248 



AGRICULTURAL INDICATORS. 



/7Z7 



1785 



'85Z | 

 1 



1909 



.55 



i? -85 



.30 



I.ZO 



1. 10 

 1.00 



1. 10 

 1.00 

 .30 

 .80 



1.10 3 

 IjOO s " 

 JO C 

 .80 * 

 .5 



ever, and investigation at present is chiefly confined to the nature and extent 

 of the coincidence between them. The outstanding fact is that our knowl- 

 edge has reached a point where it seems increasingly possible to employ the 

 sun-spot cycle as a method of anticipating the coincident or related changes 

 in climate and vegetation. 



Evidences. — The evidence of the cycle of sun-spots has all the certainty 

 of astronomical data. The number of sun-spots has been recorded for every 

 year since 1750, and the dates of the maxima and minima are definitely known 

 as well as their intensity. Cycles 

 in the annual growth of trees have 

 been found by Douglass in a num- 

 ber of diverse regions, in Europe as 

 well as in America. It is obvious 

 that trees growing in the most fa- 

 vorable conditions will not exhibit 

 cycles, since there is no limiting 

 factor to produce variations in the i 7ZB 

 width of the rings. Moreover, the *" 

 same tree sometimes fails to show 

 cycles throughout its life, or does not 

 show them with equal clearness. This 

 is not difficult to understand when 

 the complex relations of factors, of 

 competition and reaction, parasites, 

 fire, lumbering, and other disturb- 

 ances are taken into account. By 

 far the greater part of the evidence 

 of existing cycles has been furnished 

 by Douglass (1919). In his study of 

 Arizona trees, he has found that, 

 during the last 160 years, 10 of the 

 14 sun-spot maxima and minima 

 have been followed about four years 

 later by pronounced maxima and 

 minima in tree- growth, and that the 

 same trees show a strongly marked 

 double-crested 11-year cycle during 

 some 250 years of their early growth. 



They likewise exhibited a relation to the temperature curve for southern 

 California, and this curve in turn resembled in form and phase the inverted 

 curve of the sun-spot cycle. 



In the investigation of trees growing in wet climates, Douglass has also 

 found conclusive evidence of cycles. The trees of Eberswalde near Berlin 

 showed the 11-year sun-spot cycle since 1830 with accuracy. In the group as 

 a whole, the agreement is marked, the maximum growth falling within 0.6 

 year of the sun-spot maximum. In six groups of trees from England, north- 

 ern Germany, and the lower Scandinavian peninsula the growth since 1820 

 shows pronounced agreement with the sun-spot cycle, every maximum and 

 minimum since that date appearing in the trees with an average variation of 

 20 per cent. 



I.ZO 

 1. 10 



1.00 



OZ4-68I0 0Z4- 

 Years 



Fig. 13. — The 11-year cycle during the last 

 250 years, as shown by the yellow pine and 

 Sequoia. After Douglass. 



, 



