With one last gulp of iced tea, I stepped out of a rented sedan onto the weedy shoulder of Forest Service Road 117 to perform my pre-fieldwork ritual. I tucked my pants into my socks, applied sunscreen and bug spray, and pressed a baseball cap over my spiky bed head. The morning temperature in western Kentucky was already approaching 90°F (32°C), unusual for late April. I grabbed my water bottle and tablet from the back seat and turned towards my experimental garden plot, which was planted with three subspecies of pink-flowered herbaceous Phlox. As a doctoral student working with Robin Hopkins, a faculty member at the Arnold Arboretum, I have returned regularly to western Kentucky and Tennessee to study the role of local adaptation in the divergence and speciation of these closely related lineages.
A skeletal dead tree stood on the opposite side of the field, a favorite perch for large birds. I recognized the familiar broad-shouldered silhouette and gleaming white head of an adult bald eagle. Surely it had long since noticed me and my car, and as I pushed through the tall grass to arrive at my modest garden, I wondered if I might now be familiar to the eagles of this area. I was relieved to find that my plants still stood, and in fact, they seemed to be thriving. The spring before, in 2018, I had worked with Robin and lab technician Matt Farnitano to plant 321 rooted cuttings at this site, each no more than four inches tall. Now many of the plants boasted dozens, even hundreds, of bright pink flowers. I set down my water bottle and turned on the tablet, ready to record herbivore damage and count flowers for as long as the daylight permitted.
This plot is a type of experiment known as a common garden. Three different taxa—Phlox pilosa subsp. pilosa, P. pilosa subsp. deamii, and P. amoena—had been planted in a random order, and because the growing conditions are consistent, any differences in traits among the three taxa can be ascribed to genetic differences rather than plastic responses to the environment. Common garden experiments have a rich history in plant biology. Botanists in the first half of the twentieth century (especially Göte Turesson, Jens Clausen, David D. Keck, and William Hiesey) made foundational contributions to our current understanding of heritable variation in natural populations using common gardens. Outside the Weld Hill Research Building at the Arnold Arboretum, other researchers are using a series of common garden plots to study the ecology, morphology, and physiology of woody plants. In fact, the entire Arnold Arboretum can be viewed as a large common garden, with plant species and varieties from around the world growing in one location.
My research in Kentucky required not one but three common gardens, one in each habitat of my three study taxa. During the summer of 2017, I had traveled throughout the native ranges of these three subspecies in the southeastern United States and collected plant material for the gardens. Perennial Phlox propagate well from cuttings, so I collected single stems from wild plants, leaving the rest of the plant in the ground. I mailed these stems back to labmates at the Weld Hill Research Building who planted them in soil so they would produce roots. After one year in the Weld Hill greenhouses, they furnished three cuttings each, allowing me to plant a genetically identical panel of cuttings in each garden. All three of my common gardens sit adjacent to a wild population of one of the three subspecies. This experimental design—plant all taxa in all habitats—is called a reciprocal transplant. I repeated any measurements taken in this garden in the other two, both within a couple hours’ drive.
A reciprocal transplant is a powerful test for local adaptation. Populations that are adapted to different ecological niches are unlikely to encounter each other in their distinct habitats, and if they do, the nonlocal taxon is likely maladapted and will not persist. Local adaptation, therefore, may contribute to the divergence of closely related lineages. In general, Phlox pilosa subsp. pilosa favors open grassy areas in full sun, while P. amoena grows in the grassy fringes of mixed hardwood forest, and P. pilosa subsp. deamii peppers the understory of similar forest edges. But because the ecological factors that differentiate the preferred habitats of my three Phlox taxa are multidimensional and not entirely obvious to my human senses, I let the wild populations guide me to appropriate sites for the experiment.
Settling into my morning work routine, I opened a spreadsheet on my tablet that contained a stack of three-digit codes in a column on the left. Each code corresponded to a unique plant identifier that was stamped into an aluminum tag and fastened in the ground at the base of each plant. In order to test for local adaptation, I designed my experiment to evaluate traits related to fitness, like susceptibility to herbivore damage and total reproductive output. My goal on this visit was to score the presence or absence of herbivore damage and count the number of open flowers on every plant. I labeled two new columns (“herbivory_2” and “flowers_2”) and eased into a cross-legged seat on the edge of my plot.
