Willow and Aspen in the Ecology of Beaver on Sagehen Creek, California

DOCTORATE DISSERTATION: A four year study was made of the introduced beaver on Sagehen Creek, a tributary of the Little Truckee River, in the northern Sierra Nevada. The purpose of the investigation was to examine and compare utilization of willow and aspen in colonies where the supplies were strikingly different. Aspen, which arose following logging fires, was cut by the beaver as long as available. The heaviest cutting occurred close to shore and was very light at distances of more than 100 feet from the water's edge. The amount of aspen, measured in terms of weight of bark and twigs, used by a beaver was correlated roughly with availability, being highest where aspen was most available, lowest where least available. The beavers cut most aspen during the late summer and early fall when they were storing their winter food supplies. In addition to this major rhythm in activity, an unexplained coincidence of minor spurts and lags in cutting was noted in two colonies over five miles apart. Although aspen sprouts after cutting, the new shoots were eaten by sheep which grazed the Sagehen basin during the summer. Consequently, the supply is constantly diminishing, primarily because of beaver cutting, secondarily because sheep kill the new sprouts. As aspen diminished in abundance the beavers cut increasing amounts of willow. Some willow, however, was cut even in areas where aspen was still abundant. Willow cutting exceeded willow growth in two of the three beaver colonies studied. Although it is a vigorous sprouter, willow may be locally exhausted by heavy use and the beaver then gradually shift to adjacent sites. Willow on the original sites can then recover. Following the elimination of aspen, the beaver population on this stream will probably stabilize itself at a level considerably below its peak and will subsist on willow, shifting its sites of cutting on a type of rotational sustained yield basis as local sites are exhausted.