Getting to know the trees of North America

Tribute to trees, in form and metaphor, have appeared in art, music, and poetry for millennia. “And this is our life, exempt from public ghosts,/Finds tongues in trees, books in running streams,/Sermons in stones, and good in all,” Shakespeare wrote. Trees of knowledge and trees of life have deep roots in human culture. And if trees really did have tongues, they could tell the story told in the introductory chapters of Smithsonian Trees of North America: from diminishing habitats, sudden climate changes and even extinction.

The book is the work of a decade for W. John Kress ’73, a distinguished scientist and curator emeritus of botany at the Smithsonian’s National Museum of Natural History. Now based in Vermont, he has spent most of his career studying tropical plants. But in the early 2000s he began thinking about compiling a book of North American trees. He was well underway with the project, crisscrossing the continent to photograph the last of the 326 native and introduced tree species featured in the book, when COVID curtailed travel. To overcome the lockdown, he writes in the acknowledgments, he enlisted the help of botanical colleagues on the other side of the map, who mailed him specimens to photograph in his home studio.

The result is stunning. Set against a rich black background, the photographs elevate tree fruits and flowers, buds and branches, leaves and seeds to an art form that rivals any other. Together with the detailed descriptions of each species, they help identify the most common trees on the North American continent. Many of the images were originally compiled during the author’s work on a free tree-identification app for smartphones called Leafsnap (no longer available; Kress instead recommends iNaturalist, a crowdsourced alternative for identifying plants, animals, insects, and fungi). But this gorgeous book is not intended as a field guide—or at least not in its 800-page physical manifestation, which weighs a hefty six and a quarter pounds (for portability, consider a second copy in one of the digital formats that will be available upon publication). Instead, Kress hopes the book will encourage readers to of trees.

Each species entry, arranged from oldest to most recently evolved, includes copious photographs, Latin and common names, a distribution map, a physical description, a discussion of uses and value, ecology, vulnerability to climate change, and conservation status. What sets Kress’s book apart from existing field guides is that the photographs are taken, selected, and presented with an eye for morphological differences—crucial for distinguishing related species. Still, identifying trees can be complicated. Kress introduces readers to leaf shape, structure, and orientation; to the appearance of bark and wood grain; and to the diversity of cones, flowers, and fruits. Leaves are the best first clue, but as he acknowledges, a consequence of evolution is natural variation that can sometimes be misleading. Fruits and seeds—”the glue that holds forest communities together”—also aid identification, but learning about their variation can be “overwhelming.” number of seeds makes the difference between a drupe (one) and a berry (many), for example, but what distinguishes a pome fruit from a follicle, nut, samara, or capsule? Kress helpfully compares these fruits by placing photos of each form side by side on a full page, as he does for cones, and also for flowers and inflorescences.

Kress, who won the 2021 Sargent Award from Harvard’s Arnold Arboretum to support his research for the book, wrote it for three reasons, as he explains in the foreword. First, he believes that familiarizing people with the names of the trees they encounter will enrich their lives. Identifying the species is a first step.

His remaining aspirations perhaps reflect his training as an ecologist. First, he answers the question: What do trees do for people? He hopes to give readers a sense of the value of trees, and to clarify their role in maintaining the health of forest communities, ecosystems, and the planet at large. In the foreword, conservation biologist Margaret D. Lowman lists their many virtues. In addition to housing an estimated half of the planet’s terrestrial biodiversity, trees “provide medicine, food, timber, shade, climate change mitigation, soil conservation, oxygen, energy production, and a spiritual heritage,” she writes, “for more than two billion people on Earth whose religions take refuge in forests.”

If recognizing the ecological benefits that trees provide to humans is one aspect of a two-way relationship, the other is human understanding of what trees need to survive. This, Kress says, is his final impulse to write the book. With exceptional clarity and authority, he lays out the case for conservation and provides a basic introduction to tree morphology and ecology. The explicit subtext of the complex diversity of tree reproductive strategies (human sexual activity is boring compared to tree sex and asexual reproduction) is that many tree species and the ecosystems they support are vulnerable to human environmental interventions.

Kress briefly delves into the nature of this vulnerability. While there are an estimated three trillion trunks (individual trees) worldwide—half the number before the advent of agriculture between 10,000 and 12,000 years ago—a small number of species account for the vast majority. In the Amazon, for example, 1.4 percent of the species account for half the trees. “It is astonishing to consider that the remaining 98.6 percent of the Amazon’s tree species are largely rare,” he writes. “Many have not yet been discovered or described by botanists.” Astonishingly, an estimated 15 billion trees are lost each year, Kress reports. The primary threats include habitat loss; commercial exploitation; environmental pollution; urbanization; invasive species; and the spread of animal and plant diseases and pests.

While the scientific case for forest conservation is compelling (a natural forest, with its intricate web of plant diversity, for example, captures 40 times as much carbon as an equivalent area of ​​tree plantation (see “Plants on a Changing Planet,” May-June 2024, page 38)), cultivating a deeper human connection with trees involves more than facts. And that’s what Kress wants to do. of Smithsonian Trees of North America in pictures and words. The introductory chapters are peppered with references from popular culture: the song “Hickory Wind,” co-written by Gram Parsons ’67 (see “Sound as Ever,” July-August 2023, page 44); a poem about poplars by Gerard Manley Hopkins—and reflections on what it means to know a tree. Chapter five, on tree names and identification, opens with a quote that neatly sums up one of the book’s central goals: “Knowing a tree’s name is the beginning of an acquaintance—not an end in itself,” writes Julia Ellen Rogers in The Tree Book. “There’s the rest of someone’s life where you can follow it. Tree friendships are very valuable things.