DISTRIBUTION OF INVASIVE PLANTS IN AN EASTERN FOREST FRAGMENT
David Bedell
SUNY ESF Global Environment Online, Spring 2009
ABSTRACT -- Douglas Park is a 17-acre woodland park in Sleepy Hollow, New York. Though the
park appears to be one of the best-preserved natural places in Sleepy Hollow, no inventory of
invasive plants has ever been done. Due to the park’s proximity to heavily developed areas,
including suburban neighborhoods and a cemetery, I hypothesize that the park should have
extensive infestation by invasive plants. An inventory of the invasive plants in the park was
performed in August 2009. 14 species of invasives were identified and their distributions mapped.
Confirming my hypothesis, there were at least some invasive plants in all areas of the park, and
their distribution was clearly linked to human disturbance pathways.
Introduction
The eastern deciduous forest once stretched nearly unbroken from Maine to Georgia. Leslie Sauer, of
Andropogon Associates, summarizes the eastern forest’s history1:
Before settlement by Europeans, most of the eastern United States was covered by what is known as
the eastern deciduous forest, a dense and multilayered forest interrupted only by rocky outcrops,
large rivers, and coastal wetlands. Native peoples cleared some land in their use of forest resources
and for agriculture and burned even larger areas to manage for game and other resources, but the
forest remained largely intact. The early European colonists, however, caved towns, pastures, and
croplands out of the forest and maintained them by constant control of natural growth. The early
forest industries, which harvested timber for firewood, charcoal, and building, further fragmented
the landscape.
Douglas Park is owned and managed by the Village of Sleepy Hollow. The park is approximately 17 acres,
approximately 16 acres of which is wooded. Formerly a part of the estate of William Douglas, a lawyer, the
land was donated to the Village of North Tarrytown, as Sleepy Hollow was then known, in 19392.
The park is surrounded by landscapes that have been heavily, often completely, changed by human activity.
See Figure 1.
The Sleepy Hollow Cemetery borders the park on the north and west, across the Pocantico River. The
cemetery was founded in the 17th century and expanded over time. Though isolated native woodlands exist
within the cemetery, most of its land is landscaped with exotic plants.
The Old Croton Aqueduct State Historic Park and the former roadbed of Gory Brook Road are on Douglas
Park’s eastern edge. The Old Croton Aqueduct was built prior to the Civil War to bring drinking water to
New York City. Forming a horizontal ledge, the Aqueduct acts as a detention area for stormwater runoff
from the farm meadows above, with water then flowing into Douglas Park via two semi-natural channels.
This is a significant change in hydrology from natural conditions.
The main entrance to Douglas Park is from New Broadway Avenue, in the Webber Park neighborhood.
Webber Park is a semi-urban neighborhood with small lots and walking access to shopping, schools, and
public transportation. Half of the housing in the neighborhood was built before World War II, and about
half after. The proximity of housing to natural places has been shown to be a strong factor in invisibility3.
As a forest fragment, the park lacks the size necessary to resist and recover from disturbances 4. Because
Douglas Park has a large proportion of edge habitat, it should be particularly vulnerable to invasion by
aggressive exotic plants. Invasives have been identified as the most significant threat to biodiversity after
1
outright habitat loss. The United States Forest Services defines a species as invasive if (1) it is nonnative to
the ecosystem under consideration, and (2) its introduction causes or is likely to cause economic or
environmental harm or harm to human health 5. Despite the likelihood that Douglas Park suffers infestation
by invasive plants, no inventory has ever been done6.
Figure 1. Douglas Park (bottom center) and surrounding lands.
2
Methods
Douglas Park was surveyed on foot in August 2009 by the author. The location of individual plants and
areas of infestation was determined using paper maps7, satellite photography8, and the author’s knowledge
of the park, and recorded on paper maps.
The data was visually copied from the paper maps into Adobe Photoshop. The distribution of each species of
plant was placed in a separate Layer, allowing multiple views of the data to be created by toggling Layer
visibility.
