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Instructors' Note
A student in Forest Ecology (fall 2002) at Georgetown University (GU), Washington, D.C., produced this report as an individual class project which had a limit of about 20 hours.
Goals of the projects included: (1) learning about a relatively natural forest (Glover-Archbold Park which is adjacent to Georgetown University), the "open urban forest" of GU Campuses, and the "urban forest" in residential neighborhoods near GU. (2) providing information about these subjects to interested parties via the Internet. These are short projects that lay groundwork for further investigation in their respective areas. Therefore, these are pilot projects. To see all of the 2002 projects, please use the keyword FE2002R on this Website. Projects of future Forest Ecology students might continue lines of investigation of the 2002 projects.
E. M. Barrows and N. Bakkour, instructors
Biodiversity in the Leavey Center Urban Forest, Georgetown University, Washington, D.C.
Jeremy S. Asnis
Forest Ecology 355
Fall 2002
Georgetown University
Abstract
This study examines the number and frequency of native and alien-invasive plants in the Leavey Center “Urban Forest” (LEUF, Leavey Esplanade), Georgetown University, Washington, D.C. I hypothesized that the roof garden is dominated by alien invasive plants (those not native to the Washington, D.C., Area). After surveying the LEUF and recording the approximate frequency of each species present, this hypothesis was at least partially supported. Regarding species diversity, slightly less than half of plants surveyed were native. However, in terms of species frequency, non-native plants greatly outnumbered native species. I then speculate on the logic of the LEUF planting strategy, considering both the potential detriment to diversity and boon to student enjoyment the current Esplanade atmosphere offers.
Introduction
While the total number of species found in the Leavey Center “Urban Forest” (LEUF, Leavey Esplanade) was over 20, more were probably overlooked. Of those examined, a few species were particularly notable, either for their abundance or general prominence in the roof garden design. One such species is the London Plane Tree (Platanus x acerifolia). In a sense, the Leavey Center urban forest is organized around the 12 London Plane Trees that transverse the main garden in two rows of six. Much garden symmetry occurs in the LEUF, for example, Liriope beds and Holly plantings.
Despite being a centerpiece of the LEUF, the London Plane Tree is actually alien to the Washington, D.C., Area. The tree is common in many urban areas, which may be due to its resistance to stressful urban conditions (Queensu 2002). Some of this resistance may be derived from the fact that the this Tree is a hybrid (P. orientalis x P. occidentalis) (Chengappa 2002). As is seen in the LEUF, the bark of the Plane Trees often flakes off in patches, leaving a somewhat mottled appearance. The leaves vary, although they are usually palmately lobated with three, five, or seven lobes and appear somewhat maple-like. The London Plane Tree can reach a maximum height of 100 feet. Despite its prominent role as a center piece for the LEUF, the Northern Red Oak (Quercus borealis) was actually first specified for the Forest, but died soon after planting. According to Mr. Michael Pontti, “My best guess is the trees perished from lack of maintenance in the contractor's holding yard prior to planting. They were irrigated very infrequently and weren’t mulched. I called it to the attention of the general contractor, but nothing much came of that warning.” When no similarly sized trees could be located, Mr. Pontti specified Platanus x acerifolia because its characteristics were well suited to a roof garden microenvironment, although it may not necessary fit in the greater Washington, D.C., environment. That is, the London Plane Tree has a “high tolerance to atmospheric impurities and it is also the most resistant to diseases” (Botany 2002), precisely the sort of conditions it might face in the LEUF. Mr. Pontti also noted that these trees were particularly tolerant of “low moisture levels and compaction of the root zone area” – features he had observed in London Plane Trees in China.
While perhaps not the most conspicuous flora on the LEUF, Liriope (L. muscari and L. spicata) was used as ground cover in the LEUF, and was by far the most frequently recorded plant in terms of square feet occupied. Also an alien that is slightly invasive, Liriope is native to the Philippines and China. In the LEUF, Liriope can grow up to 1 foot tall. When Liriope flowers in the summer and fall, tall stalks with bell-shaped lavender or white flowers appear (Botany 2002). Like the London Plane Trees, Liriope seems particularly suited to the LEUF’s shallow planters because it is tolerant of dry soil.
In six symmetrically-placed planters along the southern edge of the LEUF, Liriope is the ground cover for another key LEUF tree – Snowdrift Crabapple trees of unknown parentage (Malus ‘Snowdrift’), which may or may not be native to the Washington, D.C., Area. Although it is a smaller in the LEUF, ‘Snowdrift’ can reach a maximum height 21 feet (Mpelectric 2002). This is also the spread of the Crabapple, but in the relatively small Leavey planters (approximately 4 by 6 feet), it is unclear if these trees will be able to reach their maximum height and spread. However, like the Liriope and the Plane Trees, ‘Snowdrift’ is particularly tolerant of most soil conditions and moisture levels (Mpelectric 2002). ‘Snowdrift’ also has white flowers and orange fruits. While these trees normally flower in the spring, Professor Edward Barrows reported having observed a large fall bloom during 2001, and I also observed flower bud and flowers during October 2002. I don’t know the reason for fall flowering, but one potential reason may be the weather. The trees may have begun to flower in response to September rains that arrived after a very dry summer.
