NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 277 



MULTIPLICITY AND RAINFALL INTERPRETATIONS 



It has been seen that sharp boundaries on growth layers do not mean 

 the end of the annual growing season and that multiplicity character- 

 izes the extreme lower forest border. The errors inherent in tree 

 growth-rainfall comparisons are quite obvious if each growth layer, ir- 

 respective of its annual or intra-annual character, is compared either 

 with annual rainfall or with the rainfall of some period within the 

 year. 



The grave error arising from the use of multiple growth layers as 

 annual increments needs no further comment. 



Without doubt, growth-layer thicknesses should be compared with 

 amount of soil moisture, if such data were available, because amount 

 of soil moisture available to plants depends upon factors other than 

 simple rainfall quantities. This is even more true perhaps in the case 

 of intra-annuals. Here a knowledge of the time of occurrence of the 

 widely spaced rains of the extreme lower forest border (text fig. 48), 

 or the fluctuations of soil moisture incident thereto, is of no help un- 

 less the exact time of formation of intra-annuals within the general 

 growing season is known. Soil-moisture fluctuations point out the in- 

 tervals when growth can occur, other factors being favorable, and, 

 conversely, a knowledge of the exact intervals when growth occurs can 

 point out when, other factors being favorable, the amount of soil mois- 

 ture conducive to growth in a given locality is present. The compari- 

 son of intra-annuals with soil-moisture fluctuations within a season is 

 of great significance and awaits close investigation. 



Insofar as the transpirational stream is concerned, a tree depends 

 upon the soil moisture immediately present in the ground when growth 

 occurs. Rain that has fallen before growth started and has run off, 

 evaporated, transpired, or percolated out of reach can have no influ- 

 ence on subsequent tree growth; rain that falls after growth has 

 ceased for the year can have no influence on the preceding growth. 

 This sounds elementary, and yet numerous cases exist wherein tree 

 growth has been correlated either with annual or less than annual rain- 

 fall, and the student making interpretations there form will speak of 

 wet and dry "years." One month or a succession of several months in 

 a year with high rainfall does not mean that other months will have 

 high rainfall or even rainfall above average. Indeed, Henry (1931) 

 says that in the year 1930 in Arkansas, January had 223 percent. May 

 200 percent, June 22 percent, and July 19 percent of normal ; the en- 

 tire year averaged 96 percent. Much tree growth in a season does not 



