4 CLIMATIC CYCLES AND TREE-GROWTH 



changes for many centuries, for the meteorologists certain drought 

 conditions and climatic changes over a similar length of time, and 

 for the botanists an opportunity for learning how vegetation reacts 

 to certain phases of its environment. In addition, various problems 

 of dating, such as the chronology of the prehistoric ruins of the South- 

 west, have received a new approach, but solar and climatic cycles 

 with an ultimate view to seasonal prediction have continued the 

 central theme. 



Prediction possibility has been one of the great incentives to recent 

 work upon tree-rings. There seem to be two approaches to long-range 

 forecasting. One is by direct tracing of the physical causes and the 

 other is by learning the history of past changes and working out 

 empirical methods. Each needs the other; so the climatic history 

 written in trees is doubly useful, for it may of itself give means of 

 foretelling the future, if such can be found, and, on the other hand, 

 if the physical causation is traced first, the derived line of causes 

 must agree with and explain this known history in trees. Thus pre- 

 diction will gain at once greater reliability. The last chapter in the 

 book deals with the various climatic cycles found in trees. 



The effort to find a basis of seasonal prediction is the modern phase 

 of an age-old problem. In our day of newspapers, calendars, and clocks 

 it is hard to realize that at the beginning of prehistoric agriculture 

 farmers knew little of the time of day or the time of year except as 

 signs in the heavens told it to the rare man who had learned the 

 language of the sky. We are now in the same stage of ignorance 

 regarding yet longer cycles and hope to find our time in relation to 

 them so that we may know better when and what to produce each 

 season for modern needs. 



DEVELOPMENT 



With a conviction of the climatic value of tree-ring studies, one 

 can see two general lines of development, roughly described as exten- 

 sion in space about the world at the present time and extension in 

 time to past eras. The former has economic and scientific value, 

 because, in this way, climatic variations in different hemispheres, 

 continents, and latitudes may, within limits, be studied, in spite of 

 absence of formal instrumental records; so also the effects of mountain 

 ranges, continental contours, different orientation of exposure, and 

 the reaction of vegetation under different conditions. A beginning 

 is made in this volume along these lines. A set of yellow pine ring 

 records has been obtained from the Western States, and especially 

 the Southwest, by which a large area can be reviewed and a first 

 estimate made of effects such as those just mentioned. 



Similar information regarding past climates is contained in fossil 

 trees. Without knowing exact dates, we can learn something about 



