Climatic control of the distribution and abundance of beech (Fagus L.) in Europe and North America

Percentages of beech pollen in networks of surface samples from Europe and E North America reflect the present distribution and landscape abundance patterns of beech trees in both continents. Response surfaces constructed by a locally-weighted averaging technique characterize the relationship between the pollen percentages and mean January and July temperatures in each continent. Both surfaces are unimodal with optima at -1 to -4°C (January) and 18°C (July) and adequately predict the patterns of beech pollen abundance in their respective continents. The North American response surface can predict the pattern of beech abundances in Europe and vice versa. This similarity implies that the continental-scale patterns of beech abundance are in equilibrium with modern climatic patterns on both continents irrespective of differences in the migrational history, competitive interactions and degree of human impact. The physiological characteristics determining the species' distributions are presumably evolutionarily conservative, having persisted through repeated orbitally-forced cycles of migration since the original (mid-Tertiary) separation of European and North American temperate forests. -Authors