The Magazine of the Arnold Arboretum VOLUME 76 • NUMBER 3 The Magazine of the Arnold Arboretum VOLUME 76 • NUMBER 3 • 2019 CONTENTS Arnoldia (ISSN 0004–2633; USPS 866–100) 2 The Hybrid Mystique is published quarterly by the Arnold Arboretum Jake J. Grossman of Harvard University. Periodicals postage paid at Boston, Massachusetts. 14 Roaming through Ranges: The Evolution Subscriptions are $20.00 per calendar year of Tree Species Distribution Maps in the domestic, $25.00 foreign, payable in advance. United States Remittances may be made in U.S. dollars, by James Ellenwood check drawn on a U.S. bank; by international money order; or by Visa, Mastercard, or American 28 Taiwan Dispatches Express. Send orders, remittances, requests to Ernest Henry Wilson purchase back issues, change-of-address notices, and all other subscription-related communica- 34 Hurried Journey: Botany by Rail tions to Circulation Manager, Arnoldia, Arnold Jonathan Damery Arboretum, 125 Arborway, Boston, MA 02130- 3500. Telephone 617.524.1718; fax 617.524.1418; 40 Sax Pine: A Hybrid Left Behind e-mail [email protected] Jared Rubinstein Arnold Arboretum members receive a subscrip- tion to Arnoldia as a membership benefit. To Front and back cover: Andrew Robeson prepared become a member or receive more information, maps for Charles Sprague Sargent’s Report on the please call Wendy Krauss at 617.384.5766 or Forests of North America (Exclusive of Mexico), email [email protected] published in 1884. This map shows “relative average density of existing forests” across the country, which Postmaster: Send address changes to was calculated in the number of cords of lumber that Arnoldia Circulation Manager could be harvested per acre. Regions marked with The Arnold Arboretum Roman numeral I were estimated to yield less than 125 Arborway one cord per acre, while those marked with VII (the Boston, MA 02130–3500 highest in the eastern United States) were estimated Jonathan Damery, Associate Editor at fifty to one hundred cords. Map from Arnold Arbo- Andy Winther, Designer retum Archives. Editorial Committee Inside front cover: Railroads allowed Ernest Henry Wilson rapid access to collecting locations in Taiwan. Anthony S. Aiello On March 12, 1918, he photographed a man stand- Peter Del Tredici Michael S. Dosmann ing on railroad tracks with a large specimen of Vitex William (Ned) Friedman quinata (a member of Verbenaceae, then known as Jon Hetman V. heterophylla) reaching overhead. Notice the Julie Moir Messervy epiphyte-laden branches. Photo from Arnold Arboretum Archives. Copyright © 2019. The President and Inside back cover: The Sax pine (Pinus monticola × Fellows of Harvard College parviflora var. himekomatsu) retains the blue foliage of the Japanese white pine, one of the hybrid parents. Cones were observed on accession 266-46*F. Photo by Jonathan Damery. GROSSMAN, J. J. 2019. THE HYBRID MYSTIQUE. ARNOLDIA, 76(3): 2–13 The Hybrid Mystique Jake J. Grossman ybrids, the results of successful breed- for the biological species concept due to their ing between two parents of different wanton tendency to hybridize (Burger, 1975)— Hspecies, occur frequently in nature, we might say that there is evidence of red oak yet are perhaps most familiar to us when they (Quercus rubra) introgression into a stand of result from human intervention. We encoun- northern pin oaks (Q. ellipsoidalis). These pin ter in our intentionally cultivated hybrids the oaks will still be pin oaks, but perhaps with utility of the mule, the stateliness of the Lon- some hidden genetic diversity and leaves or don plane tree, and the sensuous smells and bark that look, well, just a little bit different. tastes of myriad vegetables and fruits, including The ubiquity of such situations has led biolo- broccolini and the tangelo. These remarkable gists to formulate the idea of hybrid complexes examples are of our own making, but hybridiza- or zones: sets of species or populations in which tion between closely related species is perhaps rampant interbreeding has produced a messy the norm rather than the exception in nature. gradient of similar organisms, rather than dis- Though hybrid offspring are sometimes sterile crete sets. Our cultivated citruses represent one and can be visually distinct from their parents such complex, in which ten progenitor species (like mules), they are just as likely to be fer- in southeast Asia and Australia have given rise, tile and to pass unnoticed by us. These cryptic through hybridization, to dozens of domesti- hybrids, diagnosable only through genetic test- cated taxa (Wu et al., 2018). ing, breed with each other or with individuals And so, when I began my own foray into of their parent species (in a process called back- the world of hybrid aspens (Populus), I risked crossing), giving rise to new hybrid progeny. wading into a thicket of biological questions Over generations, such interbreeding consoli- that could have been difficult or impossible to dates novel hybrid traits, sometimes leading to resolve. Fortunately, I was a first-year graduate the formation of new species. Because what