Restoring New York City
Proposals for Improving Ecological and Human Health
Edited by Dr. James A. Danoff-Burg
Department of Ecology, Evolution, and Environmental Biology, Columbia University

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The NYC Time Capsule:

An Urban Genetic Diversity Preserve


Emily Muhlhausen


It’s alarming.  Over the past century, tens of thousands of alien invaders have crept past our borders and settled here.  They often move into undesirable areas, reproduce, and spread out, using a disproportionate share of resources.  But this isn’t politics – it’s ecology.  Introduced, exotic, and invasive species are competing for resources and “jobs” in ecological communities – and they’re winning.  An invasive species can be defined as a species that achieves reproductive success and some measure of dominance in an ecosystem to which it has been introduced – usually through human intervention. (So, ecologically speaking, humans are the ultimate invasive – worse than kudzu, zebra mussels, or cane toads - no matter where they’ve immigrated from.)

Commonly, a species is considered exotic or introduced until it wreaks some ecological havoc and damages its adopted ecosystem, at which point it is an invasive. Carla D’Antonio and Jeanne Chambers include a further distinction in their chapter of Foundations of Restoration Ecology – terming species that have caused significant ecological or economic damage “invasive and damaging,” which seems like a useful but fine distinction, resting on the question of significance.  Invasive species can damage an ecosystem several ways – direct or indirect competition with native species for resources; predation or parasitism on native species; interfering in the natural successional regime; and altering the ecosystem beyond the native species’ tolerance levels. 

Parasitism and predation on native species can be one of the easiest ways for exotics to claim the “invasive and damaging” title – just as smallpox devastated the (naturalized) Native Americans while the European settlers who introduced the disease to the New World were blissfully immune, native species have no defensive immunities or adaptations to many foreign parasites and predators that may have been kept in check in their own native habitats.   For example, several of the native tree species that once were dominant or highly prevalent species in the forests of the Eastern US have been nearly wiped out by exotic parasites or predators. 

The American Elm, once a dominant species in four major forest types as well as the most popular street tree in “Main Street” America, is now reduced to isolated stands, solitary survivors, and immature understory in the forests  over which it used to loftily preside.  The culprit behind this arboreal genocide is an Asian fungus, commonly known as Dutch Elm Disease. 

 Asian elm species are adapted to resist this parasite, but when it was introduced in shipments of lumber in the 1920s and 1930s to Europe and subsequently North America the native elms of those areas were devastated.  By the 1970s, the fungus’ destruction was nearly complete – residents of small towns and cities alike watched as the elms of their tree-lined streets and avenues died like dominos. The fungus had spread throughout nearly the entire native range of the elms.  Populations from the extreme north and south of the range, in Florida and Canada, and a few isolated trees and stands throughout were all that survived.

American Elms still grow in the understory of many forests throughout the Eastern US, but the vast majority of the saplings are infected with the fungus before they reach maturity, and the chance of a tree achieving its full lifetime is vanishingly slim.  Some of the rare stands of mature American Elms grow in what seems like an unlikely place – the parks of New York City.  Planted in the late 19th century by Frederick Law Olmstead as he was designing and creating the parks, the stands in Central Park, Riverside Park, and Tompkins Square Park survived in providential isolation while the elms in the residential areas of the city died out completely. 

Now, there is hope that Dutch Elm Disease is waning; the fungus having nearly exhausted its host species.  Although currently, nearly all American Elms are still vulnerable, there may come a time when elms can once again flourish – in 20 years, 50 years, or 100 years.  When that time comes, however, too much of the genetic diversity of the species may have been lost to restore American Elms to their natural extensive range. 

The American Chestnut, once “the hallmark of the Eastern Woodlands” (American Chestnut Cooperators’ Foundation) and an important economic resource for both its wood and nuts, has been devastated by another Asian fungus, the chestnut blight.  Where Chinese and Japanese Chestnut species have adapted to the presence of this fungus, and are thus resistant, American chestnuts had/have no such evolutionary defenses.  Although many chestnut trees still sprout in forests where mature trees used to stand, growing from the root systems of trees that were destroyed in the blight, these saplings are struck down before they reach reproductive age. 

The Eastern Hemlock can live to 800 years old, and commonly surpasses 400, making it one of the best indicators of the ecological history of an area.  Hemlocks can be studied to determine climate variation, historic land use, acid rain levels, and disturbance regimes.  Due to an extremely high shade tolerance, the trees can survive as understory trees for decades or even centuries until space opens up in the forest canopy.   The Hemlock Woolly Adelgid, a species of aphid native to Asia, feeds on the sap of hemlocks at the base of the needles, causing needle drop-off that weakens and eventually kills the tree.  As a result of the amazing reproductive success of the adelgid, and the hemlock’s lack of evolutionary adaptation against the disease, death tolls of the hemlocks are threatening to rival that of the American Elm or Chestnut. 

