Natural hybrids in native trees: why oaks, hickories and maples cross
Which native trees hybridize and why, how red and white oaks keep apart, which natives have close kin in eastern Asia, and which seed grows true to its kind.
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How two species of one genus crossOaks cross only within their own groupWhich oaks crossGenes that pass between oak speciesWhy an odd oak is hard to nameHickories: pecan and its many crossesMaples: Freeman maple and the sugar maple groupOther natives that crossNatives with close kin in eastern AsiaButternut and the Asian walnutsBlue ash and the Asian ashesOther natives with Asian kinWhen a native crosses with an introduced relativeWhat crossing means for a planterCommon questionsSourcesOaks, hickories and maples cross with other species of their own genus where the two grow close and bloom together, and the offspring are hybrids. Oaks cross only within their own group, red oak with red oak and white oak with white oak, and most oak hybrids are single odd trees with leaves between their parents’ shapes. Pecan crosses with at least four other hickories, and red maple with silver maple. Over generations a hybrid can breed back into one parent and carry the other species’ genes with it, yet the species stay distinct, because each holds to its own ground. Some natives, butternut among them, also cross with Asian relatives planted here.
How two species of one genus cross
A hybrid forms when pollen from one species reaches the flowers of a close relative and sets seed. Oaks and hickories hang their male flowers in catkins, and hickories make huge amounts of pollen that the wind carries. The five eastern white oaks of one study are interfertile and often grow in mixed stands. The barriers that do exist include flowering at different times and a flower’s preference for pollen of its own species.
A hybrid that breeds back with one parent, and its seedlings with that parent again, carries the other species’ genes into a population that still looks pure. That seeping of one species’ genes into another is introgression. Where two species meet over a wide area, a hybrid swarm of trees mixed in every degree can grow up, as Ohio buckeye and its kin form in places.
The species hold together anyway, because each keeps to its own ground. In mixed oak stands studied in Europe, each species clustered on the soil and water it suits, and hybrids grew mostly where the parents’ ground met. A smaller share of the adult trees than of the seeds were hybrids, a sign that selection weeds hybrids out.
Oaks cross only within their own group
Oaks cross in the wild only with oaks of their own group, and hand pollinations between oaks of different groups have failed with very few exceptions. A hybrid of two red oaks still has a bristle at the tip of each leaf lobe, and a hybrid of two white oaks still has none. Most hybrids are single odd trees or small scattered clusters, though in places a population of mixed trees spreads over a wide area.
Which oaks cross
The crosses on record:
- Bebb oak. White oak and bur oak cross where their ranges overlap, and Bebb oak is one of the commonest white oak hybrids.
- Bur oak and swamp white oak. They cross often where they meet, and the hybrids have more deeply lobed leaves than swamp white oak.
- Dwarf live oak. Where dwarf live oak meets Chapman oak along the east side of Florida, the two form hybrid swarms.
- Northern red oak. Northern red oak crosses readily with other red oaks, among them pin oak, willow oak, Shumard oak and bear oak.
- Lea oak. Black oak and shingle oak make the Lea oak pictured above.
- Other red oaks. Blackjack oak crosses with Georgia oak. Myrtle oak, Arkansas oak, sandhill oak and runner oak each have hybrids on record with nearby red oaks.
Genes that pass between oak species
In a 1991 study, A.T. Whittemore and B.A. Schaal sampled five white oaks of the eastern United States, every tree typical of its own species:
“Restriction site markers in the chloroplast DNA reveal several clear cases of localized gene exchange between species, showing that there is appreciable gene flow between sympatric species in this group.” (Whittemore and Schaal, Interspecific gene flow in sympatric oaks, 1991)
One nuclear gene variant stayed with a single species, which suggests that nuclear genes pass between the oaks less freely. The one North Carolina stand of Georgia oak has crossed with other red oaks, one of the ways native trees become rare.
Why an odd oak is hard to name
An oak whose leaves match no species may be a hybrid, or simply an odd tree of one species. Leaves vary within a species and with the site, so a botanist judging a suspected hybrid first weighs that variation, then looks for parents among oaks of the same group growing nearby. Which oak is this leaf sorts the eastern oaks by lobes, bristles and undersides.
Hickories: pecan and its many crosses
Hickories are known for their variability, and many natural hybrids among them are on record. Pecan crosses with at least four of its kin. Water hickory crosses with it where the two grow near each other, and the offspring fall between the two.
