CLIMATIC CYCLES AND TREE-GROWTH. Ill 



SUMMARY. 



In the foregoing investigation the following conclusions have been 

 reached : 



(1) The variations in the annual rings of individual trees over considerable 

 areas exhibit such uniformity that the same rings can be identified in nearly 

 every tree and the dates of their formation established with practical certainty. 



(2) In dry climates the ring thicknesses are proportional to the rainfall 

 with an accuracy of 70 per cent in recent years and this accuracy presumably 

 extends over centuries; an empirical formula can be made to express still 

 more closely this relationship between tree-growth and rainfall; the tree records 

 therefore give us reliable indications of climatic cycles and of past climatic 

 conditions. 



(3) The tree's years for such records begins in the autumn. 



(4) Double rings are caused by spring drought and are indicative of the 

 distribution of rainfall throughout the year. 



(5) Tree records may be used in the intensive study of the location of homo- 

 geneous meteorological conditions and in outlining meteorological districts. 



(6) Certain areas of wet-climate trees in northern Europe give an admirable 

 record of the sunspot numbers and some American wet-climate trees give a 

 similar record, but with their maxima 1 to 3 years in advance of the solar 

 maxima. It is possible to identify living trees giving this remarkable record 

 and to ascertain the exact conditions under which they grow. 



(7) Practically all the groups of trees investigated show the sunspot cycle 

 or its multiples; the solar cycle becomes more certain and accurate as the area 

 of homogeneous region increases or the time of a tree record extends farther 

 back ; this suggests the possibility of determining the climatic and vegetational 

 reaction to the solar cycle in different parts of the world. 



(8) A most suggestive correlation exists in the dates of maxima and minima 

 found in tree-growth, rainfall, temperature and solar phenomena. The preva- 

 lence of the solar cycle or its multiples, the greater accuracy as area or time 

 are extended, and this correlation in dates point toward a physical connec- 

 tion between solar activity and terrestrial weather. 



(9) The tree curves indicate a complex combination of short periods includ- 

 ing a prominent cycle of about 2 years. 



(10) An instrument has been constructed which promises special facility 

 in the analysis of such periods. 



The items enumerated above point to the general conclusion that 

 near at hand and readily available in our forest areas is written a story 

 of climatic cycles and solar relationship which in part at least is inter- 

 preted by the methods illustrated in the foregoing pages. 



