Oaks ‘n’ Folks – Volume 6, Issue 1 – April 1991
Plantings of blue and valley oaks, both acorns and nursery stock, face several problems that threaten survival. Weed control may be one of the most important of these. But predation by small mammals and insects also causes significant losses in new plantings.
Among small mammals, pocket gophers are one of the chief threats. They take seeded acorns and can destroy oak seedlings up to several years old by cutting roots and girdling stems. Poisoning and trapping are successful control methods, but they usually must be repeated several times each year to provide protection; gophers are territorial, and vacant territories may be reoccupied if there are other gophers in the area.
Continued vigilance is necessary when employing conventional control methods. The rodents may go unnoticed until depredations begin, especially where grasses and forbs obscure evidence of burrowing activity.
During 1990, at Lopez Lake County Park in San Luis Obispo County, gophers destroyed 15 percent of seedlings that emerged from a 1987 fall seeding of valley oak acorns. By fall 1990, mortality measured in a 1988 seeding at the same location showed that gophers had taken an average of two-thirds of all emerged seedlings in two treatments and were responsible for over 90 percent of all mortality. In this seeding, 90 percent of gopher predation occurred where weeds were uncontrolled, nearly half again as much as took place where weed control was practiced.
Seedlings that survive weed competition may face a greater threat from gophers than seedlings growing with weed control. Where control of herbaceous growth removes most of the food, the site may be less attractive.
Nursery stock (2-3-month-old valley oak seedlings) at Lopez Lake also has suffered loss from gophers in spite of early control efforts. During 1990 alone, gophers destroyed 12 percent of the seedlings transplanted in early March 1988.
These losses encouraged application of a protective measure not widely used. In 1989, half of all nursery stock planted was protected with individual underground tubes, 2 inches in diameter and 18 inches long, made from rolled and stapled aluminum window screen. Planting holes were augered to a depth of 18 inches, the tubes placed in the holes and partially filled with tamped soil, and the seedlings planted in the last 6 inches of the tubes. Manufacture and installation of each tube represented little more time than that required to set a gopher trap.
The screen tubes, extending well below the 6-8 inch depth of most foraging burrows, have proven highly successful. By fall 1990, over 90 percent of the unprotected seedlings had been lost, but the loss among protected seedlings was less than 10 percent. Of this mortality, gophers were responsible for nearly 100 percent among unprotected seedlings and less than 15 percent among those protected.
Although gophers are a clear threat to oak seedling survival at Lopez Lake, small mammals on the surface, such as rabbits, also cause damage. Seedlings currently protected with rigid plastic protectors (screens) or protected early during establishment with these screens now are more than 50 percent taller, on average, than those unprotected.
Threats to oak seedling survival are many. Weeds, grasshoppers in some locations, and small mammals, especially gophers, can combine to devastate new oak plantings. Control of one or more of these problems usually will be necessary, if oaks are to survive.
prepared and edited by John M. Harper, Richard B. Standiford, and John W. LeBlanc