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Plant Focus

Quercus peninsularis
A Red Oak (Section Lobatae) endemic to inland ranges of northern Baja California, Mexico

Conservation Efforts for Quercus cedrosensis

Dario Berrini 1and Greg Bluffin 2 

1 Senior Horticulturist, San Diego Zoo Wildlife Alliance
2 Senior Horticulturist, San Diego Zoo Wildlife Alliance


Quercus cedrosensis, commonly known as Cedros Island Oak, is a rare oak in the Protobalanus or Golden Oak section, inhabiting a portion of the Peninsular Ranges, which stretch from Southern California to the southern tip of the Baja California peninsula in Mexico. The name was chosen in reference to Cedros Island, off the Pacific coast of Baja from where it was first described.

Quercus cedrosensis tre
A medium to large shrub with a dense canopy, Quercus cedrosensis can sometimes attain tree form; Otay Mountain Wilderness, June 2023

The presence of the species in California was not known to science until 1995. Given the lack of published in-depth studies, it is now imperative to learn more about this oak’s habitat to better understand its growth patterns and future challenges to its survival.

Selecting a branch for airllayering
Dario Berrini collecting cuttings for propagation trials

Thus far, in California, only a handful of occurrences have been discovered near the US-Mexico border, in and around the Otay Mountain Wilderness. This is a rugged area of San Diego County, characterized by deep canyons with steep, rocky, meta-volcanic walls, often shrouded in fast-moving fog. It’s a sky island with a seasonal ebb and flow of running water critical to the oaks growing along these seasonal drainages. Increasing periods of drought, wildfires, and border activities are serious threats to an oak that is already experiencing very low recruitment. These challenges highlight the need for both in-situ and ex-situ conservation efforts to insure the survival of this species.

Quercus cedrosensis leaves
Quercus cedrosensis leaves showing typical mature morphology

Quercus cedrosensis usually presents itself as a medium to large shrub with a dense canopy but can, on occasion, attain a true tree form. The leaf morphology of this species is an important factor in the process of identification. The small-sized leaf with a glabrous underside and white venation, a pronounced pointed tip, and smooth or only marginally serrated leaf edges at maturity are some distinctive traits to look for. For the past several years, conservation specialists from the San Diego Botanic Garden (SDBG) have explored the region and collected important data on population distribution, acorn production, and growth patterns, which they have generously shared with several conservation partners.

Quercus cedrosensis acorn
One of the few viable acorns found in 2021 

In December of 2019, The Global Conservation Consortium for Oak (GCCO) was launched with the goal of saving oaks by coordinating oak conservation efforts throughout a network of institutions working together. The GCCO plays an important role in organizing and supporting several institutions having common objectives at both a regional and international level. In October of 2020 the San Diego Zoo Wildlife Alliance (SDZWA) joined forces with other partners to create future ex-situ collections of six oak species of conservation concern; among these species was Q. cedrosensis. The creation of ex-situ collections will provide ideal conditions for monitoring the species up close and facilitating propagation efforts.

Sleeve over twig
Sliding a plastic sleeve over the branch selected for air layering

The goal of propagating significant numbers of Q. cedrosensis has proven to be extremely challenging, however, as viable acorn production has been very low for reasons not yet fully understood. Fortunately, SDBG has been successful at sprouting a handful of the precious acorns while SDZWA has been working on in-vitro germination and tissue culture. Propagation from cuttings is also being attempted but with very limited success, so SDZWA staff decided to try air layering as well. This process began in October 2021 when two air layers were started on a Q. cedrosensis in the Otay Mountain Wilderness.

Removing bark for airlayering
The bark and cambium are removed from a roughly 2-cm-long section of the branch around the entire circumference

Air layering is a form of clonal propagation that has the advantage of leaving a chosen branch attached to the plant while it develops roots at the site of the air layer. Once root development is sufficient, the branch can be cut off from the parent plant and grown in a pot until it is ready to be planted in the landscape.

Sleeve sealed with ties
Both ends of the plastic sleeve are tied off using plastic cable ties

This technique begins by selecting a suitable branch, often about the thickness of a pencil, sliding a plastic sleeve over the branch, and removing the outer bark and cambium at the chosen site of the air layer. The bark and cambium are removed from a roughly 2-cm-long section of the branch around the entire circumference. At this point, rooting hormone is often added, and in our case (though not shown in the photos), we covered the wound with a gel rooting hormone using a cotton swab. We then covered the wound with pre-moistened sphagnum moss, pulled the plastic sleeve over the moss, and tied both ends off using plastic cable ties.

Aluminium foil covering
Aluminium foil is used to povide darkness and protection from animals

This was then covered by aluminum foil to provide darkness for root growth as well as protection from direct sunlight and small animals.

Removing rooted branch
Once root development is sufficient, the branch is cut off from the parent plant

The air layers were subsequently monitored every few months by both SDZWA and SDBG to check progress and maintain hydration. This was initially done by cutting a small slit in the plastic sleeve, just large enough to spray water inside, and covering with a layer of plastic wrap afterwards. Later the method was modified and a large syringe attached to a very thin cannula was used to puncture through the plastic. After a very long 19 months, the first air layer was cut from the parent plant and potted up at the San Diego Zoo, where it is currently thriving and awaiting planting on Zoo grounds. This success holds promise not only for Q. cedrosensis but also for other endangered oak species that prove difficult to propagate due to a lack of viable acorns.

Rooted plant
Visible roots of air layer 16 weeks after removal from parent plant

Recent collaboration with GCCO partners in Baja California renewed enthusiasm and strengthened international relations among conservationists. It’s clear that populations of Q. cedrosensis in Baja California are facing the same struggles for survival as populations here in California and any tool that can be found to mitigate that struggle is welcome. While the successful air layering of Q. cedrosensis was both arduous and time consuming, the fact that we now know that it can be done provides us all with one more tool that can be used to secure the future of this rare and unique oak.

Rooted plant potted up
The propagated plant potted up at the San Diego Zoo awaits planting out on Zoo grounds

Photos © Greg Bluffin