As part of a long-term blue oak thinning study in the Southern Sierra (see “Thinning Enhance Blue Oak and Interior Live Oak Acorn Production”), 49 trees were felled. Each tree was aged, and fire scars were dated. Fire scars were identified on 8 of the sample trees (16 percent of the population), allowing us to determine fire history in the stand from 1850 to the present.

Results and Conclusions:
Figure 1 below shows the dates of the 10 fire events that occurred between 1851 and 1996 on the study site in Tulare County near California Hot Springs. The mean fire return interval was 12.8 years (+/- 2.3 years), with a range from 6 to 25 years. There were no fires on the site since 1966.

Based on the age structure of the sample trees on this study site, less than 25 percent of the trees became established from 0 to 3 years after the fire, and another 55 percent were established 4 to 11 years after fire (Figure 2). These results show that the date of fire was much less important than the McClaran and Bartolome (1989) and Mensing (1992) studies, where most of the trees were established immediately following fire events.

Fire suppression has changed the the pattern of fire in blue oak stands. Fire, from both prescribed burns and from natural sources, is much less common in blue oak stands today. This results in large fuel build-ups, higher stand density and crown cover, and different stand structures. Instead of relatively frequent, low intensity fires, we now anticipate infrequent, high intensity fires. Strategies to re-introduce fire, or to achieve restoration of fire-resilient stand structures, is appropriate as part of a long-term sustainability strategy for blue oak woodlands.

Literature:
McClaren, M.P. and J.W. Bartolome. 1989. Fire-related recruitment in stagnant Quercus douglasii populations. Can. Journal of Forest Research 19:580-585.

McCreary, D.D. 2004. Fire in California’s oak woodlands. UC ANR on-line publication. 8 pp. http://ucanr.org/sites/oak_range/files/59574.pdf

Mensing, S. A. 1992. The impact of European settlement on blue oak (Quercus douglasii) regeneration and recruitment in the Tehachapi Mountains, California. Madrono 39(1):36-46.

Many blue oak trees in California foothills might be more accurately described as “silver oaks” this year. From a distance, they shimmer with a silvery halo. On closer inspection the outermost leaves are coated with a white to gray powdery fuzz.

The cause, according to Doug McCreary of the Integrated Hardwood Range Management Program at UC Berkeley, is powdery mildew. Powdery mildew, a group of fungi that causes a white, flour-like growth on the surface of leaves, is common on roses, begonias, grapes and many other ornamental plants and agricultural crops.

Powdery Mildew on an Apple Shoot

“People have called us worried that the affected trees may be showing signs of SOD, but this is clearly not the cause. SOD symptoms are far different, blue oak is not a SOD host and SOD is restricted to coastal forests,” McCreary said.

McCreary assures oak lovers that powdery mildew rarely kills the majestic trees. Even small seedlings that have all of their leaves severely infected usually survive and recover.

“Powdery mildew makes it more difficult for the affected leaves to photosynthesize and produce food, and if it’s severe enough, it can also result in the leaves distorting, curling up, dying and falling to the ground,” McCreary said. “But most affected trees will simply grow a new crop of leaves later in the summer or the following spring. And if weather conditions return to a more normal pattern next year, with little or no rainfall after March, it is unlikely that powdery mildew would continue to be severe or widespread.”

Some people may be inclined to treat affected trees with fungicides. However, these treatments are most effective when the symptoms first appear, which occurred weeks or months ago. It is also generally not recommended to treat trees in wildland settings. There are too many trees to treat and the potential environmental risks of applying fungicides across a large landscape can outweigh the benefits. Above all, McCreary said, don’t panic and cut down the trees, even if all their leaves fall off.

“The trees are still very much alive,” McCreary said. “Losing their foliage is just the oak’s way of dealing with an unwanted pest. By this time next year they should again be leafed out without that silver covering currently observable.”

The unusually wet March and April is at least partially responsible for the higher-than-normal incidence of powdery mildew in blue oaks, he said. Increased incidence of powdery mildew has also been reported on California black oaks and coast live oaks on the coast.

“Powdery mildew doesn’t need rainfall, but it is favored by warm conditions, high humidity and low light and it loves young, succulent foliage,” McCreary said. “Because California was blessed with above average rainfall this past spring, there has been – and continues to be – considerably more moisture in the soil. Under these conditions, oak trees will grow a ‘second flush’ of leaves, usually in May or early June, that is very susceptible to powdery mildew.”

