5. Acer saccharum Marshall (sugar maple, hard maple)
Pl. 196 m–o; Map
808
Plants
monoecious or sometimes dioecious, medium to large trees to 30 m tall with ascending
to spreading branches, the bark of young trees smooth and gray to dark brown,
becoming furrowed and/or separated into narrow, thick plates on older trees.
Twigs gray to reddish- to orangish brown, the winter buds elliptic-ovate,
sharply pointed at the tip, with 6–12 overlapping scales. Leaf blades 7–15 cm
long, broadly triangular-ovate in outline, sometimes wider than long, the
undersurface light green and sometimes glaucous, glabrous, or often with minute
tufts of hairs in the axils of the main veins, less commonly hairy on the
surface or along the veins, with (3)5 main lobes, these tapered to sharply
pointed tips and with the sinuses rounded or U-shaped, the margins undulate or
more commonly toothed and with smaller lobes, the central lobe cut 1/3–1/2 of
the way to the blade base, broadest at the base or with nearly parallel sides.
Inflorescences produced during expansion of the leaves, umbellate staminate and
pistillate clusters from buds at or near the branch tips, the individual
flowers with long, drooping, hairy stalks. Calyces 2.5–6.0 mm long, the sepals
fused more than 1/2 of the way to the tip, with 5 shallow lobes, greenish
yellow, usually hairy toward the tips. Corollas absent. Staminate flowers with
5–8 stamens inserted on the margin of a nectar disk. Pistillate flowers with
the ovary glabrous or nearly so. Fruits dispersing after the leaves are mature,
the samaras 2.5–4.0 cm long, glabrous, the wings 2.0–3.5 cm long, spreading at
about 90–120° (rarely much narrower and nearly parallel). 2n=26. April–May.
Common
throughout Missouri (eastern U.S. west locally to Idaho
and Arizona; Canada,
Mexico).
Mesic to dry upland forests, margins of glades, ledges and bases of bluffs, and
banks of streams, rarely bottomland forests; also moist to dry, shaded,
disturbed areas.
This species is
a major timber tree in the eastern United States. However, as noted by
Settergren and McDermott (1962), most of the “hard maple” that is sold in Missouri (principally as veneers and boards for flooring)
is imported from elsewhere, because Missouri
sugar maples tend to have unsightly discolorations in their wood. The species
is the principal source of maple syrup and also is cultivated as a shade tree.
The leaves turn various shades of yellow, orange, and red in the autumn.
The A.
saccharum complex consists of a bewildering assortment of names and taxa,
many of which have been treated as separate species at different times in the
past. It is possible that this is an example of a species whose distribution
became fragmented at some point in the past (perhaps in response to
glaciation), with subsequent morphological divergence of geographically
isolated groups of populations over time. These segregates then presumably came
into contact again and hybridized as secondary hardwood forests developed
following the rapid destruction of primary forests in the eastern United States
for agriculture and logging during the past two centuries. The present
treatment follows that of Desmarais (1952), whose detailed study of
morphological variation provides a good starting point for further
investigations into interrelationships among taxa in the group. Readers should
note that there is marked intergradation between the four subspecies recognized
below.
The analysis of
Desmarais (1952) is notable for documenting the large range of specimens that
are intermediate between the black and sugar maple morphotypes, which many
botanists continue to treat as distinct species. Regional floras of midwestern
states, including Voss (1985) and the present volume, also have noted the
existence of large numbers of sugar maple trees with at least some black maple
characteristics. In addition to morphological features, such trees seem to
combine the faster growth rate of sugar maple with the greater drought
resistance of black maples. The decline of oak regeneration and concomitant
invasion of the oak-hickory-dominated forests by sugar maples in Missouri has been of
grave concern to foresters and ecologists (Wuenscher and Valiunas, 1967; Nigh et
al., 1985; Pallardy et al., 1988, 1991). It has been attributed to fire
suppression following European settlement of the region that promoted
succession to a more mesic forest type. It also may have been aided by the
ecological adaptations of sugar maple stocks potentially containing genetic
materials of black maples conferring greater drought resistance on such trees.