REPRODUCTION OF WESTERN YELLOW PINE. 29 



tween seed production and rainfall occurs only when a seed year im- 

 mediately precedes a period of ample moisture. The 1908 and 1909 

 seed crops both failed to give results because they were not followed 

 by sufficiently long periods of immunity from severe drought. Simi- 

 larly, the favorable moisture periods of 1911-12 and 1918-19 6 were 

 unavailing because they were not preceded by good seed crops. The 

 seed crop of 1913 was effective because no really serious drought oc- 

 curred until the foresummer of 1916. It would have been still more 

 effective if there had been more precipitation during the summer and 

 fall of 1915 or if the seed had been available a year earlier, thus tak- 

 ing advantage of the entire rainy period, which extended from July. 

 1913, to August. 1915. The grand climax came with the heavy seed 

 crop of 1918 and the extraordinary precipitation of 1919. followed 

 by reasonably favorable moisture conditions in 1920. This period 

 promises to give the best reproduction experienced on the Coconino 

 in 40 years. Success would have been still more certain, however. 

 if the seed which matured in 1918 had been available a year earlier, 

 thus profiting by the two consecutive wet summers of 1918 and 1919. 

 During the entire period of 12 years, two good seedling crops of gen- 

 eral distribution have originated on the Coconino and Tusayan For- 

 ests, and only one of these, that of 1919, was adequate to give full 

 stocking over extensive areas. 



SOIL. 



The general characteristices of soils which influence plant growth and by 

 which they can be most logically grouped are determined by climate. The 

 effect of the climatic conditions prevailing over the western yellow-pine 

 belt of Arizona and New Mexico has been to produce soils, in general, low 

 in their content of humus or completely decomposed organic matter; com- 

 paratively high in content of mineral elements of plant food ; high in content 

 of lime carbonate at the lower altitudes or localities of lowest precipitation 

 and generally alkaline (neutral at the higher altitudes) in reaction; and gen- 

 erally coarse, since rock decomposition has not kept pace with rock disinte- 

 gration. These general characteristics are modified locally by the nature 

 of the rock from which the soil is derived, by the slope of the land, and by 

 the age of the soil. The rock underlying the yellow-pine belt is, over the greater 

 part of the area, of volcanic origin, mainly basalt, but limestone, sandstones, 

 conglomerate clays, and old granitic rocks are also present. Physiograph- 

 ically. this is a region of mountains and high plateaus. The plateaus are 

 deeply incised by streams, resulting in regions of comparatively level or 

 rolling table-land and rough broken topography adjacent to the plateau rims 

 and in the canyons. The geology and topography of the region are varied, 

 and owing to the influence of these factors there are a number of soil types 

 and locally important differences in mechanical composition, structure, and 

 percentages of chemical compounds in the soil. (J. O. Veatch. 7 ) 



8 A heavy seed crop matured in 1018 and germinated in 1010. but no germination oc- 

 curred during the prolonged rainy summer of 1918, because.no seed was produced in 1017. 



7 A full discussion of the geology of the San Francisco Mountains region is given by 

 Robinson (21). See also soil description in the following pages. 



