When no pollen does not mean no trees

Abstract

The question of what signal, if any, appears in the pollen record when trees are present in the vegetation without producing pollen or with no pollen being recorded, is addressed. Four scenarios are envisaged: (i) the number of trees in the landscape are very few and scattered, (ii) the trees are too young to produce pollen, (iii) climate conditions are unfavourable for the trees to produce pollen and (iv) the trees are cut or damaged so that they do not flower. Each of these is considered in terms of pollen accumulation rates (PARs) and present theories and models of pollen dispersal. Examples are provided for the forest limit areas of the northern boreal trees in Finnish Lapland using data of pollen deposition monitored by pollen traps and results from the high temporal resolution (near annual) analyses of peat profiles. The relevance of the results to questions such as finds of spruce macrofossils in the Swedish mountains, the 8200 cal b.p. cold event, the migration of species/vegetation succession, and widespread damage to trees are all considered. It is concluded that although these situations are sometimes ‘invisible’ or misrepresented when pollen assemblages are expressed in the traditional percentage manner, they are often revealed by PARs. The fact that the pollen assemblage reflects a much wider regional area than is often understood can strengthen signals which have a regional impact, such as those which are climate induced, but may obscure events which affect only a limited spatial area or occur as small patches in the landscape.