12 BULLETIN 1200, U. S. DEPARTMENT OF AGRICULTURE. 
The amount of seed destroyed by rodents also enters into the final 
result of the seed crop. Mice, chipmunks, and squirrels are very 
active in collecting and storing seeds and cones, their chief source 
of food, especially during the period of ripening. It has been noted 
that a white-footed mouse will eat 300 and a chipmunk 600 Douglas- — 
fir seeds a day when in captivity.° In regions where rodents are 
abundant, light crops of seeds are entirely consumed, as are also the 
seeds produced by scattered seed ‘trees. 
The average Douglas-fir tree produces about 24 bushels of cones 
per tree, with an average of 1,000 cones per bushel. Sometimes the 
number of cones produced is the direct cause of the variation in the 
amount of seed produced. However, diseased trees on poor soil 
produce almost as many cones as do sound trees and trees on good 
soil, and the variation in effective seed production is due entirely 
to the quality of seed produced. Other factors that effect the pro- 
duction of cones also effect the production of seed in direct propor- 
tion. The yield of cones per tree is greatest with medium-aged, 
good-sized trees that grow in open stands in warm localities. 
SEED DISTRIBUTION. 
The chief agents of distribution, or the determining factors in the 
migration or extension of the range of the species, are wind, animals, 
man, gravity, and sometimes in mountainous regions landslides and 
snowslides. | 
Wind had always been considered the prime agent in the migra- 
tion of conifers in the Pacific Northwest until investigations showed 
that its influence is not so far-reaching as was formerly believed. 
_An instance was noted of seed distribution from Douglas-fir trees 
about 125 feet tall at a time when the wind was blowing 15 to 20 
miles an hour. It appeared that at least 90 per cent of the seeds 
fell within 1 chain (66 feet) of the parent tree, and not over 5 
per cent were carried more than 2 chains. Occasionally seeds were 
carried great distances. Several seeds that were carried by the 
wind from 6 to 10 chains were gathered and found to be only 
empty shells, incapable of germination. This no doubt accounted 
for their separation from the other heavier seeds, which because of 
their weight continued their spiral course downward. Wind is a 
definite and persistent agent of distribution and occasionally car- 
ries seeds for long distances, but it usually is effective for dis- 
tances of only 3 to 5 chains from the parent tree. No dense stands 
of young growth, resulting from wind distribution, have been 
found at greater distances than these from seed trees. 
Rodents are among the important factors affecting the distribution 
of Douglas-fir seed, although in an indirect way. Rodents gather 
seed and store it in the forest floor for food, but do not carry it far 
from the seed trees. When they return, perhaps after a period of 
rain or snow,. they fail to find a good deal of the seed. Tests 
have shown that mice detect seed by scent, and that they are 
better able to find seed in mineral soil than in duff, especially when 
the duff is more than three-fourths of an inch deep. The seed 
buried by rodents furnishes the supply of stored seed needed for 
10 Willis, C. P. The Control of Rodents in Field Sowing. Proceedings of the Society 
of American Foresters, Vol. IX, No. 3. 1914. 
i 
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