Predation in Restoration and Mitigation

Oaks ‘n’ Folks – Volume 11, Issue 1 – June, 1996

From acorn to tree is a “tough haul” for most oak seedlings.Both survival and growth of blue and valley oaks can be severely limited by uncontrolled competition from annual grasses and forbs. But interference from “weeds”, while a major problem, is only one threat and, perhaps, not the first that potential seedlings must face. Large herbivores, such as livestock and deer, are also acknowledged problems. These mammals eat both acorns and seedlings. In the fall, acorns are an important source of food for deer, and their foraging can severely deplete acorn crops. Wild pigs represent another threat.

Populations of this animal are increasing rapidly, and pigs are efficient harvesters of fallen acorns. Pigs also dig up planted acorns. In one blue oak regeneration study, acorns were planted before erection of a protective fence around the study site. In an overnight attack, wild pigs dug up and ate most of the acorns. Fencing, either permanent or temporary, can be used to protect planted sites from large herbivores,and management of livestock can help limit the impacts of grazing by these animals. As a result, large mammals may be less a threat than smaller creatures.

Foraging by small mammals accounts for considerable loss of acorns. Ground squirrels have been observed climbing oak trees to cut small branches with attached acorns. After the acorn clusters fall to the ground, individual acorns are collected and stored. Planted acorns may also be dug up by any of several rodents and other small mammals. In experimental plantings, ground squirrels and deer mice have taken all or nearly all unprotected (by screen cages) blue oak acorns before seedling emergence; and in one case, raccoons were suspected of destroying a blue oak planting.
Transplanted seedlings also are targets. In one planting, 3-month old valley oak nursery stock was decimated by ground squirrels. While half of these seedlings were protected by window screen cages, the rodents destroyed many of the cages to get at the “protected” seedlings. Control of the local squirrel population was necessary to save the planting.

During the first few growing seasons, attacks on oak seedlings by the larger rodents and jack rabbits have been discouraged by closed window screen cages and open, rigid plastic seedling protectors with a larger mesh. The latter protection is adequate where the threat from insects, primarily grasshoppers,and small meadow mice, or voles, is minimal. In experimental plantings of blue and valley oak, protection from small mammals and insects has made a great difference in survival. However, when seedling growth exceeds the height of protection, closed window screen cages must be opened. Seedlings are then exposed to attack, but early protection has generally allowed them to develop and grow large enough to survive.

The described problems with small mammals represent above ground threats.Pocket gophers can be a serious below ground threat , and where these rodents are common, oak seedling losses will occur.

Because pocket gophers live underground, their threat usually requires more effort to address. At sites where these animals are abundant, tubes of rolled window screen placed in augured holes have been used successfully to protect blue and valley oak nursery stock. Losses among unprotected seedlings in these plantings were as high as 95 percent the first season. Observations suggest that young oaks may be vulnerable to gopher attack until they are at least 2 inches in diameter at the soil surface.
When oaks are planted for restoration and mitigation, consideration must be given to all potential threats to success. Weed control is a primary requirement, but protection from herbivores is also important. Management and fencing usually are adequate to control large animals. But more intensive efforts may be required to protect from rodents, other small mammals and insects. Such protection may represent the difference between success and failure.

prepared and edited by Richard B. Standiford and Pamela Tinnin