Introduction

It is a widely accepted principle of plant geography that the distribution of vegetation and of plant species is primarily controlled by climate, but that climate changes with time (Cain, 1944). The basis of this principle is the broad correlation of vegetation with latitude, essentially a correlation with temperature, and the modification of this relationship by the availability of water.

These correlations are well illustrated by the boundaries of the climax woodlands of northern Europe, which run latitudinally across northern Russia and Finland but then have a southwestern trend across Scandinavia and Britain (Sjörs, 1965). These boundaries are correlated with various measures of summer warmth. The northern limits of many of the constituent species are similarly correlated with summer warmth but also with the vegetational zones themselves.

The possibility that temperature controls the boundaries through its effect on reproduction rather than vegetative growth is suggested from the common observation that heavy crops of fertile fruit of some of the dominant species of tree follow, or coincide with, years of exceptional warmth (Matthews, 1955). There is, however, very little information about the production of fertile fruit at the northern limits of species, nor are there many studies of the population dynamics of species at their limits, so that the relation between reproductive capacity and regeneration is unknown.

Studies of species in northwestern England

Recent studies on the distribution of vegetation in Britain for the National Vegetation Classification (Rodwell, 1991) provide many examples of plant communities that are restricted either to the warmer and drier southeast or to the cooler and wetter northwest of Britain.