376 INTRODUCTION TO PLANT GEOGRAPHY [CHAP, 



will be protected from at least the strongest insolation and may bear 

 at the same altitude deciduous forest with hygrophilous ground- 

 vegetation. On the other hand in moist northern regions the most 

 luxuriant vegetation may be developed on the southern and western 

 slopes which receive the greatest benefit from the sun. The tendency 

 is of course reversed in the southern hemisphere. However, north- 

 and-south-running mountain ranges may show instead another 

 aspect effect, namely, marked differences in rainfall on their two 

 sides. Thus of New Zealand the eastern side, sheltered from the 

 prevailing westerly winds, has in places no more than one-tenth of 

 the rainfall of the forested western side, and supports only poor 

 tussocky grassland over considerable areas. 



The general tendency towards cooler and damper conditions as 

 we ascend mountains usually leads to marked attendant changes in 

 the vegetation. These may run the gamut from arid plains or 

 lowland forests, dealt with above, all the way to high-alpine regions 

 of perpetual snow. With the tundra and other communities lying 

 above (as in latitudes beyond) the tree-limit, we shall be concerned 

 in the next chapter. Those communities developed between such 

 extremes and the general run of lowlands in temperate regions 

 usually involve faciations or extensions of one or other of the forest 

 types already described. However, in some instances, as most 

 notably the desert uplands of parts of temperate Asia, there may be 

 instead almost plantless wastes or expanses of moving sands — with 

 salt tracts of various extent, and only occasional oases supporting 

 deciduous trees such as Poplars. 



A good example of the usual forested sequence in mountainous 

 districts is seen in western North America, where, above a basal 

 zone of ' improved ' plains vegetation, the ' montane forest ' extends 

 from the arid foothills upwards into the mountains through an 

 altitudinal range of often some 2,000 metres. The main dominants 

 are Ponderosa Pine {Piniis ponderosa), White Fir {Abies concolor), 

 and Douglas Fir [Pseudotsiiga taxifolia), though many others occur, 

 the closest relationship being with the Pacific ' coast forest '. 

 Extending above, through an altitudinal belt of commonly some 

 1,000 metres, comes the ' subalpine forest ', which is related 

 primarily to the boreal forest but also to the coast and montane 

 forests, the main dominants being species of Picea and Abies — 

 particularly P. engelmannii (Engelmann Spruce) and A. lasiocarpa 

 (Subalpine Fir), with often some Lodgepole Pine {Pimis contorta 

 var. latifolia) and allied species. W' hile the multiplicity of dominants 



