REPRODUCTION OF WESTERN YELLOW PINE. 115 



GRAZING. 

 REVIRW OF PREVIOUS INVESTIGATIONS. 



As early as 1904, Leiberg {IJf) expressed his conviction that 

 sheep grazing is responsible for the failure of western yellow pine 

 reproduction in the San Francisco Mountains. 



Investigations by the Avriter {15) on the Coconino National Forest 

 in 1908 showed that the number of seedlings browsed under average 

 grazing conditions ranged from 10 to 41 per cent for the current 

 season, while on sheep driveways and bed grounds the percentage 

 was considerably higher. This damage was attributed almost wholly 

 to sheep, and therefore it was recommended that sheep be excluded 

 from cutting areas on which reproduction was deficient during a 

 period of 20 years, which was the estimated average time required 

 for reproduction to become established and grow beyond serious 

 danger of injury from sheep. 



The above study was continued on a more extensive scale by the 

 Fort Valley Experiment Station in 1910. This investigation sup- 

 ported the conclusions arrived at in 1908, but also pointed out the 

 need for more detailed consideration of damage under various con- 

 ditions prevailing on the range, with the object of ascertaining the 

 effect of such factors as abundance and character of forage, intensity 

 of grazing, and season of grazing. Accordingly a more compre- 

 hensive working plan was developed, and the project was assigned 

 to Grazing Examiner Hill, under the direction of the Office of 

 Grazing. Hill pursued the investigation vigorously during the 

 years 1912, 1913, and 1914. He made from four to five examinations 

 annually on 250 plots, distributed over a radius of 25 miles about 

 Flagstaff. The following is a summary of his findings {11) : 



Out of 8,945 trees of a size subject to grazing injury 16.7 per 

 cent were severely damaged and 16.1 per cent moderately damaged 

 each year. For seedlings below 6 inches in height the number 

 seriously injured amounted to 21 per cent. These figures refer to. 

 current damage. Hill's work, as well as that of- Chapman, West- 

 veld, Kimball, and the author, shows that the cumulative effect of 

 such damage over a long period of years may nullify all progress 

 in reproduction. 



The average amount of damage per week during various seasons 

 was as follows : April 15 to May 20, 0.07 per cent ; May 21 to July 8, 

 2.5 per cent; July 9 to September 12, 1.9 per cent; September 13 to 

 November 6, 0.9 per cent. From these figures it appears the amount 

 of damage prior to May 20 was negligible. The greatest damage oc- 

 curred during the latter part of June and early in July, or during 

 the dry foresummer. Contrary to prevailing opinions, the damage 



