The postglacial vegetation development of an area in northwest Poland and its chronological course was reconstructed based on four pollen diagrams from two sites in the eastern Lower Oder valley and the southern Lower Warta valley. Great emphasis was laid on the immigration and spread of beech, which reaches its natural borders in this region. Primary the Lower Oder valley represents an area with a mixed deciduous forest, dominated by lime and elm. After immigrating in the Subboreal around 4300 cal. B.C. beech spread very slowly in this area. The same concerns the southern Lower Warta valley, where the beech arrived around 3500 cal. B.C.. It is shown that the spread of beech in the mesocratic forests around 2600 cal. B.C. coincides with the appearance of anthropogenic indicators. The largest extension of the distribution of the beech in the eastern Lower Oder valley was reached in the Migration Period with pollen percentages of 18 %. In the southern Lower Warta valley the pollen percentages of beech never exceeded values of 5 %. Obviously only a few kilometres southeast of the Lower Oder valley beech has entered an area of unfavourable environmental conditions beyond its ecological optimum. The increase of frost events in spring and especially of summer drought reduced the competitive strength of beech in regard to other trees. Unlike in the north-west of the eastern Lower Oder valley thus beech was prevented from becoming the dominant tree. Besides, other ecological conditions characteristic for the gentle sloping ground moraine landscape limited the spread of beech forest to small isolated populations. The spread of hornbeam occurred much earlier in the southern Lower Warta valley than in the eastern Lower Oder valley. Already in the Bronze Age pollen percentages of 15 % were reached in the southern Lower Warta valley. Although the competitive strength of beech was reduced, no extended hornbeam forests were established as it was the case in the Great Poland-Kujawy lowland. Hornbeams probable built the second tree layer in a mixed deciduous oak forest, strongly degraded by human impact.