VEGETATIONAL TYPES OF TEMPERATE LANDS 337 



by thousands of miles, or, within reason, wherever in the world 

 they may be. For whereas a single factor of the environment may 

 result in a characteristic vegetational feature, it usually takes the 

 entire habitat complex to stamp the community fully. Consequently, 

 to the extent that distant habitats may be comparable in supporting 

 similar vegetation-types, we may deal with them on a world basis. 

 In this the first division is climatic, followed by local diflFerentiation 

 caused by edaphic or other diflferences : and as the vegetational 

 types of temperate and adjacent lands are the most familiar to the 

 greatest number of us, we may conveniently start with them. 



Deciduous Summer Forests 



Summer-green forests, dominated by broad-leafed trees which 

 lose their leaves for the unfavourable period of winter, constitute 

 the main climax formation over much of temperate Europe, eastern 

 Asia, and North America, reappearing in some comparable regions 

 of the southern hemisphere. From the physiological point of view 

 the cold winter tends to be a dry period, owing to the fact that low 

 temperatures often hinder absorption of water by the roots : this 

 is counterbalanced by the leafless condition during winter, for it 

 is chiefly from the leaves that loss of water takes place, and it is 

 mainly such loss which has to be made good by absorption from 

 the soil. If active transpiration continued when the resultant 

 water-deficiency could not be made good by absorption, owing for 

 example to warm weather when the soil remained frozen, serious 

 injury and even death might result. As it is, these deciduous broad- 

 leafed forests occupy many of the most populous regions of the 

 world, and although in such areas we ip'ay now see around us only 

 patches of anything approaching a climax, this situation is largely 

 due to human disturbance — to clearance for agricultural or other 

 purposes, or to the depredations of Man's domestic animals. For 

 the conditions that have favoured the growth of such forests have 

 also favoured the development of the most prosperous agricuhure 

 and grazing, including widespread cultivation of cereals, and con- 

 sequentlv of some of the highest stages of human civilization. 

 Owing to the deciduous habit of the main dominants and the char- 

 acteristic dying down of many of the associated plants, these forests 

 look entirely different in winter (Fig. 94) and summer (Fig. 95). 



Deciduous summer forests have their main development (i) m 

 eastern North America in the temperate beh northwards to the 



