392 ME. M. p. PBIOE ON THE VEGETATION OF 



the west- of Lake Ubsa, the Larch forest occurs between 6000 and 7000 feet. 

 Some 40 miles to the south of this again, on the Kundelun mountains, the 

 Larch forest occurs between 6300 and 7600 feet, while on the right bank 

 of the Upper Kobdo River and round the Kobdo Lakes, the Larch-forest 

 zone occurs between 7000 and 8000 feet. Thus the zone gradually rises 

 in altitude as the southern latitudes are reached. But in the case of the 

 Larch-forest association, latitude and altitude are not the only factors which 

 control its distribution, for it appears that shelter from insolation, and 

 perhaps wind, is a third factor to be taken into consideration. This is 

 suggested from the fact that the Larch- forest association all over the Xortli- 

 "West Mongolian plateau is found only on tliat side of the valley which has a 

 northern aspect, while it reaches its best development in the shady glens of 

 the mountains. Moreover, the complete absence of any Larch forest in the 

 country between Achit Nor and the Kobdo River seems to suggest that a 

 large area of open down country, even though it may lie at a favourable 

 altitude and latitude, if too exposed to wind and snow, is unfavourable for the 

 development of Larch-forest association. In this area, therefore, which is 

 drained by the left tributaries of the Kobdo River, namely, the Suok, the 

 Taharty, the Olonor, and the Tsagangol Rivers, the Plateau-desert association 

 merges into the Alpine association without an}' intermediate Larch-forest 

 association. The following are its most characteristic species : — Larix 

 sihirica, Vacciinnm Vkis-ld'i a^ Acoiiitum sp., Potentilla Infurca. Plmis 

 sihiriixi also occurs in this association in the extreme north, and is found as a 

 straggling specimen as far south as the western Tannu-ola mountains, where 

 it reaches the summit. It appears that the excess of sun and light is 

 responsible for its absence in latitudes further south of this. 



3. The Alpine Association is distributed in North- West Mongolia in the 

 following areas : — lu the Kemchik valley of the Upper Yenisei plateau it is 

 found on several isolated mountain masses between 6000 and 6500 feet. On 

 the Kundelun mountains it begins at an altitude of 7000 feet, and further south 

 still, on the right bank of the Kobdo River, it commences at 8000 feet. 

 It reaches the snow-line at about 11,000 feet in the last two places, wherever 

 there is suiScient soil. The best development of the Alpine association is 

 found on the left tributaries of the Upper Kobdo River, namely, on the Suok, 

 the Taharty, the Olonor, and Tsagangol Rivers. Here a large area of rollino- 

 down country between 8000 and 10,0'J0 feet alt. is the geographical feature 

 and is peculiarly favourable for the development of this flora. It is interesting 

 to know that it is just here that Oois amnion, the great mountain sheep of the 

 Altai, is found in considerable numbers, and its distribution seems to coincide 

 with the widespread distribution of the Alpine association. The charac- 

 teristic species of the Alpine meadows of North-West Mongolia are : — Gentiana 

 ahaim, Gentiana prostrata, Saxifraga Hirculus, Primula nivalis, Trollius 

 /tsiaticiif, Papaver nvdicaule, Betula nana. 



