Associated Forest Cover
provided by Silvics of North America
Throughout its range, American elm seldom grows in pure stands and
is usually found in mixture with other species. It is a major
component of four forest cover types: Black Ash-American Elm-Red
Maple (Society of American Foresters Type 39), Silver
Maple-American Elm (Type 62), Sugarberry-American Elm-Green Ash
(Type 93), and Sycamore-Sweetgum-American Elm (Type 94). It is a
minor component in 20 other forest types.
Black Ash-American Elm-Red Maple (Type 39) appears throughout the
Northern Forest and into the Boreal Forest in Canada, and
throughout the Lake States and into the northern edge of the
Central Forest. In this type the most common associates, other
than the type species, are as follows: In the Lake States and
Canada, balsam poplar (Populus balsamifera), balsam
fir (Abies balsamea), and yellow birch (Betula
alleghaniensis); in Ohio and Indiana, silver maple (Acer
saccharinum), swamp white oak (Quercus bicolor), sycamore
(Platanus occidentalis), pin oak (Quercus palustris),
black tupelo (Nyssa sylvatica), and eastern
cottonwood (Populus deltoides); in New England and
eastern Canada, sweet birch (Betula lenta), paper birch
(B. papyrifera), gray birch (B. populifolia), silver
maple, and black spruce (Picea mariana); and in New York,
white ash (Fraxinus americana), slippery and rock elms
(Ulmus rubra and U. thomasii), yellow birch,
black tupelo, sycamore, eastern hemlock (Tsuga canadensis),
bur oak (Quercus macrocarpa), swamp white oak, and
silver maple.
Silver Maple-American Elm (Type 62) is common throughout the
Central Forest and extends into Canada. Major associates in this
type are sweetgum (Liquidambar styraciflua), pin oak,
swamp white oak, eastern cottonwood, sycamore, green ash (Fraxinus
pennsylvanica), and other moist site hardwoods.
Sugarberry-American Elm-Green Ash (Type 93) is found throughout
the Southern Forest within the flood plains of the major rivers.
Hackberry (Celtis occidentalis) replaces sugarberry (C.
laevigata) in the northern part of the range. Major
associates are water hickory (Carya aquatica), Nuttall
(Quercus nuttallii), willow (Q. phellos), water
(Q. nigra), and overcup (Q. lyrata) oak,
sweetgum, and boxelder (Acer negundo).
Sycamore-Sweetgurn-American Elm (Type 94) appears as scattered
stands throughout the Southern Forest region and lower Ohio River
Valley. Common associates include green ash, sugarberry,
hackberry, boxelder, silver maple, cottonwood, black willow (Salix
nigra), water oak, and pecan (Carya illinoensis).
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Climate
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Within the natural range of American elm, the climate varies from
warm and humid in the southeast to cold and dry in the northwest.
Average temperatures are as follows: January, from -18° C (0°
F) and below in Canada and 16° C (60° F) in central
Florida; July, from 16° C (60° F) in Manitoba to 27°
C (80° F) in the Southern States; annual maximum, 32° C
(90° F) to 35° C (95° F) in the Northeast and 38°
C (100° F) to 41° C (105° F) in the South and
West; annual minimum, from -40° C (-40° F) to -18°
C (0° F) in the North and -18° C (0° F) to -1°
C (30° F) in the South.
Average annual precipitation varies from a scarce 380 min (15 in)
in the Northwest to a plentiful 1520 mm (60 in) on the gulf
coast. Over the central part of the species range there are about
760 to 1270 min (30 to 50 in) per year. Throughout the range most
of the precipitation comes during the warm (April-September)
season. Average annual snowfall generally varies from none in
Florida to about 200 cm (80 in) in the Northeast. A few areas,
mainly around the Great Lakes, get 254 to 380 cm (100 to 150 in)
of snow per year.
The average frost-free period is about 80 to 160 days for the
northern tier of States and Canada to about 200 to 320 days for
the gulf coast and Southeastern States.
