IOS Tour of California, Part 2: Southern California

I joined the group for the Southern California portion of the Tour—my first time here. Southern California, as I now understand it, is a place of richly diverse gardens, walled highways, and back roads that lead, almost inevitably, to an oak. In this land, International Oak Society lanyards unlock free admission and secret tours, and nobody is fazed when conversation turns to leaf shape or the trichome density in possible hybrid species. But it’s not just oaks—from behind freeway barriers, palms and eucalyptus watch over the movements of millions, while further afield the urban sprawl gives way to desert yuccas, manzanitas, island chaparral, and stunning mountain shrubscapes at the US–Mexico border.

Amidst all this, there are those working to preserve the region's botanical richness, even as the human footprint expands. Their efforts—a mix of cultivation, protection, and restoration—help sustain areas under threat, alongside places where nature still thrives, left to its own devices.

We began with the places most controlled, the gardens. In the carefully curated landscapes of the Huntington Botanical Gardens, every tree, shrub, and rock seemed purposefully placed. Curator Tim Thibault led us from Quercus phellos (Eastern US) to Q. splendens¹(Pacific slope, Mexico). As we hurried past rows of non-oaks, Sairus² jokingly lamented that it was “terrible to just walk past trees like animals,” sweeping his hand toward a stately pine. But we were here for the oaks, and we saw many, including an impressively large Q. rysophylla and a young Q. brandegeei from a recent collecting trip.

At the Los Angeles County Arboretum & Botanic Garden, we found ourselves in a native grove of Q. engelmannii where many trees had fallen but continue to grow, propped up on their elbows. A lovely dappled light filtered through the twisting branches onto a field of pink flags marking oak seedlings. This was not reintroduction but “augmentation” as Jim Henrich, our guide and the Curator here, described it. From somewhere outside the grove, a Red-whiskered Bulbul (Pycnonotus jocosus) sang softly, a reminder of the exotic gardens we had just passed through, where frangipani (Plumeria sp.) and Madagascar palms (Pachypodium lamerei) lined the paths. Our visit encompassed oaks from around the world—Q. falcata of the Eastern US, Q. variabilis from East Asia, and a striking Q. resinosa from Mexico.

At the California Botanic Garden, the landscapes flowed more naturally, as if the plants had been left to arrange themselves. The birds seemed to appreciate this naturalism as much as we did—California Thrasher (Toxostoma redivivum), California Towhee (Melozone crissalis), and California Scrub-Jay (Aphelocoma californica) flitted through the brush, their common names perfectly matched to the garden’s sense of place. Though naturalistic and open, a careful design was still present as garden staff, led by Ashlee Armstrong, guided us through local ecosystems, pausing to admire native oaks like Q. palmeri and Q. pacifica, a preview of many species we’d later see in situ. The garden transported us further south still, leading to boojum trees (Fouquieria columnaris) and Mexican blue fan palms (Brahea armata) in the Baja California section.

Somewhere between the gardens and the next stop, our van made an unexpected detour—to REI. One of our fellow travelers emerged with a new duffle bag, soon to be filled with over twenty pounds of acorns. The group shared a laugh before the lively van conversations resumed.

As the trip progressed, these conversations became a highlight. From the front row, Sean Hogan, our leader and driver, noted growing waves of laughter and conversation as the trip progressed. By my second day, refrains of "E-eye E-eye E-eye" had become our unofficial theme song, thanks to Jean-Baptiste’s³ playful ribbing of our English pronunciation of Latin names ending in "ii "—a joke that Roderick⁴ backed up by noting that Latin doesn’t actually have an "eye" sound. These van conversations reflected the rhythm of our trip itself—botanical intensity paired with lighthearted moments, always driving us forward in a shared curiosity and a growing camaraderie.

The next morning, we boarded a catamaran, and were soon speeding across the ocean at 33 knots (60 km/h) toward Santa Catalina Island, one of the Channel Islands. Five dolphins (Delphinus delphis) briefly surfaced alongside, effortlessly keeping pace as the buildings of Long Beach faded into the horizon. Eventually, the island appeared with puffy white clouds hovering above. It looked like a wrinkled creature, stretched out in shades of brown and dotted with green, as if a long piece of the dry California hills had broken free and floated out to sea. We docked in Avalon, a charming town of about 3,300, where staff from the Catalina Island Conservancy greeted us, ready to guide us through the island’s unique ecosystems.

We drove between restoration sites on dirt roads that wound through island chaparral, dotted with prickly pear cactus (Opuntia sp.) and the occasional large stand of Q. pacifica, their branches heavy with pale yellow acorns that sparkled in the midday sun.

