Managing for Sustainable Blue Oak Populations

Blue oaks (Quercus douglasii Hook & Arn) are native to California and are found as far north as the Sikiyou mountains (Siskiyou and Del Norte counties) and as far south as the Tehachapi mountains (Kern County; McCreary et al., 2011; Fryer, 2007). They can be found in areas receiving only 10 inches of rain and as much as 60 inches of rain ( UC Oaks - Blue Oak Woodlands ) and from 1,600 to 5,900 feet in elevation (Fryer, 2007).

Blue oak woodlands are estimated to cover more than 2.9 million acres in California (Christensen, Campbell, and Fried, 2008). They provide food and shelter for over 270 wildlife species ( UC Oaks - Blue Oaks Wildlife ) and are a critical part of watershed function. Blue oaks anchor the soil and reduce erosion. Over 80% of oak woodlands are privately owned and a majority of them are working landscapes actively used to produce forage for livestock production. Oak woodlands are a favorite place for recreation and provide a valued viewshed. When compared to grasslands, oak savannahs and oak woodlands hold more carbon in the soil, support a greater diversity of species, and are more resistant to invasive species (Eastburn et al., 2017).

However, in some areas of California blue oak regeneration is not keeping pace with attrition (McCreary et al., 2011) and the majority of oak trees are over 50 years old, with some estimated to be over 600 years old ( UARK ). The primary concern centers on a lack of saplings. In most blue oak woodlands there are older trees and seedlings sprout from acorns, however the seedlings often do not make it to the sapling stage (McCreary 2009). After decades of research, five primary potential causes of the slow regeneration have been identified (McCreary 2009):

1. Introduction of non-native annual grasses (lack of available soil moisture)
2. Grazing management
3. Increased rodent populations
4. Altered fire regime
5. Changing climate

But like many things in science...it depends. The causal factor restricting the transition from seedling to sapling is most likely a combination of multiple factors that vary from site to site. For example, grazing intensity and timing influence cattle behavior, species selection and overall grazing pressure. Spatial factors influencing regeneration could include soil, precipitation, slope and aspect.

Attempts to artificially regenerate oaks have met with mixed success. Like regeneration, the mixed success is most likely attributable to multiple factors that vary from location to location. However, careful weed management and protecting the seedlings from animal damage have proven to be the most successful. Removing vegetation in a circle (about four to six feet) around seedlings for two to three years is recommended with follow-up as necessary to reduce insect and rodent damage. Weed control around oak seedlings and plantings has the added advantage of reducing desirable habitat for insect and rodent herbivory (McCreary 2009). In some cases, summer irrigation of seedlings increased survival and growth. Clearing surrounding vegetation and laying mulch down around the seedling may be just as effective as irrigation (McCreary 2009). In some cases top pruning of seedlings enhanced seedling growth and survival, but this treatment has not be well-researched in California. Oral history from Native Americans suggests that oak seedlings were regularly browsed by deer (R. Goode, 2021)

Anecdotally, many land managers across California have experimented on their own with planting oaks, planting acorns, and protecting oak seedlings that have naturally sprouted. All of these attempts have met with mixed success.

During and after the most recent severe drought (2012-2016) inquiries to UC Cooperative Extension regarding oaks increased. In response to these inquiries, Julie, Devii and, Rebecca initiated a survey to gain feedback from landowners regarding their oak concerns. Detailed results of the survey can be found  here . Out of 49 responses, 22 were ranchers, 9 were agency staff, 2 were consultants, 2 were academics and the remainder were other or unknown. When asked why oaks were important over 30 respondents cited them as important habitat. Other reasons stated were shade, beauty, ecosystem services, California heritage, old/disappearing, erosion control, native, and firewood. Primary interests of survey respondents focused on why oaks die, how to prevent them from dying, how to encourage new oak growth, the impact of fire, historic oak population dynamics, and controlling invasive species in oak woodlands. While responses to this survey were limited, results show a clear trend that land managers are interested in learning more about oaks, oak woodlands, and how to manage them sustainably.

As a result of the severe drought, many land owners witnessed oak trees die on their land. We received many questions regarding the cause of oak mortality and if there was anything that could be done to keep their oaks alive. An oak expert from UC Riverside, Tom Scott, shared that oaks grow for about half their life and are slowly dying for the other half of their life. In other words, for many of the oaks that died as a result of the severe drought, there was nothing that could be done.

Oaks die for many reasons including drought, competition, pests, disease, and of course, natural senescence. Sweicki and Bernhardt (2020) looked at the most recent mortality among blue oaks following the severe drought and concluded that the causes were primarily drought-related. Das et al. (2019) sought to quantify blue oak losses due to the severe drought and determined that blue oak basal area dropped by about 26% and up to 19% of dead oaks died recently.

