Senior Editor

When W. John Kress was in college and pondering what life was all about, he used to climb up into a treetop and stay there for hours at a time. “I wanted to be away from everything else and be with nature in some way,” he says now, speaking to me from his home office in leafy Vermont.

Kress is the author of a new book, an 800-page tome called Smithsonian Trees of North America. It’s an incredibly thorough guide to just about every leaf, needle, flower, seedpod and pinecone you’re likely to come across as you walk around the United States or Canada. Kress—a research botanist emeritus at the National Museum of Natural History and former interim Under Secretary for Science at the Smithsonian Institution—wrote the text and took most of the photographs.

He notes that the book doesn’t cover all the tree species in North America—a global tree assessment published in 2021 estimated that there are 1,432 of them. But the 326 species the book does include account for 98 percent of the trees on this continent, north of Mexico. (The U.S. and Canada share many more species of trees with each other than they do with Mexico, so it’s common for botanists to consider the lands south of the border as a separate region.)

“We take trees for granted a lot,” Kress says, as I glance out the window at a flowering crepe myrtle in my own backyard. “And that was the point of the book. Not every tree is the same. Another point of the book is that we’re losing that diversity. We need to start paying attention.”

When it comes to the animal kingdom, you’ll hear people talk about “charismatic species”—the elephants, pandas, lions and dolphins that never fail to attract zoogoers or sell plush toys. Conservationists hope these alluring creatures will serve as ambassadors, making people care about entire habitats and all the other forms of life within them.

With the notable exception of Groot from Guardians of the Galaxy, you don’t usually see tree toys or arboreal characters in children’s cartoons. (Let’s not talk about the dismembered heroine of Shel Silverstein’s The Giving Tree.) And yet trees are all around us if we’re lucky, an underappreciated backdrop of shade and greenery. Kress wants people to care about the individual trees in their neighborhoods, form relationships with them and, through that, build a deeper connection with nature.

Preview thumbnail for 'Smithsonian Trees of North America

Smithsonian Trees of North America

An indispensable illustrated source of information for hundreds of species of North American trees

Ahead of his book release this Tuesday, September 3, we spoke about the botany, beauty and companionship of trees.

Red ironbark eucalyptus inflorescence
Red ironbark eucalyptus inflorescence

Did this book grow out of any particular event or research project of yours?

Most of my work as a researcher at the Smithsonian has been focused on tropical plants—herbs, bananas, gingers, these sorts of things. But I wanted to re-engage people with nature, because I think we’re losing that. When I’d walk down the streets of Washington, D.C., I’d see everybody looking at their phones, particularly children or young adults. And I said, “I’ve got to do something to get people back into nature, or we’re doomed.”

You helped create a plant-identifying smartphone app a few years ago.

Yes, Leafsnap. The idea was that people would want to use their phones to identify a tree, and then they’d become engaged in the tree and not just their phones. That’s when I started gathering images of all those parts of trees. Then Yale University Press said, “Why don’t you take those photos and write about the trees of North America, and we’ll make it into a book?”

One thing you encourage people to do in the book is to form real relationships with specific trees. It makes me think about how people get attached to their own dogs and cats—their golden retriever isn’t interchangeable with their neighbor’s golden retriever. But we don’t always stop to notice how every tree on a street has a different character.

Every maple is different from an oak, but also, every oak is different from another oak. Their barks are different, their leaves are different, their acorns are little bit different. Trees are shaped by their environments, and there are also genetic differences between individuals in the same species, just like there are between us—dark hair, blond hair, blue eyes, brown eyes. If you study trees carefully, you’ll see that there’s quite a bit of variation.

When you were saying that just now, I found myself thinking about a tree on a hill in Iowa, where I grew up. It was a cottonwood tree, and I always used to notice it because when a breeze blew, it looked like it was flickering, or shimmering. I found out later that it had to do with the flat shape of its stems.

Do you know the Latin name of the quaking aspen? It’s Populus tremuloides. Because the leaves are always doing that same thing, trembling with the slightest breeze. Good for you that you noticed!

Your book is full of pictures of every little part of a tree. Most of us don’t really notice those parts unless we step on a pinecone, or an acorn falls on our heads.

The flowers and fruits are really what define the species of a tree. It’s not really the leaves, because there’s a lot more variation in the leaves than there is in the flowers and fruits. Back in 1753, botanists decided that we would classify plants based on their flowers, fruits and bark. As I explained in the book, it’s not just petals. There’s anthers and stamens and carpals, and you have to open the ovary and see how many different little seeds will develop in there.

Netleaf oak infructescence
Netleaf oak infructescence Smithsonian Trees of North America

Until I looked through your book, I never really thought about the fact that an oak tree, for instance, has flowers.

Yeah, people will notice a magnolia tree flower, but nobody looks at oak flowers except when they sweep up those little things that fall from oaks in the spring. Those are the male flowers. I wanted to show all these parts of the tree, as beautifully as I could. Most field guides are sketches, and not very good sketches at that. So taking the time to make those photographs was not trivial.

Tell us a little bit about that process.