A cross of pecan with another hickory is a hican, and a few were named and grafted for their nuts. Shellbark is probably the other parent of most named hicans, and Pleas came from pecan and bitternut. Burton is thought to be pecan and shagbark. The crosses on record:
| Cross | On record |
|---|---|
| Pecan and water hickory | Fairly common where their ranges overlap |
| Pecan and bitternut hickory | Named hicans include Pleas |
| Pecan and shellbark hickory | Likely parents of most named hicans |
| Pecan and shagbark hickory | Five named clones |
| Pecan and mockernut hickory | Reported |
| Shagbark and shellbark hickory | Three named cultivars |
| Shagbark and bitternut hickory | Seven named cultivars |
| Bitternut and pignut hickory | Described in 1937 from northeastern Arkansas |
Sources disagree on pignut and red hickory. Flora of North America treats red hickory as the northern form of pignut, which appears to cross with the typical form where the two meet. The Forest Service’s silvics notes that red hickory has been treated both as a hybrid of pignut and shagbark and as a variable species of its own. Chemical analyses of hickory nut oils suggest a long history of crossing among them.
Maples: Freeman maple and the sugar maple group
Red maple and silver maple bloom at overlapping times, and red maple crosses with silver maple in the wild. Their hybrid is the Freeman maple, named for Oliver M. Freeman of the U.S. National Arboretum. Its leaves fall between its parents’ in shape.
Sugar maple and its kin grade into one another, and botanists disagree on where one species ends. Black maple keeps its identity in the west of its range, while in the Northeast it crosses readily with sugar maple:
“In the Northeast, black and sugar maple are separate populations with distinct characters that are united by a large intermediate population which includes a variety of recombinations from the two taxa.” (USDA Forest Service, Silvics of North America, Black Maple)
The cooler, moister climate there favors the hybrids that lean toward sugar maple, so sugar maple’s genes are working into black maple. Florida maple grades into sugar maple through intermediate trees from Maryland to northern Florida and in east Texas, and a limited exchange of genes between them still goes on. Some botanists rank black, Florida and chalk maple as species, and others as subspecies or varieties of sugar maple.
Other natives that cross
Crossing runs through many other native genera:
- Chestnuts and chinquapins. All the chestnut species can cross, and hybrids of American chestnut and Allegheny chinquapin are scattered from Tennessee to New Jersey, as chinquapin or chestnut explains.
- Buckeyes. Red buckeye crosses with yellow buckeye, Ohio buckeye and painted buckeye where it meets them.
- Slimleaf and woolly pawpaw. Slimleaf pawpaw crosses freely with woolly pawpaw, and their hybrids have showy flowers with pink or red centers.
- Serviceberries. Bartram’s serviceberry crosses readily with other serviceberries growing nearby.
- Plums. Where Chickasaw plum meets American plum the two cross, and the hybrid has a name of its own.
- Wild azaleas. Piedmont azalea crosses readily with other wild azaleas, and Alabama azalea crosses freely with it in the wild.
- Hollies. Dahoon and American holly cross, and several hollies registered as American holly cultivars, Foster and Savannah among them, are in fact the hybrid.
Natives with close kin in eastern Asia
Asa Gray, the leading American botanist of the 1800s, drew notice to a pattern: many trees of the eastern United States have their nearest relatives in eastern Asia. Some still make fertile hybrids with those relatives after millions of years apart.
Butternut and the Asian walnuts
Butternut is one. Its genes place it beside the Asian butternuts, Japanese and Manchurian walnut, though its chloroplasts match those of the American walnuts, so its exact place is still argued. It cannot cross with black walnut, but it crosses easily with Japanese walnut, planted on farms in the East since about 1850.
The crosses, called buarts, grow faster, bear more and tolerate canker better. Canker had killed more than 80 percent of the butternuts in many states by the 1990s, and in some places buarts and their offspring are nearly the only butternuts to be found. In one survey of 1,363 trees at 48 places, mixing reached nearly 70 percent on settled land but stayed under 8 percent in forest.
Blue ash and the Asian ashes
Blue ash is the opposite case. The ash family trees place it in a small group with two ashes of the Southwest and Mexico, apart from the Asian ashes. It stands up to the emerald ash borer, or EAB, far better than other American ashes.
