Introduction
Oak trees are an important California natural resource, but in many areas they are not regenerating naturally. In addition, clearing land for economic reasons is reducing existing stands in many locations.
Artificially regenerating oaks is one way to help conserve this important natural resource. In pursuit of this objective, regeneration techniques for blue and valley oak are being studied in six counties: Mendocino, Yuba, Contra Costa, Madera, San Luis Obispo, and Monterey.
The regeneration studies employ both acorns and nursery stock planted within fenced exclosures that exclude deer and livestock. This tactic is necessary to prevent seedlings damage that large herbivores can cause, when unmanaged, and permit examination of other factors that reduce seedlings survival.
Threats to Seedling Survival
Competition from annual grasses and forbs for soil moisture has been shown to be major threat to seedling survival. Controlling weeds with herbicides greatly increases emergence and survival of oak seedlings from directly seeded acorns. Transplanted nursery stock cannot be expected to survive without weed control.
However, weeds are only one of many problems facing a young oak seedling. Small mammals and insects such as ground squirrels, gophers, and grasshoppers, also are threats. In many locations screen protection must be provided to reduce seedling losses. When used in combination, weed control and above ground screen protection give oak seedlings a good chance to survive. The screens protect against depredation. Weed control, in addition to reducing moisture stress, eliminates small mammal and insect habitat in the immediate vicinity of oak seedlings.
Gophers, found nearly everywhere, present a special problem. Above ground screens offer no real protection against their depredations. But weed control, especially elimination of tap-rooted forbs such as filaree and mustard, helps to discourage their presence by eliminating a food source.
At this time real control of gophers demands a more active program. Poisoned grain placed directly in burrows has been effective at the several study sites. However, this tactic is effective only so long as the program is maintained; when gophers are killed, new individuals move in to claim the territory. As part of the ongoing research program, screen protection below ground is being tested as a barrier against gophers.
Research Results
Above ground growth of surviving seedlings in our study was slow during the first two growing seasons. Such slow growth may reflect efforts to establish adequate root systems. Low rainfall in 1986-87 probably had some influence also. Slow growth was common to both blue and valley oak, although the latter grew faster. This was possibly due to the more favorable sites on which it grows or a result of inherent characteristics.
During the third growing season (in this case the dry 1987-1988 season) both species demonstrated dramatic above ground growth with many individuals more than doubling their height. Depending on location, some blue oak seedlings now are 2-3 feet tall, and a few valley oak seedlings in San Luis Obispo County are over four feet in height. With a few more years’ protection, surviving plants in our study should be well enough established to reach maturity.
Artificial regeneration of oaks in California requires a significant investment of time, labor, and money, but if carefully done, it can supplement nature’s efforts and help to conserve a unique vegetation heritage.
prepared and edited by John M. Harper and Richard B. Standiford