The use of annual arboreal pollen deposition values for delimiting tree-lines in the landscape and exploring models of pollen dispersal

In reconstructing past vegetation by pollen analysis there is an increasing need to quantity plant abundance and to specify the distribution of vegetation units in the landscape. Fundamental to such reconstructions is an understanding of the pollen-vegetation relationship. Monitoring pollen deposition by a standardized method in different vegetation situations provides numerical data for this relationship. Here monitoring is by modified 'Tauber' traps following the standardization of the Pollen Monitoring Programme (PMP). Results are presented for an 18-year period (1982-1999) at ten localities within the latitudinal boreal forest zones of northern Fennoscandia. Particular focus is placed on the tree-lines as these are regarded as climate-sensitive boundaries in the landscape. At an annual temporal resolution, pollen deposition reflects climate, primarily the temperature of the growing season of the year before pollen emission. It is the long-term average pollen deposition which reflects the presence/absence and abundance of trees in the surroundings of the monitoring site, Pollen influx values are given for varying degrees of coverage of Betula, Pinus and Picea. The tree-lines of these three taxa are crossed when pollen influx rises above 500, 500 and 50 grains cm(-2) year(-1) respectively. The comparable pollen influx thresholds for the presence of forest are: 1000, 1500 and 100 grains cm(-2) year(-1) respectively. These numerical values apply to openings in the forest cover of c.l ha. However, inside the forest, pollen deposition values can be some three times higher. These data contribute towards the validation of models of pollen-vegetation relationships and the results enable a more objective interpretation of fossil pollen assemblages from terrestrial deposits in terms of both tree-lines and climate. (C) 2001 Elsevier Science B.V. All rights reserved.