Plants were identified as invasive if they appeared on the USDA National Invasive Species Information
Center invasives list or if they were exotic and appeared to be exhibiting invasive tendencies 9.
Results
The survey found 4 exotic invasive tree species, 2 opportunistic native species, 6 exotic invasive shrub and
woody vine species, and 2 invasive herbaceous species. See Figure 2.
Trees
Acer platanoides
Ailanthus altissima
Aralia elata
Robinia psuedoacacia
Morus alba
Catalpa speciosa
Norway Maple
Tree-of-Heaven
Devil’s Walking Stick
Black Locust
White Mulberry
Hardy Catalpa
Shrubs and Vines
Rubus phoenicolatus
Rosa multiflora
Berberis thunbergii
Celastrus orbiculatus
Euonymus alatus
Lonicera japonica
Wineberry
Multiflora Rose
Japanese barberry
Oriental Bittersweet
Winged Euonymous
Japanese Honeysuckle
Herbs and Grasses
Microstegium vimenium
Fallopia japonica
Japanese Stiltgrass
Japanese Knotweed
A great diversity of native trees grow in Douglas Park. The most common are Liriodendron tulipifera (Tulip
tree), Fagus grandiflora (American beech), Betula lenta (Black birch), Quercus rubra (Red oak), Quercus
alba (White oak), Acer rubrum (Red maple), Acer saccharum (Sugar maple), Carpinus caroliniana (American
hornbeam), Sassafras albidum (Sassafras), Platanus occidentalis (Sycamore), and Tsuga canadensis
(Hemlock).
The southern part of the park is dominated by large tulip trees. Bands of heavy stands of beech run roughly
east-west in several places. The center and north of the park have an overstory of large white and red oaks.
Few native shrubs are left in Douglas Park. Virtually all of these are Hamamelis vernalis (Witch hazel) and
Viburnum acerifolium (Arrowleaf viburnum). These dominate the northwest corner of the park but are only
sporadically found elsewhere.
3
Figure 2. Distribution of invasive plants in Douglas Park.
4
In addition, several trees not native to the eastern forest were noted that were planted by the Village:
Trees
Acer platanoides
Pyrus calleryana 'Bradford’
Picea pungens
Norway Maple
Bradford Pear
Blue Spruce
The pear and spruce were planted along the park’s central path. The Norway maples were planted near the
park’s entrance. One additional planted tree, an Acer rubrum (Red maple), which is native to this area, was
planted too deeply and showed significant bark damage 10.
The following pages contain distributions for the invasive species identified, as well as for Acer palmatum
(Japanese maple), which had a significant range in Douglas Park and should be monitored over time for
invasiveness.
5
Acer platanoides (Norway maple)
The Norway maple was introduced to the United States as an ornamental in the mid-18th century11. It is a
vigorous competitor to native trees, growing faster than native Acer saccharum (Sugar maple). The canopy
of Norway maple is very dense, denying light to native plants, and its roots secreting allelopathic chemicals
that prevent native seedlings from germinating. It seeds prolifically and seedlings can grow in moderate
shade.
Figure 2. Distribution of Acer platanoides (Norway maple).
Norway maple grows in all parts of Douglas Park, but particularly on its eastern border. A long, evenlyspaced line of evenly-aged trees grows between the Old Croton Aqueduct and the former roadbed of Gory
Brook Road, which is now a carriage trail leading into the Greenrock Property. This is a clear example of an
intentional human pathway: These were likely planted as street trees. Numerous younger trees, varying in
size from seedlings to 35-foot trees, surround these original plantings on either side of the Aqueduct. As
evidence of the dense shade and allelopathic qualities of Norway maple, few plants grow under them. Less
than a dozen younger native trees can be found in 500 meters on the Gory Brook Road side, most American
beech and sassafras.
6
Within Douglas Park, the Norway maples found vary from mature trees over 50 feet in height to seedlings.
