A M E R 1 C A J: sylviculture 



The most important representative of a xerophilous character 

 is the Yellow Pine. The hecistothermal zone shows Spruces, Birches, 

 Cottonwoods. 



V. Wind. 



Wind brings moisture and drought, heat and cold; it covers 

 or uncovers vegetation with sand or snow drifts, tumbling, at 

 prior geographical eras, whole mountains into the valleys (Loess 

 formation). Severe wind dwarfs tree growth and forces branches 

 to grow in leeward direction only. The influence of a slight ob- 

 struction, preventing the access of wind at high latitudes, is splen- 

 didly illustrated by the growth of Spruce and Fir on Pisgah Ridge. 

 On high mountains, the possibility of growth is often entirely 

 determined by wind. 



Wind is responsible for the crooked growth of Catalpa in the 

 prairies, the shake in Hemlock on exposed ridges, the fungus dis- 

 eases in many trees. 



The rainfall and, consequently, the existence of forests depends 

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

 Gulf and the Great Lakes. A cross- section through North America 

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

 inter-dependence between the lowest gaps in the mountain chains 

 and the forest on the next mountain chain lying to leeward. For 

 instance: lowest gap in Coast Range at 3,000 feet above sea level; 

 no forest in Cascades below 3,000 feet; lowest gap in Cascades at 

 4,000 feet above sea level; no forest in Blue Mountains below 4,000 

 feet; lowest gap in Blue Mountains at 5,000 feet above sea level; 

 no forest in Rockies below 5,000 feet. 



The east slopes of the Coast Range, Cascades, Blue Mountains 

 and Rockies, below the gaps in these chains of mountains, show 

 little or no forest, and the lowlands to the east of the mountain 

 chains are deserts and prairies. 



Moist sea winds, after passing one chain of mountains allow 

 the forest to grow on the next chain only above the altitude of the 

 gaps in the first chain. 



Picea alba and dwarf pines like Pinus pungens show great 

 strength in resisting wind. In the West Tsuga mertensiana, Pinus 

 albicaulis, further Western Juniper rank first among the trees brav- 

 ing severe storms. 



Wind is essential for the breathing and for the perspiration of 



leaves and bark; for driving pollen on stigma to fertilize the seed; 



for trimming the branches, thus forming clear boles; for distributing 



seed. The investigations conducted by Fliche (French forester) 



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