Several recent inventories of California oaks have indicated that for several native species there is insufficient natural regeneration. That is, there are not enough young seedlings becoming established to take the place of older trees that will eventually die. One of the species reported to have this regeneration problem is Quercus douglasii, commonly called blue oak. This species grows in vast ranges in the foothills surrounding the central basin of California. While the amount of natural regeneration of blue oak is highly variable and is greatly influenced by specific site factors, the recognition that there are areas where few young seedlings are starting to grow has prompted studies to test alternative seeding and planting techniques. These studies have revealed how difficult it is to establish blue oak seedlings and have shown that there is a wide variety of pests that damage acorns and young plants. Grazing from wild and domestic animals often prevents seedlings from developing into saplings. Predation of planted acorns and young shoots by rodents and birds can wipe out entire planted areas. And competition from grasses and forbs can severely limit soil moisture and result in seedlings mortality from drought.
Advantages of Stump Sprouting
While there is an obvious need to develop successful artificial regeneration techniques so that blue oaks can be re-established on lands where they have been eliminated, an alternative management approach in areas where they are currently growing is to incorporate a system of partial harvesting, with sprouting from cut stumps as a means of establishing new trees. From a regeneration standpoint, sprouting has several potential advantages over artificial regeneration.
First, since no acorns are involved, there is less risk of damage from insects, rodents or birds. Second, since the root systems of established trees are massive in comparison to those of planted seeds or seedlings, they have a greater ability to tap soil moisture and maintain a more favorable water balance during the hot, dry summers characteristic of the blue oak region. And finally, the initial growth rate of sprouts should be greater than that of planted seedlings since stored carbohydrates in the roots provide an abundant food reserve unavailable to small seedlings.A rapid initial growth rate may be critical to successful establishment since it should allow plants to grow above the level where they are susceptible to clipping or repeated browsing.
Unknown Details of Sprouting
While there is evidence that blue oaks can start to grow from a cut stump, little is known about the details of sprouting. For instance, it is not known if the ability of blue oaks to sprout is consistent over its range, or if sprouting varies with the season of cutting. It is also not known if stump height influences sprouting response.
To answer some of these questions, a statewide blue oak sprouting study has been initiated. Five regional study sites have been established, ranging from San Luis Obispo to Mendocino County. On each site, selected trees will be harvested in the summer, fall, winter and spring to determine what is the best time of year to cut trees to produce the greatest sprouting response.
In addition, trees will be cut at either ground level or at 3 feet. Since both deer and livestock will eat new shoots, browsing can prevent sprouts from elongating and growing into trees. We will compare the sprouting from different stump heights to determine if higher stumps produce sprouts that are less susceptible to animal damage. Finally, this study will compare sprouting in different regions of the state. Perhaps in some areas, stump sprouting might provide a reliable and easy method of establishing new trees, while in other regions it may be ineffective.
prepared and edited by John M. Harper and Richard B. Standiford
Douglas D. McCreary
Natural Resource Specialist, U.C. Sierra Foothill Range Field Station