A more diverse group of younger native trees can be found here, including hornbeam, American beech, black
birch, and shagbark hickory. In three places, Norway maple has made deep inroads into Douglas Park.
Within these wedge-shaped infestations, few or no native trees grow, and under the middle infestation,
virtually no plants of any kind survive.
7
Celastrus orbiculatus (Oriental bittersweet)
Oriental bittersweet is a woody, twining vine native to China and Japan. It was introduced to the United
States as an ornamental in the late 19th century.
Oriental bittersweet can climb and kill mature native trees of all sizes. Host trees are strangled by the
twining stems, shaded by mature vines, and are at higher risk of being toppled in storms because of the
added weight of the vines.
Figure 3. Distribution of Celastrus orbiculatus (Oriental bittersweet).
In Douglas Park, seedlings of oriental bittersweet are common. Larger vines are more rare, but two areas of
Oriental bittersweet exist where trees are being smothered. One is along the Pocantico River in the south of
the park, and the other is on the western edge of the open field near the park’s main entrance. Along the
park’s central path, there are several instances of young vines climbing through young Canadian hemlock
and black birch.
8
Oriental bittersweet is extremely common in the Rockefeller State Park Preserve and along the Old Croton
Aqueduct12. Many are large enough to smother mature trees and set fruit. Smaller vines are common in the
suburban neighborhood to the south of Douglas Park, especially within hedges and along fences.
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Ailanthus altissima (Tree-of-Heaven)
Tree-of-Heaven is a tree native to China. It was introduced to the United States as an ornamental in the
late 19th century. Tree-of-Heaven is strongly allelopathic.
Figure 4. Distribution of Ailanthus altissima (Tree-of-Heaven).
Three Tree-of-Heaven were found in Douglas Park. Two trees grew close together on the park’s border with
the Old Croton Aqueduct. One was large – about fifty feet tall – and the other medium-sized. A small tree
grew within a larger area dominated by multiflora rose. None of the trees showed evidence of fruiting.
Since Tree-of-Heaven is dioeceous, these trees may have been male.
Tree-of-Heaven is common along the Metro North Railroad right-of-way along the Hudson River, and within
the former General Motors assembly plant property at the western end of Beekman Avenue in Sleepy
Hollow13.
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Aralia elata (Devil’s Walking Stick)
Aralia elata is native to China. It strongly resembles the native Aralia spinosa. The differences are minor
(the native tree has longer inflorescences) and taxonomists at the Brooklyn Botanic Garden originally
believed that trees found in the vicinity of New York City were Aralia spinosa expanding its range
northward. Research indicates that the trees are in fact Aralia elata introduced unintentionally.
Figure 5. Distribution of Aralia elata (Devil’s Walking Stick).
Devil’s Walking Stick can be found in most less-shaded areas of northern part of the park, and along the
Pocantico River. These trees are young and have not formed thick stands yet, though some were observed
growing in small groups with many trees in flower.
Devil’s Walking Stick is extremely common in the Rockefeller State Park Preserve 14. Mature, thick stands
crowd out all native plants in numerous areas of the park.
The management of the Rockefeller State Park Preserve has identified Devil’s Walking Stick, along with
Fallopia japonica (Japanese knotweed) to be the two highest-priority invasives management targets15.
11
Acer palmatum (Japanese maple)
Acer palmatum is a tree native to Japan and has been a popular ornamental tree in the United States since
the 19th century. Acer palmatum is not generally considered an invasive plant, but its behavior in Douglas
Park is an example of how invasiveness is dependent on a plant’s behavior in a particular environment.
Figure 6. Distribution of Acer palmatum (Japanese maple).
Because its seedlings can survive within the shade of a closed canopy, Japanese maple is spreading in
Douglas Park, perhaps from seeds of trees planted across the Pocantico River in the Sleepy Hollow
Cemetery. Most examples found are young, with only a few exceeding two meters in height.
Japanese maple is widely planted in the area surrounding Douglas Park 16. There are many examples in the
Sleepy Hollow Cemetery, the Webber Park neighborhood, and in other suburban areas nearby.
