The Evolution of Trees of California: From Jepson to Today

In 1909, botanist Willis Linn Jepson released Trees of California, a groundbreaking field guide that would go on to inspire a century of botanical exploration. At the time, California was still being defined—not just politically and geographically, but ecologically. Jepson’s work provided a unifying framework for understanding the rich and varied forests of the Golden State.

The book was received with enthusiasm by plant lovers, amateur naturalists, and academics alike. It was one of the first accessible guides to California’s arboreal diversity and quickly became a trusted resource. Nature clubs, teachers, and university students eagerly adopted Jepson’s clear language and detailed descriptions. It was praised in botanical circles and became part of a larger movement in the early 20th century to connect Americans with their natural environment. Even today, many seasoned botanists recall Jepson’s work as their entry point into the world of California flora.

The Challenge of Documenting the Trees of California

Jepson’s Trees of California was a marvel of its time. With no access to digital photography, GPS, or internet databases, he relied on black and white photographs, hand-drawn illustrations, meticulous note-taking, and physical specimens gathered over years of travel. Much of this was done on foot or by rail, with handwritten letters serving as the primary form of communication between fellow botanists and institutions.

Despite these limitations, he described 92 tree species in remarkably vivid and accurate terms. He documented habitats, leaf shapes, bark textures, and growth forms with scientific precision and clarity. Each entry reflected not only taxonomic knowledge but a deep personal relationship with California’s forests. The book invited readers to slow down, look closely, and pay attention.

A Century Later: California Trees Builds on the Legacy

More than a hundred years later, California Trees, the new field guide I co-authored with Matt Ritter, picks up the torch. Our book is a modern-day reflection of Jepson’s vision—expanded, updated, and reimagined for a new generation of tree lovers.

We now describe 95 native tree species, aided by tools Jepson couldn’t have imagined. Our photographs are full-color and high-resolution. Our distribution maps are built from thousands of georeferenced observations contributed by community scientists. Our taxonomy reflects over a century of research, with DNA studies revealing new relationships and previously unrecognized species.

Technology has changed everything—but the intent remains the same: to deepen the public’s connection with the trees of California.

What Counts as a Tree? Defining the 95

One of the most surprisingly complex parts of compiling California Trees was deciding what exactly qualifies as a tree. Many woody plants walk the line between shrub and tree depending on context, habitat, and individual growth form. To set clear criteria, we applied what we call the “tree majority rule.” If, in recorded surveys, a species was observed growing as a tree more than 90% of the time, we considered it a tree for the purposes of this book.

This allowed us to confidently include 95 true native tree species—those that, across most of their range, consistently exhibit the growth habit and ecological role of a tree.

We were also inspired by David Sibley’s elegant definition of a tree: “If you can walk under it, it’s a tree; if you have to walk around it, it’s a shrub.” This image—the solitary trunk rising upward, crowned with a distinct canopy—captures the essence of what people intuitively see when they think of a tree. It’s a useful, if imperfect, guidepost in an ecosystem full of edge cases.

The Power of Modern Tools and Collective Knowledge

The difference between 1909 and today is not just a matter of visuals. With platforms like iNaturalist and Calflora, we now have access to an unprecedented flow of real-time plant data, often collected by everyday observers. This has helped refine our understanding of species ranges and ecological niches across California’s diverse landscapes.

Honoring the Past, Inspiring the Future

In many ways, California Trees is a love letter to Trees of California—not just the title, but the idea. Jepson helped people name the trees around them. And with that naming came awareness, and with awareness came care.

Our hope is that readers of California Trees will walk into the forests and woodlands of California and see more than green silhouettes. We want them to know the names, learn the stories, and feel a connection. Every tree has something to say—about resilience, climate, place, and time.

Jepson’s legacy is not just in his words but in the generations of people who’ve come to love California’s forests because of them. With California Trees, we strive to carry that spirit forward.

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Interested in exploring the native trees of California?

Pick up a copy of California Trees from us or tell your local bookstore about it!