o ^ 



J2 



O 

 -C 0) 



<u ri 



5g - o 

 376 



fc 



the greater the separation of individual rains 

 (text fig. 48), the greater the soil-moisture 

 fluctuations, the greater the variability of 

 the growth-layer record, the greater the 

 number of partial groMrth layers, and the 

 greater the factor of multiplicity. Trees on 

 steep, rocky slopes with little soil depend 

 rather directly upon rainfall, to be sure. 

 However, individual rains, widely spaced 

 in time as they are in the lower forest 

 border, can bring about variations whereby 

 conditions suitable for tree growth, from 

 the standpoint of soil moisture, are termi- 

 nated and reinitiated one or more times 

 during a general growing season. 



If the upper limits of the ponderosa pine 

 zone in the Southwest represent the forest 

 interior with near optimum soil moisture, 

 then we may observe the increase of vari- 

 ability and all it implies downward through 

 the ponderosa pine zone, the piiion, juniper, 

 and oak, to the desert mesquite. Increase of 

 multiplicity does not necessarily accompany 

 increase of variability but, if the work on 

 the southern High Plains is a criterion, the 

 two do increase together in the Southwest. 

 Arizona with its two rainy seasons — winter 

 and summer — would be expected to break 

 into multiplicity very soon with decreasing 

 rainfall, especially because of the intense 

 drought of late spring-early summer, which 

 separates the end of the winter rainy season 

 from the summer rainy season. 



The loblolly pine (Con T), introduced to 

 the Lubbock area from a wetter climate, 

 grew multiple growth layers in both trunk 

 and branches even when watered periodi- 

 cally throughout the summer. 



All the present investigations emphasize 

 the intimate relationship between excellent 

 cross-dating and amount of multiplicity — 

 both, along with other features, being highly 

 characteristic of the lower forest border in 

 the Southwest. 



