136 BTJLLETIlSr 1105, U. S. DEPARTMENT OF AGRICULTURE. 



In the foregoing discussion, eight more or less interrelated factors 

 or complexes have been recognized. They are seed supply, climate, 

 soil, herbaceous vegetation, cutting, brush disposal, grazing, and 

 miscellaneous enemies. 



SEED SUPPLY. 



An abundant supply of seed is needed to offset consumption by 

 rodents and an extremely high mortality among young seedlings. 

 The minimum seed requirement for average conditions on the Coco- 

 nino and Tusayan National Forests has been placed at 8 pounds 

 per acre in a good seed year. In other localities this figure might 

 possibly be lowered. On most of the Coconino and Tusayan, about 

 three such seed crops, occurring at intervals of from 3 to 5 years, 

 will normally be required to complete restocking. Under more 

 favorable conditions, such as occur on the Sitgreaves, Apache, 

 Santa Fe, Carson, Lincoln, and, in fact, practically all of the Other 

 National Forests in Arizona and New Mexico, two or even one seed 

 crop of the above proportion will usually suffice. All observations 

 agree that the best seed trees are large, sound, healthy individuals 

 which have grown in an isolated position, thus acquiring well- 

 developed crowns and wind firmness. Sufficient emphasis has not 

 heretofore been placed on size. It has been customary to regard 

 all healthy trees down to a diameter of about 11 inches as seed 

 producers. Eecent investigations have shown that, as a class, the 

 trees below a diameter of approximately 21 inches contribute rela- 

 tively little toward the needed seed supply. This is due partly to 

 immaturity and partly to. the fact that the great majority of 

 the smaller trees occur in groups, where they are poorly exposed to 

 light and where a large part of their product is ineffective because 

 of poor dissemination. For the above reasons it seems advisable 

 in fixing a minimum limit as to the required number of seed trees 

 to disregard those below 21 inches in diameter breast high. The 

 average minimum for the Coconino and Tusayan National Forests 

 has been placed at 4 trees above 20 inches diameter breast high 

 per acre. Possibly this number can be reduced to three per acre 

 where soil and climatic conditions are more favorable. 



CLIMATE. 



Climate is perhaps the largest natural complex of factors affecting 

 reproduction in the Southwest, and the greatest single element in this 

 complex is moisture. It is safe to say that an addition of 2 inches to 

 the normal rainfall during the month of June would practically elim- 

 inate the reproduction problem in this region. The rainfall of July 

 and August is usually adequate for abundant germination ; but often 