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Damaging Agents
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Since 1930, when Dutch elm disease
reached the United States in a shipment of elm logs from Europe,
it has spread to 41 States from coast to coast. The causal
fungus, Ceratocystis ulmi, is introduced into the sap
stream of twigs or small branches during feeding by the smaller
European elm bark beetle, Scolytus multistriatus, and the
native elm bark beetle, Hylurgopinus rufipes. Dutch elm
disease is characterized by a gradual wilting and yellowing of
the foliage, usually followed by death of the branches and
eventually the whole tree (5,14).
In addition to Dutch elm disease, several other diseases also are
responsible for losses in shade and forest elms. Phloem necrosis,
caused by a virus (Morsus ulmi) is detected by flagging
or browned leaves and butterscotch-colored phloem with a
wintergreen odor. It is transmitted by the whitebanded elm
leafhopper (Scaphoideus luteolus) and through root
grafts. Trees usually die within a year after symptoms appear.
Verticillium wilt (Verticillium albo-atrum) is soil borne
and usually enters host plants through the roots. Trees show
dieback symptoms similar to Dutch elm disease (10). Other
diseases include diebacks caused by Cephalosporium spp.
and Dothiorella ulmi; a leaf black spot (Gnomonia
ulmea); twig blight (Cytosporina ludibunda); cankers
(Nectria spp., Sphaeropsis ulmicola, and Phytophthora
inflata); elm wetwood (Erwinia nimipressuralis); and
elm mosaic virus (3,4). Some of the common wood rot fungi
are Pleurotus ulmarius, P. ostreatus, Armillaria mellea,
Ganoderma applanatum, Phellinus igniarius, and numerous
species of Polyporus.
American elm is attacked by hundreds of insect species including
defoliators, bark beetles, borers, leaf rollers, leaf miners,
twig girdlers, and sucking insects. The carpenterworm (Prionoxystus
robinae) bores into the sapwood and degrades the wood. Among
the insects that defoliate elm are the spring cankerworm (Paleacrita
vernata), the forest tent caterpillar (Malacosoma
disstria), the elm leaf beetle (Pyrrhalta luteola), the
whitemarked tussock moth (Orgyia leucostigma), the elm
spanworm (Ennomos subsignaria), and many other
leaf-eating insects that attack elm and other hardwoods. The elm
cockscombgall aphid (Colopha ulmicola) forms galls on the
leaves but does little damage to the tree. Several scale insects
attack elm and may cause damage. Both the elm scurfy scale (Chionaspis
americana) and the European elm scale (Gossyparia spuria)
are widely distributed. Among the leafhoppers, the
whitebanded elm leafhopper is classed as a serious pest since it
is the vector for phloem necrosis (15).
Besides insect and disease losses, animal damage, and fire,
climatic factors also can have an impact on survival and growth
of American elm. Young forest trees may sunscald when exposed by
harvesting or thinning operations. Open-grown American elm forks
and develops a widespread crown that is susceptible to injury by
heavy, wet snows and glaze storms. Of 37 tree species examined
after an ice storm in Illinois, American elm ranked fourth in
susceptibility to ice damage. In dense stands, such injuries are
less severe and are not generally a management problem. Although
American elm is shallow rooted in wet soils, it is fairly
windfirm because the roots are widespread.
The species is reasonably drought resistant, but prolonged drought
reduces growth and may cause death. During the drought of 1934,
in the Midwest prairie region, losses of American elm and
associated species ran as high as 80 to 90 percent. The 1951-54
drought also caused severe losses in the bottom lands of the
South where American elm was more susceptible to drought than the
lowland red oaks. Prolonged spring floods may cause death or
growth loss. Despite suitable temperatures, in Minnesota bottom
lands root elongation does not begin until the spring floods
recede and soil aeration increases. On these sites and where
trees are planted between street and sidewalk, buttress roots
often are a result of inadequate soil aeration.
Fire damage is not a major management problem in the North;
however, in southern bottom lands, fall and sometimes early
spring fires are extremely damaging. Fires can kill seedling- and
sapling-size trees and wound larger trees, thus admitting
heartrot fungi.
Animal damage to American elm, from the sapling stage to maturity,
is not a serious problem except for sapsucker injury that
degrades the wood.
