30 SEEDING AND PLANTING 



and by rendering the air in contact with the foliage drier. 

 Winter killing of foliage often results from the desiccating action 

 of high winds during periods when the loss of water cannot be 

 made up by a supply from the soil. 



The nature of the deviation from normal growth caused by 

 high winds and the degree of injury depend largely upon the 

 species; therefore, special attention should be given to the wind- 

 resisting qualities of species planted in prairie regions, along sea 

 coasts, on high elevations, and in other exposed places. 



On the whole, coniferous species are more resistant to injury by 

 high winds than broadleaved species during the season when the 

 latter are in full foliage because of the smaller surface exposed to 

 the action of the wind. In winter, however, the opposite is 

 true because the exposed surface of all deciduous trees is greatly 

 lessened by the casting of the foliage. As the most violent winds 

 usually occur between the late autumn and the following spring, 

 conifers on an average for the entire year suffer more than broad- 

 leaved species. 



16. Minor Atmospheric Factors in Their Relation to the 

 Choice of Species. In the vicinity of volcanoes and hot sul- 

 phur springs, all vegetation' suffers to a greater or less extent from 

 the poisonous gases which escape from them. Sulphurous acid 

 gas is the chief factor in causing this injury. Because of the very 

 limited areas over the earth's surface where this manner of in- 

 jury naturally occurs, it would scarcely warrant its discussion as 

 an atmospheric factor were it not that the same kind of injury 

 occurs where large quantities of coal are burned. Mason 1 states 

 that smelter fumes cause serious damage on the Deerlodge National 

 Forest. It has been estimated by chemists that at least 2500 tons 

 of sulphur dioxide are annually thrown into the atmosphere from 

 the smelters near this forest. Experiments have shown that as little 

 as one part of sulphur dioxide in a million parts of air will kill pine 

 seedlings after prolonged exposure, while 80 parts to a million 

 parts of air are often present ten miles or more from the smelter. 

 The sulphurous acid is absorbed in the gaseous form through the 

 foliage and is oxidized into sulphuric acid and acts as a poison. 

 As different species exhibit varying degrees of resistance to this 

 gas, it should be taken into account when plantations are made 



1 Mason, D. T. : The life history of lodgepole pine in the Rocky Mountains. 

 (U. S. Forest Service, Bui. 154, p. 22. 1915.) 



