MODELING HOLOCENE CHANGES IN THE LOCATION AND ABUNDANCE OF BEECH POPULATIONS IN EASTERN NORTH-AMERICA

Maps of Holocene changes in pollen percentages show changes in the magnitude and location of taxon abundances. They also show changes in the patterns of decreasing abundance around the areas of maximum abunances. Transport equations with source/sink, advection and diffusion terms provide analogs for modeling these different components of continent-scale population dynamics. We used recently developed numerical techniques to fit such a transport equation to Holocene changes in beech (Fagus) pollen percentages along a transect between Tennessee [USA] and southern Quebec [Canada]. A constant coefficient model explained 91% of the variance for the changes in beech pollen percentages between 8000 and 500 yr B.P. Estimated values for the coefficients were 9.6 km2/year for diffusion, 0.05 km/year for advection, and 0.18/1000 yr for the exponent in the source/sink term. Perturbation and residual analysis showed these values to be robust and statistically sound. A second version of the transport model with temporally varying parameters for the source/sink term yielded small but significant improvement by explaining 94% of the variance. A plausible interpretation of the model results is that the changes in beech populations reflect spatial shifts in the multifactorial environmental conditions that are most favorable for the growth of beech trees.