Dutch Elm Disease, the chestnut blight, and the Hemlock Wooly Adelgid are invasive and devastating species, causing severe ecological and in some cases economic damage to the communities and ecosystems they strike.  Losing these significant and historic tree species entirely is unthinkable, but all-too-possible.  It is essential that the genetic diversity of these species be preserved against the time when the alien invaders can be defeated – by breeding, engineering, or encouraging adaptations for resistance, developing chemical and biological controls, or simply extirpating the invasives from the natural range of the trees. 

New York City, due to its unique geographical situation and urban ecological features, has an opportunity to become a Genetic Diversity Preserve, a time capsule containing a range of genes and alleles to be “opened” when each species can resume its place in the broad ecology of the eastern forests.  Using the island effect and strategic placement of individuals in each meta-population, trees can be isolated from outbreaks and infestations of their predatory species.   The high visibility of New York’s urban ecosystems, enjoyed by millions of New Yorkers and tourists, also presents an advantage – by engaging the public in the effort to protect these heritage species, “foothold situations” can be more readily detected and eliminated. 

Of course, the very urban setting that provides these benefits presents a number of challenges – the ecosystem must tolerate perpetual disturbance, increased exposure to pollution, and provide “ecosystem services” to the human population of the city.  Also, the frightening persistence of the invading species mandates that the success of a Genetic Diversity Preserve will require continued and rigorous observation and maintenance. 

Despite these pitfalls, the Preserve will benefit the economy, ecology, and community of New York City, and eventually, the entire Eastern US. 

Elms, Chestnuts, and Hemlocks will be introduced to the city’s urban ecosystems in three ways.  First, through planting as street trees, individuals will be provided an isolated and conspicuous growing environment, where fungus transmission and insect dispersal are unlikely, and invasions will be readily obvious.  Second, stands of trees will be introduced to barren areas – new parkland, remediated areas, and land where cleanup efforts and invasive removal have left no remaining functional ecosystems.  Third, the desired tree species will be introduced to existing forested areas in city parks.  This last method of introduction requires the most consideration in drafting plans, so as not to disturb what are already functioning ecosystems in their own right. 

Highbridge Park has been selected as an initial proof of concept site for the introduction of these trees.  Although each park site will require a custom restoration plan, Highbridge has the potential to serve as a pattern card for future sites, and provide much needed progress information that will allow for plan modification as the ecological ramifications of the restoration work become clear. 

Highbridge has a motley history compared to the towering vision and scope of city parks such as Central Park and Prospect Park, where Olmstead and Vaux designed urban oases engineered as perfect English gardens.   Most of the park’s extensive acreage was acquired around the turn of the 20th century, as old buildings were condemned and torn down (New York City Department of Parks & Recreation 2000).  The park is named after the High Bridge, a pedestrian walkway built along the Croton Aqueduct (1848), part of the city’s first reliable water supply. Park features include the bridge and the historic water tower, also part of the 19th century water system.  The site is ripe for restoration because it is large (119 acres), underutilized, highly colonized by invasive species, and has extensive “natural areas.”  It is also publicly visible because of an existing restoration project working along the Hudson River shoreline, and reports of petty crime, vagrancy, and prostitution.

“Not far away, in Highbridge Park, which stretches for two miles across Upper Manhattan, the scene was even more grim on a recent weekend. Huge sections of the 119-acre park set aside as natural areas have been taken over by homeless people who have built permanent shacks made of sheet metal and steel pipes driven into the earth. One of the park's residents is a heroin addict and prostitute who would give her name only as Joanne. Her makeshift house has a bed and a nightstand. She said she had lived there for 13 years. Men smoked crack cocaine a few feet from where a youth baseball game was being played….

[Parks Commissioner] Mr. Benepe, who expressed both skepticism and surprise at the park's condition when told about it, said the city's plan was: "Let nature take its course." "Trees are growing, insects are buzzing, oxygen is being produced, and there's nothing wrong with that," he said.”   (Williams 2005).  

Each restoration/introduction plan for a city park will be just that – a two-pronged approach that combines the cleanup and restoration of the parkland with the introduction of time capsule species.  The plan will include a comprehensive site survey, cleanup, removal of invasive (and in many cases exotic) species, and re-introduction of natives and time capsule species in particular. 

