- Author: Douglas McCreary
- Editor: Sophie Kolding
Towards the end of January, an unusual phenomenon occurred in the Sierra foothills - some of the blue oaks began leafing out. While the vast majority of trees remained leafless, one could see occasional light-green canopies interspersed with their bare neighbors. This is unusual in that the earliest leaf-out most people recall observing doesn’t start until late February or early March.
So what is causing this? Clearly the weather pattern this past winter has been very unusual in terms of precipitation. There was abundant early rainfall in October and November, followed by a December and January that were some of the driest on record. However, dormancy and its subsequent breaking are much more controlled by temperature than by rainfall. That is, a significant period of cold is necessary before most temperate plants resume growth in the spring. Photoperiod – or the relative daily lengths of light and darkness - also often plays an important role but this obviously doesn’t vary from year to year.
Has the temperature pattern been abnormal? The CIMIS (California Irrigation Management Information System) long-term data was compared with this past winter’s averages and nothing stood out as particularly different, although there is apparent, large variability from year to year. There was a day or two of somewhat colder-than-average temperatures in mid-January, but below-freezing temperatures in the foothills occur almost every year. Therefore, at this point, the cause of early leaf-out remains a mystery.
What about the impacts? It is likely that some of these “early leafers” will suffer frost damage. This occurred several years ago in mid-spring when there was an unusually late freeze and many blue oaks had their foliage killed back. Most of this occurred at mid-elevations, presumably because trees at higher elevations hadn’t leafed out yet, and those at lower elevations were so far advanced that their foliage was hardier. While this certainly wasn’t good for the trees, they were able to recover and produce a new crop of leaves within a month or 6 weeks and no increase in mortality was noted. It seems likely that, as a species, blue oaks have evolved individuals that leaf out at different times of the year so that at least a portion of their population can take advantage of different weather conditions. The “risk takers” might get hammered now and then, but under the right conditions, they can take better advantage of unusual weather patterns than their more conservative colleagues. In the event of significant warming in California as a result of climate change, these may even be better adapted to survive and prosper in a changing environment.
- Posted By: Sophie Kolding
- Written by: Jim Sullins and John Maas
Four head mature cows have been reported dead in Tulare County foothills from 2500 feet to 1000 foot elevations. All were in the vicinity of Blue Oaks with heavy crop of acorns.
Neil McDougald, Livestock Range Advisor of Madera County indicated that this year is similar to other years he has seen where there have been incidents of acorn toxicity. There is a very large acorn crop, and recent rains has led to some early green-up under Oak Trees which can result in some cows camping and eating an overload of acorns resulting in acorn poisoning.
Most cattle in California spend at least part of the year in areas where oak trees abound. Health problems due to ingestion of oak leaves or acorns are certainly not an everyday problem; however, when problems do occur they can be catastrophic. Several years ago, in a few northern California counties, about 2,700 cattle died due to oak toxicity1.
Some years have much higher production of oak acorns than others and this year, 2011 in the Foothill Ranges of the Southern Sierras, has been reported as one of the highest production years observed. In addition early rains may increase the risk of acorns by reducing the nutrient value of last year’s forage, as well as causing an early greening of grass under the oaks where acorns are accumulating.
During the week of October 10, 2011 four head mature cows were reported to be found dead in Tulare County foothills from 2500 feet to 1000 foot elevations. All were in the immediate vicinity of Blue Oaks with heavy crop of acorns. These cattle were not available for necropsy and cause of death was not confirmed, however oak acorn toxicity is suspected.
Oak toxicity symptoms usually appear when cattle eat 50% or more of their diet as oak (acorns, leaves, buds). Toxicity can be prevented by supplementing cattle with hay or other supplements when forage conditions are poor and acorns are abundant. A higher risk may occur if early or late snowstorms covers available forage and knocks down oak limbs with large amounts of buds and young leaves or acorns, be sure to start hay supplementation immediately. A delay of only a day or two could result in many deaths.
There are two recommendations to avoid cattle loss due to acorn toxicity. One is to provide supplement to draw cattle away from the acorns, so that their exposure decreases. The other is to remove cattle observed eating acorns from fields with high levels of acorns.
It is recommended that producers work with their local veterinarian and/or diagnostic laboratory, to be certain of the actual cause of death in all livestock losses. There are many other factors that can cause sudden death and although oak toxicity may be likely and immediate actions should be taken to prevent further deaths, a follow up with a veterinarian is often the best course of action to prevent further losses. For a more comprehensive discussion on identification and recommended actions for Oak Toxicity and Acorn Calf syndrome in cattle please refer to the link below.
Dr. John Maas, UC Davis Extension Veterinarian, informative article on oak toxicity (OAK TOXICITY UCD Vet Views California Cattleman, Jan 2001):
Blue Oak Acorn
Cattle Pasture amongst Blue Oaks
- Posted By: Richard B Standiford
- Written by: Neil McDougald, UCCE Range Advisor, Fresno/Madera Counties; Richard Standiford, UCCE Specialist, Berkeley; Ralph Phillips, UCCE Livestock Advisor Emeritus, Kern County
Fire is an important part of blue oak ecosystems (See: Fire in California's Oak Woodlands). Although we recognize that fire suppression on oak woodlands has increased the fire return interval, there are only a limited number of studies documenting pre-suppression fire frequency (see McClaren and Bartolome, 1989; Mensing 1992).
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.
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./span>
- Posted By: Jaime Adler
- Written by: Douglas McCreary and Jeannette Warnert
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.
“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.”
- Posted By: Richard B Standiford
- Written by: Rick Standiford, Forestry Specialist, UC Berkeley; Ralph Phillips, UCCE Livestock Advisor Emeritus; Neil McDougald, UCCE Natural Resource Advisor
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).
(lbs./10 sq. ft. of canopy)
No acorns visible on tree
Acorns visible only after close examination. Maybe 1 or 2 acorns are observed
Acorns are readily visible, but do not cover entire tree
Acorns are readily visible, cover entire tree. Limbs appear to sag from weight of acorns.
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.
Figure 2. Interior live oak acorn production by thinning levels.
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.