Japanese maple has a slow growth rate and few trees in Douglas Park are mature. Most are seedlings
growing in dense shade.
12
Berberis thunbergii (Japanese barberry)
Japanese barberry is a shrub native to Japan. It is a popular ornamental plant in the United States, and
can be found in suburban yards, public parks, and office complexes.
Birds eat and spread its seeds, and it can grow in dense shade. Barberry leafs out early in spring, shading
out native herbs.
Figure 7. Distribution of Berberis thunbergii (Japanese barberry).
Japanese barberry was widely distributed in Douglas Park. It was mostly found as an individual plant, but
forms dense thickets along the trail at the northeast corner of the park and on the trail at the southwest
corner of the park.
It is difficult to separate the impact of Japanese barberry on the herbaceous layer of the forest from that of
heavy browsing by deer. There is little evidence of native herbs anywhere in the park.
Japanese barberry was widely planted in the Sleepy Hollow Cemetery and the Webber Park neighborhood,
and dominates the understory layer in many parts of the Rockefeller State Park Preserve 17. Perhaps the
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worst infestation was found along Peggy’s Way, a Preserve trail about 500m north of Douglas Park along the
Old Croton Aqueduct. Large thickets cover acres of the forest floor.
14
Euonymus alatus (Winged Euonymus)
Winged euonymus is a popular ornamental shrub native to China. It is a widely planted ornamental in the
United States.
Figure 8. Distribution of Euonymus alatus (Winged euonymus).
Winged euonymus exists as single plants or small groups across Douglas Park, though it is not as
widespread as Japanese barberry.
Winged euonymus is planted in Sleepy Hollow Cemetery and the Webber Park neighborhood.
15
Catalpa speciosa (Hardy Catalpa)
Hardy catalpa is large tree native the American Midwest. Its natural range has been extended by human
disturbance of natural areas and by human planting. Its large leaves cast a dense shade that outcompetes
native plants endemic to the northeast.
Figure 9. Distribution of Catalpa speciosa (Hardy catalpa).
Hardy catalpa is rare in Douglas Park. A fairly large tree exists along the Old Croton Aqueduct to the east
of the park. Two younger trees grow near the banks of the Pocantico River.
Several large hardy catalpa grow along the Old Aqueduct Trail about 500 meters to the south of Douglas
Park18. It is not unusual to find an isolated tree in the Rockefeller State Park Preserve.
16
Fallopia japonica (Japanese knotweed)
Japanese knotweed is a tall perennial herb, native to Japan, that can tolerate heavily disturbed sites and
has a rapid rate of growth19. It was introduced to the United States as an ornamental plant in the early
1900’s.
Figure 10. Distribution of Fallopia japonica (Japanese knotweed).
Japanese knotweed is found in dense stands along the Pocantico River, as isolated plants in several parts of
the park near the Pocantico, and as a large, dense stand along the central path.
The stand along the central path likely developed from seeds or root fragments brought in fill used around
an electrical box that was installed by the Village of Sleepy Hollow. It is spreading downhill along the path
of one of the park’s streams. No other plants were observed growing within the Japanese knotweed; the
stand is a pure monoculture.
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Discussion
Human disturbance pathways can either be intentional (the planting of Norway maples as street trees) or
unintentional (the spread of invasive shrubs planted in suburban neighborhoods). The path system in
Douglas Park consists of Village-created and maintained gravel carriage trails and unauthorized “outlaw”
trails. The authorized trails are wide and effectively form long gaps in the forest canopy, creating
additional edge habitat that is favorable to invasives. Outlaw trails link the official Douglas Park trail
system with the Old Croton Aqueduct, the Sleepy Hollow Cemetery, and to several parts of the park along
the river. Outlaw trails can cause accelerated erosion by altering natural drainage patterns.