The site survey will provide the basis for the rest of the plan.  Using a BEF perspective, ecologists will gather data about the primary productivity, nutrient cycling, carbon budget, and decomposition cycles of the existing ecosystem (Naeem 2006).  Imperatives will include evaluating the soil structure, determining the extant plant and animal species, estimating population sizes, and classifying species as native, exotic, invasive, or invasive and damaging.  Research and observation will provide a basis for determining the role each species plays in the ecosystem.  Particular care will be paid to determining whether exotics or invasives now play a functional role in ecosystem processes – for example, whether a native species may have adapted to the presence of the exotic as a food source or habitat.  If this is the case, the restoration efforts may leave an exotic or an invasive species in place, or search for a niche replacement for an invasive and damaging species. 

At Highbridge Park, surveyors will determine the species composition of the forested areas, and select areas for restoration based on the presence of invasives, state of degradation, and the similarity of habitat to that preferred by the time capsule species.  30 acres in total will be marked for restoration in the initial plan, using plots of approximately 3 acres each.  Sites will be designated as one of five types: control, remediated, invasives removed, natives re-introduced, and time capsule species introduced.    Success of the restoration will be monitored using a BACI time series approach to determine changes in the biodiversity and ecosystem functioning of the five different levels of restoration.  This should provide sufficient data to not only adapt the management plan for each type of site, but to determine best practices for future time capsule restoration plans in Highbridge and other city parks. 

Cleanup and removal of invasive species will need to be intensive.  The New York Restoration Project, working on the Highbridge Shoreline, recovered over 8,000 old tires (New York Restoration Project 2003).  Sites designated as ‘remediated’ will only be cleaned up, while the other three types of sites (excluding the control sites as well) will also undergo invasives removal. 

Norway maple and other invasives like garlic mustard have been observed in the park and are notoriously difficult to remove.   In the case of Norway Maple, physical removal seems to be the best strategy (Webb 2001).  Canopy trees will be girdled or sawn down, while seedlings will be clipped.  Webb, Pendergast, and Dwyer (2001) showed that Norway Maple responds favorably to soil disturbance, so neither mature trees or saplings will be uprooted.  Both top-down and bottom-up controls (D’Antonio and Chambers 2006) will be necessary to ensure invasives are fully removed and cannot re-colonize the ecosystem.  Sites designated as ‘invasives removed’ will only be restored to this point, in order to determine the successional structure of the sites.  The other two types of site will have native species re-introduced. 

After the removal and cleanup processes, there will be available niches and ecosystem roles that were previously occupied by the invasive species.  To keep the ecosystem functioning as fully as possible, these niches will be filled with native species that were outcompeted by the invasives, but share the same ecosystem role.  For example, Norway maple will be replaced with sugar maple, its close relative that has been largely vanishing from forests where Norway maple has taken hold.  The similar biology of the two trees should insure that any species (of Lepidoptera, primarily) that feed on or inhabit the Norway maple will be able to utilize the sugar maple instead.   Using data collected through the BACI time series experiment, comparing the Lepidoptera species in the control sites and in the sites where Norway maple has been removed and replaced with sugar maple, we can determine if the sugar maple is an appropriate replacement. 

The three time capsule species previously mentioned share a similar, very broad range and tolerance levels, and so can be introduced throughout a wide range of the acreage available at the park.  Small differences, such as the Hemlock’s preference for higher altitudes and cliff sides, the Chestnut’s for well-drained soils, and the Elm’s for lowlands and floodplains, will determine where to preferentially place higher concentrations of the trees.  The  extensive topographical heterogeneity of Highbride Park will allow extremely detailed site selections. 

The time capsule species, in addition to being a living genetic diversity preserve, will restore important ecological functions to the park.   The Eastern Hemlock provides highly specialized habitat to certain types of birds, including warblers and wood thrushes (Save Our Hemlocks).  The American Chestnut provides food for numerous species of wildlife, and can be expected to stimulate the food-web interactions at the park by providing another food source for birds, squirrels, and other small mammals.  This in turn will stimulate the higher trophic levels such as the pair of peregrine falcons and the population of red-tailed hawks that nest in the park.   All these time capsule species can be expected to benefit the New York portion of the migratory Flyway, as their natural ranges coincide with a great number of pathways for migratory birds. 

Other time capsule species that will be introduced where appropriate to fill the ecosystem roles left by the removal of garlic mustard and Japanese knotweed, among others, are two of New York’s six threatened or endangered plant species. Northern wild monk's-hood, a herbaceous perennial, shares many habitat requirements with the Hemlock, preferring cool stream banks or cliff sides.  Hart's-tongue fern, which thrives in moist, sheltered locations, will be planted alongside the elm trees (New York State Department of Environmental Conservation).  