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Flowering and Fruiting
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The process of flowering, seed
ripening and seed fall in American elm takes place in the spring
throughout the range. The glabrous flower buds swell early in
February in the South and as late as May in Canada. The flowers
appear 2 to 3 weeks before leaf flush. Soon after wind
pollination occurs, the fruit ripens, and seed fall is usually
complete by mid-March in the South and mid-June in the North.
American elm flowers are typically perfect and occur on long,
slender, drooping pedicels, about 2.5 cm (1 in) long, in 3- or
4-flowered short-stalked fascicles. The anthers are bright red,
the ovary and styles are light green, and the calyx is green
tinged with red above the middle. With controlled pollinations,
floral receptivity is greatest when stigma lobes are reflexed
above the anthers. The trees are essentially self-sterile. A test
in Canada showed only 1.5 percent viable seed from
self-pollinated flowers. Pollination may be hampered in a wet
spring since the flower anthers will not open in a saturated
atmosphere (9).
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Genetics
provided by Silvics of North America
The study of genetics in American elm has been primarily directed
toward combining resistance to Dutch elm disease with desirable
growth Characteristics. Only a few selections from American elm
look promising at this time. Noteworthy is the "American
Liberty" elm, a multiclonal variety selected from
second-generation crosses of the most resistant parents. Despite
high selection intensity, their resistance is still inferior to
resistant cultivars derived from Asian or European sources.
A few horticultural forms have been recognized. These are Ulmus
americana columnaris, a form with a narrow columnar head,
U. americana ascendens, with upright branches, and U.
americana pendula, with long pendulous branches.
Hybridization within the genus Ulmus has been aimed
primarily at breeding for Dutch elm disease and phloem necrosis
resistance. Because of the difficulty of hybridizing American
elm, which has a chromosome number twice that of all the other
elms (56 versus 28), most of the breeding and selection work does
not include American elm. Thousands of attempts to cross the
American with the Siberian elm have failed. Reports of successful
artificial hybridization and verification of hybridizing American
elm with other elms are rare.
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Growth and Yield
provided by Silvics of North America
American elm seldom grows in pure stands
and there is no information on stand yields. On good sites in
dense forest stands American elm may reach 30 to 38 m (98 to 125
ft) in height and 122 to 152 cm (48 to 60 in) in d.b.h., with a
15 m (49 ft) clear bole. On medium sites, heights of 24 m (80 ft)
are common. On very wet soils or on the very dry soils of the
Plains, however, the species is often only 12 to 18 m (40 to 60
ft) tall at maturity. In open-grown or sparse stands, the trees
usually fork near the ground and form wide arching crowns.
American elm is a long-lived species, often reaching 175 to 200
years, with some older than 300 years.
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Reaction to Competition
provided by Silvics of North America
American elm is classed as
intermediate in shade tolerance among the eastern hardwoods.
Usually it responds well to release, often growing more rapidly
than its associates at advanced ages. Once it becomes dominant in
a mixed hardwood stand, it is seldom overtaken by other species.
It can persist in the understory of pioneer species such as
eastern cottonwood, black willow, and quaking aspen (Populus
tremuloides) but dies if suppressed by tolerant sugar maple
or American beech (Fagus grandifolia).
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Rooting Habit
provided by Silvics of North America
The depth of rooting varies with soil
texture and soil moisture. In heavy, wet soils the root system is
widespread and within 0.9 to 1.2 m (3 to 4 ft) of the surface. On
drier medium-textured soils, the roots usually penetrate 1.5 to
3.0 m (5 to 10 ft). In deep, relatively dry sands in the Dakotas,
American elm may develop a taproot reaching 5.5 to 6.1 m (18 to
20 ft) down to the water table.
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Seed Production and Dissemination
provided by Silvics of North America
Seed production in
American elm may begin as early as age 15 but is seldom abundant
before age 40. When mature, American elm is a prolific seed
producers Trees as old as 300 years have been reported to bear
seeds. In closed stands, seed production is greatest in the
exposed tops of dominant trees. The winged seeds are light and
readily disseminated by the wind. Although most seeds fall within
91 in (300 ft) of the parent tree, some may be carried 0.4 kin
(0.25 mi) or more. In river-bottom stands, the seeds may be
waterborne for miles. Cleaned but not dewinged seeds average
156,000/kg (70,900/lb).