Two thinning studies in the southern Sierra Nevada resulted in enhanced acorn production. A thinning experiment evaluating three levels of stocking for blue oak (Quercus douglasii Hook. & Arn.) was initiated in 1994 near California Hot Springs, California in Tulare County. The goal of the thinning was to evaluate how oak canopy overstory levels affected individual and stand tree growth, forage production, grassland diversity, and oak regeneration. The control level for the unthinned stand was close to 100 square feet per acre with a tree canopy level of 55 percent. The stands were thinned to 1/3 and 2/3 of the existing basal area level. In the first year after thinning, in addition to tree and grassland measurements, there was a measurement of acorn production by rating each tree using the four-class system developed by the Department of Fish and Game (see below).

Figure 1 shows the impacts of the thinning on blue oak acorn production. For ease of presentation, we show the proportion of trees with no acorns (Acorn Class 1), to the proportion of trees with any acorns (Acorns Class 2, 3 or 4). This shows that for 5 of the 7 years we surveyed acorn production, the thinning treatments had a significant impact in increasing the proportion of blue oak trees with acorns. The two years with non-significance due to the thinning (1997 and 2002) were generally poor acorn years for the region.

Figure 1. Blue oak acorn production by thinning levels.

A second thinning study was carried out on interior live oak (Quercus wislizenii A. DC.) near North Fork, California in Madera County. This was a dense (close to 100 percent canopy cover) that developed after a wildfire 40 years previously. The same three thinning treatment levels were carried out, namely control/unthinned (75 square feet per acre), and a 1/3 and 2/3 thinning. In addition to tree growth, regeneration, and forage production data, acorn yields were also evaluated using the same procedure described above. Figure 2 shows the results of three years of observations on interior live oak acorn production. Thinning resulted in a significantly higher proportion of trees with acorns in all three years surveyed.

Interior Live Oak Acorns

Discussion
We evaluated the fire history of the blue oak thinning area using fire scars, and found the fire return interval from 1851 to 1966 was 12.8 years (+2.3 years), with no fires on the site since 1966. We might speculate that the historical density of blue oak stands was much lower during the period of regular fires, and the thinning created more open conditions that mimic the effects of a period of frequent, low intensity fires. The live oak stands were quite dense, following 40 years of fire exclusion. The more open stand conditions created by the thinning for both species in these studies resulted in a higher probability that a tree would produce acorns. Blue oak produces acorns that mature in the fall of the first year after spring flowering, while interior live oaks produce acorns that mature in the fall of the second year after flowering. Despite the differences in the physiological processes guiding acorn formation between these two species, these two studies show that thinning may play a key role in the yield of acorns from our oak woodlands.

For almost a century there has been concern that blue oak (Quercus douglasii Hook. & Arn.), a California endemic, is not regenerating adequately in portions of its range. For the last 20 years, there have been concerted efforts to develop successful procedures for artificially regenerating this species so that it can be planted in areas where natural regeneration is unsuccessful, or in areas where it once grew but has been lost. These efforts have been successful in identifying procedures that will work, but unfortunately, such procedures often require intensive management and are costly. Since 80% of the oak woodlands in California are privately owned and the principal activity is livestock grazing, many large woodland owners and managers have marginal incomes and are reluctant to spend a great deal to regenerate oak trees.

An alternative, but untested, approach is to use naturally regenerating oak seedlings and take measures to promote their advancement to the sapling stage. This could be critical since research has demonstrated that the bottleneck for successful regeneration is often getting seedlings to grow into saplings. If successful, using natural seedlings could result in considerable savings because no effort or cost would be expended to collect acorns, or to grow and plant seedlings. An additional advantage would be that only genetically adapted plant material would be used, alleviating concerns about using “offsite” planting stock. Because of these economic, ecologic, and low input (i.e., less work) advantages, the development of techniques to advance natural regeneration holds great promise for being adopted and implemented by landowners.

To test this strategy, a study was initiated in 2007 at six field sites in the range of blue oak throughout the state. At each site, 144 naturally occurring blue oak seedlings were identified. Half of these were under the canopy of onsite trees and half were in the open. In addition, treatments included protecting seedlings with tree shelters and controlling weeds. When the plots were established, the height of each seedling was recorded. Yearly assessments of survival and height growth have been made every fall since establishment to evaluate the efficacy of the treatments.

After three years, seedlings in treeshelters have consistently grown taller than unprotected seedlings. In addition, the survival of those receiving an annual weed treatment has been significantly higher than those not receiving it. These results suggest that utilizing existing natural seedlings could contribute to increased blue oak regeneration at lower cost, thus improving the chances that this important species can be managed sustainably and conserved for future generations.