11KITISII I ukF.ST TREES 2<; 



in beech, hornbeam, aspen, and birch ; others, like the 

 alder, throw out strong side-roots, whence proceed deep- 

 reaching rootlets ; and others again develop strong side- 

 roots confining their whole root-system practically to the 

 upper layers of the soil, as is especially the case with 

 spruce. 



The following table expresses, only however in general terms, the 

 depth of soil necessary for the various species of forest trees to 

 thrive : 



Up to \ ft. Spruce, aspen, mountain ash, birch, mountain pine. 



1 to 2. ft. Beech, hornbeam, alder, Austrian and Weymouth pines. 



2 t 3 ft- Scots pine, silver fir, Douglas fir, elm, maple and 



sycamore, ash, lime, chestnut. 



3 to 4 //. Oak, larch. 



Comparative data are not yet available to show the 

 absolute quantities of soil-moisture necessary for the various 

 species of forest trees to enable them to maintain throughout 

 the annual period of vegetation the requisite evaporation 

 and transpiration through the foliage. Experience however 

 shows that the demands for soil-moisture practically vary as 

 follows : 



Greatest alder, ash, willow, maple, elm ; considerable oak (pedun- 

 culate), hornbeam, birch, aspen, larch, Weymouth pine, spruce ; 

 moderate beech, oak (sessile), lime, silver fir ; least Scots, Austrian, 

 maritime and Cembran pines. 



On the whole the broad-leaved, deciduous trees demand 

 more soil-moisture than the conifers. The absolute demands 

 of the various species of forest trees on the nutrient strength 

 of the soil has also not yet been determined, the difficulty 

 being that the mineral constitution is not the only factor, but 

 that all the physical qualities exert important influence under 

 every circumstance and in the most complicated concrete 

 conditions. 



