This article was from: http://www.arboretum.harvard.edu/LC/Harvyard/Elm_yard.htm
Ulmus americana - 100ft. x 22ft.
Conway, New Hampshire
Photographed in 1930
Before one can answer the question of whether or not to replant the
American elm in historic landscapes, one should first ask how this tree
came to be so widely planted throughout eastern North America in the first
place? The answer can be found in the horticultural literature of the 1800s,
the most significant of which is a beautiful book by Lorin Dame and Henry
Brooks, published in 1890, Typical Elms and Other Trees
of Massachusetts (see excerpt in Arnoldia 42(2): 49-59, 1982). Because
this book was written long before Dutch elm disease had spread to North
America, and because it is profusely illustrated, it can be considered
a portrait of the species at the pinnacle of its landscape influence. The
book also contains important information on the growth rate of the species
in landscape situations that can never be recreated. Other important writers
from the 1800’s include F. A. Michaux (1819), A. J. Downing (1841), D.
J. Browne (1846), F. J. Scott (1870), G. B. Emerson (1875), and C. S. Sargent
(1890). In reading over this literature, it is clear that the American
elm was widely planted for a number of reasons, only one of which was its
great size and beauty. Other, more pragmatic reasons, and can be summarized
as follows:
Dutch Elm Disease
The Dutch elm disease fungus, along with its dispersal agent, the European
elm-bark beetle,
arrived in North America in 1930, hidden under the bark of a shipment
of European elm burl-logs that had been imported into the United States
for the manufacture of veneer (Roueché, 1961). The disease killed
American elms so quickly and spread so rapidly that people were at first
afraid that the American elm was headed for extinction. Fortunately, this
dire prediction has not materialized. The fact is that the species still
thrives and reproduces as a wild tree in wet woods and along stream banks
throughout eastern North America (Burns and Honkala, 1990). As a landscape
plant, however, the American elm is close to extinction. The grand old
specimens that once graced virtually every town common in New England,
four to five feet in diameter, have been replaced by trees that are seldom
more than two feet across.
Since the 1960s, there has been considerable publicity surrounding
efforts to "bring back" the American elm. The long-term approach has involved
either the selection of DED-resistant American elm cultivars or the hybridization
of DED-resistant elm species with one another. In contrast, the short-term
approach focuses on the preservation of existing specimens of the American
elm and includes a program of spraying for the beetle, injection of infected
trees with fungicides, and the vigilant removal of diseased limbs. While
such extraordinary treatments have succeeded in saving selected individuals
for up to twenty years, the inevitable infirmities of old age are now catching
up with older trees. Such a preservation strategy is, at best, a temporary
solution.
Case Study: The Elms in Harvard Yard
The American elm has been the main stay of the Harvard Yard landscape
for well over a hundred years. While there have been many threats to the
integrity of this planting during this time, none of them have been as
serious as the introduction of Dutch elm disease. In 1979, when Harvard
University began to implement an integrated elm protection program, there
were 285 elms on campus. Most of them were American elms, but there were
also a
An old American Elm in
Harvard Yard
number of English and European elms of uncertain identity mixed in.
Most of the trees had been planted during the early 1900s, and were about
seventy or eighty years old when the protection program began. By 1994,
after fifteen years of treatment, there were only 165 elms left. This amounts
to a 42% mortality. The average cost of the total protection program over
this time period has been approximately $25,000 per year, broken down as
follows:
A mixed planting of old and and new trees in
Harvard Yard
Essentially the numbers indicate that the cost of planting two hundred
new trees was roughly equal to cost of maintaining two hundred and eighty-five
elms for twenty years, after which time only half of them will still be
alive and the other half will be in a state of decline. To put it another
way, twenty years of maintenance of one large elm, with only a 50% chance
of survival, equals the cost of planting one new four to eight inch caliper
tree.
There is no absolute answer to the question of how much one should
invest in an elm protection program. All that can be said with certainty
is that any elm protection program should always be undertaken in conjunction
with a tree planting program of species other than elms. Planting new trees
is an investment in the future that softens the blow when a big elm inevitably
dies.
Achieving Diversity
Unfortunately very few, if any, trees have the same combination of
graceful form and great size that the American elm does. The honey locust,
Gleditsia triacanthos, comes about as close as any tree, but is much slower
growing. Zelkova serrata has roughly the same shape, but is much smaller.
As Koller and Weaver (1982) point out, there is no exact replacement for
the American elm. The key to successful substitution is to look for species
that have the same landscape impact or stature as the American elm, not
necessarily the same structure.
By working within a given genus or family, one can achieve diversity
and uniformity simultaneously. In the Tercentenary Theater part of the
Yard, for example, we chose to group various legumes together which all
share the characteristic arching trunk and
A classic Harvard combination:
Bricks and Elms
flat-topped crown, but are clearly different in other aspects of their
habit. These include: Gleditsia, Sophora, Gymnocladus, and Cladrastis.
Similarly, one can also group different oaks together to achieve a measure
of uniformity amidst diversity. In the oldest section of the Yard, we used
a variety of Quercus species, including rubra, palustris, phellos, coccinea,
alba, bicolor, and acutissima.
