Populus tremuloides (Trembling Aspen) and Betula populifolia
Master Naturalist Program, Cornell University,
Populus tremuloides (Trembling Aspen) and Betula populifolia (Poplar-leaved Birch) near the meadow, Arnot Forest, New York, with a key to some other species of Populus (Aspen, Cottonwood, Poplar)
Naturalist Program, Cornell University,
P. M. Eckel
May 14, 2013
Volunteer hours to produce this worksheet
Research and analysis: literary sources [8 hours], herbarium examination [8 hours],
Illustration [8 hours]: Total approximately 27 hours volunteer time.
I am grateful for Kristi Sullivan, director of the New York Master Naturalist Program, for introducing myself and my other classmates to the richness of the forests of the central New York uplands.
When walking in the moist to dry clearing or meadow around the pond near the teaching lodge at the Arnot Forest, two similar-appearing young trees were noted: Poplar-leaved Birch (Betula populifolia) and Quaking Aspen (Populus tremuloides).
A number of tree species near the lodge in the Arnot Forest that are in the Birch Family (Betulaceae) include Yellow Birch (Betula alleghaniensis), Hop Hornbeam (Ostrya virginiana), and Blue Beech (Carpinus caroliniana). Their leaves resemble each other, as well as Beech (Fagus grandifolia) in the Fagaceae. Both Betulaceae and Fagaceae belong to the Order Fagales because of their close relationships.
One Birch species at the Arnot, however, resembles an entirely different family due to similarity in leaf-shape and bark characteristics. Poplar-leaved or Gray Birch (Betula populifolia) is named due to its resemblance to the leaves and bark of species in the genus Populus, in the family of the Willow (Salicaceae).
There are two groups of trees in the Betula genus: one group has dark, shining yellowish or reddish gray curly-peeling bark, the other light (white to whitish gray) bark that peels in layers like little sheets of paper. Betula populifolia is distinctive in the genus because it has close bark, not separable easily into layers except perhaps when it is an older tree.
Betula populifolia and Populus tremuloides often occur together in the northeastern United States and Canada as both are pioneer or opportunistic species, readily colonizing moist or dry, cleared or burned-over land, as in the Arnot Forest meadow associated with the pond.
Betula populifolia has a restricted range in the northeastern United States and Canada, occurring only in southeastern Ontario east to Nova Scotia, and south to Pennsylvania and New Jersey.
Populus tremuloides on the other hand, is the most widely distributed tree in North America and is characteristic of the Canadian boreal forest from the Atlantic to the Pacific, extending north to the limit of permafrost, south to the Great Lakes and down through the Rocky Mountains.
The two tree species are also different in that Betula populifolia is said to be a short-lived tree, but Populus tremuloides may live for extended periods. One population in south-central Utah in the Colorado Plateau (Fishlake National Forest) is considered to be the oldest living organism, being around 80,000 years old.
Populus tremuloides and other species in the genus Populus have suckering root systems. One root gives rise to another out of which arises another tree, and then another, and another. The resulting stand of trees is considered to be a single organism, a clonal colony composed of genetically identical individuals. The Utah clone-population of genetically identical individuals of P. tremuloides, called Pando (Latin for “I spread”), weighs approximately six million kilograms (6,600 short tons) - the heaviest organism in the world (see Wikipedia: “Pando”).
These trees around the pond may all share the same root structure if they all derive from one (the first) tree. Because a Poplar tree is either a male or female tree, the species often relies on suckering because a suitable tree of the opposite sex is not available for fertilization. The population at the pond will expand as it propagates through root sprouts.
All of these trees and those listed in the key below develop aments - long, cylindrical flower clusters possessing male or female flowers or both. Another word for an ament is catkin, meaning “kitten” in German “kätzchen,” or in Dutch “katteken,” because the aments look like the furry tail of a kitten. In Betula populifolia, male and female catkins occur on the same individual tree. In Poplar and Willow species, only the catkins of one sex (male or female) occur per individual tree.
Catkins of Betula and Populus are wind-pollinated, but those of the Willows (Salix) are insect pollinated.
All species of genera in the Betulaceae have both sexes (male and female) on one individual tree. A tree of Betula populifolia is, hence, hermaphroditic, having both sexes developed on one tree. It is monoecous (one house, one tree) and may need only itself to sexually reproduce.
A dioecous tree species needs two houses, that is, two trees, one male and one female, to sexually reproduce. There is a Mr. Willow and a Mrs. Willow, a Mr. Poplar and a Mrs. Poplar. The dioecous population of Populus tremuloides around the pond may be only one sex - either male or female. All the clonal descendants will also be of the same sex as the original tree: if one is male, all the trees in the clone and their descendents will be male, if one is female, all the trees will be female.
Key to Betula populifolia and four species of Poplar
Betula populifolia Marsh. Poplar-leaved Birch; Gray Birch (Betulaceae); monoecious
Populus deltoides Bartr. ex Marsh. (Cottonwood) (Salicaceae); dioecious
Populus grandidentata Michx. . (Big-tooth Aspen) (Salicaceae); dioecious
Populus tremula L. (European Aspen) (Salicaceae); dioecious
Populus tremuloides Michx. . (Quaking or Trembling Aspen) (Salicaceaea); dioecious
NOTE: Leaf descriptions are for normal leaves; stump-sprout leaves appear quite different in the characters described, as do any hybrids between species.
