NORTHERN EUROPE 267 



is a sufficiency throughout the year and the atmosphere 

 remains fairly moist, chiefly, perhaps, by reason of the 

 low temperatures generally prevailing. The biting, dry 

 arctic winds become here more moderate, and there is an 

 abundance of snow on the ground in winter ; but the 

 uncertainties of the early and late growing period limit 

 still more the actual time available for the develop- 

 ment and work of the broad leaves, whilst the night 

 frosts, which occur at the beginning and towards the 

 close of the period of growth, endanger their very exist- 

 ence. Ultimately there is hardly time enough for the 

 maturing of the fruits of the temperate vegetation as 

 we know it in central and western Europe. As climatic 

 conditions become more severe, the number of tall trees 

 able to withstand them is necessarily much reduced. 

 Hence the uniformity of northern forests, where, contrary 

 to what occurs in warmer latitudes, a single species, or 

 two or three species, may prevail exclusively over exten- 

 sive tracts of country. 



The chief timber species are, in the western districts of 

 Scandinavia, the Norway spruce, the European larch, 

 and the Scots pine, to which in the eastern part of 

 northern Russia, east of the Ladoga and Onega lakes, 

 the Siberian spruce and fir, the Siberian larch and the 

 Siberian stone pine are added. Thus as we go eastward 

 the variety of coniferous forms increases. The spruce 

 generally thrives best in moist situations and on heavy 

 soils, while the pine and the larch are not so particular 

 in their requirements, though they do n6t grow so well 

 in damp surroundings. Among the very few hardy 

 summer-green trees that accompany the conifers in these 

 northern regions are the white birch, the aspen, and the 

 rowan. Indeed, a stunted form of birch constitutes low 

 woodlands in a broken belt north of the limit of conifers : 