At each stop, we saw varying levels of protection from mule deer, whose introduction for hunting decades ago has profoundly altered the landscape. Their unchecked browsing has prevented many native species from regenerating, turning much of the island into a interplay between what once was and what could be again. At one site, young Santa Catalina Island ironwood trees (Lyonothamnus floribundus ssp. floribundus) were individually caged—an effective measure, but one that is not scalable to the island as a whole.

Our next stop, perched at 1,300 feet, revealed what the island could become if the deer population is reduced—a goal the conservancy is actively working toward. Here, behind a 0.8-mile exclusion fence, nature seemed to exhale. A cool, salty breeze blew up from the ocean as we stepped into the enclosure and saw a thriving diversity of flora—endemic forms of Lyonothamnus, Eriogonum, Arctostaphylos, and more. White-throated Swifts (Aeronautes saxatalis) darted overhead, and Ravens (Corvus corax) croaked in the distance as we walked toward a magnificent specimen of Q. pacifica. Unlike the more typical shrubby forms, this one had taken on a twisting, multi-trunked tree shape. With permission, the group’s urge to collect acorns from this island endemic was satisfied.

The next day, we drove south to San Diego, where the freeway walls of Los Angeles eventually fell away, revealing glimpses of ocean and scrubland. For the remainder of the trip, we explored less controlled, less cultivated landscapes. Fred Roberts, a local expert with boundless enthusiasm and an endless stream of knowledge, guided us through this shifting terrain. Armed with his pocket microscope and decades of experience exploring the region, Fred approached each oak as if seeing it for the first time, carefully examining even those he had encountered before. It felt as though he was learning alongside us, verifying his knowledge, sharing his thoughts, and uncovering new details with each oak we encountered in eastern San Diego County.

We came across more white-oak hybrids than pure species, with many crosses between Q. engelmannii and local shrubby oaks (Q. cornelius-mulleri, Q. berberidifolia, Q. dumosa). A common refrain as we moved from oak to oak was, “OK, Fred… now what’s this?” It was a relief when Fred would occasionally declare we were looking at a "pure" species. But of course, the oaks themselves paid no mind to the labels we put on them. These hybrids thrived in the in-between spaces, from forest understories to desert edges, to a Q. dumosa stronghold at the edge of a parking lot in an upscale neighborhood. Without the intense restoration efforts we had seen on Catalina Island, these oaks persisted on their own terms, evolving in response to the environment rather than under human guidance.

Our exploration eventually led us to Santa Ysabel Preserve, where the landscape shifted once again. Among the brown and yellow grasses and granitic boulders, we stepped into an oak savanna dominated by Q. engelmannii. The mid-morning air was hot and still—no birds sang, and the only sound was the rumble of cars along a nearby road, people headed east and west. For me, this was the quintessential California landscape: the motion of cars juxtaposed with the stillness of ancient trees on dry, open hills. Walking among these elder trees offers a sense of their architecture, their presence—something a photograph cannot fully capture.

Though it has taken them decades to reach this state, there is an energy in their branching, the irregular crowns connecting the hot, still air to something deep within the land and time itself. Up close, the cool blue-green leaves, with their yellow petioles and mid-rib, seemed like the notes of a relic currency—symbols of a time when this was the formula for success, back when these oaks were far more widespread. This species, the northernmost of the subtropical oaks, was once far-reaching but is now isolated by the drying of US Southwestern deserts. Though they seem timeless, I saw few signs of regeneration. Scattered among the living giants were equally tall gray skeletons, their great limbs cracked under the weight of time, brittle branches slowly returning to the earth.

We continued further east, into the Anza-Borrego Desert, pulling off the road where Q. cornelius-mulleri grew abundantly among yucca (Yucca sp.), juniper (Juniperus sp.), manzanita (Arctostaphyllos sp.), cactus (Cactaceae), and agave (Agave sp.). This was my personal favorite of the shrubby oaks, thriving here, left to its own devices between a barbed wire fence and a road cut straight through the desert for miles. Its ashy-white bark, flecked with red, mirrored the silvery undersides of its leaves, while dense, energetic branches held up clouds of small, dull green leaves. The oaks were as much a part of the desert as the rocks and sand around them, enduring under a vast, indifferent sky.

