14 CLIMATIC CYCLES AND TREE-GROWTH 



all of the growth-moisture for the trees. The result is that the east 

 and west sides of a large mountain have a distinct difference in climate 

 which shows in the trees (see shadow effect, p. 108). At corresponding 

 levels the west side is wet and the east side is dry. Around the San 

 Francisco Peaks, in northern Arizona, the pines extend to 1,000 or 

 1,500 feet lower elevation on the west than on the east. Pines on 

 westerly slopes are to be preferred as less likely to be altered by local 

 conditions. 



North and south mountain slope — Snow lingers longer on north 

 slopes, and pine trees are able to live under such conditions at lower 

 altitudes. But in the middle elevations of the pine belt no sensible 

 difference has been noted in ring record between minor north and south 

 slopes. 



CONDITION OF TREE 



Lightning scars — In standing timber this commonly appears as a 

 white streak from top to bottom of the tree, about 1 inch in width 

 where the bark has been blown away and the wood revealed. The heat 

 of the electric flash has suddenly vaporized the sap and exploded the 

 tree along this narrow line. This usually heals and has no important 

 effect on the climatic record in the tree. The scar is easily recognized 

 on the stump. It is very common in the "buried trees" found in the 

 valley terrace above Flagstaff, which doubtless means that summer 

 thunderstorms were more common in that particular past climate 

 than they are now. Lightning scars are rarely seen in petrified wood, 

 but the writer has a photograph of one in a beautiful specimen from 

 Tertiary levels in Yellowstone Park. 



Injured and fire-scarred trees — The major injury to western trees 

 is from fire. This is not always caused by the careless camper or 

 «moker, for the greater number of forest fires come from lightning. 

 A single summer storm at Flagstaff has been seen to start fires in four 

 different trees. In a precipitous country it is the up-hill side of the 

 tree which is more likely to have fire injury, for it is the brush and 

 leaves and needles collected there which hold the fire till it injures the 

 tree. The fire scar is a large burnt area covering from 10° to 150° of 

 the circumference and extending from 3 to 20 feet or more above 

 ground. The tree may recover by covering a small area with new 

 growth or by abandoning all attempts to reclothe the burnt section 

 and using only the root system on the normal side. 



Different trees and fire — Fire injuries rarely give trouble in the 

 yellow pine, for they are largely on nearly level ground and there is 

 little vegetation about them. Hence, there is little accumulation of 

 rubbish and a general fire does not finger about an individual tree. 

 The sequoias represent an enormously greater interval of time and so 

 .are more likely to show fire scars. Their ages are from 700 to 3,000 



