Bigtooth Aspen is a native short-lived deciduous tree growing 30 to 60 feet high with a diameter of 1 to 1-1/2 feet, an irregular thin crown and a straight trunk with gently ascending branches. This is a typical size, but for Champion Sizes, see the text below the photos - the largest tree in Minneapolis is in this garden and the largest in the nation is in Minnesota.
The bark is thin, smooth, and whitish, gray or olive-green to milky-brown on young stems becoming a darker gray-brown and furrowed into flat, scaly ridges with diamond shaped lenticels and splits on older trunks.
Twigs are reddish-brown in color, slender and hairy when young, becoming reddish-gray by the third year. The buds are ovate and pointed with a gray to red-brown color and those that over-winter with some fine whitish hair. Flowering buds are usually on small branchlets or on distal clusters. Leaf scars are raised and heart-shaped, and like the Cottonwood, crushed twigs have a bitter aspirin taste.
Leaves on this species are quite variable (next paragraph) but in general, they are alternate, simple, broadly ovate with a short pointed tip and a rounded to slightly heart-shaped base, 3 to 4 inches long, on long, slender, vertically flattened stalks (at a right angle to the plane of the leaf). 1 to 2 glands may be present of the leaf stalk. The upper surface is a dull green, paler beneath. The leaf margins have coarse, curved teeth. The entire leaf is covered with fine hair when young which disappears with leaf maturity. Fall color is pale yellow.
P. grandidentata is strongly 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 have smaller and more numerous teeth and are the last to fall from the tree in Autumn.
Flowers: Bigtooth Aspen is dioecious, that is male and female flowers occur on separate trees. Flowers are in dense 2 to 3 inch long brownish fuzzy drooping catkins. Male flowers have a basal disk with 6 to 12 stamens. Female flowers have a basal disk, a 2-chambered ovary, the pistil with a pair of stigmata at the tip, each of which is deeply split (bifurcated). Both flowers are about 1/8 inch wide and have a small bract whose upper edge is deeply cut and hairy. 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 a narrowly conical, light green, 1/4 inch long, slightly curved and slight hairy, 2 chamber seed capsule. Then the capsule splits into two and each chamber releases between 3 and 9 small 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. Trees need to be about 10 to 20 years of age to produce seed but begin flowering much earlier. Seed germinates immediately. The catkins are technically referred to as 'aments'.
Habitat: Quaking Aspen and Bigtooth Aspen are quite similar in habitat. The root system of Bigtooth Aspen is a series of widespread laterals with descending sinker roots. The tree forms clones, The tree is not shade tolerant and must have well drained soils which gives it a wide variety of sites where it can grow, plus Bigtooth Aspen can occupy drier sites than Quaking Aspen. It regenerates from seed and from root sprouting.
Names: The genus Populus is the Latin name for the Poplar. The species grandidentata means 'having big teeth'. The author name for the plant classification, “Michx.”, refers to Andre Michaux (1746-1802), French botanist who made many exploring expeditions in the U.S. collecting and cataloging 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 large teeth on the leaf of Bigtooth Aspen provides the common name.
Comparisons: The bark of young trees in this genus is similar but the older bark near the base of the tree varies from species to species. Bigtooth Aspen does not have smooth buds or finely toothed rounded leaves like P. tremuloides (Quaking Aspen) and does not have the long pointed resinous buds of P. balsamifera (Balsam Poplar). Compared to P. deltoides (Cottonwood), the leaf shape and twig shape are different.
Above: Bigtooth Aspen bark - 1st photo - younger branches; 2nd photo - the lower trunk with age begins to scale and create ridges; 3rd photo - the lower trunk of old tress is furrowed and ridged.
Below: 2nd photo - The horizontal flattening at the base of the leaf stalk produces heart-shaped leaf scars. 3rd photo - Twig- buds in spring on an older twig. The overwintering large terminal bud has fine fuzzy hair.
Below: 1st photo - Cluster of male (staminate) catkins at the tips of last year's twigs appearing before the leaves. 2nd photo - The Staminate Catkin, the clusters of 6 to 12 stamens are greenish-white.
Below: The stamens are covered with a dark brown bract that has deep cuts on the forward edge and fine hair on the margin.
Below: 1st photo - A cluster of female catkins. 2nd photo - Detail of the female flowers
Below: Fertile flowers developing into seed pods. Note the fine hair on the green pods.
Below: 1st photo - The leaf of Bigtooth Aspen is broadly ovate and has coarse rounded teeth, a short pointed tip and a rounded to slightly heart-shaped base. 2nd photo - The leaf stalk is vertically flattened near the blade (at a right angle to the plane of the leaf blade) and then flattens horizontally near the base. This allows the 'flutter' of aspen leaves.
Below: 1st photo - The underside of the leaf shows the network of fine veins. 2nd photo - Young leaves may have a coating of fine whitish hair which disappears in maturity.
Below: A leaf comparison of the four Minnesota native species of Populus plus the introduced P. alba. Images not to scale.
Notes: Bigtooth Aspen is indigenous to the Garden. Eloise Butler catalogued it on April 29, 1907. This species and Quaking Aspen are the only two species of aspen in North America - Bigtooth Aspen is less widespread than Quaking Aspen. Its range in Canada is from Manitoba eastward except Labrador and Newfoundland. In the U.S. it is found in the NE Section with Minnesota being the northwest corner, then south to Missouri and eastward to the coast. Within Minnesota it is found in 2/3rds of the state mostly absent in the SW section.
Heritage Tree: Just SE of the Garden's front gate is a large Bigtooth Aspen listed as a Minneapolis Heritage Tree for its champion size of 58 feet in height and 30 inches in circumference. This tree scores 93.5 points but the U.S. National Champion also happens to reside in Minnesota at Sandstone. It scores 241 points with a height of 111 feet and a circumference of 116 inches.
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: Bigtooth Aspen wood is light colored, straight grained and soft. It is used primarily for pulp, making particle board and other structural panels. It provides food and cover for wildlife. In Minnesota it covers about 5% of all timber lands compared to Quaking Aspen, which covers 55% and Balsam Poplar which covers 7%.
Two centuries ago, it was not so well thought of. Andre Michaux's son, Francois, in his 3-volume North American Sylva of 1817-19: "The wood is light, soft, and unequal to that of the Virginian and Lombardy Poplars; the tree, also is inferior to these species in size, and in the rapidity of its growth. It thus appears to promise no advantage to the arts, and to be valuable only for its agreeable foliage. While it is less than fifteen feet in height it has a pleasing appearance, and it is entitled to a place in ornamental gardens."
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"