280 ON THE SOILS AND SUBSOILS 



ir. Aspect or inclination of the hillside towards one of the 

 points of the compass. 



III. Angle of inclination of the hillside towards the horizontal. 



1. Height above the Level of the Sea. — Trees arrive at the limit 

 of their growth much sooner in a vertical than in a horizontal 

 direction towards the Poles (according to Hundeshagen in the 

 ratio of 1 to 450 for every 1° Fahrenheit). Trees groAving on 

 suitable soils at the sea-level produce the greatest quantity of 

 timber and the most seed, but the quality is not so good, owing 

 to the rapidity of growth. Among important forest trees, the 

 following may be accepted as average heights in Germany. 



1200 feet to 1300 feet—Hornbeam (Carjnnus hetulus) and 

 Scotch fir, which latter, in unmixed forests, is met with at 

 a much greater elevation, but being comparatively inelastic, it 

 frequently suffers from snow-break. 



1600 feet to 1700 feet — ?>irch, elm, and, in favourable circum- 

 stances, the oak. 



2200 feet to 2400 feet — Beech, maple, ash, birch {Betula 

 picbesce^is). 



8000 feet, and often higher — Silver fir (Abies 2yectinata). 



Spruce fir {Picea exeelsa) and larch {Larix europmo.) — The 

 heights to which these trees ascend the mountain slopes are 

 modified by the following circumstances. 



2. TncUnation of the hillside towards one of the jwints of the 

 comjKiss. — The greatest quantity of timber is produced on gently 

 sloping eastern, north-eastern, and northern hillsides, which are 

 most cool and moist. The more southern aspects, however, 

 produce the most durable timber; Ijut young plantations or 

 natural undergrowth are exposed to the burning rays of the 

 summer sun, and partly to dangerous late frosts. The N.E., E., 

 and S.E. aspects are most exposed to late frosts. The W. aspect, 

 when exposed, produces the least quantity of timber, for it is 

 from the W. that violent winter storms in general come. 



3. The A7igle of Inclination of the hillsides totvards the horizontal 

 also exercises a most powerful influence on the growth of trees, for 

 the "reater the anole of inclination, the more marked are the differ- 

 ences in the quantity of the timber produced. An inclined plane 

 Q\\ovi\(\., cceteris paribiis, yield more timber than a horizontal plane. 



{a) 15ecause it is greater in the ratio of the secant to the radius, 

 and the trees do not stand quite perpendicular to the horizontal ; 

 and, 



(/>) Because each tree receives more light in consequence of 

 the gradual slope. 



In reality, however, this is not the case, for on the hillsides 

 the soil is usually Avanting in depth, and also in moisture. After 

 heavy rains some of the soil is always carried Clown into the 

 fertile valleys, thus enriched from the surrounding hills. Accord- 