When I started working on this book, I set up a portable photography lab, and then I started going to arboretums and botanic gardens, and to my backyard, to find all the species I needed. Then the damn pandemic hit, and I couldn’t go anywhere. So I just tapped all my friends and asked them, “Can you send me fruits and flowers of X?” I spent almost two years of the pandemic in my photo lab here in my house, getting a FedEx package every day. I was just astounded at how well some of those plant parts survived a trip from Oregon or a trip from Washington state.

One thing a book can’t capture is the unique smells of different trees. How do you think smell plays into our relationships with them?

I was trying to figure out how to capture that, if there was some way I could put perfume samples in there or something. But I do try to describe the fragrances of different trees. There are also the auditory elements—the whisper you’ll hear when a breeze blows through those aspens we were just talking about. Or that sound when you’re walking down the street and the acorns are falling. And there are some fruits you don’t want to bite into, but a lot that you do. Maybe at some point we’ll have a tasting field guide. That would be fun.

How many of our trees in North America come from Europe or from other places?

Of the 326 species in the book, only about 50 of them are exotic—though that number is growing.

I was just out in Northern California, and I always notice all the eucalyptus trees there. Are the eucalyptus trees in California the same as the eucalyptus trees in Australia?

The eucalyptus in California are all imported from Australia. They’re not native. The three most common types of eucalyptus were brought there because people wanted them for either ornamentals or for timber trees. They don’t take as long as an oak tree to grow. Though unfortunately, those plantations don’t sequester as much carbon out of the atmosphere. They don’t do the same things to offset climate change that natural forests do.

What about redwoods? What is it about the West Coast that’s conducive to such enormous trees?

That’s the part of the world where they evolved, and they had this abundance of moisture—some rain, but primarily fog—that allowed them to just keep growing. You also get really big trees in the tropics where there’s no winter, there’s no season when things stop growing. In Miami, you see these giant fig trees and so on. So again, the environment and the climate have a lot to do with what you’re going to see.

Hollyleaf cherry branch with infructescence
Hollyleaf cherry branch with infructescence Smithsonian Trees of North America

Are you involved with the BiodiversiTREE program at SERC [the Smithsonian Environmental Research Center on the Chesapeake Bay]?

Oh, yes. When I was Under Secretary of the Smithsonian, I actually funded that project. John Parker, who runs it, is a great fellow. They’re doing wonderful stuff. They’ve probably explained to you that they’ve planted 18,000 trees, in plots with different types of species—some with eight species and some with 16 species—and then they can compare how those plots develop over time. It’s a big experiment. There’s also ForestGEO [the Smithsonian’s Forest Global Earth Observatory, a worldwide network of researchers and forest sites].

Visitors who come to Smithsonian museums might not know about that whole other part of what we do, those huge experiments that cover enormous areas of land.

The other thing is that unlike a lot of other institutions, we can do projects that are long-term. The BiodiversiTREE experiment is going to outlive John Parker. It’s designed to last not just a year or two years or ten years, but 50 years or longer, if they can keep it going. And trees change over time, to say the least. Whole forests change as they mature. So they’ll see what they can do.

Trees obviously have a dramatic effect on our quality of life. Even little kids know that they absorb carbon dioxide and give us oxygen. And in a city like D.C., the neighborhoods with shade are like 10 degrees cooler than the neighborhoods—usually less affluent ones—where trees are scarcer.

You bet. Trees also give character to neighborhoods. There’s a photo in the book from Tallahassee, with the live oaks and the Spanish moss hanging onto them. It sets the ambiance for a city or a countryside. When I was an undergraduate working in the tropics, I had a professor who classified trees according to their architecture—whether they went straight up, whether their branches went out horizontally. He wasn’t an artist. He was a scientist just trying to understand how these trees were shaped and how they grew. But the beauty of it influenced me, and it still does 50 years later.

It works the other way around, too. When you’re sketching or painting a picture of a tree, you notice the mathematics and geometry of it.

In the book, you probably saw that I have two drawings by my grandchildren. I wanted to see what they thought a tree was at 6 years old, 8 years old. And, I mean, they’re glorious. People start appreciating early on what a tree is. Some people maintain that, and other people don’t.

What about the recent science that says trees communicate with each other underground and send each other nutrients?

You know what, I have a hard time with all that. It’s too much anthropomorphizing for me. I do think trees can communicate in various ways, but they don’t talk to each other. They don’t mother their saplings. That’s all fantasy. In some ways, I can see why you’d want to make people feel connected with trees by anthropomorphizing them. But I think it sends the wrong signal. All life out there is not based upon what we see as humans, or the way we act, by any means. So I try to stay away from that as much as possible.

Jennie's kids and their favorite tree
Writer Jennie Rothenberg Gritz's children hug their favorite tree at the Hirshhorn Museum and Sculpture Garden Jennie Rothenberg Gritz

I have to admit that I enjoy hugging trees. There’s a tree in the Hirshhorn Museum and Sculpture Garden that my kids and I used to hug every morning before I dropped them off at Smithsonian Summer Camp.

That doesn’t mean you’re anthropomorphizing the tree. I think you’re just appreciating it.

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Jennie Rothenberg Gritz is a senior editor at Smithsonian magazine. She was previously a senior editor at the Atlantic.