136 CLIMATIC CYCLES AND TREE GROWTH 



midsummer minimum that reminds one, and is probably part, of the July 

 minimum between spring and autumn maxima of equatorial regions. Thus 

 it seems highly probable that Mississippi Valley summer rains will be studied 

 in connection with torrid zone conditions and cycles, which may be sub- 

 stantially different from those in the temperate zone. 



Thus far we have approached the Mississippi Valley forecasting problem 

 by a method of comparison with Arizona conditions; a method that is indis- 

 pensable on account of the long records in Arizona. But there is another 

 approach to these Valley variations and that is by tree-ring growth in the 

 Valley itself. In connection with reclamation and power development in 

 certain areas, much-desired climatic work has been carried on by tree-ring 

 methods. Many modern and some ancient specimens have been collected, 

 measured and plotted and compared with meteorological records. 1 Personal 

 examination of specimens has assured me that cross-identification exists to 

 some degree in areas tested, although some of its features depart substantially 

 from the simpler reactions in our dry areas. Though caution is needed in 

 working them out, long sequences of value seem possible of establishment. 

 This conforms with results obtained some years ago by Dr. Robbins of the 

 University of Missouri. Furthermore a direct correlation between Valley 

 tree growth and crops should be sought with care, since its value is more than 

 obvious. 



COAST STATES 



Regarding the Atlantic Coast states, the situation is in some respects 

 similar to that in the Valley, but complicated by the presence of the Atlantic 

 Ocean, so that a slight change in the direction of the wind may have a far 

 greater effect than in the Central Valley, both in rainfall and on temperature. 

 The latitude of storm tracks is more stable but slow variations in ocean 

 temperatures become a factor. Locations close to the coast are likely to be 

 the most difficult. It is hard to find cycles in the long rainfall records at 

 Boston and New Bedford, Massachusetts, but tree-ring records in 250-year 

 old hemlocks in Vermont and rainfall records close by — Hanover, New Hamp- 

 shire — give excellent analyses in terms of cycles. 



On the Pacific coast the situation is also complicated by the ocean. 

 Precipitation depends extensively on whether storms pass across the Rockies 

 at a northerly point or move down the coast. 2 This perhaps is a latitude 

 effect and may be part of Kullmer's theory. At any rate, there is in Cali- 

 fornia rainfall a short cycle averaging 5f years and giving the effect which 

 has been named Hellmann Cycle. It has been sufficiently regular since 

 1851 to give great promise of development as a basis of long-range prediction. 

 But California rainfall values are strongly subject to the two-year cycle of 

 higher amplitude which has a good chance of rendering worthless a predicted 



1 By Dr. Florence M. Hawley and Mr. Roy Lassetter, students of tree-ring work. 



2 Compare Blake, 1935; see Bibliography. 