1. Leaf margins lobed, doubly and sharply serrate; leaves appearing varnished (glossy) and gummy under the microscope, papillae absent; leaves often pubescent in the vein axils beneath; trees monoecious (male and female catkins on the same individual tree)
Betula populifolia Marsh. (Poplar-leaved Birch; Gray Birch)
The triangular leaves are abruptly acuminate-caudate (margins concave below the narrow apex); main veins arising along the central, primary vein, appearing pinnate; this species is distinctive in its genus for the bark is close, not separating, as is the bark of all the other members of the genus; the caudate-tipped leaf is also distinctive in the genus Betula and this characteristic makes it look like Populus deltoides and P. tremuloides, both of which have similar apices. The white trunks have highly contrasting black triangular patches where the branch meets or has met the trunk.
1. Leaf margins unlobed, singly and bluntly serrate to dentate; leaves appearing dull or matt under the microscope due to a low layer of whitish papillae (dots); leaves glabrous or pubescent across the leaf when young, becoming glabrate with age; trees dioecious (male and female catkins on different individual trees).
2. Leaves abruptly acuminate-caudate in the apex (margins concave below the narrow apex); leaf veins palmate or pinnate.
Note Betula populifolia also has this type of apex, the nerves pinnate along the main vein.
3. Leaves 1-3 inches long; many leaves orbicular (round); main leaf veins (3) arising from the central base of the leaf, appearing palmate; marginal teeth finely and regularly crenate-serrate, numerous (5-) 6-10 (-11) per 2 cm.
Populus tremuloides Michx. (Quaking or Trembling Aspen)
The name means “resembling Populus tremula” a Eurasian tree; one thought is that this species is only a subspecies of the European tree (Populus tremula L. ssp. tremuloides (Michx.) A.& D. Löve); winter buds glabrous and lustrous.”
3. Leaves 3-5 inches long; most leaves deltoid (triangular); main leaf veins numerous, arising along the central, primary vein, appearing pinnate; marginal teeth medium sized, dentate-serrate, curving toward the apex, fewer, 4-5 (-6) per 2 cm.
Populus deltoides Bartr. ex Marsh. (Cottonwood)
Winter buds glabrous, lustrous, viscid or glutinous.
2. Leaves gradually acute to the apex (margins not concave below the apex); all leaf veins appearing pinnate from the primary vein.
4. Leaves 1-3 inches long; leaves orbicular to wider than long, marginal teeth coarsely dentate-serrate, rather small, curving toward the apex, numerous, (3-) 5-6 per cm.; winter buds glabrous or nearly so
Populus tremula L. (European Aspen)
Eurasian; rare in North America), to be looked for; known only from Missouri and Massachusetts (USDA database), a rare escape from horticulture, but also not often planted.
4. Leaves 3-4 inches long; leaves ovate, longer than wide, marginal teeth remote, coarsely deltoid-dentate, larger, pointed outward, fewer, 2-3 per 2 cm.; winter buds canescent-pubescent
Populus grandidentata Michx. (Big-tooth Aspen)
This is the only Poplar species treated here with canescent-pubescent winter buds and young growth - all the others are glabrous, or nearly so.
“Frequent throughout [Cattaraugus Co.] in and along borders of woods; sparse; most obvious as leaves open in May when they appear silvery-gray and stand out as one looks from a distance across a valley to a wooded hillside (Eaton and Schrot 1987).
Note: All the species mentioned have leaves that are reinforced along the margins with a thickened, cartilaginous border.
Note: The whitish papillae in Poplar species look just like those of the White Ash (Fraxinus americana).
Note: The bark in all species represented, including the Betula, is close, not peeling in layers. In Betula populifolia, the close bark is a chalky-white; those of Populus are greenish to yellowish to whitish-gray and sometimes cannot be used to distinguish trees, especially when young.
Note: All these Poplar species, and also Betula populifolia, have tremulous leaves.
Note: Voss (1985) indicated that there was a strong cartilaginous border to the leaves of Populus deltoides, but I have found such borders on all the other species too, including Betula populifolia. The same is true of the thickened or callose tips of the teeth - this character is evident to a greater or lesser extent on all the Populus species discussed. All species had some sort of ciliate edge, especially near the callose tooth tips, none, however, as strong as in P. deltoides.
Note: All species of Populus discussed have a pair of glands on the leaf, on the dorsal side above the apex of the petiole - P. deltoides nearly always, but all the others display them. In Populus tremuloides they seem infrequent. Frequently the distal end of the petiole is rugose-papillose.
Eaton, Stephen W. and Edith Feuerstein Schrot. 1987 A Flora of the Vascular Plants of Cattaraugus County, New York. Bulletin of the Buffalo Society of Natural Sciences, Buffalo, N.Y. Vol. 31.
Voss, Edward G. Michigan Flora. 1985. Part II. Dicots (Saururaceae-Cornaceae). Cranbrook Institute of Science Bulletin 59 and University of Michigan Herbarium, Ann Arbor.