Quaking Aspen is a native short-lived deciduous tree, generally found growing 30 to 40 feet high but can reach 100 feet with a diameter of 1 to 1-1/2 feet. It has a narrow rounded crown of thin foliage and often forms thickets. With that said, it is to be noted that the National Champion Quaking Aspen is 130 feet high and 152 inches in circumference and scores 291 points. It is located in the Coronado National Forest in Arizona. The largest in Minnesota is near Emily MN and scores 230 points. It is 96 feet high and 123 inches in circumference which is over twice the circumference of a typical tree of the species.
The bark is smooth when young, whitish to light yellow-gray or yellow-green and thin with small black rings on the surface, becoming with age a dark gray with thick furrows, especially near the base of the tree.
Twigs are slender, hairless, and shiny brown often with a waxy film. Twigs become grayish-yellow by the third year. Buds are conical, reddish brown and may have a slight resinous coating. Flowering buds are usually on small branchlets or on distal clusters.
Leaves on this species are moderately variable but in general, are alternate, simple, 1 to 3 inches long, ovate, nearly round, with an abrupt short point, rounded to slightly heart-shaped at the base with a finely saw-toothed edge which has very fine hair - the only hair on the leaf. The leaf stalk is flattened laterally at a right angle to the leaf plane, which allows the "trembling". 1 to 2 glands may be present of the leaf stalk. The surface is shiny green above and dull green under. Fall color is golden-yellow.
The leaves are variable because P. tremuloides is moderately heterophyllous, that is, it has two types of leaves - those that are formed in the winter bud for early spring growth, known as preformed or early leaves and leaves produced later in the season known as neoformed leaves. The later leaves will be different in size and number of teeth and are the last to fall from the tree in Autumn.
Flowers: Quaking Aspen is dioecious, that is male and female flowers occur on separate trees. Flowers are in dense 1 to 3 inch long brownish-gray drooping catkins. Male (staminate) flowers have a stalked basal disk with 6 to 12 stamens. Female (pistillate) individual small flowers have a basal disk, a 2-chambered ovary and the pistil has a pair of stigmata at the tip. Both flowers have a small brown bract obscuring them, whose upper edge is deeply cut. Flowers appear before the leaves and are wind pollinated.
Seed: After pollination, the female catkins elongate up to 4 inches in length and the fertilized flowers produce an elliptical light green 2-chambered tufted seed capsule. Then the capsule splits and each chamber releases 3 to 9 seeds embedded in fine white cotton-like hair which are dispersed far and wide by the wind in early summer. A single catkin can have 70 to 100 capsules. Seed maturity varies - sometimes they are ready for release as the leaf buds are breaking or sometimes just after the leaves have formed. Trees need to be about 10 to 20 years of age to produce seed but begin flowering much earlier. Seed germinates immediately.
Habitat: The root system of Quaking Aspen is a series of widespread laterals with descending sinker roots. The tree forms clones, occupying a few acres in our geographic area. In the western states clones have been found occupying 17 acres. This system of cloning results in this species being considered the world's largest organism. The tree is not shade tolerant and must have well drained soils which gives it a wide variety of sites where it can grow. It regenerates from seed and from root sprouting.
Names: The genus Populus is the Latin name for the Poplar. The species tremuloides means 'resembling the trembling poplar'. The author name for the plant classification - “Michx.” is for Andre Michaux (1746-1802), French botanist who made many exploring expeditions in the U.S. collecting and cataloguing many species. Two important works of his are the Histoire des chênes de l'Amérique septentrionale (1801 - Oaks of North America), and the Flora Boreali-Americana (2 vols., 1803, published posthumously and contained the description of this tree.). His son Francois, traveled with him and the father’s notes were later used for the 3-volume North American Sylva, for which Thomas Nuttall provided some supplements. The manner in which the leaf stalks are flattened causes them to flutter in the wind, giving the name of "Quaking Aspen."
