230 



during the dry, liot days of May and June, before tlie summer rains begin. 

 Daring the wet season it will almost take care of itself, but when the 

 rains cease in August the principal danger of the whole year comes, and 

 as day follows day of hot, dry. scorching weather, the tree must be lib- 

 erally' supplied with moisture to take the place of that absorbed by the 

 thirsty air. 



This region does not iweive the benefit it might from the rain-fall, 

 on account of the deep, gravelly, porous soil, through which the rain 

 sinks quickly, leaving but a small portion behind, a large part of which 

 is soon taken up by the air, and but little remains for the tree. 



ETAPOKATIOX. 



The evaporative power, or thirstiness, of the air is increased by high 

 temperature, low humidity, low pressure, and high rate of motion. It 

 is most powerful in summer in the sun and wind, and only slight in 

 winter in shade and when the air is calm. In any place like Colorado, 

 where are found many upturned faces of broken rocks and large uncov- 

 ered areas of sand and gravel, evaporation and radiation will be much 

 greater than in places where the surface is protected from sun and Wind 

 by vegetation. This is shown by the entire absence of dew on the 

 plains and the rocky slopes of the mountains, and its formation when • 

 trees and lawns are planted. 



Trees are injured by rapid evaporation, because it produces an unnat- 

 ural activity in the roots and stem to supply the drain from the cells of 

 the leaves and bark, and unless the tree has a large root-system and 

 great vigor in all its functions, it will be unable to keep up the proper 

 circulation. This is a reason for the fact already cited, that the trees 

 which succeed best here are those of rapid, vigorous root growth, with 

 a solid leaf structure. Evaporation is more rapid in winter, and 

 therefore fall planting is objectionable. Imported trees always thrive 

 best if they are cared for in a nursery for a year or two, until their 

 roots, leaves, and bark become accustomed to the new conditions of 

 climate. 



WINDS. 



The winds of this region, which, from the level character of most of 

 its surface, are very constant and often of great velocity, have a strong 

 influence upon the production of increased evaporation. In the day- 

 time throughout the year the prevailing winds are from the south and 

 southeast, while at night they are from the north. This is an advantage 

 to trees, for the south and east winds, always more or less laden with 

 moisture, come at a time when evaporation is most active, and tend to 

 relieve its evil effects. The north wind, direct from the region of per- 

 petual ice and snow, is the \^ ind of blizzards and storms, and is the 

 most injurious of all, froni the sudden changes of temperature, already 

 described, which it produces. The warm west or northwest winds, 



