Finding Goldilocks: a survey of the factors limiting natural oak recruitment.

Final report for FW23-419

Project Type: Farmer/Rancher
Funds awarded in 2023: $20,005.00
Projected End Date: 05/31/2024
Host Institution Award ID: G283-23-W9982
Grant Recipient: Far View Ranch Inc.
Region: Western
State: California
Principal Investigator:
Alex Palmerlee
Far View Ranch Inc.
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Project Information


The blue oak woodlands of California account for one-third of the grazed land in California and harbor more wildlife than any other ecosystem in California (Borchert et al 1991, Pavlik et al 1991). There is also a clear lack of new oak recruitment (Adams et al 1992, Swiecki and Bernhardt 1997). Recently, Lopez-Sanchez et al. (2014) concluded that “further research on the conditions that would ensure tree regeneration in Californian rangelands is needed in order to determine thresholds of grazing density and timing.” 

We hypothesis that many factors that impact oak recruitment do so as a correlate of grazing pressure. We propose, therefore, that there is a  ‘Goldilocks’ grazing condition where grazing and recruitment can coexist with minimal costly interventions. Our proposal combines rancher knowledge and statistical analysis of comprehensive field surveys to find better recommendations for grazing management to aid natural oak recruitment.

Because of the large scale of blue oak woodlands (3-million hectares: Bollsinger 1988) it is imperative that we study landscape-scale solutions to the problem of recruitment. Much of the blue oak range coincides with cattle grazing and grazing remains one of the few tools that can be applied at a large spatial scale. Ranchers need recommendations for supporting the health of their blue oak woodlands as it provides valuable shade and forage for livestock.

Our field days, technical bulletin, rancher fact sheet will aide ranchers in making land-management decisions that can benefit their oak woodlands and their grazing operation, ensuring another generation of young oaks for the future.We will be reaching out to many large landowners in addition to those listed above, some affiliated with NRCS and some not. While the range and scale of the survey is broad, the technical aspects are simple—making the project feasible for producers.

Project Objectives:
  1. Locate natural oak recruitment microsites on 15-20 ranches
  2. Within microsites, sample conditions known to limit oak recruitment
  3. Interview landowners regarding ranch management
  4. Compile data and analyze for trends
  5. Recommended management changes or areas for future research based on results
  6. Draft and submit publication
  7. Draft and distribute a technical bulletin with a summary of findings for agencies (RCD and NRCS offices) and non-profits (Audubon Ca., The Nature Conservancy).
  8. Draft and distribute a fact sheet for landowners and interested groups (California Cattlemen's Association)
  9. Host two field days for stakeholders at participating ranches
  10. Present findings at the annual conference of the California Association of Resource Conservation Districts and/or at meeting(s) of the California Cattlemen's Association


Click linked name(s) to expand/collapse or show everyone's info
  • Pelayo Alvarez
  • Carrie Wendt - Technical Advisor
  • Truman Young, Miles DaPrato


Materials and methods:

This project is innovative because, whereas past studies have crossed and combined a multitude of factors assumed to limit recruitment in order to propose better land management strategies, the practicality of the suggested practices is often incompatible with large-scale impacts because of costly interventions (irrigation, tubes, fencing). These interventions are essentially creating micro-sites favorable to oaks, but there is a gap in the understanding of why micro-sites already exist on many cattle ranches where oaks recruit naturally. Furthermore, because blue oaks are so slow growing, initiating a study to control for various variables doesn't prove that seedlings can make it into the critical sapling stage and onto maturity. Our study looks at the natural experiment which has already taken place and shown us where oaks recruit naturally. Therefore, our only job is to tease out the factors that make one microsite preferable and another microsite next door not.

Because our study took place along a continuum of grazing pressure, we were able to assess site qualities that promote natural oak regeneration. From these results we developed suggestions for grazing management practices. Ultimately, the goal is to help landowners take advantage of the specific conditions on their ranches that encourage recruitment without expensive intervention.