As regards the genus Ulmus itself, it must be remembered that not all
elms are equal. Just because a tree is an elm does not mean it can be used
as a replacement for Ulmus americana. The Siberian elm (Ulmus pumila),
for example, is highly resistant to DED but is very messy and quite graceless
in form. The lacebark elm (Ulmus parviflora) is a handsome tree, but much
smaller in stature than its American cousin. Some of the more recent hybrid
elms may eventually turn out to be excellent replacements, but they have
not yet been thoroughly tested (Dirr, 1990; Santamour and Bentz, 1995;
Townsend et al., 1995).
It must also be remembered that there are several other diseases besides
DED that can kill American elms (Sinclair et al., 1987). In particular,
phloem necrosis and elm yellows can be lethal to many of the elms that
have been selected for their resistance to DED (Santamour and Bentz, 1995).
It should also be remembered that the elm leaf beetle, along with a host
of other insects, had been decimating elm plantings long before DED arrived
on the scene. If the American elm is to make a return to the modern American
landscape-be it either as a hybrid or as a disease resistant selection-then
it should be on an equal footing with other trees, never as an all encompassing
landscape theme.
One can summarize the advantages to increasing landscape diversity
can be summarized as follows:
Ulmus americana - Chelmsford Center, Massachusetts
Photographed in 1925
needs to ask, "Bring it back at what density?" Clearly the high density
of American elms in many cities and towns that existed during the first
third of the twentieth century (Dawson and Khawaja, 1985) is not a situation
that ought to be recreated. Indeed, it was the high density of American
elms that allowed the elm bark beetle population to build up rapidly. leading
to the epidemic spread of DED. One sees more elms surviving these days
Ulmus americana - 90ft. x 10ft.
Mansfield, Massachusetts
Photographed in 1925
than in the past not because trees are more resistant to Dutch Elm
Disease than they used to be, but because the reduced elm population has
resulted in lower elm bark beetle populations. This, in turn, allows more
elms to escape detection by their predators.
If "restoring" a given historic landscape means replanting the American
elm-or any of its resistant selections or hybrids-to the density it occupied
historically, then that is a mistake. In the popular literature on elms,
the unspoken assumption always seems to be that if one could only solve
the disease resistance problem, then one could recreate the grand, elm-lined
streets of the past. This idea is biologically unsound. Because of the
dynamic nature of the interaction between host and predator, however, disease
resistance must always be considered a relative phenomenon-always in flux-rather
than a fixed genetic trait. Historical accuracy not withstanding, it is
not in anyone’s interest to bring the American elm back at its former position
of landscape preeminence.
References
Browne, D. J. 1846. Trees of America. Harper & Bros., New York.
Burns, R. M. and B. H. Honkala, eds. 1990. Ulmus americana, in Silvics
of North America, vol. 2,
hardwoods. U. S. D. A. Forest Service Agriculture Handbook 654.
Dame, L. L. and H. Brooks. 1890. Typical Elms and Other Trees of Massachusetts.
Little Brown and
Co., Boston.
Dawson, J. O. and M. A. Khawaja. 1985. Change in street-tree composition
of two Urbana, Illinois
neighborhoods after fifty years: 1932-1982. Journal of Arboriculture
11(11): 344-348.
Dirr, M. 1990. Manual of Woody Landscape Plants, fourth ed. Stipes
Publ. Co., Champaign, Illinois.
Downing, A. J. 1875. A Treatise on the Theory and Practice of Landscape
Gardening, ninth ed.
Orange Judd, Co., New York.
Emerson, G. B. 1875. A Report on the Trees and Shrubs Growing Naturally
in the Forests of
Massachusetts, 2nd ed. Little Brown and Co., Boston
Koller, G. L. and R. E. Weaver, Jr. 1982. Replacing the American elm:
twelve stately trees. Arnoldia
42(2): 88-100.
Michaux, F. A. 1819. Ulmus americana, in The North American Sylva,
vol. 3, translated by A. L.
Hillhouse. Paris and Philadelphia.
Roueché, B. 1961. Profiles: A great green cloud. The New Yorker
July 15, 1961: 35-53.
Santamour, F. S., Jr. and S. E. Bentz. 1995. Updated checklist of elm
(Ulmus) cultivars for use in North
America. J. Arboriculture 21(3): 122-131.
Sargent, C. S. 1890. Ulmus americana, in Silva of North America, vol.
7. Houghton and Mifflin,
Boston.
Scott. F. J. 1870. The Art of Beautifying Suburban Home Grounds of
Small Extent. D. Appleton,
New York.
Shaw, J. 1994. Every tree doomed. Harvard Magazine 96(4): 46-53.
Sinclair, W. A., H. H. Lyon, and W. T. Johnson. 1987. Diseases of Trees
and Shrubs. Cornell U. Press,
Ithaca, New York.
Townsend, A. M. , S. E. Bentz, and G. R. Johnson. 1995. Variation in
response of selected American elm
clones to Ophiostoma ulmi. Journal of Environmental Horticulture 13(3):
126-128.
Van Valkenburgh, M. and P. Del Tredici. 1994. Restoring the Harvard
Yard Landscape. Arnoldia 54 (1):
2-11.
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