student, a neophyte far more optimistic than I counts as a species is, after all, merely conven- am now when it comes to tackling a new proj- tional, it could be said that we humans, the ect. What follows is a story of a journey through descendants of interbreeding between Nean- which a team of ecologists and evolutionary derthals and early Homo sapiens, are just as biologists, myself included, tried to track down much hybrids as the most luscious of tangelos. the truth about a putative hybrid. To do so, we Perhaps foremost among the natural world’s traveled throughout the Midwest and dug deep “hopeful monsters”—a term that geneticist into the natural history of the Niobrara River Richard Goldschmidt coined in 1940 for evo- Valley, a relictual ecoregion left behind by the lutionary transformations that occur through retreat of glaciers at the end of the last Ice Age. rare but large-scale mutation—hybrids often captivate and delight human observers.1 A biotic crossroads: Yet hybridization does not always precipitate The Niobrara River Valley the formation of a new species. In natural popu- Heading into the Nebraska Sandhill region lations, hybrids are frequently formed, only to along the state’s border with South Dakota, new be subsumed, through backcrossing, into their visitors might be surprised to plunge from corn- parental stock. This process—called introgres- fields and pastures stretching as far as the eye sion—results in the enrichment of the gene can see into forested canyons hugging a cool, pool of the predominant species with genetic inviting river. Originating on the eastern edge material from close relatives. So, in the case of Wyoming, the Niobrara River runs from west of oaks—described as particularly problematic to east across the northern quarter of Nebraska Smith Falls Aspen 3 NATIONAL PARK SERVICE PARK NATIONAL SERVICE PARK NATIONAL Smith Falls aspens (Populus × smithii, right) along the Niobrara River, in northern Nebraska, are natural hybrids of two common North American aspen species. It shares habitat with northern species like paper birch (Betula papyrifera) and western species like ponderosa pine (Pinus ponderosa), both shown at left. before being subsumed in the Missouri River in pines are among over a dozen western vascular the northeastern corner of the state. The can- plant species whose distributions extend all the yons found between the river’s banks and the way into Nebraska, following the biotic east- surrounding matrix of arid short- and mixed-grass west highway formed by the Niobrara River. A prairie constitute the Niobrara River Valley. second look confirmed that this traffic moved For an eager observer, this valley presents in both directions: bur oaks (Quercus macro- more than just a respite from Nebraska’s hot, carpa) and silver maples (Acer saccharinum) dry uplands. Rather, as Midwestern botanist made unusual appearances for species that Charles Bessey first observed in 1887, the Nio- generally cannot be found in great abundance brara River Valley is a “meeting-place for two further west than the meeting of the prairies and floras,” a unique location in North America forests in Minnesota and Iowa. It also became in which East and West comingle. Born and clear that the cooler, north-facing slopes of the raised in Arizona, I had spent about five years valley, in particular, offered suitable habitat for living in the Midwest by the time I first vis- species generally found further north, includ- ited the valley. When I arrived by car from St. ing cosmopolitan but drought-intolerant paper Paul, Minnesota, where I had just begun my birch (Betula papyrifera). The same pattern— doctoral studies as a plant ecologist, it felt a confluence of biota typical of the montane like I was seeing old friends again after a long West, the deciduous forests of the East, and the absence. Most noticeably, stands of ponderosa boreal forests of the North—holds for herba- pine (Pinus ponderosa), a decidedly western ceous plants, insects, and vertebrates as well species, greet visitors to the Niobrara. These (Kaul et al., 1988). 4 Arnoldia 76/3 • February 2019 NATIONAL PARK SERVICE PARK NATIONAL The hills and canyons of the Niobrara River Valley have provided shared habitat for an unusual combination of eastern and western species—plants and animals, alike. Of aspens, poplars, popples, and out west (P. fremontii), and northwestern black cottonwoods cottonwood (P. trichocarpa), the first tree spe- Any exhaustive flora of the Niobrara is bound cies to have its genome sequenced. Most spe- to mention aspens, meaning species of the cies in the genus share traits with each other genus Populus. These trees go by a plethora of and with other members of the willow family common names: aspens, poplars, popples, and (Salicaceae): they are dioecious, meaning that cottonwoods—names which do not neatly map male and female flowers are borne on separate on to the current phylogenetic characterization trees and have simple leaves and relatively short of six sections within the genus (Hamzeh and lifespans (often less than one hundred years).