Adverse weather may reduce the seed crop. Spring frosts can injure
and kill both flowers and fruit. Observations in Minnesota showed
that while nearly ripe seeds were not injured by night
temperatures of -3° C (27° F) for several successive
nights, most were killed a week later when the temperature
dropped to -7° C (19° F) and remained below freezing
for 60 hours.
Mammals and birds also may reduce the seed crop. The flower buds,
flowers, and fruit are eaten by gray squirrels. The seeds are
also eaten by mice, squirrels, opossum, ruffed grouse, Northern
bobwhite, and Hungarian partridge.
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Seedling Development
provided by Silvics of North America
Germination in American elm seed is
epigeal. It usually germinates soon after it falls, although some
seeds may remain dormant until the following spring. While
germination may extend over a period of 60 days, most of the
seeds germinate in 6 to 12 days. Germination is best with night
temperatures at 20° C (68° F) and day temperatures of
30° C (86° F). However, germination is almost as good
when daily temperatures range between 10° C (50° F) and
21° C (70° F). Seeds can germinate in darkness, but
germination increases in light. Seeds also can lie on flooded
ground for as long as 1 month with little adverse effect on
germination, except possibly where siltation occurs in flooded
bottoms.
American elm seedlings can become established on moist litter,
moss, and decayed logs and stumps, but do best on mineral soil.
Although they do grow in full sunlight, seedlings perform best
with about one-third of full sunlight during the first year.
After the first year or two, they grow best in full sunlight.
Seedlings that develop in saturated soils are stunted and
characterized by early yellowing and loss of the cotyledons,
extremely short internodes, and small leaves.
American elm can withstand flooding in the dormant season but dies
if the flooding is prolonged into the growing season. Compared
with other bottomland species, American elm is intermediately
tolerant to complete inundation. Some may be killed by early fall
frosts, but those that survive soon are hardened by temperatures
alternating between 0° C (32° F) and 10° C (50°
F). A constant temperature of 0° C (32° F) for 5 days
also hardens the seedlings enough to avoid frost killing (7).
Studies in Iowa and southeastern Michigan on wet lowland and
upland mesic sites show that despite high mortality from Dutch
elm disease, the next generation will be much like the last.
Although American elm has been essentially eliminated from the
overstory, it is a significant part of the understory and
seedling layers. Some observations suggest that there will be a
shift toward more intolerant species under the dead elms.
American elm may be perpetuated for generations, even though the
average life span of the trees is likely to be reduced. Where
seeds are available, American elm is a prominent early invader of
abandoned fields. On upland sites in the Midwest, fire, as a
natural component of the environment, has kept American elm from
invading the prairies (1,2,12,13).
In determining vegetational patterns and succession, allelopathy
is apparently not as important for species coming in under
American elm as it is for species coming in under sycamore,
hackberry, northern red oak, and white oak. In a test in
Missouri, there was lower productivity and higher percent soil
moisture under all test species but American elm. This apparently
was due to toxic leaf leachate present from the four test
species, but not present in leachate from American elm (11).
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Soils and Topography
provided by Silvics of North America
American elm is most common on flats and bottom lands throughout
its range but is not restricted to these sites. On the southern
bottom-land region, it is found widely in first bottoms and
terraces, especially on first bottom flats, but not in deep
swamps. At higher elevations in the Appalachians, it is often
limited to the vicinity of large streams and rarely appears at
elevations above 610 in (2,000 ft). In West Virginia, however, it
does appear in high coves at elevations of 760 in (2,500 ft). In
the Lake and Central States, it is found on plains and morainal
hills as well as on bottom lands and swamp margins. Along the
northwestern edge of the range, it is usually restricted to
valley bottoms along watercourses.
Although American elm is common on bottom-land soils, it is found
on many of the great soil groups within its range. The soils
include well-drained sands, organic bogs, undifferentiated silts,
poorly drained clays, prairie loams, and many intermediate
combinations.
