4 EMORY OAK IN SOUTHERN ARIZONA. 



daily for periods of a week or more at a time, starting usually about 

 the end of the first week in July. While these storms furnish most 

 of the soil water for tree growth, the evaporation, at low altitudes, 

 is much greater than the precipitation. 



In the whole region there is but little tree growth in the winter 

 rainy season, only moderate growth in the spring dry season, much 

 growth in the summer rainy season, and slight growth in the late 

 dry season. 



SOIL AND MOISTURE. 



Emory oak so adapts itself to conditions of soil and moisture 

 that it is able to grow on a variety of sites, though it does best in 

 deep alluvial soils with abundant water. In the best sites in the 

 broad valleys the soil may be 20 feet deep and have a constant 

 supply of underground water within reach of the roots. In the 

 narrow valleys and on the slope bases and bench lands the soil may 

 vary, on the best sites, from 3 to 15 feet in depth, with moderately 

 good soil-moisture conditions, and, in the best situations, some peri- 

 odic surface flow of water. On the slopes the site has a very decided 

 influence on growth, and there is marked advantage to the trees in 

 the deeper soils of the lower slopes and in the greater moisture of 

 north and east slopes. 



ASSOCIATED SPECIES. 



In broad, open valleys the commonest associates of Emory oak 

 are mesquite (Prosopis juliflora velutina), acacia (J.cacm greggii), and 

 desert willow (CJiilopsis linearis), though on the most moist situations 

 there will be ash (Fraxinus velutina), sycamore (Platanus uniglitii), 

 and willow (Salix taxifolia). Although scarcely ever found in the 

 bottoms of the valleys, blue oak (Quercus oblongifolia) is an intimate 

 associate on the bases of the slopes that border these valleys. 



In the narrow valleys, on slope bases, and on benches the stand 

 may be pure or mixed with any one or all of the following species: 

 Arizona white oak (Quercus arizonica) , white-leaf oak (Quercus 

 hypoleuca), Cliihuahua pine (Pinus chihualiuana) , and alligator 

 juniper (Juniperus pachypld&a) . Even when all are present Emory 

 oak usually predominates. 



Since slope stands are transitional, they present the greatest 

 diversity of associated species. Arizona white oak and white- 

 leaved oak are the commonest associates on north and east slopes; 

 on south and west slopes, blue oak, mountain mahogany (Cer co- 

 carpus parvifolius) , cliff rose, manzanita (Arbutus xalapensis), and 

 numerous shrubs. At elevations of from 6,000 to 7,000 feet there 

 is a considerable mixture of Mexican pinon (Pinus cembroides) . 



