Introduction

The fact that low summer temperatures restrict the distribution of plants is an everyday experience. Gardeners who try to grow exotic species find that they will not set ripe fruits except in unusually warm summers. After unusually cold summers buds and fruits may fail to ripen and are damaged by early frost. Such early frosts have sometimes destroyed the grain harvest over large areas, resulting in famine. By comparing the latitudinal distribution of cultivated species in Europe a characteristic pattern of distribution limits is found towards the north. Some species are restricted to the southernmost parts of the area with the warmest summers. Typical examples are the traditional cultivation of grapes, going north to the Rhine Valley in Germany, into southern England but not reaching northern England; and the cultivation of maize north to central England and southern Sweden, of wheat north to Scotland and in the valleys of southern Scandinavia north to Trondheim in Central Norway, and potatoes which can be grown all over the British Isles and north to the inner fjords of North Norway.

Summer temperatures limit the distribution of species altitudinally as well as latitudinally. The altitudinal limits of plants in the Alps tend to be higher than in Fennoscandia. This is shown in Fig. 21, where the altitudinal limits of plants in southern Norway are plotted against the same species in the Alps. It is seen that the altitudinal limits in the Alps are, on average, 1000 m higher than in southern Norway.