36 LOCALITY AND FOREST VEGETATION. 



S2)eci<'s which require fjrcater deptli : Scotch pine, Cembran 



pine, elms, Norway maple, sycamore, white poplar. 

 Species irldclt rcindre (jreatest depth : Silver tir, larch, ash, 

 lime, sweet chestnut, and especially oaks. 

 At the same time, the roots of these species go rarely to a 

 :lepth of 4 feet below the surface, unless they do not find 

 sufficient moisture in the upper layers of the soil, a case 

 occurring in countries with a long dry season. 



2. .1 suitable Deyree of Porosity. 



Neither too firm nor too loose soils are favourable for tree 

 growth. Too firm soils make the penetration of the roots 

 difficult, if not altogether impossible, prevent the admission 

 of the necessary air, interfere with the movement of water, 

 and incline towards swampiness, accompanied by increased 

 danger from frost, and by strong shrinking and cracking in 

 summer. Too loose soils endanger the stability of the trees, 

 are liable to be carried away by water or wind, suffer from too 

 rapid drying and too rapid decomposition of the humus, and 

 the plants growing in it are subject to frost lifting. The best 

 soils are of middling consistency, such as loam and calcareous 

 soils with a good layer of humus. 



3. A suitable Degree oj Moisture. 



By a suitable degree of moisture is here understood that 

 degree which corresponds to the natural requirement of any 

 particular species, which it is desired to grow. The more 

 uninterruptedly that degree is maintained throughout the 

 growing season, the more favourable will be the development 

 of the tree. 



Apart from climate and subsoil, the condition of the soil 

 itself, its depth, porosity, the nature and proportion of its 

 component parts, affect the degree of moisture. The forester 

 can d(j much, either to preserve moisUue in the soil by 



