New View Points in Silviculture. 207 



southern hemisphere, it includes the months of November, De- 

 cember, January and February. Wherever the average temper- 

 ature during the vegetative period is below 50 degrees Fahren- 

 heit no forest can exist. The line connecting the points which 

 have during the four vegetative months an average temperature 

 of 50° is the limit of forest extension or the "isohyle." 



Another climatic factor which determines the existence of the 

 forest is the humidity of the air. The basis of his repeated 

 studies in the prairies of North America and Eastern Asia during 

 the years 1885-87 led him to consider the following figures as 

 the minimum amount of moisture in the air and in the ground 

 (precipitation) which are necessary for the existence of the 

 forest : 



If less than 1.95 inches of rain falls in one region during the 

 months of May, June, July and August on the northern hemi- 

 sphere, and on the southern hemisphere during the months of 

 November, December, January and February, the natural estab- 

 lishment of a forest is impossible, even though the moisture of 

 the air may be ever so great. Such an un forested coast line 

 upon which the precipitation is not sufficient — even though the 

 moisture in the air is sufficient and, in fact, in some places quite 

 high — extends in North America from the fortieth degree north 

 latitude southward through Central and South America to Pata- 

 gonia. Such a coast line lies also on the west side of Africa 

 south of the equator. It lacks forest because the necessary water 

 is lacking in the soil ; if this water is supplied artificially, these 

 prairies can be transformed into luxuriant grain fields, orchards, 

 and forests. 



Regions which during the four months receive more than 1.95 

 and less than 3.9 inches of rain, also do not support a forest when 

 the humidity in the air during the same time sinks lower than 

 50 per cent. The great prairies between the Rocky Mountains 

 and the immense stretch of forest near the Atlantic Ocean in 

 North America receive less than 3.9 inches of precipitation with 

 an average relative humidity of less than 50 per cent. To this 

 may be added the extensive steppe region of southern Russia 

 and western Asia; the steppes of the interior Mongolia of east- 

 ern Asia ; the steppes of Uraguay, Paraguay, southeastern Africa, 

 and Australia. At any time such tracts of country in which the 



