It is generally agreed that the natural vegetation, without interference by man, comes in the long run to reflect the climate, or at least to reach a biotic climax, but Britain has been interfered with since the earliest historical times. In all probability the natural vegetation, recovering from an Ice Age that came to a gradual end in Britain only about 15,000 years ago, was forest, deciduous for the most part. Some small areas, for example, the Chalk Downs, were perhaps edaphic grasslands, the ill-drained swampy regions were fen and marsh, and the highlands, above a tree-line at about 1,000 ft, were moorlands deteriorating to arctic Alpine formations on the very tops. The snow-line, it is said, is only just above the summit of Ben Nevis and the Cairngorms. Patches of snow sometimes linger here (on northern slopes) throughout the year, and in most years ski-ing is to be had in the Scottish hills, even into April.
Little is left, however, of the natural vegetation, the forests have been cleared for agriculture and pasture is now to be found, either permanent or in short leys, enjoying a climate that is luxuriantly wet for natural grass (grassland is a semi-arid formation in the natural state). Cereal crops, too, are cultivated grasses and, under the cultivation conditions of today, produce yields that would startle their ancestors. Nowhere is it too dry for them and in few places too wet, but oats grow better in the wet western and barley does best in the drier eastern counties, though harvest weather can be critical. But temperature limits crops more severely, and the effective northern limit of successful wheat cultivation lies along the line of the northern boundary fault of the Midland Valley from Dumbarton to Stonehaven. As would be expected each crop has need of a certain quantity of heat during its growth and although temperature is not a unit of heat a helpful guide to the crop-growing potentiality of the land is given by the map of day degrees or accumulated temperatures above a threshold figure (42° F.) at which most cool-temperate crops begin actively to grow. There are many ways of calculating this, but the simplest is to sum the temperature (above 42° F.) for each day of the year. Clearly in the far north the early sunrise of summer takes the temperature above the threshold figure earlier in the morning and holds it there later in the evening. Thus there would seem to be a good reason for counting the 'hour-degrees'. But in fact this is allowed for in using the mean temperature of the day. Many crops and especially the cereal crops, require a minimum number of daydegrees between sprouting and ripening, but as growth clearly depends partly on the amount of solar energy received by the plant the approach can only give an approximation to the truth. The matter is complicated, but in English practice it is usual to regard the average duration of the period between the passing of the threshold (42° F.) in spring and the repassing of this value in autumn, not without good reason, as defining the growing season.