The UCANR Integrated Hardwood Range Management Program conducted a great deal of the research on oaks until it dissolved in 2010 ( UC Oaks - About Us ). In addition to developing a large body of research, they disseminated the information in a practical way to land managers through workshops, newsletters, and field days. The results of our recent oak survey suggests that there may be a need to begin conducting educational workshops and field days again. As a first step towards achieving that goal, UC updated the website with a fresh look and improved navigational features.

Livestock producers and other land managers have expressed interest in continued research to address oak management challenges including how to diagnose and prevent causes of oak mortality, prolong mature oak lifespans and increase young oak recruitment and success. There is tremendous interest and opportunity for oak outreach in California.

The 8th California Oak Symposium is scheduled to be held in March of 2022. Beginning in 1979, the California Oak Symposium is held every five to seven years and is a place where the latest research and outreach on oaks in California can be shared. While it would be a new tradition, the symposium may be a venue for outreach to groups and individuals interested in learning more about oak regeneration and hands-on skills to implement planting projects. The California Naturalist program is another potential venue for outreach. It is a statewide program that "uses a science curriculum, hands-on learning, problem-solving,  citizen science , and community service to instill a deep appreciation for the natural communities of the state and to inspire individuals to become stewards of their local resources." We are currently in discussion with the California Naturalist program regarding the potential for partnering to conduct outreach at the California Oak Symposium and statewide through the California Naturalist program.

Our research seeks to achieve two primary goals: 1) Assess the current status of regional oak populations and track oak mortality over time and 2) Determine which methods are most effective regionally for regenerating oaks and supporting the transition from seedling to sapling.

We will establish 1/10 acre plots in Kern, Tulare, Fresno, Madera, San Benito, Santa Cruz, and Monterey counties. Plots will be span elevational gradients whenever possible and will be sampled every five years in late summer. Trees within plots will be rated on a scale of 0 to 4, where 0 represents a dead tree and 4 represents a tree that has 91 - 100% of it's canopy green (growing/thriving). One tree within each plot will be selected to facilitate tracking trends in growth from year to year. This tree will be visited and sampled each year. Individual branches will be marked and measured so that stem elongation can be measured the following year when we return. This will provide site-specific insight into the impacts of weather variation from year to year.

Methods for supporting oak regeneration regionally will be tested across multiple sites that represent multiple soil types, slope, aspect, and shading. These methods are not currently written. Before these methods are developed we will speak with local land managers who have tried to facilitate the oak transition from seedling to sapling and hear from them about what they did, what worked and what didn't. We will seek to learn more about weather patterns and grazing management at the time when each land manager was conducting their work with oaks. We also hope to partner with local tribes to learn cultural practices that may benefit oak seedling success.

Current climate change predictions for California forecast an increase in droughts and other severe weather. The simple truth is that there is only so much we can do to support the current population of oaks. We need young, new trees to take their place and effective management practices to achieve this goal. Ultimately, our goal is to develop research projects that address current gaps in knowledge and provide practical solutions land managers can implement to support their current oak populations and improve oak recruitment success.

Christensen, Glenn A.; Campbell, Sally J.; Fried, Jeremy S. (2008).  "California's forest resources, 2001–2005: five-year Forest Inventory and Analysis report" .  United States Forest Service , Pacific Northwest Research Station: 40–46. Gen. Tech. Rep. PNW-GTR-763. (pg 43)

Das, A.J., N.J. Ampersee, A.H. Pfaff, N.L. Stephenson, T.J. Swiecki, E.A Bernhardt, P.K. Haggerty, and K.R. Nydick. 2019. Tree Mortality in Blue Oak Woodland during Extreme Drought in Sequoia National Park, California. Madrono 66:4 164-175

Eastburn DJ, O’Geen AT, Tate KW, Roche LM (2017) Multiple ecosystem services in a working landscape. PLoS ONE 12(3): e0166595. doi:10.1371/journal.pone.0166595

Fryer, Janet L. 2007. Quercus douglasii. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: https://www.fs.fed.us/database/feis/plants/tree/quedou/all.html [2021, January 30]

Goode, R. 2021. Native American Burning for Cultural Resources. California Rangeland Conservation Coalition Annual Summit. 1/28/21. (link coming soon)

McCreary D, Tietje W, Davy J, Larsen R, Doran M, Flavell D, Garcia S. 2011. Tree shelters and weed control enhance growth and survival of natural blue oak seedlings. Calif Agr 65(4):192-196.  https://doi.org/10.3733/ca.v065n04p192 .

McCreary, D. 2009. Regenerating Rangeland Oaks in California. UCANR Publication 21601e. https://oaks.cnr.berkeley.edu/wp-content/uploads/2018/09/Regenerating-Oaks.pdf

Rao, D. 2020. Here's What People Want to Know About Oaks. UCANR Blog post 39546. Published March 1, 2020. https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=39546

Swiecki, T. and E. Bernhardt. 2020. Recent Increases in Blue Oak Mortality. Phytosphere Research. https://oaks.cnr.berkeley.edu/wp-content/uploads/2020/04/Swiecki-Bernhardt-Oak-Mortality-4.2-2020.pdf