VIII. ENVIRONMENT 



This chapter deals with the effects of climate, topography, and 

 other external agencies on ring-growth in trees; after which the point 

 of view is reversed and the observed effects are listed as indicators of 

 past climates. 



EFFECTS IN TREES 

 CLIMATE 



The common factor over large areas is climate. A heavy winter 

 snowfall in Northern Arizona, which supplies abundant moisture for 

 the trees there, extends over hundreds of miles and supplies abundant 

 moisture in northwestern New Mexico, 225 miles away, or over on the 

 coast mountains, a matter of 400 miles in the opposite direction. A 

 dry winter in Flagstaff is usually dry in the other places also. Even 

 at much greater distances the resemblances are enough to enable us 

 to carry dates across in the trees. 



Rings a climatic phenomenon — This is not surprising, for the ring 

 is a climatic phenomenon. It begins with large, white rapid growth 

 in the late spring when the sap flows. The usual time of this at Flag- 

 staff (elevation 7,000 feet) is in late May or June and is well observed 

 by the dendrograph, which magnifies the diameter of the trunk and 

 shows its daily and hourly variations. In this arid climate, spring 

 growth depends on the precipitation of the preceding winter, for the 

 months of April, May, and June are exceedingly dry. In July and 

 August come the heavy summer rains with a large run-off and little 

 benefit to the trees. When the season closes, there is a gradual cessa- 

 tion of the activity of the tree, owing to lowered temperature and 

 diminished water-supply. This causes the deposition of harder 

 material in the cell-walls, producing in the pine the dark, hard autumn 

 part of the ring and the protecting bark. The growth stops altogether 

 in winter. 



Small single rings — If the winter and spring have been unusually 

 dry, the Arizona tree may stop growing by summer. The resulting 

 ring will consist of a small white spring growth and a threadlike red 

 outside growth. In old trees the ring may become microscopic or 

 appear as a thickening of the red ring of the preceding autumn, and 

 even disappear altogether in parts of the circuit of the trunk. In 

 some extreme cases, sections could be found in which a ring or two 

 is absent from the entire circuit. Very likely it was active for a time 

 but not long enough to leave white cells. 



Double rings — On the other hand, if the winter precipitation has 

 been normal, the tree passes through the spring drought and reacts 



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