The Leaves of Maples vs. Maple-leaved Nannyberry
The Leaves of Maples vs. Maple-leaved Nannyberry
Naturalist Program, Cornell University,
P. M. Eckel
May 13, 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.
During one of the weekends required for the Master Naturalist certificate, I and numerous students were introduced to various trees and shrubs while ambling through the rich forest near the teaching lodge. It was June and there were many young saplings that could be confused with shrubs. One sterile plant, for example, was identified by a person as a Red Maple, but another expressed doubt and suggested it might be Maple-leaved Viburnum. In the absence of fruit and flowers to settle this question, we had to pass by this plant and its identification.
It was thought, then, to be of some value to introduce a general guide to the identification of these two species, and other Maple and Viburnum plants as to leaf characteristics alone.
If the fruits are present, there is no problem: Viburnum shrubs are in the Caprifoliaceae, Maples in the Aceraceae, so one might guess that in some ways these plants bear no relationship to one another. For example,
Viburnum: fruit is a drupe, a ‘stone fruit,’ the fruits berry-like, with a hard kernel, a fleshy part surrounding it and a skin-like surface (like a tiny peach).
Acer (Maple): fruit is a samara, a winged nutlet: a dry fruit composed of a nutlet at one end of a dry, membranous wing. In Maples, the samaras occur in pairs: two samaras are joined along one side of the kernel, with the wings symmetrically out-spread. Each individual in the samara-pair is called a ‘key.’ There are then two ‘keys’ to a maple fruit. Other trees, such as the Ashes (Fraxinus) and Elms (Ulmus) also have samara-fruit, but these are individual and not paired. Such fruits are also called ‘key-fruits’ since when clustered together on the tree they resemble a bunch of keys. This type of fruit literally uses its ‘wings’ to create a spin as the ripe fruit leaves the tree - in maples the fruit are also called ‘helioschizocarps’, ‘heli-‘ as in ‘helicopter.’ Children call maple fruit ‘helicopters.’ The word ‘samara’ that the botanists use is the one used by the Romans for the fruit of an elm tree.
Viburnum flowers are white and organized in an erect, umbel-like cluster (actually in something called a ‘compound cyme’).
Maple flowers are rather small and relatively inconspicuous, with petals absent or when present colored red or yellow or greenish, in small, dense drooping clusters, although Acer spicatum (Mountain Maple) is unique in its narrow, erect axis of flowers (a long-peduncled panicle).
Both families are characterized by having plants with opposite (not alternate) leaves.
Some major families and genera may be remembered by a the mnemonic device for plants with opposite leaves: MADCapHorse: Maple, Ash, Dogwood, Caprifoliaceae, Horse-Chestnut.
Silver Maple, although not observed, is probably in the Arnot forest near streamsides and other low, wet areas, since the teaching lodge is in moist lowland in the valleys near the bases of the uplands that surround the camp.
Silver and Red Maples are common wetland indicators. Although there are other Maples associated with woods, these two are dominant or co-dominant tree species in low, wet woods - it is really their difference from Viburnum acerifolium that was thought important in the present report.
Since Viburnums are all shrubs, one of the easiest distinctions to differentiate them from Maples are to compare them with mature Maple trees: their size will separate them. The problem comes when the Maples are only very young that they resemble shrub species.
The problem for the naturalist is that leaves of Maple-leaved Viburnum look uncannily like those of Red Maple - even the variation in their shape and hairiness across long geographic areas is similar.
Quick-key to leaves of Red Maple and Maple-leaved Viburnum
Note, when a name is used informally it is printed in Roman characters; when used as a taxonomic name, it is italicized.
1. Sinuses between the principal leaf lobes acute at their base; petioles of leaves without basal stipules; leaves more or less pubescent when young (early in the season) - soon glabrescent (generally smooth and hairless); leaves often 5-lobed (with a pair of small lobes at the leaf-base)
Red Maple (Acer rubrum L.)
1. Sinuses between the principal leaf lobes rounded at their base ( all lobed Viburnums); petioles of leaves with basal stipule-pair; leaves always softly and persistently downy beneath with clustered hairs and tiny scattered brown to blackish resinous dots; leaves always 3-lobed (there may be teeth at the leaf base, but never with a pair of small lobes at the leaf base);
Maple-leaved Viburnum (Viburnum acerifolium L.)
Key to the leaves of both sterile adolescent Maple-trees and lobed Viburnum-shrubs:
The three-lobed species of Viburnum are Viburnum acerifolium (Maple-leaved Viburnum), Viburnum edule (Low-bush Cranberry) and Viburnum trilobum (also called V. opulus var. americanum) (High-bush Cranberry). These are the only three species in the genus in our area with 3-lobed leaves - all the others have no lobes and are egg-shaped to nearly circular.
NOTE: What is the lobe of a leaf, and what is a sinus?
A lobe is a great big tooth along the margin - a lobe may have other, smaller, subordinate teeth as well. The sinus is the empty space between the lobes.
Leaves that are unlobed are flat, planimetric shapes such as circles, squares, triangles.
All Maple leaves have lobes. Most Oak trees (genus Quercus in the Beech Family (Fagaceae) are also distinctive by their striking lobes and sinuses.
All Viburnums, except the lobed species mentioned above, are generally ovate, or egg-shaped, as are the Dogwoods in the genus Cornus (Cornaceae) - another set of shrubs with opposite leaves, but all of which are without lobes.
Key to the leaves of both sterile adolescent Maple-trees and lobed Viburnum-shrubs.
NOTE: The petioles of the leaves of Maple trees are frequently (usually) (5-) 6-7 cm long; those of Viburnum are mostly 1-2 cm long (infrequently 3-4 cm long; and never 5-7 cm or longer).
