Dry And Wet Hills

Mountains upon the older granitic, siliceous, or schistose rocks that reach a considerable altitude are frequently the habitats of true moorland or bog plants. There are, in fact, upland moors and lowland moors. In each case it is a sine qua non that peat of a considerable depth be formed. When the peat is waterlogged there is also a Sphagnum or bog-moss association, and a moss is formed. But since these conditions are really dependent largely upon soil characters, they have been treated separately under bogs. A peaty dry surface gives rise to a moorland with erica-ceous plants, such as Ling, Heath, and Mat-weed, whilst a Cotton-grass moor is largely intermediate in regard to the supply of moisture. The heaths that are developed on thin stony or gravelly soil at lower elevations correspond to the lowland moors, as do the upland moors to the upland heaths.

Excluding these special types there are wet mountains or hills and dry ones, and the plants that are found upon the one do not as a rule occur on the other.

Dry hills furnish such plants as Dyer's Weed, Rock Rose, Hairy Violet, the chalk or limestone on which they grow being naturally well drained. Marsh Mallow also grows on sandy rocky hills, and Hare's Foot Trefoil, Kidney Vetch, Silky Mountain Vetch, Sainfoin, Oak-leaved Mountain Avens, Salad Burnet, Ploughman's Spikenard, Cotton Thistle, Wild Thyme, Clary, Sheep's Sorrel, Box, Musk Orchis, Sheep's Fescue are found also on dry hills. Yellow Balsam, Gentian, Felwort, and Fragrant Orchis are found on wet hillsides, and Field Scabious, Dropwort, on damp clay, often, though not invariably, at high altitudes.

Climate And Hills

Hills have an important bearing upon climate. As one ascends 300 ft. there is a difference of 1° F., so that the thermometer in the lowlands at 560 F. would read at the summit of a hill of 4500 ft. (the limit in the British Isles) at 410 F., a difference of 15 degrees. This has naturally a great effect upon plants in performing their various life functions. Germination to begin with is slow, and may never take place in many ill-developed seeds. Growth is maintained slowly by impoverished powers of assimilation, respiration, and transpiration, for nutrition is scanty, and therefore reproduction must be hazardous, so that it is inevitable that montane plants differ in their seasons of flowering from those within the plains. Fresh types of plants succeed each other at different altitudes owing to these variations in temperature, etc. Thus the trees disappear at 1000-1500 ft., sub-Alpine plants flourish up to 2000 ft, and above this range the Arctic and Alpine species.

Rainfall And Hills

Hills exercise a decided influence in causing the moisture in the atmosphere to be condensed and precipitated as rain. The position of the range of hills in relation to the wind is important, rain being precipitated to the lee of the hills. When the axis of the range is oblique, the effect upon the air current is similar to that of a less steep slope, and more so than in the case of a range at right angles, and the air ascends more slowly, rain being precipitated over a wider area. The snow-line on a hill necessarily also affects the atmospheric conditions and rainfall in such regions.