28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. Ill 



It is clear that the relations of tree growth to rainfall are highly 

 complex not by themselves alone but also by the interplay of the 

 entire range of growth factors, a circumstance emphasized by plant 

 physiologists and ecologists.^'^ The problem calls for much more 

 work — it is far from finished. The observational method of field se- 

 lection and laboratory analysis remains secondary to direct experiment 

 on growing trees under controlled conditions. 



In any event, the present work comparing tree growth and rainfall 

 characteristics strongly suggests that trees selected from certain sites 

 and from the proper rainfall or soil-moisture zone can be sensitive 

 indicators of rainfall changes through the use of average variation, 

 average departure, and average departure from mean variation. 



CORRELATION OF TREE GROWTH AND RAINFALL 



Range of tests. — Fairly extensive correlations were made between 

 tree growth and Chacon and Santa Fe rainfall both for groups and 

 for individual trees. The trend method was applied in its complete 

 form until it was determined that variations of i or 2 years did not 

 unduly distort the results. In addition to the more extensive corre- 

 lations, selected tests were made between tree growth and the records 

 of other rainfall stations. 



Tree growth (groups) and Chacon rainfall. — Groups i to 9 were 

 correlated with Chacon rainfall for the month-intervals shown in 

 table 15. However, the table includes only those groups which were 

 most significant. 



The table shows that correlations with March-July and January- 

 August rainfall are the highest, and of these two intervals March- July 

 is the more important. July rainfall is necessarily included as is 

 indicated by the lower correlations of March-June. Of the 5 months, 

 March- July, the rainfall of May- June is more important to tree 

 growth than that of March-April and the rainfall of April is of 

 less importance than that of May, June, or July. Apparently tree 

 growth, as represented by the trees selected, responds directly to the 

 rain which falls during and the several weeks immediately preceding 

 the actual growing season. 



The most striking fact obvious at first sight is the correlation be- 

 tween group 7 (all trees) and March- July rainfall, the trend coeffi- 

 cient being 0.965 and the ratio of opposed trends 0.12. A trend 

 ratio of 0.12 means that the trend of rainfall was opposite to the trend 



^1 Bot. Rev., vol. 7, pp. 651-655, 1941. 