For this study, we secured funding for a baseline of 15 ranches. Adding this SARE grant allowed us to reach 9 more ranches with identified recruitment microsites. This provided us with critical redundancy in our data. Because so many factors are at play, the more microsite replicates we had, the more we were able to tease apart the various factors limiting recruitment. Indeed, even these data, over 60 control and 60 recruitment sites, was at the low end of what was necessary.

For our purposes, we defined a ‘recruitment area’ as any site containing individuals of sapling size (>140cm) with a density greater than 5 individuals per 20 square-meters. A sapling was any individual with a basal diameter of one centimeter or more and a height >140 cm and <6m, based on research suggesting that these thresholds are common with survival through the sapling stage (Sweiki and Bernhardt 1998,  Phillips et al 2007). We used an already awarded NRCS Conservation Innovation Grant to survey ranches for suitable microsites. With microsites identified, we returned to collect data from each site. SARE funding allowed us to expand this experiment to additional ranches and additional microsites providing more strength to identified patterns as well as adding a workshop.

Methods and Objectives:

  1. Identify landowners.
    • Working with our partners at local RCDs and NRCS offices, along with several non-profit partners (Audubon California, Point Blue Conservation Science), as well as rancher referrals, we developed a list of willing landowners. We estimated at least two recruitment sites per ranch based on previous observations. In order to build statistically significant models, we located and surveyed 60 individual recruitment locations. Reaching a large number of ranches (defined by property ownership), allowed for large geographic differences and ensured at least 60 sample locations.
  2. Set up survey
    • Within the participating ranches we relied on the landowners’ knowledge, personal experience, satellite imagery, and field scouting to identify ‘recruitment areas’ that fit our definition for natural recruitment.
    • In recruitment areas, we measured many factors (vegetation, rockiness, slope, aspect, shade, number of mature trees, count of dung) including a destructive sample to age the small trees. We noted if the recruitment area had multiple age classes (in particular the rare sapling and young tree stages), in this way we know that not only do oaks recruit in those areas, they thrive into maturity.
    • Following the methods of Sweiki et al. 1993, we used the presence of particular size classes as the key determinant for a ‘recruitment area.’
    • Recruitment areas varied in size. To ensure that sampling didn't over report the same phenomena, replicates were at least .25 miles apart, to minimize spatial auto-correlation.
    • Each ‘recruitment replicate’ was paired with a ‘non-recruitment replicate.’ Non-recruitment replicates were located at the nearest site where no recruitment was visible. We repeated all measurements at both recruitment and non-recruitment sites.
  3. Measurements
    • Landowner interview: land use history: grazing, discing, other events such as fires.
    • Document water and supplement (e.g. salt lick) locations
    • Create a Cumulative grazing score (c.f., Swieki et al 1997): sum of
      (months grazed) × (relative stocking) × (season factor) for the period 1990-2022. Relative stocking ranges from 0 (none) to 3
      (high). Season factor is 1 for winter and 2 for summer or year-round.
    • Dung counts, as a measure for grazing pressure, taken by transect survey after the rainy season (c.f., Young et al 2005). This will help determine if static barriers (fences, rocky outcroppings) are impacting cattle pressure.
    • Percent bare ground, rock cover, and plant cover by visual estimate.
    • Shade using a percentage (full sun, partial sun, partial shade, full shade).
    • Presence/absence and species of potential nurse plants.
    • Proximity to mature oak tree using a range finder
    • Soil type via NRCS soil map (
    • Slope via percentage estimate
    • Aspect by compass app on smart phone
    • Confounding factors: proximity to road or ditch, salt licks or water troughs, presence of browsing obstacles (relic fences, debris, rock outcroppings), or other anomalies.
  4. Analysis
    • Statistical analysis will take place in collaboration with the UC Davis Graduate Group in Ecology, and will include ... [multivariate correlation techniques, and model building with Akaike's information selection criterion (AIC)]
Research results and discussion:
  1. All surveys completed at 24 ranches. Good relationships were built with many landowners, including ones that the PI now regularly visits to collect seed for habitat restoration, and who contact the PI for questions regarding their oak resources.
  2. Two workshops were conducted with two partners (Audubon California and The Nature Conservancy). Both were well attended and both partner organizations have expressed interest in continuing an annual workshop on the issue. Many attendees have reached out to the PI about issues seen on other ranch properties, ideas for restoration projects, and with their own ideas on how to make things better.
  3. Many landowners expressed interest in an option to cage individual trees on their property to allow for escape from browse pressure. This was a practice identified by our research as an important next step. We have also put a new SARE request for funding in to research the efficacy of this practice with the hope that we can demonstrate the value of cages and also understand more about the duration of grazing a seedling would need.
  4. The fact sheet we produced based on our research was distributed to over 50 individuals directly via our workshops and sent electronically to a much broader audience.
  5. Our hypothesis: that natural refugia on cattle ranches exist due to factors that artificially limit the grazing pressure on that area. This hypothesis did show statistical significance because there was a positive relationship between natural oak recruitment and rockiness, AND a negative relationship between dung presence and rockiness. Recruitment graphs Therefore, oaks recruit more in rocky areas and cattle spend less time in rocky areas. While we cannot determine direct causality (there may be other factors about rockiness that are good for oaks) there is certainly a very real suggestion that cattle harm oaks and less cows means more oaks. This is also borne out in the seedling growth rate data, which show that on lightly grazed ranches saplings grow faster than on moderate or heavily grazed sites.Growthrate_x_Grazingpressure
  6. One landowner who heard about out study but did not participate, has decided to fence off a large area of the ranch and only graze it once every three years, one of the anecdotal findings we found.
  7. Another landowner who heard about our study paid $7000 to have their little oaks caged and study the resulting growth.
  8. Our findings on oak growth rate have been extremely eye opening. Through the process of aging our samples we made a connection with a dendrochronology lab at University of Arkansas. Dr. David Stahle has a 30+ year career of aging these oaks and was impressed by the data we collected on small trees, since he has only cored trees at breast height (meaning all the rings from the tree's early life are not present). There has only been an assumption of how old it took these individuals to reach breast height. Now, he has already visited the PI's ranch to collect more data and taken back another batch of samples to study at his lab.
  9. The issue of oak regeneration is certainly of great concern to all of the constituents involved in this project. From landowners and non-profits to government agencies, all want solutions to this problem and at the greater spatial scale the better. We're working to continue to learn more about the early lives of these long-lived oaks so that we can better inform land management, restoration, and conservation.


