THE WOODLAND CLIMAX. 



195 



in western Nebraska northward into Saskatchewan and westward to Oregon. 

 It also occurs frequently with chaparral and scrub on the Edwards Plateau 

 in Texas. 



Unity of the formation. — As has already been indicated, the geographic 

 unity of the woodland climax is less than in any other western formation. 

 This is necessarily the case because of its regular occurrence on the lower 

 levels of mountain ranges and plateaus. As a consequence, it is characteris- 

 tically fragmentary, though consistent in being generally southern and at 

 middle altitudes of 7,000 to 8,000 feet. Climatically, it is intermediate between 

 chaparral and sagebrush on the one hand and the montane forest on the other. 

 The average rainfall is 15 to 20 inches and the average evaporation about 20 

 to 25 inches. The summer temperatures are high and the winters moderate 

 for the most part, though snow lies for some time over most of the climax 

 regions (fig. 8). 



Fig. 8. — Monthly and total rainfall for representative localities in the associations of 



the woodland climax. 



Floristically, the woodlands show a high degree of unity. The dominants 

 all belong to three genera, Pinus, Juniperus, and Quercus. While the number 

 of minor species or varieties is large, amounting to 19, the actual species are 

 but 8. The most important of these are Pinus edulis, Juniperus californica, 

 and J. occidentalis. These three with their varieties range throughout the 

 region, and one or the other occurs more or less regularly in each association. 

 The subdominants are relatively few, but it is significant that Rhus, Ceano- 

 thus, Cercocarpus, Purshia, etc., are found over the major portion of the forma- 

 tion. 



Ecologically the formation is essentially uniform in its composition, con- 

 sisting of small trees with evergreen leaves. While the latter are present in 

 three forms, scale, needle, and broad leaf, it seems clear that these are ecologi- 

 cally equivalent or nearly so. The constant mixture of cedar and pifion in 

 the central association leaves no doubt of their essential equivalence, though 

 the scale-leaved cedar is clearly the more xerophytic. The same is true of the 

 southern association of cedar, pifion, and oak, and apparently also of the 

 California association of digger pine and oak. In all of these communities, 

 the dominants have marked ability to adjust themselves to more xerophytic 

 conditions by becoming shrubby or by growing in an open stand. A direct 



