i6 



F. H. W. GREEN 



would have been omitted, and almost certainly the Alpine and Alpine 

 foreland areas. Why is this? The summer rainfall in the latter areas occurs 

 mainly in heavy showers, with long dry periods in between ; these allow for 

 aeration of the soil. 



Rainfall and PE have been measured in recent years at Cwm Dyli at the 

 foot of Snowdon. Here the annual rainfall is of the order of 150 in., and in 

 all calendar months in i960, for instance, rainfall exceeded PE. Thus, if 

 assessed by calendar months, Cwm Dyh had no PWD in that year. If 

 however, the assessment is made by wet and dry periods, Cwm Dyli is 

 seen to have had about 6 in. PWD in i960. At Cwm Dyh there is good 

 grassland ; it has been possible to drain the soil and prevent it from being 



NORTH 

 WEST 



SOUTH 

 EAST 



SEA-LEVEL 



Fig. 3. Diagram illustrating the suggested altitudinal distribution of certain different 

 'water balance regions' in a section across Europe (not to scale). A is fell (Montane 

 Zone), B is the extreme 'oceanic' type, witli no significant potential water deficit, C is 

 the main humid region of north-west Europe but with significivit periods of potential 

 water deficit each year, D is the Mediterranean type, with dry summers and wet winters, 

 E is steppe. (See text.) 



permanently waterlogged during the growing season. The chmatic situa- 

 tion in summer in the pastures of Switzerland and Norway is comparable. 

 It is different in the truly humid areas of north-western mainland of Scot- 

 land, which in this sense can be described as the most 'maritime' in Europe. 



This brief paper has attempted to deal only with one threshold which can 

 be studied, and defined, in this manner. It needs to be investigated whether 

 light can be shed on other chmatic limits. Figure 3 shows diagramatically 

 the nature of the possible Hmits of the Mediterranean type of vegetation; 

 the relevant numerical values of PWD and PWS, and their seasonal 

 distribution, need to be determined. There is great scope similarly for 

 investigating 'finer' hmits, for natural plant communities, and individual 

 cultivated crops, between these very broad divisions. 



It may be useful to make reference here to some other physiological effects 

 on plants-and animals-and of the lack of a period of PWD. As a first 



