6 



The prevailing winds are from the south and southeast during 

 the spring and summer and from the northwest in fall and winter. 

 Their average velocity is high. The southerly winds of summer fre- 

 quently come as hot waves, which, by causing excessive transpiration, 

 injure tree growth. Moreover, the strong northwest winds of winter 

 are more injurious to trees than is commonly supposed. Transpira- 

 tion goes on at all seasons, and since the northerly winds blow when 

 the ground is frozen, the trees are unable to secure the soil water and 

 suffer or even die from drought as certainly as in summer. 



Occasional prolonged hot or cold periods occur, which are a very 

 severe test to trees, and species which have been introduced from 

 other localities sometimes die. In the unusually cold winter of 1898-99, 

 for example, a large percentage of the Osage orange hedges were killed, 

 not only in Nebraska, but thruout the central Mississippi Valley. 



PRECIPITATION. 



The following table shows for eastern Nebraska the total annual 

 precipitation, the total for the growing season, and the ratio of the 

 seasonal to the annual rainfall: 



Altho the average annual precipitation for Nebraska for the period 

 from 1876 to 1904 was but 23.57 inches, the above table shows that 

 more than two-thirds of the total falls during the growing season, thus 

 giving the State a very favorable showing against States farther east, 

 where the total annual rainfall is considerably greater but more 

 equally distributed thru the different seasons. This fact explains, in 

 large measure, the success which has attended tree planting. 



On the other hand, the wide deviation in the amount of precipitation 

 from year to year is a factor which tends to restrict the number of 

 species that can safely be recommended. For example, the precipita- 

 tion in 1881 was 30.91 inches. In 1893 the precipitation was only 

 16.18 inches, and the years 1894 and 1895 also experienced a shortage 

 in rainfall, making three consecutive dry years. This period was very 

 severe on planted timber, and some species proved unadapted to cer- 

 tain localities or situations where they hitherto had been successful. 



[Cir. 45] 



