the fall of 1956 was warm and dry. Thus, it is not entirely clear if weather conditions, 

 size of the Douglas- fir crop or a combination of these two, abundance of other desirable 

 foods, or other factors might have accounted for the annual differences in pine cone 

 cutting by squirrels. 



The period of squirrel cutting extended from mid-July to the period of maximum 

 seed dissemination in September. During 1953, squirrels cut 15 percent of the crop 

 before August 3. Most of the cones cut early in the season were used immediately, but 

 the cones cut just before maximum seed dissemination were cached (Squillace 1953). 

 Halvorson (1965) indicated that squirrels use cones from these caches over periods of 

 2 years or longer. 



Abortions also reduced the number of surviving cones substantially during the 

 second season of cone development. Even though the number of abortions dropped consider- 

 ably from that in the first season, abortions still accounted for a surprisingly high 

 22 percent reduction over the 4 cone years studied--exceeding 10 percent every year 

 when cones remained at the beginning of the second growing season. 



Like squirrels, insects played a more important role in the second year than they 

 did in the first year of cone development. Unidentified insects accounted for an 

 average of 8 percent (ranging from 5 to 12 percent) of the mortality during the second 

 season of cone development. Although not measured in these studies, birds and chip- 

 munks probably harvested some seeds from the mature cones before the seeds were all 

 disseminated. 



Based on the total potential cone crop present in the original ovulate buds, only 

 6 percent of the cones survived the 2-year development period to disseminate seeds 

 (fig. 5). Abortions were by far the most important limiting factor (70-percent reduc- 

 tion) ; most of this loss occurred in the first year. Squirrels were an important factor 

 in the second season, but in relation to the total original potential they accounted 

 for only 14 percent of the total reduction. Insects, bud failures, and other unidenti- 

 fied factors accounted for the remaining 10 percent of the reduction. 



Total Cone Development Period 



Figure 5. — Factors affecting the 



survival of ponderosa pine cones 



dialing their entire 2-yeaT cone 

 development period. 



POOLED FIRST- AND SECOND-YEAR DEVELOPMENT 



8 