American elm grows best on rich, well-drained loams. Soil moisture
greatly influences its growth. Growth is poor in droughty sands
and in soils where the summer water table is high. In Michigan,
on loam and clay soils, growth is good when the summer water
table drops 2.4 to 3.0 in (8 to 10 ft) below the surface, medium
with summer water table at 1.2 to 2.4 in (4 to 8 ft), and poor
when topsoil is wet throughout the year. On sandy soils underlain
with clay, growth is medium to good where the summer water table
is 0.6 m (2 ft) or more below the soil surface. Organic soils are
usually poor sites, but those with a summer water table at least
0.6 m (2 ft) below the surface are classed as medium sites for
American elm.
In the South, American elm is common on clay and silty-clay loams
on first bottoms and terraces; growth is medium on wetter sites
and good on well-drained flats in first bottoms (8). In the and
western end of the range, it is usually confined to the silt or
clay loams in river bottoms and terraces. In shelterbelt
plantings on the uplands, however, survival is generally best on
sandy soils where the moisture is more evenly distributed to
greater depths than in fine-textured soils. American elm most
commonly grows on soils of the orders Alfisols, Inceptisols,
Mollisols, and Ultisols.
Soil acidity under stands of American elm varies from acid on some
of the swamp margin sites in the Lake States to mildly alkaline
on the prairie soils. A soil reaction considered suitable for
this species ranges from pH 5.5 to 8.0.
Leaf litter of American elm decomposes more rapidly than that of
sugar maple (Acer saccharum), shagbark hickory (Carya
ovata), white oak (Quercus alba), and northern red
oak (Q. rubra). Under Missouri conditions, the
leaves crumble readily after 18 months on the ground. They have a
relatively high content of potassium and also of calcium (1 to 2
percent). Because its litter decomposes rapidly and contains many
desirable nutrients, American elm is considered a "soil-improving"
species.
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Special Uses
provided by Silvics of North America
Before the advent of Dutch elm disease, American elm was prized
for its use as a street tree. It was fast growing, hardy,
tolerant to stress, and appreciated for its characteristic
vaselike crown. Beautiful shaded streets in many cities attested
to its popularity.
The wood of American elm is moderately heavy, hard, and stiff. It
has interlocked grain and is difficult to split, which is an
advantage for its use as hockey sticks and where bending is
needed. It is used principally for furniture, hardwood dimension,
flooring, construction and mining timbers, and sheet metal work.
Some elm wood goes into veneer for making boxes, crates, and
baskets, and a small quantity is used for pulp and paper
manufacture.
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Vegetative Reproduction
provided by Silvics of North America
Small American elm trees produce
vigorous stump sprouts. Although not documented, some
observations suggest that replacement in dense, undisturbed
bottom-land stands in Minnesota may be by root suckers of mature
trees.
American elm can be propagated by softwood cuttings taken in June
and treated with indolebutyric acid or by leaf bud cuttings. In a
test, greenhouse-grown stock rooted easier than field-grown
stock. Propagation by dormant root cuttings has not been
effective.
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Distribution
provided by Silvics of North America
American elm is found throughout Eastern North America. Its range
is from Cape Breton Island, Nova Scotia, west to central Ontario,
southern Manitoba, and southeastern Saskatchewan; south to
extreme eastern Montana, northeastern Wyoming, western Nebraska,
Kansas, and Oklahoma into central Texas; east to central Florida;
and north along the entire east coast.
The native range of American elm.
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Brief Summary
provided by Silvics of North America
Ulmaceae -- Elm family
Calvin F. Bey
American elm (Ulmus americana), also known as white elm,
water elm, soft elm, or Florida elm, is most notable for its
susceptibility to the wilt fungus, Ceratocystis ulmi.
Commonly called Dutch elm disease, this wilt has had a tragic
impact on American elms. Scores of dead elms in the forests,
shelterbelts, and urban areas are testimony to the seriousness of
the disease. Because of it, American elms now comprise a smaller
percentage of the large diameter trees in mixed forest stands
than formerly. Nevertheless, the previously developed silvical
concepts remain basically sound.
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