While staking locations for new plants this spring, I paused to marvel at one of my favorites in the Arboretum’s collection of around 16,000 plants. Accession 103-31*A is a European hornbeam (Carpinus betulus) that we received as a mere whip in 1931 from the famous French nursery of Léon Chenault. The multi-stemmed species produces numerous, low-branching trunks that do not just grow up, but outward as well. After almost a century, this tree has layered into the earth near Spring Brook in a linear fashion, making it a bit wider in one direction (65 feet) than the other (48 feet). With seven stout stems splaying along a central beam, the tree resembles a ship or some leviathan from the ocean’s depths, cresting waves of yellow flowered lesser celandine every spring. The form is so incredible that we bestowed upon it the status of Signature Specimen, an elite Arboretum category recognizing plants that rise above the rest.

The tree differs in other ways from its 1930s look. Initially, it bore the cultivar epithet ‘Quercifolia’ because of its incised leaves akin to an oak’s. Yet within its first decade, the tree reverted to wild type, first producing both normal and oakleaf forms on adjacent branches. Now bereft of any oak-like-leaves, we simply call it Carpinus betulus. Trees, and tree collections, change in remarkable ways.

An important lesson for the botanical garden curator is to accept the change that occurs to the objects under your care. After sixteen years curating the Arboretum, my mantra is that you don’t just accept changes, you lean into them. Living collections are special. With all due respect to my colleagues in other museums, our objects are not rocks to be assembled, cataloged, and placed in a drawer. While ill-advised, one could leave that cabinet unopened for decades, and those geology specimens would be about the same as the day they went in.

You cannot ignore trees the same way. Some do fine under benign neglect, but the majority—particularly in their first few years—need special attention to establish and remain healthy. We do everything we can to keep our plants that way, but realize that even mature and established plants can be brought to their knees (or pneumatophores, if a bald cypress) by pest, pathogen, or hurricane. Living organisms quietly senesce on their own, too—it is called old age.

In my first year (2007), the Arboretum saw a lot of change. We removed 576 plants from the permanent (non-nursery) collections. Some died of old age. Others were removed (after evaluation) because they no longer aligned with our updated collections policy. Some removals followed single-accession reviews, others were systematic audits of entire collections. The Forsythia on State Lab Slope were the most prominent. After decades of growing into themselves to the point that many individuals’ identities (and their backstories) were no longer guaranteed, we grubbed out whole masses of the canary-flowered plants. All told, we removed 64 that year, and another 29 since. (We later planted 50 new shrubs, and the collection received National Accreditation in 2018.)

I think back to other removals, particularly of old, resolute trees. In 2010 we took down a Magnolia acuminata (15157*E) near Arborway Gate. This cucumber tree—an 1890 plant from J. J. Dixwell—had a DBH of nearly 40 inches and a height of 70 feet. But, decline followed the damaging 1997 April Fool’s Day Blizzard, and fruiting bodies of wood-decaying fungi poured out of its crevices. It simply wasn’t going to make it.

The youngest plants always dominate a collection, no different from a forest floor overrun by seedlings.

This past winter, on February 4th, temperatures dipped down to -11.5°F. The last time it got as cold was 1957 (-12°F). We’ve started removing plants killed outright, and I know others will struggle for a few years before succumbing. Removals will be most prominent among the youngest, particularly species marginally hardy to begin with.

Between January 1, 2007 and December 31, 2022, we removed 6258 plants from the permanent collections (an annual average of 391, or 2.4%). While I was sad to see them go, I lean into the change, the losses, and call them planting opportunities. Play the deletions out over a decade and you’ll see how fast a collection shrinks if you don’t have a new plant pipeline. Most new plants do become middle-aged; many survive to dotage. In 2007, 805 plants were at least 100 years old. Thanks to rapid collection growth between 1907 and 1923 (including four expeditions to east Asia by Ernest Wilson), now there are 1348 centenarians.

Due to similar, recent collecting efforts, we’ve planted 5397 since 2007, an annual average of 337. Eighty percent are still alive, and account for 27 percent of the current collection. Astonishing to think that over a quarter of the plants in the Arboretum have been planted under my watch—until I calculate the number at the start of 2007 that had been planted the preceding 16 years: 25 percent. The youngest plants always dominate a collection, no different from a forest floor overrun by seedlings. They don’t all survive, and only a percentage mature to become centenarians.

As I write this, about 100 trees have been transplanted from the nurseries by our horticultural team this spring; another 180 trees and shrubs in containers await planting. The vast majority were collected as seed from far-away lands: Acer pycnanthum from Japan, Larix laricina from Wisconsin, Ostrya carpinifolia from the Republic of Georgia. With each planting (and removal), our dynamic collection changes in composition and character.

As I get back to pounding wooden stakes into the ground (after marking each with the accession number and scientific name), I think about the ghost trees that came and went in the exact same spot, and also about my predecessors, including whoever sited that European hornbeam (perhaps John George Jack or William Judd). Did they expect it to be around in one hundred years? Would they be surprised that despite losing its defining value (oakish leaves), it changed to become the grandest artwork hanging in Harvard’s tree museum’s Carpinus gallery?

Glancing over the list of what will go out in 2024 or 2025, I see exciting things waiting in the wings. Newly sourced Davidia involucrata from China, devilish Acer diabolicum from Japan, and the rarely-found-in-fruit Cladrastis kentukea from Tennessee. Rather than wait for change (i.e., canopy gaps) to occur naturally, I may nudge a bit of change (i.e., early removals) to open things up in advance. This will give each new plant—like 103-31*A has done for nearly a century—the opportunity to lean into openings created by trees that came and went.

Michael S. Dosmann is the keeper of living collections at the Arnold Arboretum.

From “free” to “friend”…

Established in 1911 as the Bulletin of Popular Information, Arnoldia has long been a definitive forum for conversations about temperate woody plants and their landscapes. In 2022, we rolled out a new vision for the magazine as a vigorous forum for tales of plant exploration, behind-the-scenes glimpses of botanical research, and deep dives into the history of gardens, landscapes, and science. The new Arnoldia includes poetry, visual art, and literary essays, following the human imagination wherever it entangles with trees.

It takes resources to gather and nurture these new voices, and we depend on the support of our member-subscribers to make it possible. But membership means more: by becoming a member of the Arnold Arboretum, you help to keep our collection vibrant and our research and educational mission active. Through the pages of Arnoldia, you can take part in the life of this free-to-all landscape whether you live next door or an ocean away.

For more tree-entangled art, science, and writing, subscribe to Arnoldia by becoming a member of the Arnold Arboretum.