Walking through the landscape to my office one morning last fall, I happened on a new sign between Willow Path and Meadow Road:
LOOK UP
You are entering
one of the most diverse collections
of maples in the world.
The word “diversity” caught my attention. I’ve spent much of my career in and around collections—before arriving at the Arboretum, I worked in libraries, archives, and museums at Harvard and elsewhere—and usually, “diversity” wasn’t the first word we used to describe them. Collections are “wide-ranging” or “focused” or “preeminent”—above all, “comprehensive.” Here in the Arboretum, what does diverse mean?
In terms of species count, the genus Acer, which comprises the maples, is a middling taxon. Quercus numbers some 500 species of oak (469, according to Plants of the World); Acacia, another tree-predominant genus, boasts more than a thousand. These genera pale in comparison to Astragulus—the milkvetches—which leads the list with more than 3000 species. With about 130 species, by contrast, Acer is far from the most diverse—if species count is our chosen marker of diversity.
We’re talking about biodiversity, which is a term of fairly recent coinage. Diversity mattered to Darwin, who used the word to describe variation in wild and domestic settings. In the 1872 edition of Origin of Species, for instance, he notes that “the number and diversity of inheritable deviations of structure, both those of slight and those of considerable physiological importance, are endless.” With the rise of ecology in the early twentieth century, “biodiversity” began to take familiar conceptual shape. Writing in 1916 about the flora of the Southwestern U.S. for Scientific American, botanist J. Arthur Harris celebrates the region’s richness and variety. To say that the desert “contains a flora rich in genera and species and of diverse geographic origin or affinity,” he writes, “is entirely inadequate as a description of its real biological diversity” (my emphasis). Harris marvels at how deep this biodiversity goes: “(D)istinctions between species are not solely of the kind that can be drawn or photographed or ascertained by inspection of that sacred and indispensable mummy, the type specimen,” he observes. “The species of plants are not merely externally dissimilar, but inherently very diverse; they are not merely morphologically differentiated but physiologically very distinct; they are to be distinguished not merely by their external form, but by their methods of reaction to the various factors of their environment.”
So diversity scales from the physiological to the morphological to the ecological. How to hold it all? “Comprehensive,” a word with roots in taking and gathering, suggests that a collection is completable. In his 1753 Species Plantarum, Carl Linnaeus estimated that there were no more than 10,000 species of plants in the world—as book historian Molly Hardy has pointed out, he designed his herbarium cabinet to accommodate this fixed number. A 2022 study found 376,366 species of vascular plants have been validly described—likely a fraction of the actual number. The herbarium needs a lot of cabinets.
Biodiversity is liquid, inventive, ephemeral, and ever on the move.
Given spiraling diversity, collectors must take a selective approach. With the addition of Acer capillipes, which Michael Dosmann and Miles Sax brought back from their recent trip to Japan (see “Science and Spirit in the Forests of Central Honshu,” beginning on page 18), our Acer collection now holds 63 species—roughly half the number in the genus. But species inventory is only part of the mission. As Harris observed more than a century ago, species “are not merely externally dissimilar, but inherently very diverse.” We are still learning that diversity runs deep, extending beyond form to physiology and ecological relation. In the Arboretum, this diversity factors into collection, propagation, and planting decisions—and the landscape becomes not only a repository, but an incubator of such diversity in turn.
In contrast to the grasping finality of “comprehensive,” “diverse” means “varied paths.” Collectors and propagators indeed take varied paths to find and grow trees for the collection—seeking seed at the top of a ridge and at its foot, taking cuttings from riverbanks and the arid edges of clearings, at the peripheries of known ranges and deep within their forested hearts. Whether in suburban Ontario or on the slopes of Mt. Fuji, collectors note where a plant is growing, under what conditions of light, terrain, and proximity to other plants. These observations suggest where to plant in the Arboretum, how a given plant might respond to different conditions of soil, climate, light—how it might bump up against the edges of what’s known about the species. As well as determining what will thrive in our garden, this genetic diversity has implications for conservation and resilience in the face of climate change and habitat loss.
Working with collections can shed light on how biodiversity unfolds in the world beyond the garden’s edge. Botanist Jake Grossman, who studied the Arboretum’s maple collection as a postdoctoral Putnam Fellow, observes that the maples of North America diverge in crucial respects from their relatives in Asia, where the genus evolved some sixty million years ago. “After all,” he writes for Silva in 2019, “maple evolution has taken place over many extreme fluctuations in climate, the formation of the continents as we know them, and the coincidental evolution of diverse plant competitors, microbial symbionts, and animal pollinators and predators.”
Earlier, I called Acer a “middling taxon” when it comes to species count—but it turns out that diversity is more than an inventory. Biodiversity is liquid, inventive, ephemeral, and ever on the move. It’s not something that proceeds from a pure origin, nor does it point toward some final form or ultimate realization. It crops up at all scales, from the landscape to the individual leaf or flower. The collection varies continuously, and always offers the possibility of wonder. And thus a living collection is more than fixed and comprehensive—it is diverse. And this diversity is ongoing and ever unfolding. So, look up!
Matthew Battles is the editor of Arnoldia.