Participation Summary
23 Producers participating in research

Research Outcomes

Recommendations for sustainable agricultural production and future research:

There are several key takeaways from this project. They are detailed in the 'Fact Sheet' distributed to partners, participating landowners, and attendees of workshops.

  1. Below 1,500 feet elevation, natural oak recruitment is rare. Where it does occur, it is most often associated with rockiness. Of all of the metrics we analyzed (slope, aspect, vegetation cover, presence of woody debris) rockiness and shade showed a statistically significant relationship to blue oak recruitment. Areas without recruitment were less rocky and less shady. This is made more interesting by the fact that rockiness also had a significant impact on dung count: there was less dung in rocky areas. These two results combined support our hypothesis that natural oak refugia on cattle ranches exist primarily as a function of reduced relative grazing pressure.Recruitment graphs
  2. Small oaks are not necessarily young oaks. Many landowners took us to areas on their ranch where they had 'young' oaks. As part of the research, we took a destructive sample, with landowner permission, at each recruitment area. After aging these individuals it was clear that many small oaks are quite old. The variability was so wide that a 1/2-inch diameter sapling could be between 3 and 32 years old. Our starkest example of this was an individual 17-inches tall that was born in 1959, making it 64-years old. These samples highlight how variable growth rate can be. Furthermore, we found significant evidence that growth rate was directly related to grazing level: oaks on ranches with lighter grazing pressure grew faster.Growthrate_x_Grazingpressure Oak Growth Rate
  3. The biggest area of concern from this research is that the problem of oak recruitment on rangelands may be far more dire than once believed. Of the sites we studied, we looked for one key metric: release from grazing, or individuals that had made it over 2-meters in height and could therefore be considered released and likely to reach maturity. Many individuals, because of what we describe as the 'browse-trap,' may be so perpetually and chronically browsed that they represent a zombie population that will die before reaching reproductive size.
  4. There are areas of hope. There was anecdotal evidence that grazing once every three years was compatible with oak regeneration. If this was found to be true, landowners could be incentivized through funding to reduce herd size and manage their properties with an annual rotation where pastures were rested 2 of every 3 years. This is an area we see much need for further research, and have already submitted another SARE application to fund that research.
  5. Another option for immediate action is that existing saplings, while rare, should be caged to allow release above browse height. Planting oaks is very expensive as it requires large investments in time and materials, namely irrigation, tubes, stakes, and protection from cattle. By caging existing oaks, landowners can provide the best opportunity to establish a new generation of oaks on their property. What we need to know, is how long trees need to be caged to escape the browse height. Certainly we expect that trees that have been suppressed the longest will show the most dramatic increase in growth as they are older and therefore have more underground biomass to support a 'bolt' or release upward. Ranchers could be incentivized to install cages and, with this additional information, move cages to new trees after the required time. Many landowners at our workshops were eager to hear about this option. We have submitted another SARE application to address this question as well.
1 Grant received that built upon this project
30 New working collaborations