In a Michigan garden planted in 2007, no blue ash had died of the borer by 2022, while 86 percent of the Chinese ash had. Why it fares so well is not known, though its inner bark shares traits with Manchurian ash’s that may hinder the larvae. Black ash, the American ash closest to Manchurian ash, is the one the borer kills first:
“Given the close phylogenetic relationship between Manchurian ash and black ash in section Fraxinus […], the stark contrast in EAB host preference and suitability of black ash seems counterintuitive.” (Mathieu and McCullough, Long-term survival and radial growth of four North American and two Asian ash species in a common garden exposed to emerald ash borer invasion, 2025)
Other natives with Asian kin
Three more natives have close kin in eastern Asia:
- Sassafras. Sassafras is sister to the genus’s two eastern Asian species.
- Witch hazels. Witch hazel has two Asian kin, Japanese and Chinese witch hazel.
- Spicebushes. Spicebush is one of three North American species in a genus of about 100, the rest in eastern Asia.
When a native crosses with an introduced relative
Four natives cross with Asian relatives planted here: butternut with Japanese walnut, American chestnut with Chinese chestnut, red mulberry with white mulberry, and American bittersweet with oriental bittersweet. White mulberry, a tree of northern China naturalized in the eastern United States, hybridizes with red mulberry, and the hybrids come out closer to the white. Pure red mulberry is now uncommon, since the white both crosses with it and crowds it out, which is why seed source matters for red mulberry.
Oriental bittersweet and American bittersweet are cross-fertile in greenhouse tests, but how often they cross in the wild is not known, partly because their hybrids are hard to identify. Oriental bittersweet also crowds the native out, and hybridization and introgression are counted among the threats to American bittersweet.
What crossing means for a planter
A seed or nut from a tree that bloomed near a related species may grow into a hybrid, so an acorn from an oak beside another oak of its group can be a cross. Seed of a wild tree in a stand of its own kind grows into its own species, as will it come true from seed explains.
A hybrid is a natural tree and can be a sound one: Freeman maples grow fast, with strong wood and good fall color. A planting meant to restore a species wants seed of that species, though, and the surest is seed from a parent tree among others of its kind, away from related species in bloom. Butternut hybrids are common on settled land and scarce in forest, so butternut’s restorers choose parent trees with care. Blue Whale Trees grows every tree it sells from seed, and its acorns, nuts and seed come from the same proven parent trees as its saplings.
Common questions
Do oaks hybridize?
Yes, within their own group: red oaks cross with other red oaks and white oaks with other white oaks where two species grow close. Most hybrids are single odd trees, and Bebb oak, the cross of white oak and bur oak, is among the commonest.
Can a red oak and a white oak cross?
No. Oak hybrids form in the wild only within one group, and hand pollinations between oaks of different groups have failed with very few exceptions.
What is a hican tree?
A hican is a cross of pecan with another hickory, most named ones probably with shellbark. A few, such as Burton and Pleas, were named and grafted for their nuts.
Is a Freeman maple a red maple?
No, it is a cross of red maple and silver maple that arises in the wild where the two meet, and its leaves fall between theirs. Autumn Blaze is one of its named cultivars.
Is black maple a sugar maple?
It is a close relative that some botanists treat as a subspecies of sugar maple and others as a species. In the Northeast the two cross readily, while in the west of its range black maple keeps its identity.
Sources
(14 references)
- Quercus: Flora of North America, vol. 3 (Kevin C. Nixon), with its treatments of Quercus alba, Q. bicolor and Q. macrocarpa, of Carya illinoinensis, C. aquatica, C. cordiformis, C. laciniosa, C. ovata and C. glabra, of Sassafras, Hamamelis and Lindera, and of Prunus angustifolia in vol. 9: oak hybrids only within a section and hand crosses between sections failing, single trees or scattered clusters, how to judge a suspected hybrid, the bur and swamp white oak hybrids, the hickory crosses, red hickory as pignut’s northern form, the Asian species of sassafras, witch hazel and spicebush, and Chickasaw and American plum.
- Black Maple: USDA Forest Service, Silvics of North America, with the chapters on white oak, bur oak, northern red oak, black oak, pecan, water hickory, bitternut hickory, sweet pignut hickory, shellbark hickory, shagbark hickory, butternut, Florida maple, red maple, silver maple, Ohio buckeye and American holly: the quotation on black and sugar maple, sugar maple’s genes working into black maple, the named oak crosses, hickory pollen on the wind and the hickory crosses, the pignut and red hickory question, butternut’s crosses with Asian walnuts and none confirmed with black walnut, Florida maple’s intermediates and names, red and silver maple’s hybrid, hybrid swarms of buckeyes, and the hollies registered as American holly that are hybrids.
