NO. l8 TREE GROWTH AND RAINFALL CLOCK 



27 



these comparisons using March-July rainfall. An increase in average 

 rainfall (as between 1850-1897 and 1898-1941) is accompanied by 

 an increase of average growth-layer thickness on dry sites (group 10) 

 and by a decrease on wet sites (group 11) ; an increase in rainfall 

 giving a decrease in its average variation is accompanied by an increase 

 of average variation among growth layers from dry sites and by a 

 decrease among growth layers from wet sites ; an increase in rainfall 

 giving a decrease in its average departure is accompanied by an in- 

 crease of average departure among growth layers from dry sites and 

 by a decrease among growth layers from wet sites ; and an increase in 

 rainfall giving a decrease in its average departure from mean variation 

 is accompanied by an increase of average departure from mean varia- 

 tion among growth layers from dry sites and by a decrease among 

 growth layers from wet sites. 



In other words, changes of average variation, average departure, 

 and average departure from mean variation among growth layers 

 from wet sites follow the changes of the same features in the rain- 

 fall whereas the growth layers from dry sites react in the opposite 

 direction. The case is reversed as regards changes in growth-layer 

 thicknesses : the growth layers from the dry sites follow the changes 

 in average rainfall amounts. However, as discussed under the study 

 of growth layers, two trees, HPC i and 7 of group 10, determine the 

 characteristics of the group. Their elimination from the complete 

 record leaves a homogeneous group consisting of the other two trees 

 of group 10 and all trees of group 1 1 (HPC 8 is not included because 

 its sequence is too short). This group of six trees, group 7 (re- 

 stricted), is conformable within itself, and the changes in its charac- 

 teristics from 1850-1897 to 1898-1941 agree with those of Santa Fe 

 rainfall. 



Nevertheless, there remains the problem of why the average vari- 

 ation of the dry-site trees increased with a decrease of average varia- 

 tions in rainfall during 1898-1941. Calculation of the average varia- 

 tion of rainfall for different months and month-intervals shows that 

 only April and May increased their average variations during the 

 period 1898-1941. It might be, therefore, that the dry-site trees were 

 more influenced by April-May, or spring, rainfall than the wet-site 

 trees were. Or the problem may concern limiting factors and optimum 

 or near optimum soil-moisture conditions in that the wet-site trees 

 grew under conditions where the trees responded directly and con- 

 sistently to changes in rainfall. 