NOTE: The petioles of the leaves of Maple trees have no stipules at their base; those of Viburnums have a pair of stipules at their base (although these often deciduous later in the season).
1. Sinuses between the principal leaf lobes acute at their base and the sinuses toothed all the way to the base (note the acute part of the otherwise rounded lobe of Acer saccharinum is a sharp notch at the extreme base);
2. Terminal, apical tooth abruptly narrowed into a long, tail-like (caudate) tip; terminal main leaf lobe broadest at the base [triangular], uniformly tapered to the apex; subordinate lobes lacking - sides uniformly coarsely dentate, (2-3(-4) teeth per centimeter)
Acer spicatum Lam. (Mountain Maple)
The bark of branches lack prominent pale stripes; leaves 3-lobed, often with 2 extra lobes near the base; year old branchlets minutely pubescent.
2. Terminal, apical tooth gradually narrowed-triangular; subordinate lobes along the sides of the main leaf lobes.
3. Terminal main leaf lobe narrowed or angled inward near the base; main, terminal lobe long (2/3 the length of the leaf
Acer saccharinum L. (Silver Maple)
3. Terminal main leaf lobe with sides parallel; terminal lobe shorter (1/2 the length of the leaf)
Acer rubrum L. (Red Maple)
1. Sinuses between the principal leaf lobes rounded at their base (all lobed Viburnums).
4. The rounded sinus minutely and finely (not coarsely) serrate all along its margin (8-10 small teeth per centimeter), terminal, apical tooth abruptly narrowed into a long, tail-like (caudate) tip; terminal main leaf lobe broadest at the base [triangular], uniformly tapered to the apex [subordinate lobes lacking - sides uniformly denticulate
Acer pensylvanicum L. (Striped Maple; Moosewood)
Sometimes one sinus of the two is acute at the base; the leaves are somewhat cordate at the base and lack subordinate lobes along the sides; leaf lobes 3; terminal main leaf lobe broadest at the base [triangular; bark of branches and young trunk with prominent pale stripes; year-old branchlets glabrous.
4. The sinus toothless (smooth) at the base (all lobed Viburnums); terminal, apical tooth gradually narrowed into a long-triangular point.
5. Terminal main leaf lobe with sides parallel [may be narrowed or angled inward near the base] [with subordinate lobes along the sides].
6. Most leaves wider than long.(rectangular, longer from left to right), lobes mostly rectilinear; lobes seven
Acer platanoides L. (Norway Maple)
Street tree native to Europe, escaping into nearby wood-lots; sap milky, especially evident during the summer; leaves very often infected with large black spots, Tar Spot of Maple, a fungus endophyte, Rhytisma punctatum or R. acerinum, that lives in the leaf tissues of Maples and is conspicuous in September and October.
6. Most leaves square (equilateral) in general outline, lobes mostly curvilinear; lobes five
Acer saccharum Marsh.
(Sugar Maple; Maple-syrup Tree)
Native tree of woodlands; sap watery; the national emblem of the Commonwealth of Canada; the emblematic tree of New York State; leaves usually with few or lacking large black spots.
5. Terminal main leaf lobe broadest at the base [triangular], tapered to the apex, [subordinate lobes lacking - sides uniformly denticulate].
7. Underside of leaves more or less sprinkled with red-orange, resinous dots,
densely pubescent beneath with mostly stellate or tufted hairs
Viburnum acerifolium L.
There are no special glands on the petioles or near them; fruit is blue-black; petioles short; slender stipules may be present, but without thickened tips; young branchlets downy; tolerates a drier habitat than the next two species.
7. Underside of leaves smooth or with scattered single (simple) hairs and no resinous dots.
Fruits are orange-red for both of these species.
8. Leaves smooth or thinly hairy; petioles with prominent, club-shaped glands toward the summit of the petiole near junction with the blade; stipules with thick tips present at the base of the petiole; lobes strongly developed
Viburnum trilobum Marsh.
(=opulus L.): (Highbush Cranberry)
Leaf margins entire to sparingly toothed; petioles are short.
8. Leaves hairy along veins and axils; petioles typically with several stalked glands along the edge of the leaf near its junction with the petiole; lacking large glands on the petioles; stipules absent; lobes often poorly developed (shallow, not deep)
Viburnum edule (Michx.) Raf. (Lowbush Cranberry)
The leaf lobes are often poorly developed (shallow, not deep); petioles short; the only Viburnum to bear flowers and fruit on the previous (not current) season’s growth, cymes below the leaf apex on opposite sides of the stem on two-leaved spurs below the current season’s leaves; a rare species in New York State, found only in Ulster and Essex counties.
Gleason, Henry A. 1952. The New Britton and Brown Illustrated Flora of the Northeastern United States and Adjacent Canada. 3 Vols. New York Botanical Garden. New York.
Soper, James H. and Margaret L. Heimburger. 1982. Shrubs of Ontario. Royal Ontario Museum Publications in Life Sciences. Queen’s Park, Toronto, Ontario.
Symonds, George W. D. 1958. The Tree Identification Book: A New Method for the Practical Identification and Recognition of Trees. Quill (publications), New York.
Voss, Edward G. Michigan Flora. 1985. Part II. Dicots (Saururaceae-Cornaceae). Cranbrook Institute of Science Bulletin 59 and University of Michigan Herbarium, Ann Arbor.
Voss, Edward G. Michigan Flora. 1996. Part III. Dicots (Pyrolaceae-Compositae). Cranbrook Institute of Science Bulletin 61 and University of Michigan Herbarium, Ann Arbor.