- Interspecific gene flow in sympatric oaks: A.T. Whittemore and B.A. Schaal, Proceedings of the National Academy of Sciences 88 (1991): five interfertile eastern white oaks in mixed stands, chloroplast DNA exchanged between neighboring species, the quotation, and a nuclear variant kept to one species.
- Interspecific gene flow and maintenance of species integrity in oaks: O. Gailing and A.L. Curtu, Annals of Forest Research 57 (2014): oak species kept distinct despite gene flow, flowering times and pollen competition as barriers, species clustered by soil with hybrids where they meet, and fewer hybrids among adults than among seeds.
- Acer x freemanii: NC State Extension Gardener Plant Toolbox, with its Acer leucoderme page: Freeman maple as a hybrid of red and silver maple that also arises in the wild, its fast growth, strong wood and fall color, its name, Autumn Blaze among its cultivars, and chalk maple’s names as a subspecies and a variety of sugar maple.
- Celastrus orbiculatus: USDA Forest Service, Fire Effects Information System, with its review of Morus rubra: the two bittersweets cross-fertile in greenhouse studies, the extent of crossing in the wild unknown and hybrids hard to identify, the threat of hybridization and introgression to American bittersweet, and red mulberry hybridizing with white mulberry naturalized in the East.
- Invasives in Your Woodland: White Mulberry: University of Maryland Extension: white mulberry’s native range in northern China, its hybrids with red mulberry closer to the white, and the native outcompeted and replaced.
- Short Prunings: Fruit Varieties and Horticultural Digest, vol. 7 (American Pomological Society, 1952): hicans as natural crosses of pecan with other hickories, shellbark the likely other parent of most named ones, Pleas from pecan and bitternut, and Burton supposedly from pecan and shagbark.
- Conserving a threatened North American walnut: a chromosome-scale reference genome for butternut (Juglans cinerea): C.R. Guzman-Torres and colleagues, G3 Genes, Genomes, Genetics 14 (2024): butternut placed beside the Asian butternuts with a North American chloroplast and its place still uncertain, Manchurian walnut as a close relative, no crossing with black walnut and easy crossing with Japanese walnut, hybrids faster, more productive and more tolerant of canker, and canker’s toll of more than 80 percent in many states by the 1990s.
- Human-impacted landscapes facilitate hybridization between a native and an introduced tree: S.M. Hoban, T.S. McCleary, S.E. Schlarbaum, S.L. Anagnostakis and J. Romero-Severson, Evolutionary Applications 5 (2012): Japanese walnut planted on eastern farms since about 1850, its vigorous, fruitful hybrids with butternut, 1,363 trees at 48 places with mixing near 70 percent on settled land and under 8 percent in forest, and Asian trees still able to cross with North American kin after millions of years apart.
- Conservation and Management of Butternut Trees (PDF): Purdue Extension FNR-421-W (Farlee, Woeste, Ostry, McKenna and Weeks): the hybrids with Japanese walnut called buarts, and places where buarts and their offspring are nearly the only butternuts to be found.
- Long-term survival and radial growth of four North American and two Asian ash species in a common garden exposed to emerald ash borer invasion: R.D.R. Mathieu and D.G. McCullough, Environmental Entomology 54 (2025): blue ash’s place in a small group with two ashes of the Southwest and Mexico, no blue ash killed by the borer from 2007 to 2022 against 86 percent of Chinese ash, black ash’s close kinship with Manchurian ash, the quotation, the vulnerable hybrid of Manchurian and black ash, and the inner bark traits blue ash shares with Manchurian ash.
- The phylogeny and biogeographic history of ashes (Fraxinus, Oleaceae) highlight the roles of migration and vicariance in the diversification of temperate trees: D.D. Hinsinger and colleagues, PLOS One 8 (2013): blue ash paired with singleleaf ash of the United States, and their group set apart from the other ash sections.
- Land Bridge Travelers of the Tertiary: The Eastern Asian-Eastern North American Floristic Disjunction: Arnoldia, the Arnold Arboretum of Harvard University (David Yih, 2012), the Internet Archive’s copy of March 24, 2025: Asa Gray drawing notice to the eastern Asian and eastern North American disjunction, and sassafras sister to its two eastern Asian species.
