NORTH-WEST MONGOLIA AND CHINESE DZUNGARIA. 391 
B. NORTII-WEST MONGOLIAN PLATEAU. 
General Notes on the Floral Zones. 
The principal floral associations of North-West Mongolia can be divided 
into three groups consisting of the following :—1. Plateau-Desert Association ; 
2. Larch-Forest Association; 2. Alpine Association. Although many species 
in these groups overlap, still, in general, each association can be said to 
occur in definite zones, which vary according to the two principal factors, 
altitude and latitude. Generally speaking, at northern latitudes and at low 
altitudes the same flora is found as in more southern latitudes at higher 
altitudes, but the Larch-Forest Association tends to thin out in southern 
latitudes, leaving the other two converging. Altitude and latitude are the 
factors which cause temperature, rainfall, and snowfall, and the degree and 
intensity of these natural conditions vary in almost exact relationship to the 
altitude and latitude all over the North- West Mongolian Plateau. 
1. The Plateau-Desert Association occurs in all the most low-lying parts 
of the valley-bottoms, plains, and evaporating basins and all districts where 
the distance from the mountains is great and consequently the rainfall small 
or non-existent. Round some of the desert lakes a very thin zone of marsh 
flora is to be found, growing where saline moisture has reached the surface. 
With the exception of this and a few straggling patches of Larch forest, the 
whole of North-West Mongolia between the altitudes of 2000 and 7500 feet 
is covered with the Plateau-desert association. At the last-named altitude 
the colder climatic conditions and higher rainfall generally enable either 
the Larch-forest association or the Alpine association to ebtain the upper 
hand. The Plateau-desert association, however, is broken in a few places 
between 3000 and 7500 feet, for where special sheltered valleys shade the 
vegetation from the fierce summer sun, there isolated and straggling patches 
of the Larch-forest association can be seen. Plateau-desert association is 
perhaps best studied in North-West Mongolia at an altitude of about 
6000 feet. It can be said to be sub-divided into two subordinate associations. 
The first of these is found more on the shady side of the valleys and in 
parts of the plains less exposed to cold or drying winds, and is characterised 
by species of the genus Caragana. The second subdivision is found in the 
driest, hottest, and most exposed plateau desert, and is the most widely 
distributed. The most characteristic species of the latter are :— P'otentilla 
sericea, Arenaria capillaris, Chenopodium frutescens, var., Stipa capillata, 
Artemisia argyræfolia, Oxytropis tragacanthoides. 
9. The Larch-Forest Association is found scattered wherever conditions 
are suitable between certain altitudes. Thus in the Kemchik valley on the 
Upper Yenisei plateau, Larch forest is found on scattered areas between 
3000 and 6000 feet. Some 80 miles to the south, in the Saklya valley on 
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