Deeper in the desert, Q. palmeri appeared like a warrior plant in contrast—its thin, angular branches resembling barbed weapons, covered in small, stiff, spiny leaves with a golden cast underneath. Where Q. cornelius-mulleri blended into the landscape, this stand of Q. palmeri stood like a defiant remnant of a harsher past. The San Diego Zoo has permits to collect here and help the species reproduce, but the plants seemed uninterested in the assistance—we searched and found only a handful of acorns with their uniquely flared caps, leaving them all on the branches. From the prickly green foliage, burnt silver limbs jutted out like twisted antlers, repeating everywhere. The plants, it seemed, had literally risen from the ashes.

We said goodbye to Fred Roberts, and on the final day of our tour, staff from the San Diego Zoo Wildlife Alliance picked us up in a caravan of four-wheel-drive vehicles as we headed south toward the Otay Mountains on the border with Mexico. After stopping to unlock a yellow gate marked “Customs and Border Patrol”, we wound our way up a dusty road into the brown, rocky terrain. In the distance, Mexico came into view—dense neighborhoods and warehouses pressed up against the border wall, a layer of fog or smog hanging over the city.

We paused briefly in an area thick with Hesperocyparis forbesii (Tecate cypress), their bark a camouflage of maroon and peeling gray flakes. Higher into the mountains, the road narrowed, clinging to the ridge so tightly that, at times, it felt like we could tumble straight into the dusty outskirts of Tijuana. Then the city disappeared from sight, and on either side of the border, the land mirrored itself—wild and seemingly indifferent to the divisions imposed upon it. From a distance, the wall appeared almost translucent, with gaps between the bars creating a strange shimmering effect, flickering like a screen in the light—an illusion that resolved into solid steel columns and plates as we drew nearer.

Our hike to the stands of Q. cedrosensis was a scramble along narrow, improvised paths following a small trickle of water, the canyon’s sheer reddish-brown rock walls rising around us. We walked single file, passing discarded water bottles, a small blue backpack—evidence of a quiet migration through the same shrubby oaks we had come to see.

We pulled ourselves onto a gently sloping shelf of pale rock, smoothed by years of water flow. Encircling us were 8- to 15-foot tall clusters of Q. cedrosensis, their small wavy leaves a vibrant light green, catching the sunlight. Softer in appearance than its relative Q. palmeri, another shrubby member of Section Protobalanus, Q. cedrosensis felt perfectly suited to this canyon. Only a handful of occurrences of this oak have been discovered near the US–Mexico border, and this was one of them. It seemed firmly established here, its roots tapped into the seasonal wash, indifferent to the passage of people, and seemingly unconcerned with reproduction. We found only a single acorn, though the San Diego Zoo Wildlife Alliance team is attempting to propagate this rare oak through air layering.

We drove north, away from the border, entering a landscape bathed in intense light. A shrubscape of blue-green, chartreuse, and earthy tones unfolded before us, composed of Arctostaphylos, Hesperaloe, and Chamaebatia (mountain misery). Our final pull-off brought us to a stand of shrubby white oak hybrids. Without Fred Roberts to guide us, we could only wonder and enumerate the possibilities.

Several Q. ×morehus hybrids stood among them, their lustrous foliage likely inherited from Q. wislizeni, which grew just behind. But where had their pointed Red-Oak lobes come from? The closest Q. kelloggii, we were told, stood on a distant mountain, just visible on the horizon. How long had these oaks been silently exchanging genes across the dusty valleys and rocky ridges?

The oaks, doing their thing, as they have for millions of years—quietly persisting and evolving, even as humans, with all our interventions, rush by in our brief timelines. It was a fitting end to a fantastic trip with a great group, meeting the region’s oaks and the people dedicated to preserving them.

Read the report of the first part of the Tour here.

¹ The correct name for this species is now Q. sororia, an oak that is very rare in cultivation. The name Q. splendens is now a synonym of Q. peduncularis. The change was published in 2020 by Susana Valencia and Allen Coombes, who noted in their article that Q. sororia was not known to be in cultivation. 

² Sairus Patel, participant from USA

³ Jean-Baptiste Bellili, participant from France

⁴ Roderick Cameron, participant from Uruguay

Photos © John Leszczynsiki

Acknowledgments

The IOS is grateful to Sean Hogan and Preston Pew for leading this Tour and to the following institutions for their generous support:

Cistus Nursery - The Huntington Botanical Gardens - Catalina Island Conservancy - San Diego Botanic Garden - San Diego Zoo and Wildlife Alliance

In particular we would like to thank Tim Thibault and The Huntington for laying on lunch for the Tour during our visit, and Narda Fargotstein for providing Acorn Bars from the Thousand Oaks Acorn Company for every Tour participant.