Education and Outreach

4 Consultations
2 Curricula, factsheets or educational tools
1 Journal articles
2 Workshop field days

Participation Summary:

4 Farmers participated
36 Ag professionals participated
Education and outreach methods and analyses:

We held two workshops, one on each side of the Central Valley for convenience. The first, hosted by Audubon California, was very well attended (40 individuals). Their Bobcat Ranch is located in Yolo County, close to the Bay Area and Sacramento. The second, hosted by The Nature Conservancy, did not draw as many attendees (11), but those that did come were adamant about their adoption of new practices. Both workshops were held in the field on cattle ranches that participated in our study. Attendees were eager to share their experiences and ask questions about their own management of oak woodlands.

Education and outreach results:
  1. Field workshop was a great format, a presentation of findings in a conference room would not have been as effective
  2. At least 4 ranchers said they would adopt some form of protection for their oak woodlands. 2 agency partners reported modifying current oak management/restoration projects based on the workshop.
  3. One NRCS employee has been in contact since the workshop with his observations in the field on other ranches
  4. 4 participants from the second event emailed with follow up questions, requests for further information and assistance.
  5. Particularly in the second workshop, because it was smaller in size, we were able to make a large portion of the workshop a 'round table' discussion where all were able to share: a) something they'd tried in protecting or restoring oaks, and b) Something they're eager to try after the workshop.
4 Farmers intend/plan to change their practice(s)
1 Farmers changed or adopted a practice

Education and Outreach Outcomes

Recommendations for education and outreach:

Our two workshops were very well received. Agency partners, landowners, and non-profits in attendance were all very eager to adopt measures to safe-guard their oaks. By far, most ranchers were interested in efforts to cage existing trees. One rancher said, "If I could get $100 to cage trees I'd do 100 per year." It is worth noting that in a recent unrelated oak restoration project we will be spending at least $600/tree, if our survival is what we hope for. Another landowner said she would immediately fence off a large area with small oaks on her property and graze it once every three years. From the interest in our workshops it is clear that oak issues are a major concern in the state and there is ample area for further outreach. While there are and have been ongoing symposia related to oak woodlands in California, these are largely attended by folks in academia and results are not always accessible to the general public. After one of our workshops, our partner Audubon California reached out to see if we could make this an annual event because they saw the demand was so high. More funding is needed to support these efforts.

4 Producers reported gaining knowledge, attitude, skills and/or awareness as a result of the project
Non-producer stakeholders reported changes in knowledge, attitudes, skills and/or awareness as a result of project outreach
10 General public
2 Students
38 Ag Service Providers
Key changes:
  • Desire to cage existing trees with existing old fencing materials around the ranch.
    Desire to have an NRCS practice to fund caging

  • Willingness to immediately adopt oak safe-havens where cattle are fenced out and allowed in every third year.

Information Products

    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.