339 
Limits of Plants in North-West Yunnan. 
the Mekong, and Pang-tsi-la by the Yangtze, which, swept by the 
wind from the high plateau, was drenched with rain day after day 
when bright sunshine (depending on the wind from the Mekong 
below) prevailed at A-tun-tsi, distant a few miles as the crow flies. 
The most pronounced result of this is that the same flowers 
open one or two months earlier in the former valley than in the 
immediate vicinity of A-tun-tsi—an important point. 
All south-facing valleys are directly exposed to the raking wind 
already mentioned, and under such conditions the tree-limit may be 
lowered by as much as a thousand or fifteen hundred feet. This is 
probably a direct result of desiccation, since from such valleys even 
the deciduous-leaved Larix is entirely absent; Abies and jfuniperus, 
both pronounced xerophytes, being the limiting species, and these 
scarcely occurring above 14,000 feet. 
Not only is the upper limit of trees thus curtailed, but the lower 
limit of the forest-belt is considerably raised. Thus in the A-tun-tsi 
valley, which is peculiarly exposed to the savage winds of the Mekong 
gorges, there is no forest at all below 11,000 feet, and deciduous¬ 
leaved trees are the exception, whereas in favoured valleys the 
forest-belt may begin below 10,000 feet, in which case there is usually 
a well-marked transition from deciduous-leaved trees in the valley- 
bottom (Populus , Betula, fuglans, Acer), to Quercus and Pinus on 
the rocks and finally Abies (often with Larix) at the valley-head. 
In the former case, as at A-tun-tsi, the place of the lower forest- 
belt is taken by another shrubby belt, composed chiefly of scrub-oak, 
but it may include, on limestone rocks, scattered pines, as above 
Pang-tsi-la. 
On the road from A-tun-tsi to Batang the Tsa-lei-la is crossed 
at an altitude of 15,800 feet, this pass facing due north and south. 
On the A-tun-tsi or Mekong side, facing south, alpine grassland 
prevails for the last 1,500 feet, broken here and there in the shelter 
of the rocks, to within a few hundred feet of the summit, by patches 
of dwarf Juniper only ; on the Batang (Yangtze) side facing north, 
forests of Abies and Larix extend to within six or seven hundred 
feet of the pass, finally giving place to a narrow belt of dwarf 
Rhododendron, which in favoured situations extends practically to 
the summit, though on the southern slope there is no Rhodo- 
dendron within a thousand feet of the summit. 
Yet the herbaceous alpine flora is but little affected—in the 
valleys alpine grassland is almost indifferent to the wide range of 
conditions—the shrub-flora is more affected, and the forest-belt 
most of all. 
