822 



SCIENCE 



[N. S. Vol. XLIII. No. 1119 



presenting an altitudinal range of over 6,000 

 feet. The gently-sloping plains or bajadas 

 about this mountain-mass are clothed with a 

 low, open vegetation, in which the creosote 

 bush (Covillea tridentata) plays the leading 

 role. Passing from the base to the summit of 

 the range the observer meets with a constantly 

 changing panorama of vegetation. Shreve 

 considers three vegetational types in this 

 series, the desert, the encinal and the forest. 

 The first type is similar to that of the bajadas, 

 becoming modified as one ascends, and is con- 

 sidered as ceasing at an altitude of about 4,000 

 feet on the north slopes and 4,500 feet 

 on the south ones. Encinal is next encoun- 

 tered, extending on the south slopes to 

 about 6,300 feet and on the north slopes to 

 about 5,S00 feet. This type is characterized 

 by a spotted and open stand of evergreen oaks 

 (Quercus oblongifolia and Q. arizonica) in its 

 lower reaches, and by nearly closed stands of 

 these and other small trees in its upper region. 

 Juniper, manzanita, and such plants as Dasyli- 

 rion, Nolina, Yucca and Agave occuj in this 

 region, and another oak (Q. emoryi) and a 

 scrub pine {Pinus cemhroides) are common in 

 the upper encinal. The upper encinal grades 

 into the lower forest, the latter dominated by 

 Arizona yellow pine {Pinus arizonica), which 

 extends to the summit of the range on south 

 slopes and to an altitude of about 7,500 feet 

 on north slopes. On the north slopes of the 

 highest points and ridges an entirely different 

 type of forest is encountered, the white-fir for- 

 est, which is dominated by Pseudotsuga mu- 

 cronata, Pinus strohiformi-s and Ahies concolor 

 (white-fir), the latter especially in the upper 

 regions. The fir forest is by far the most meso- 

 phytic of all the habitats considered. 



As to floristics, the flora of the desert and 

 encinal regions is closely related to that of the 

 Mexican deserts lying to the south, while the 

 relationships of the forest flora are partly with 

 the flora of the Mexican Cordillera and partly 

 with that of the Rocky Mountains. "The 

 Mexican group is more conspicuous in the 

 make-up of the vegetation, while the Eoeky 

 Mountain contingent is apparently preponder- 

 ant in number of species" (page 40). 



Shreve's discussion of the climatic features 



of this mountain range constitutes the most 

 important part of the study. The duration of 

 the frostless season here receives adequate at- 

 tention, for the fijst time in work of this kind, 

 and the two rainy and two dry periods that 

 characterize the year in southern Arizona are 

 also thoroughly dealt with. A pair of graphs 

 (Fig. 3) with ordinates representing the win- 

 ter and the summer precipitation as percent- 

 ages of the annual, plotted to a geographic 

 base (13 stations, from Los Angeles to Mesilla 

 Park, N. M.), have, roughly, the appearance of 

 the letter X. The downward-slanting line re- 

 fers to winter, and the other to summer rain- 

 fall. Los Angeles has high winter precipita- 

 tion and practically none in summer, while 

 Mesilla Park has much more rain in summer 

 than in winter. The center of the X, where 

 precipitation is about equally divided between 

 winter and summer, lies between Casa Grande 

 and Tucson. Rain-gauges and cylindrical 

 porous-cup atmometers were operated at vari- 

 ous different altitudes, for a number of sum- 

 mers, and the results furnish much more satis- 

 factory information than has hitherto been 

 available, on the relation between precipitation 

 and evaporation, on the one hand, and altitude 

 and vegetational character, on the other. Soil 

 moisture, soil temperature and various other 

 climatic conditions also receive attention, and 

 all of these are correlated with altitude, direc- 

 tion of exposure and the character of the vege- 

 tation. 



Probably the most definite advance in 

 method to be found in this book is the em- 

 ployment of the ratio of evaporation to soil 

 moisture-content, for the period of the arid 

 fore-summer. This ratio exhibits a very satis- 

 factory correlation with the character of the 

 vegetation at different altitudes and on dif- 

 ferent exposures. Such a novel and very 

 promising method should attract the attention 

 of plant ecologists generally. " The ratio of 

 evaporation to soil moisture comprises a meas- 

 urement of all the external factors which af- 

 fect the water relations of plants, except the 

 influence of radiant energy on transpiration 

 and the possible effects of soil temperature on 

 this function" (page 93). For a wealth of in- 



