Late-Quaternary Migrational Strategies of Tree Species

Abstract

The widely held notion that modern distributions of taxa can be used to interpret their past history underestimates the complexity of shifts in range margins, centers of dominance, and changes in community composition through the lateQuaternary interval. Modern patterns of distribution of species along environmental gradients are often assumed to be in equilibrium with prevailing climatic conditions and disturbance regimes (e.g., Whittaker 1975). In a conceptual model representing a three-dimensional map view of the pattern of relative dominance of a taxon in equilibrium (Fig. 6.1), the population center coincides with the geographic center of distribution. The population center is located in the optimal portion of the environmental gradient, and the dominance of the taxon decreases symmetrically away from its optimum in all directions. This assumption is incorporated within forest-stand simulation models such as FORET and JABOWA (Shugart 1984). In cross-section view, percent dominance of the taxon along an environmental gradient appears as a symmetrical bell-shaped or Gaussian curve.