682 JOURNAL OF FORESTRY 



but also by the poor success of planting. The years 191 5 and 19 17 are 

 examples of the second type of spring. The winter precipitation was 

 only 9.4 inches in 1914-15 and 6.1 inches in 1916-17, but in both years 

 the precipitation between April i and May 15 was around 6 inches. 

 As a result, the growth of saplings was above normal and plantings 

 were very successful. 



It will be noted that the growth curve, while fluctuating from year 

 to year, has a gradual upward trend from 1909 to 19 17. This is due 

 partly to the high precipitation in the springs of 1915 and 1917, but 

 mainly to the fact that the growth rate of yellow-pine saplings nor- 

 mally increases with age. For this reason it would not be fair to com- 

 pare the gro^yth in 1909 with that of 191 7 without making allowance 

 for this increased growth rate. In recognition of this, among other 

 factors, the saplings measured were selected as nearly as possible from 

 a single age class of about 25 years. 



The slight divergence between the growth curve and the spring pre- 

 cipitation curve in 1909-11 is accounted for by several circumstances. 

 The decline in spring precipitation may be disregarded, since the 

 amount in any year, being less than i inch, is too small to have any 

 appreciable effect. Any retarding influence exercised by the slight 

 decrease in precipitation is more than offset by the normal tendency of 

 the saplings to increase their growth rate with age. This tendency is 

 further augmented by another condition. As explained under "Biotic 

 Factors," the saplings were seriously injured by grazing prior to 1909. 

 In the fall of 1908 almost every sapling was broken, bruised, or par- 

 tially defoliated. Observations at this time indicated that this condi- 

 tion had prevailed for several years, and that the plants were in a sub- 

 normal state of vitality due to a reduction of leaf surface. After 

 grazing animals were excluded the foliage began to increase, and the 

 response is reflected in increased height growth. 



The precipitation for June has been omitted because normally it is 

 a negligible quantity. An exception to the rule occurs in 1914, when 

 the amount recorded for June is 1.22 inches. Undoubtedly the June 

 precipitation played an important role in the increase of growth during 

 this year. 



A further correlation of height growth with moisture is found in the ' 

 records of evaporation, relative humidity, wind movement, length of 

 rainless period, and number of prevailingly cloudy days in April, May, 

 and June. All of the above factors are a more or less accurate index 

 of atmospheric moisture conditions. Evaporation may be regarded as 



