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



the typical lower forest-border association of juniper-pinon at eleva- 

 tions of about 6,000 feet. Area rainfall is concentrated in the summer 

 months, especially July and August. 



YUMA, ARIZONA 



The citrus trees, whose branches were sent to us, grew at the Uni- 

 versity of Arizona Experiment Station on a flat, mesalike area 4 

 miles south of Yuma. They were dependent almost wholly upon irri- 

 gation and therefore had an excellent opportunity of showing the 

 effect of such irrigation on growth. The elevation of Yuma is 141 

 feet, and the Experiment Station is somewhat higher. Yuma has ex- 

 tremes of high temperature, low rainfall (annual average of 3.33 

 inches), and low soil moisture, so much so that plant life, except for 

 the native desert association of creosote bush and cactuses, is abso- 

 lutely dependent upon irrigation. The soil is light and very sandy. 

 Competition was not a factor in the growth of the trees. 



THE VICINITY OF WASHINGTON, D. C. 



The trees used were planted and grew on the grounds of a private 

 residence in Maryland, half a mile outside of the northwest boundary 

 of the District of Columbia. They stood at an elevation of 230 feet 

 on the slightly rolling surface of the Piedmont Plateau. The soil is a 

 heavy clay loam. 



The average annual rainfall of about 42 inches is much greater, and 

 it is more evenly distributed, than that in the Lubbock region. Tem- 

 peratures are in general somewhat lower and soil moisture fluctuations 

 less; in spite of this, the trees suffered from drought efifects while 

 they were under daily observation. Killing frosts occur, and one was 

 observed in the spring of 1944, one of the three springs during which 

 the trees were under study. Cold spells in the spring periodically de- 

 layed growth. The specimen trees grew in an area where the mixed 

 hardwood and pine forest had been cleared away. Competition did not 

 affect any of them. 



TREE DESCRIPTIONS 



In order to refer to the specific locality and to a specific tree from 

 that locality, the following symbols were adopted. 



LOCALITY SYMBOLS 



CC Coopers Canyon near Post, Tex. 



CM Chisos Mountains, Tex. 



Con Conservatory, trees grown in, for one winter. 



LV Las Vegas, N. Mex. 



M Mackenzie Park, Lubbock, Tex. 



