PLANTING INDICATORS. 357 



jured or killed on lightly grazed plots, 31.8 per cent on moderately grazed areas, 

 and 65 per cent on heavily grazed plots. During 1915 and 1916 the average 

 percentage of injured and killed sprouts by cattle browsing was 1.6, 2.4, and 

 26.8 on lightly, moderately, and heavily grazed plots, respectively. On clear- 

 cut lands, where the reproduction is conspicuous and the stand even, the 

 annual mortality due to sheep grazing is exceedingly heavy. As a rule, three 

 years of successive sheep grazing on such lands results in the destruction of 

 the entire stand. " 



Cycle indicators. — Trees, and shrubs also, may serve as indicators of climatic 

 cycles by virtue of their growth, seed-production, or reproduction. In addi- 

 tion, there appears to be a certain correlation between the frequence and in- 

 tensity of forest fires and the dry and wet phases of the cycle. The growth 

 of trees as recorded in the annual rings is the classic material for the studies of 

 Douglass, Huntington, and Kapteyn upon climatic cycles. The width of the 

 ring indicates the varying rainfall of different years so clearly that Douglass 

 (1919) has found it possible to cross-identify rings from trees grown many 

 hundreds of miles apart. He has also found that the yellow pines of central 

 Arizona often indicate two growing periods in one year by the formation of a 

 double ring, and Shreve (1917:706) states that this appears to be regularly 

 the case with trees at 6,000 feet in the Santa Catalina Mountains. It seems 

 almost certain that height-growth and volume will likewise show cycle cor- 

 relations, and this is suggested by Pearson's results in the study of the re- 

 lation of height-growth to spring precipitation in northern Arizona (p. 351). 

 The suggestion that seed-production is related to climatic cycles is based upon 

 its well-known periodicity (Zon and Tillotson, 1911: 133), as well as upon the 

 fundamental fact that as a growth response it is controlled primarily by water 

 and temperature. It seems probable that the seed-production cycle of pines 

 especially is a response to the interaction of the 11-year cycle and the excess- 

 deficit cycle of 2 to 3 years. 



Reproduction reflects more or less faithfully the variations in rainfall dur- 

 ing the 2 to 3 year, the 11-year, and the 22-year cycles. This correlation is 

 clearly seen in the case of woodland and montane forest, especially at the 

 lower limit, but it is naturally less evident in climaxes with a higher rainfall. 

 It is most striking where woodland or forest is in contact with a community 

 of lower water requirements, such as grassland, sagebrush, or chaparral, and 

 shows less in the reproduction on the forest floor. All the cases of tree savan- 

 nah and "natural parks" so far investigated warrant the working hypothesis 

 that reproduction in such areas is cyclic and corresponds as a rule to the 

 11-year cycle, though minor variations conform to the 2 to 3 year cycle. 

 There is also considerable evidence that the success or failure of planting 

 operations has often been determined by their accidental coincidence with the 

 wet or dry phases of the 11-year cycle, while it is obvious that in the future 

 planting should be carried out with reference to the phases of the 2 to 3 year 

 and 11-year cycles (plate 90). 



PLANTING INDICATORS. 



Kinds. — Indicators of sites for planting are of two kinds: (1) those that 

 indicate the former presence of forest; (2) those that suggest the possibility 

 of developing forest in grassland or scrub areas. The first are indicators of 

 reforestation, the second of afforestation. The obvious indicators of reforesta- 