Comparisons: The bark of young trees in this genus is similar but the older bark near the base of the tree differs from species to species. Quaking Aspen does not have hairy buds like P. grandidentata (Bigtooth Aspen) and does not have the long pointed very resinous buds of P. balsamifera (Balsam Poplar). Compared to P. deltoides (Cottonwood), the leaf shape and twig shape are different.
Above: The narrow crown of this tree is typical. The bark (3rd photo) near the base becomes darker gray with furrows on older trunks while the bark above (2nd photo) still remains whitish-gray with many small black rings.
Below: Twigs are slender, hairless, and shiny brown often with a waxy film. Twigs become grayish-yellow by the third year. Buds are conical, reddish brown and may have a slight resinous coating. 1st photo - 2nd year spring twig, 2nd photo - older spring twig with brown 2nd year growth below the buds.
Below: Leaf bases can be rounded as shown, or slightly heart-shaped. Edges have a fine saw-tooth, the entire leaf presenting a more oval to rounded appearance.
Leaf characteristics. 1st photo - the motion of the leaves is caused by the leaf stalk being flattened at a right angle to the leaf plane where it joins the leaf. Stalks will also have small glands. 2nd photo - The saw-tooth leaf edge has fine hair and the underside is paler in color.
Below: Male flowers 1st photo - the male catkin elongated with flowers developed. 2nd photo - the male flowers with stamens attached to the stalked basal disk and with the small brown hairy bract.
Below - Female flowers: 1st photo - the female catkin in elongated form. 2nd photo - the individual fertilized female flowers with the greenish-yellow basal disk, the green ovary, now enlarged, and pistil with the 2 withering stigmata at the tip and also the withering small brown hairy bract. 3rd photo - Female catkins with fertilized flowers beginning seed pod development.
Below: Note that the flowering and seed maturity is all occurring either before the new leaves have opened or as they are opening. 1st photo - Fertilized female flowers have formed seed pods, with the new leaves just opening. 2nd photo - Female catkins already dispensing mature seeds with the leaf bud just breaking.
Below: A leaf comparison of the four Minnesota native species of Populus plus the introduced P. alba. Images not to scale.
Notes: Quaking Aspen is considered indigenous to the Garden Area. Eloise Butler noted it in her Garden Log on April 29, 1907 along with Bigtooth Aspen. Some new plants were added in 1989 by Gardener Cary George. This is considered the most widely distributed tree in North America. It is found in all Canadian Provinces except Nunavut and in the U.S. it is absent only in the SE states. Within Minnesota it occupies more forest land than any other species (55% of all timber land) and is found in all counties except for those of the SW section of the State.
There are only four species of Populus that are native and commonly found in Minnesota: P. balsamifera, Balsam Poplar; P. deltoides subsp. monilifera, Plains Cottonwood; P. grandidentata, Bigtooth Aspen; and P. tremuloides, Quaking Aspen. Two others are reported: One is a native cross - P. X jackii, Jack's Cottonwood, which is usually sterile [The DNR does not track county populations of it]; and the other is the introduced P. alba, White Poplar.
Uses: Quaking Aspen wood is light, soft and with straight grain. It is a most important timber tree, providing wood for particleboard, pulp, boxes, crates, building studs and it is the preferred species for oriented strandboard. It provides 50% of the cords cut annually in Minnesota of all wood species. For wildlife it is a valuable browse and shelter tree. In Minnesota it is of particular importance for Ruffled Grouse. Aspen is well suited for erosion control. However, when the country was young and there were large forests of other trees, the opinion was different - as this comment explains:
Michaux's son, Francois, in his North American Sylva of 1817-19 wrote: "The wood of the American Aspen is light, soft, destitute of strength, and of no utility. These defects are not even compensated by an ample size and rapid growth, and the tree is so much neglected that it is felled only to disencumber lands that are clearing for cultivation. Observation. Since the publication of the French edition of this work, I have been informed that the wood of the American Aspen has been successfully divided into very thin laminae, for the fabrication of women's hats. These hats were for a short time fashionable in several towns of the United States." (Ref. #27c), How things have changed!
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"