NO. l8 TREE GROWTH AND RAINFALL CLOCK 



^3 



Chacon rainfall. Good correlations in general result under three 

 circumstances : In the lack of summer rainfall in the intervals com- 

 pared, in the proportion of overlap between the two intervals, and in 

 the length of the intervals. For instance, November-May versus 

 January-May has a trend coefficient of 0.99 and a trend ratio of 0.06 

 whereas, contrariwise, May-August versus March-April has values 

 of — 0-33 and 0.66. The table as a whole shows great variation and 

 indicates the necessity of comparing tree growth with different rain- 

 fall intervals. It goes farther than this. If tree growth is found to 

 correlate with one particular month-interval, then a great quantity of 

 xylem (as a thick growth layer) formed during a certain season 

 suggests copious rainfall for that month-interval ; it does not neces- 

 sarily suggest that the entire year is a wet one. 



Rainfall characteristics. — For a study of the influence of a single 

 factor, such as rainfall, on tree growth it is necessary to have long 

 records at the immediate site of the trees. Short records taken a 

 matter of several miles distant can be highly indicative but not neces- 

 sarily conclusive. In the present case the record at Chacon, 7 miles 

 away, begins with 1909. Therefore, the longer records of Santa Fe 

 and Albuquerque were used, in spite of greater distances, in order to 

 determine possible differences in rainfall characteristics between the 

 periods 1850-1897 and 1898-1941. 



Table 11, in the first place, gives the ]\Iarch-July and January- 

 August rainfall for 1909-1941 at the several stations. As will be 

 shown later, the rainfall of March- July is a significant factor in tree 

 growth. In the second place, table 11 gives the rainfall of Santa Fe 

 and Albuquerque for the periods 1850- 1897 and 1 898-1 941 set out 

 for various month-intervals. Two points must be considered. First, 

 there is the striking fact that the average rainfall of March-July for 

 both Santa Fe and Albuquerque was less during the period 1850- 1897 

 than during that of 1898-1941. The same is true for the average 

 rainfall of Albuquerque for January-August. Second, there is the 

 fact that the average rainfall of January-August at Santa Fe was 

 greater during 1850- 1897 than for the following 44 years. The reason 

 for this inconsistency with the intervals mentioned in the first point 

 above was suspected as soon as it was determined that the average 

 rainfall of January-May, in contrast to the rainfall of January- August, 

 was less during the earlier period, 1850-1897. Therefore, the average 

 rainfall was computed for March- April, May- June, April, May, June, 

 July, August, and September. Only July and August showed greater 



