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 homogeneous 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 prevalence 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 connection between solar activity and terrestrial weather.
(9) The tree curves indicate a complex combination of short periods including 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 interpreted by the methods illustrated in the foregoing pages.