The alder [ Alnus glutinosa (L.) Gaertn.] problem in the British Isles: a third approach to its palaeohistory

summary The previous approaches (rational and empirical limits for the alder pollen curve) are discussed in relation to the immigration/establishment and spread/expansion phases. For the former, consistent ‘tail’ values for alder pollen of over 3 % (total land pollen) are considered requisite, lower values being ascribed to long–distance transport. For the latter, a mapped plot of the radiocarbon dates for the initial marked rise in the alder pollen curve, to over 10 %, is presented, using only those diagrams from sites considered stratigraphically reliable and for which the dated samples derive from specific points on the curve (start, mid–rise or finish). Certain ecological considerations, which appear to have been either overlooked or neglected, are dealt with in relation to the thermoclimatic and hygroclimatic demands of alder. Alder establishment seems highly probable by 8000 BP for at least two sites in southeastern England. This supports the view that alder immigrated from Western Germany or Holland. The spread of alder shows a clear regionalism, with a major increase after about 7500 BP, mainly after 7000 BP. The failure of alder to spread north and west from its initial footholds is ascribed to the seasonal, prolonged periods of drought during the Boreal chronozone (c. 7800—‐75 00 BP), i.e. the hygroclimatic conditions were unfavourable. The initial establishment was restricted to the south east of England on account of the thermoclimatic demand and the source areas for seed. The delays shown in its eventual spread north and west are ascribed to unfulfilled thermoclimatic demands, possibly due to low air and sea temperatures. A plea is made for the use of primary sources only, not computer‐stored data, when plotting these phenomena, and for future investigations to create a denser network of reliably dated sites, with unequivocal stratigraphic conditions, continuous pollen diagrams, and with macrofossil investigations when appropriate (excluding wood), at all altitudes.