The Rate of Spread and Population in Increase of Forest Trees During the Postglacial

The last major ice-sheets in northern Europe and northern North America reached their maximum extent about 18000 years ago. With the retreat of the ice-sheets, changing climates permitted many plants to expand their ranges onto freshly deglaciated ground. The spread of these plants can be followed across continents by means of the pollen preserved in suitable sediments. Two aspects of this spread will be discussed. First, at individual sites, it is possible to compare the way in which the populations increase with theoretical models of population growth. It appears that, for forest trees, the populations increased exponentially, doubling every 20-500 years. Second, by estimating, from the pollen curve, when populations of each taxon began to increase at a series of sites across a continent, it is possible to estimate annual `rates of spread'. These may be up to 2000 m a<sup>-1</sup>, but it appears that spread may be achieved at population densities too low to be detected in the pollen record. The `spread' observed may be due solely to the initial abundance/distance gradient and the rate of population increase.