The pattern of invasive plant establishment is strongly linked to canopy density. With the exception of the
lawns at the south of the park that are regularly mowed by the Village of Sleepy Hollow, the areas of the
park with the most open canopy have the densest invasive shrub cove. Multiflora rose dominates in larger
gaps where more sun is available, which wineberry is most common where sun is less abundant.
Shade-tolerant trees such as maples and beech had a much higher recruitment than trees such as oak and
hickory that require fire and significant canopy gaps. The understory under beech in particular was often
extremely dense. In combination with the denser shade, invasives are much less prevalent in beech stands.
Oak and hickory recruitment was poor in Douglas Park. Few young trees were identified, with even fewer
saplings and seedlings. This is typical of poor recruitment for these species in eastern forests in general.
Lack of recruitment by oaks and dense invasive shrub understory comprise a positive feedback loop: As older
oaks die and open ever larger gaps in the canopy, the additional light fosters invasive growth, inhibiting the
growth of young oaks which might eventually close the canopy.
Conclusion
The eastern deciduous forest exists now only in fragments. Douglas Park, in Sleepy Hollow, New York, is a
small, 17-acre woodland park that has survived relatively intact as the lands around it have been developed
for human use. The park has been extensively infested by invasive plants that have been spread by birds
from adjacent lands, along waterways from upstream infestations, and along authorized and unauthorized
hiking trails. For Douglas Park to retain its character as a natural place for the people of Sleepy Hollow to
enjoy, invasive plants that interfere with natural forest succession must be managed.
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References
1
Sauer, Leslie. The Once and Future Forest. Island Press, 1998.
Steiner, Henry. The Place Names of Historic Sleepy Hollow and Tarrytown. Heritage Books, 1998.
3
Gavier, Gregorio; Stewart, Susan; Huebner, Cynthia D.; Radeloff, Volker C. 2009. Is housing a factor of
invasive plants distribution at coarse and fine scales?. In: McManus, Katherine A; Gottschalk, Kurt W.,
eds. Proceedings. 19th U.S. Department of Agriculture interagency research forum on invasive species 2008;
2008 January 8-11; Annapolis, MD. Gen. Tech. Rep. NRS-P-36. Newtown Square, PA: U.S. Department of
Agriculture, Forest Service, Northern Research Station: 27-29.
4
Sauer, Leslie. The Once and Future Forest. Island Press, 1998.
5 United States Department of Agriculture Forest Service Invasive Species Program,
http://www.fs.fed.us/invasivespecies/definition.shtml.
6
Anthony Giaccio, Sleepy Hollow Village Administrator. Verbal communication, July 2009.
7
Prepared by the author using data from the Westchester County GIS: http://giswww.westchestergov.com/
8
Google Maps, http://maps.google.com/. Data copyright © Google, Inc., and others.
2
9
USDA National Invasive Species Information Center.
http://www.invasivespeciesinfo.gov/plants/main.shtml
10
Noted by Lou Sebesta, NYS DEC Urban and Community Forester, Region 3, during an inspection of the
park in July, 2009. The inspection was requested by the Village of Sleepy Hollow at the behest of the
author.
11 Nowak, David J. and Rowan A. Rowntree. History and Range of Norway Maple . Journal of Arboriculture
16(11): November 1990.
12 Author’s observations, August 2009.
13
Author’s observations, August 2009.
14
Aleks Jarosz, NYS Office of Parks, Recreation, and Historic Preservation, Rockefeller State Park Preserve.
Verbal communication during invasive plant volunteer orientation, August 2009.
15
Aleks Jarosz, NYS Office of Parks, Recreation, and Historic Preservation, Rockefeller State Park Preserve.
Verbal communication, August 2009.
16
Author’s observations, August 2009.
17
Author’s observations, August 2009.
18
Author’s observations, August 2009.
19
USDA Forest Service 1999. Japenese Knotweed (Pest Alert). NA-PR-04-99. Newtown Square, PA:U.S.
Dept. of Agriculture, Forest Service, Northern Area State & Private Forestry.
19