams indicate the existence of pure birchforests at least during the middle part of the Preborealperiod (pollen zone IV in the Danish zonesystem). Although the identification as to speciesof birch pollen on the basis of size measurements isdifficult or sometimes doubtful, it is most likelythat Betula pubescens sens. lat. formed the pioneerbirch forests. Minor constituents in this forest wereaspen (Populus tremula) and later on pine (Pinussilvestris). Probably the birch forest offered resistanceto the invasion of pine.The structure of the Preboreal forests is littleknown. In the first part of the period the amountof non-arboreal pollen is still significant. Obviouslytreeless patches with bare mineral soil enabled atleast some of the heliophilous herbs, grasses, anddwarf shrubs of the Late-glacial to grow in the newenvironment.Map b, Fig. 2, is intended to show the ice marginat 7500 B.c. (according to the clay varve chronology)and the corresponding forest situation. North ofSkane the Preboreal birch forests still prevailed.In Ska,ne the landscape was just changing to theBoreal situation (see below).The next forest-forming tree to appear was thepine (Pinus silvestris), which obviously came fromsouth and southeast (but may have had another,later path of invasion from the northeast intoLappland). Almost at the same time the hazel(Corylus avellana) appeared. It probably invadedthe fertile soils of South Sweden. Especially in thesouthwestern parts of the country it formed largewoods (cf. map c, Fig. 2). Evidently they werealmost pure, only locally mixed with elm, a typeof wood which has no natural equivalent in Swedenof today. The pine certam!y was the predominanttree on poor or very dry soils. The birch becamemore restricted, probably to open sites. However,north of the hazel area, in Central and NorthSweden, birch forests were formed (see below).This hazel-pine-birch period (pollen zone V) or theOlder Boreal time lasted about one thousand years,at least in the south. According to radiocarbondatings, recently published by T. NILSSON (1964a),it began as early as about 7900 B.c. in Skane. Itseems plausible to assume a northward retardationowing to the northward migration of the hazel.In the absence of radiocarbon datings referableto the beginning of the Corylus pollen curve (zoneborder IV/V) farther to the north, it is difficult toestimate the magnitude of this retardation. Anyhow,there was a delay of probably a few hundredyears in the province of Dalsland and still morein the area north of the Vanern-Hjalmaren-Malarendepression.The Post-glacial, middle partThe invasion of alder (Alnus glutinosa) 6800-6600 B.c. (Skane, T. NILssoN 1964a) and evenmore thermophilous trees like elm ( Ulmus glabrassp. scabra), oak (Quercus robur), lime or linden(Tilia cordata), ash (Fraxinus excelsior) and maple(Acer platanoides) implied a considerable changeof the vegetational landscape. The beginning of thisprocess, with less lime and ash than later, is oftenassigned to the Boreal and may be called theYounger Boreal time (pollen zone VI or BO 2according to T. NILSSON 1961, p. 11, 1964a, p. 47).Possibly these thermophilous trees first met someresistance from the dense Boreal hazel forests (cf.B. LINDQUIST 1938, pp. 131, 266). Therefore theywere perhaps not able to keep pace with the rapidclimatic amelioration. However, between 6800 and6200 B.C. the alder and the other broad-leavedthermophilous trees occupied most of Skane (T.NILSSON 1964a) and with some retardation the restof South Sweden.The change in the forest composition, as mentionedabove, was the vegetational response to thebeginning of a period of climatically favourableconditions, viz. the Post-glacial warm period. ItActa Phytogeogr. S1wc. 50