102 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



ADVANTAGES OF LOCATION. 



The pine tree of northern Arizona lends itself peculiarly well to the investigation here 

 contemplated. Not only is its situation favorable because of the absence of other vege- 

 tation and of all pests which might seriously alter the growth of the tree, but because the 

 soil is of such a nature that variations in precipitation are quickly felt in the trees. Of 

 still more importance is the fact that the relatively open and unobstructed character of 

 the country makes the meteorological elements relatively homogeneous over a consider- 

 able area, and tree records from widely separated locahties show similar features. The 

 importance of this is illustrated by the conditions near Flagstaff. To the south of the 

 town, where the tree records were obtained, the altitude averages about 7,000 feet, and varies 

 only a few hundred feet from place to place. North of the town, however, the San Fran- 

 cisco Peaks rise about 12,700 feet, and the yellow pine extends up their slopes to about 

 9,000 feet. On the side of the mountain, exposed to the prevalent westerly storms, the 

 snowfall is heavy. There are found all the springs and ranches, and the overland stage 

 line to the Grand Canyon goes that way in spite of heavy grades, for there water can be 

 obtained for the horses. On the east side there is little snow, barren park lands abound, 

 and the traveler has a run of 25 miles between watering-places. This represents the 

 disadvantage of the very mountainous region, for difTerent sides of a high range present 

 different meteorological conditions. 



Southeast of Flagstaff the plateau country extends nearly 100 miles to the so-called 

 rim, where the land drops off to the lower levels of southern Arizona, while to the south- 

 west the rim is 50 miles distant. On the slopes of the rim the trees go down to an elevation 

 of about 5,500 feet. Here the trees are peculiarly sensitive to changes in rainfall, since 

 they live under severe conditions due to the decrease of the rainfall with decreasing altitude. 

 They are so sensitive, indeed, that in extremely dry years the older trees sometimes omit 

 the formation of any ring whatever. Such an omission is of course significant, but it is 

 an exaggeration of the actual conditions and it leads to grave errors. Besides the trees from 

 near Flagstaff, others were collected from the mountain around Prescott, southwest of 

 the rim across the deep Verde Valley. Among the high and broken ridges of that region 

 the rainfall on opposite sides of a ridge may vary greatly. Hence nearly 60 trees from 

 various localities were measured before a growth was found close enough to Prescott to 

 be compared minutely with records of precipitation at that place. 



SEASONAL CONDITIONS AND TREE GROWTH. 



The climate of this part of Ai'izona possesses the general characteristics described in an 

 earlier chapter of this book. Because of the altitude, the winter temperature often falls 

 from 15° to 20° F. below zero. Shallow valleys are especially subject to low temperatures, 

 for in the absence of general or storm winds, such as prevail over the eastern part of the 

 country, the cold air settles in the lowest places. Even in summer the temperature is 

 often low and snowstorms not infrequently occur in May, and during the last 18 years 

 one occurred in June. These conditions favor very perfect ring production, but the divi- 

 sion of the rainfall into a winter and summer season is a disadvantage in the attempt to 

 investigate climate by means of the growth of trees, for the spring drought naturally checks 

 growth and some of the trees often act as if winter were approaching, and form a layer of 

 hard wood like that characteristic of the fall. Usually such trees begin to grow again when 

 the summer rains come, and thus form a double ring, but some stop growing entirely. 



Meteorological records in northern Arizona are necessarily meager, yet not so deficient 

 as might be expected. The country was first settled in the fifties, when gold was dis- 

 covered in Arizona as well as in California, and lines of travel were established from Santa 

 Fe westward across the plateau. The "blazings" on the pine trees marldng the earlier 

 roads are still to be distinguished. Soon after the opening of the country the government 



