THE OAK-CEDAR WOODLAND. 



201 



broides. This is nearly universal in the mountains of southeastern Arizona 

 and adjacent New Mexico, and doubtless in those of northern Mexico as well. 

 Quercus oblongifolia is regularly present in the lowest part of the zone, and a 

 shrubby form of Q. reticulata in the uppermost portion. To the north and east 

 Pinus edulis and Juniperus monosperma enter the mixture also. On the 

 Guadalupe and Davis Mountains of trans-Pecos Texas the grouping is Quercus 

 arizonica, Q. emoryi, Pinus edulis, Juniperus pachyphloea, J. monosperma, 

 and J. sabinoides. In the Chisos Range to the south, Pinus cembroides and 

 Juniperus fiaccida occur as well. East of the Pecos River, the number of 

 dominants decreases abruptly, and the rough areas of the Staked Plains and the 

 Edwards Plateau show only Juniperus sabinoides, J. monosperma, Pinus edulis, 

 and Quercus arizonica, single or in varying mixture. Quercus arizonica in par- 

 ticular becomes reduced to a shrub and mingles with the live-oak chaparral. 



Factor relations. — The relative requirements of the dominants are shown 

 by their altitudinal positions. In the mountains of southern Arizona, the 

 lowest oak is Q. oblongifolia, followed by Q. emoryi, this by Q. arizonica, and 

 then by Q. hypoleuca. They drop out in about the same order, except that 

 Q. arizonica is represented at the highest elevations by the shrubby form of 

 Q. reticulata. Juniperus pachyphloea begins above the lower oaks, while 

 Pinus cembroides enters still later. Shreve (1915 : 24) places the lower limit 

 of the woodland or "encinal" zone of the Santa Catalina Mountains at 4,300 

 feet and the upper at 6,000 to 6,500 feet. Quercus oblongifolia and Q. arizonica 

 are the first to appear at the lower edge of this zone, followed by Juniperus 

 pachyphloea. Quercus emoryi and Pinus cembroides enter at 5,000 feet, and 

 Q. hypoleuca at 5,600 feet. The cedar and pinon reach their maximum abun- 

 dance between 5,500 and 6,500 feet. Quercus oblongifolia disappears at about 

 5,200 feet and the typical form of Q. arizonica at 6,500 feet. Quercus emoryi 

 reached its upper limit at 6,300, while Pinus and Juniperus cease to be domi- 

 nants between 6,500 and 7,000 feet. 



Summer rainfall in inches. 



Average daily evaporation in cubic centimeters for north and south exposures. 