Collecting these data was a comprehensive sensory experience. As I pushed and pulled inflorescences aside to reveal more clusters of bright pink, my fingers reluctantly harvested the sticky secretion that protects the flowering branches of Phlox pilosa subsp. pilosa. Each time I agitated a bunch of flowers, a small flare of sweet fragrance mixed with the sharp scent of spring grasses and forbs soaking in the midmorning sun. The exaggerated buzz of a carpenter bee hummed under the exclamations of chattering songbirds. A jumping spider tickled across my wrist. Sitting quietly, eye-level with the asters (Erigeron philadelphicus), I immersed myself in the dense fabric of interactions that contributed to the deceptively neat figures in my spreadsheet.
This common garden, in the full-sun habitat of Phlox pilosa subsp. pilosa, is tucked into the northern tip of a 170,000-acre inland peninsula (the largest in the United States), which spans the border between Kentucky and Tennessee. When the Tennessee Valley Authority completed the two dams that isolated this strip of land, aptly named Land Between the Lakes, the residents were forced to move, leaving their properties to be reclaimed by mixed hardwood forest. The house that complemented this yard and surrounding fields has long since been demolished, but a patch of feral bearded irises (Iris × germanica) and a single mature post oak (Quercus stellata) hint at where it once stood. These days, the property is mowed annually and burned periodically by the United States Forest Service as part of a scattered network of restored prairie patches, important habitat for the robust deer and turkey populations in this National Recreation Area. My research permit with the local Forest Service office, however, guaranteed that this field would not be burned from spring 2018 through fall 2020, and my garden plot, demarcated with pink marking flags, would not be mowed.
The Phlox that I study are the hangers-on of a much more audacious long-term experiment—the conversion of southeastern prairie into farm and forest. Through the conversion into farmland, suppression of fire, and elimination of grazing bison, humans removed the sources of periodic disturbance that once precluded large trees and favored communities of resilient herbs and grasses. These changes have been compounded by the ebb and flow of fertilizer and pesticide use, an evolving system of hunting regulations, and a rapidly changing climate, creating a volatile experiment with few constant variables. Each species has borne witness to the arc of human impact in its own way. The bald eagle, once suffering, now thrives. The same is true for white-tailed deer and wild turkey. Free-ranging bison have not returned, but Land Between the Lakes supports two small populations of reintroduced bison that graze on fenced-off grasslands, an allusion to the millions that roamed widely until the early 1800s. Dozens of species of prairie-dwelling plant have retreated to small patches of suitable habitat and are threatened or endangered. Today, the closest approximation to the lost prairie disturbance regimes is often the roadside, periodically grazed by a fleet of Department of Transportation mowers. These parallel ribbons are precious refugia for what remain of the remnant prairie species in this part of the world, including the Phlox that I study.
Research on the presence and strength of local adaptation may be especially relevant as humans continue to modify the environment. As the southeastern prairies shrank, these three Phlox withdrew into smaller and smaller patches of suitable habitat. If the Phlox were forced into shared fragments, their chances of contacting one another, hybridizing, and melting into one shared gene pool likely increased. Yet, with the exception of a half mile of roadside in western Tennessee, I have never found any of my study taxa living together. After my initial round of spring observations, I would return to these bustling common gardens every few weeks to track flower output as well as the number of fruits each plant produced, the number of seeds in a subset of those fruits, and the aboveground biomass at the end of the growing season. These traits quantify survival, growth, and reproduction, all aspects of fitness that would allow me to test my prediction that these subspecies are locally adapted to distinct ecological conditions. If so, it would help explain how they kept their ecological distance, even as they were concentrated into small pockets of prairie-like habitat.
By seven o’clock, the tall grass around me glowed pink. The yellowthroats and gnatcatchers resolved their conversation for the day, and I strained to distinguish the Phlox flowers from one another. I had counted more than six thousand flowers on about two-thirds of the plants in the garden—a tedious but satisfying task—and would finish the rest before the next day’s lunch. I gathered my water bottle and tablet and swished back through the tall grass. Standing next to the rental car, I shook off the tunnel vision of counting flowers and let my eyes wander over the rolling field. My gaze landed again on the large dead tree. A bald eagle leapt from an upper branch, circled the field once, and slipped out of sight behind the canopy.
Ben Goulet-Scott is a doctoral candidate in the Department of Organismic and Evolutionary Biology at Harvard University and a fellow of the Arnold Arboretum.