SYi.\ ICULTUEE. 



light demandeis. The high atmospheric moisture of high altitudes 

 causes accumulation of large quantities of leaves. In the tropics 

 there is little litter on the ground. 



C. Root system. 



It is the decaying root which allows the precipitations to trickle 

 down to the lower strata of soil. Hence tap rooters seem more 

 efficient than flat rooters in converting a rapid surface drainage into 

 a slow underground drainage. Decomposing the rock by chemical 

 action, the tap root forces it to yield its soluble salts. 



D. Soil improvement through root-bacteria and fungi. 



The upper layers of forest soil are densely peopled with the 

 liyphae of basidiomycetes, living on humus. Leguminovis trees 

 (Locust, Kentucky Coffee tree, etc.) by their root-bulbs increase the 

 fertility of the soil, and the Alders seem to act in a similar way. 

 On abandoned fields in Pisgah forest the soil is improved by Su- 

 mac, Sassafras, Locust, etc. These species act as ushers for more 

 exacting forest growth, improving the physical conditions of the 

 soil. 



Very little is known about the nature of the improvements. 



Paragraph IV. The North American Sylva. 



TJie northern limit of the forest coincides with the isothermal 

 line, 30 degrees Faht., which lies on the west side of the continent 

 at 70 degrees latitude in Alaska (under influence of the .Japan cur- 

 rent), and on the east side at 5.5 degrees latitude in Labrador and 

 at tlie Hudson Bay. 



The rainfall and, consequently, the existence of forest depends 

 on the moist sea winds supplied by the Pacific, the Atlantic, the Gulf 

 and the Great Lakes. A cross-section through Xorth America at 

 the latitude of Lake Michigan and Portland, Oregon, shows the 

 inter-dependence between the lowest gaps in the moimtain chains and 

 the forest on the next mountain chain lying to leeward. For in- 

 stance: lowest gap in Coast Range at 3,000 ft. above sea level; no 

 forest in Cascades below 3,000 ft.; lowest gap in Cascades at 4,000 

 ft. above sea level; no forest in P.lue Mountains below 4,000 ft.; 

 lowest gap in Blue Mountains at 5,000 ft. above sea level; no forest 

 in Rockies below 5,000 ft. 



Tlie east slope of the Coast Range, Cascades, Blue Mountains and 

 Rockies shoMs little or no forest, and tl;p lowlands to the east nf 

 the mountain chains are deserts and prairies. 



17 