Materials and Methods
A main purpose of this study was to catalog both the diversity and frequency of the plants in the LEUF. I surveyed the plants, and after identifying the representative species with help of Professor Barrows and Mr. Pontti, I tallied the frequency and approximate area covered by many species. Results I found 25 species in the LEUF (Table 1). This table lists all species found in the LEUF, their frequency, and their status as native, alien, alien invasive. For trees, I approximated the area under their canopies. If frequency is listed as low, single or small groups of the species may have been observed as “weeds” in otherwise uniform patches of planted plants. Native and alien invasive plants each covered about 40% of the area, and alien, noninvasive plants covered about 20% of the area (figure 1).
Discussion
My survey showed that non-native (alien or alien invasive) plants greatly outnumbered native plants in the LEUF. This distribution appears to have two major impacts on the students of Georgetown and the greater Washington, D.C., environment. The first impact is practical in nature and concerns the alien invasive subgroup – planting flora with the ability to spread and out-compete native plants in close proximity to native forests such as Glover-Archbold Park is worrisome. Although these invasives are currently contained to the LEUF, animals, wind, or other disperal agents could potentially spread these plants beyond the LEUF and into the surrounding environment. Particularly in the case of Japanese Wisteria and English Ivy, the chance spreading from the LEUF and into surrounding areas could have unfortunate results because of the vigorousness of these species. While careful attention to the maintenance of the LEUF might help this problem, it would seem that there is a chance of such a spread happening. The second major concern that the LEUF brings up is mainly philosophical. That is, does is make sense for a centerpiece garden on Georgetown’s Main Campus to be mainly devoted to plants that do not represent the surrounding native diversity? In short, the author believes the answer is yes. The LEUF is primarily utilized as a retreat for students and a venue for gatherings and events, not as an ecological study lab. Thus, the LEUF must primarily meet that objective. According to students interviewed for this paper, it does. Of eight students polled, all had attended at least one event in the LEUF, and a few had repeatedly returned to relax or study there. Because the LEUF seems to have facilitated this involvement to at least some degree, it appears to be doing its “job” successfully. That said, this function could ideally be accomplished by native species, satisfying both concerns. However, economic and plant survival concerns may or may not dictate otherwise, and further investigation would be needed to assess the feasibility of populating a major campus space with only native plants. While this might not impact the current LEUF, it could be useful when planning the next campus garden.
Acknowledgement
I thank Dr. E. M. Barrows and Mr. M. Pontti for helping me identify all of the plants in the Leavey Center Urban Forest.
Literature Cited
http://www.queensu.ca/pps/grounds/arboretum/London-planetree.htm. (2002 10 29)
http://www.chengappa.demon.co.uk/planes/text/acerifolia.html. (2002 10 29)
http://www.botany.com. (2002 10 29)
http://www.mpelectric.com/treebook/fact58.html. (2002 10 29)
Table 1. Frequencies and statuses of plants growing in the Leavey Center Urban Forest.
|
Common Name |
Taxon
|
Frequency |
Status |
|
Virginia Three-sided Mercury |
Acalypha virginica |
Low |
Native |
|
Ash-leaved Maple |
Acer negundo |
Low |
Native |
|
Tree-of-heaven |
Ailanthus altissima |
Low |
Alien invasive |
|
Wild Garlic |
Allium vineale |
Low |
Alien invasive |
|
Porcelainberry |
Ampelopsis brevipedunculata |
Low |
Alien invasive |
|
Moss |
Bryophyta |
Low |
Native |
|
Smoke Tree |
Cotinus sp. |
5 trees @ 5 ft. tall |
Native |
|
Honeyvine |
Cynanchum leave |
Low |
Native |
|
Yellow Nut Sedge |
Cyperus esculentus |
Low |
Alien invasive |
|
English Ivy |
Hedera helix |
10 x 30 ft. patch |
Alien invasive |
|
Holly |
Ilex aquifolium x Ilex cornuta |
13 trees @ 8 ft. tall |
Alien |
|
Winter Jasmine |
Jasminum nudiflorum |
3 x 20 ft. patch @ 3 ft. tall |
Alien invasive |
|
Eastern Red Cedar |
Juniperus virginiana |
10 x 10 ft. patch @ 8 ft. tall |
Native |
|
Turf-lily |
Liriope muscari |
550 sq. feet |
Alien invasive |
|
Crabapple |
Malus ‘Snowdrift’ |
6 trees @ 5 feet |
Alien |
|
White Mulberry |
Morus alba |
Low |
Alien invasive |
|
Virginia Creeper |
Parthenocissus quinquefolia |
Low |
Native |
|
Eastern Sycamore |
Platanus occidentalis |
Low |
Native |
|
London Plane Tree |
Platanus x acerifolia |
12 trees @ 20 – 50 ft. |
Alien |
|
Wild Black Cherry |
Prunus serotina |
Low |
Native |
|
Black Locust |
Robinia pseudoacacia |
Low |
Native |
|
Spirea |
Spirea sp. |
150 sq. ft @ 3 ft tall |
Alien |
|
Dandelion |
Taraxicum officinale |
Low |
Alien invasive |
|
|
Viburnum cv. |
150 sq. ft @ 5 ft tall |
Alien |
|
Wisteria |
Wisteria floribunda |
6 vines @ 10 ft. tall |
Alien invasive |
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