Introduced individuals will be sourced as seeds from as wide a source population as possible, with NYC as the epicenter.  In order to restore a large meta-population of each time capsule species that would represent a sufficient gene pool to help reestablish the species in the future and preserve its evolutionary potential, seeds will be harvested from as many surviving trees as possible.  Stands in the city parks and throughout the tri-state area will be preferential source populations, with the hope that they may have favorably adapted to the area, but seed collection will extend as far as possible within the species’ range to collect a suitable sample size.  Trees will likely be sprouted in nurseries or greenhouses, to protect from early predation and trampling, and transplanted. 

It has been determined through a number of common garden experiments in which thousands of elm saplings were inoculated with the fungus that an extremely small number of American Elm individuals (approximately 1 in 100,000) are naturally resistant to or tolerant of Dutch Elm Disease.  These individuals have been reproduced many times to create DED-resistant cultivars.  It is unlikely that this desirable genetic variation will breed true, so these cultivars are cloned using purely vegetal reproduction.  There are about six of these cultivars that are not patented (and therefore do not incur a royalty if cloned) including the Valley Forge and New Harmony varieties.  Using a proportion of these cultivars to actual elm seedlings in this restoration effort might prevent catastrophic losses in the event of a severe DED outbreak, but will also reduce the genetic diversity of the meta-population.  Therefore, DED-resistant clones will be most useful interspersed with seedlings in areas where spacing of less than 50 feet is desirable, to create a traditional elm-lined street or visually cohesive stand.  In this case, using a mixture of clones and seedlings will allow for genetic variation but minimize losses in the event of a severe outbreak.  To preserve genetic diversity, clones will make up no more than 15% of the planted trees.   

Ongoing maintenance and bottom-up control regimes will be developed, similar to the plan detailed below for the American Elm.  Fungicide, pesticide, and hypoviral injections, along with biological controls (Pt beetles for the Hemlock) and physical maintenance of the trees will be used to control any outbreaks of dutch elm, chestnut blight, or wooly adelgids.

Dutch Elm Disease is caused by the micro-fungus Ophiostoma ulmi, or more frequently by a more virulent species of the same fungus, Ophiostoma novo-ulmi.  The fungus infects the vascular system of the elm, causing the tree to grow tissues that clog its phloem and prevent it from transporting water.

DED can be spread in two ways.  The first, direct transmission through root grafts, causes a domino effect among trees that have bonded their root systems at one or more points, extremely common among elms grown less than 50 feet apart.  The disease is transmitted from one infected tree directly into the vascular system of its neighbors.  This, then, is the root cause of the rapid demise of the elms on the tree-lined streets – the monoculture’s fatal flaw.    This method of transmission spreads the disease throughout the tree, causing a full crown wilt when the water columns are blocked, and inevitably causing the death of the tree.  This is a key feature in the plan to preserve elms in NYC’s urban forest – no susceptible elms will be planted within root-grafting distance of each other or in monocultures.  Hopefully, this will ensure that this deadly and swift method of transmission is eliminated. 

The second way DED is spread between American Elms is through the movements of elm bark beetles.  There are two species of beetles that specialize on elm bark and act as vectors for the micro-fungus: the smaller European elm bark beetle (the larger of the two) and the native elm bark beetle.  These beetles spend their entire life cycles in elm trees – the European beetle feeds on twig crotches and the native beetle feeds on the inner bark of lower stems.  Both lay eggs by tunneling into dead or stressed elm wood, where the larvae can feed on the sapwood and inner bark.  Adult beetles carry the spores of the micro-fungus from diseased trees to healthy trees, where they enter the vascular tubes the beetles chew through in their feeding.  Although the beetle populations can rapidly reach large numbers, and can introduce the fungus to several different sites on a previously healthy tree (behavior more common to the European beetle) infected branches can usually be identified as they begin to wilt, and the tree can often be saved. 

Frequent monitoring of the translocated trees is vital.  Disease incursion sites must be identified and remediated as quickly and efficiently as possible, before the disease has a chance to spread.  Obviously, the city maintenance staff from the Department of Parks and Recreation does not have the extra person-hours to inspect 1,000 trees on a daily or weekly basis, so a public awareness campaign will need to be introduced.  It will feature a small reward for each person who correctly identifies DED on one of the city elms, and include information on recognizing elms, identifying Dutch Elm Disease, and on the historical and ecological significance of the American Elm.  Similar plans will be introduced for the Chestnut and the Hemlock. 

Reproductive success of the time capsule populations will depend heavily on the success of these control regimes in maintaining a healthy, parasite-and-predation free, invasive-free population.     Manual propagation may be necessary to ensure reproductive success for the more isolated trees, and also to foster resistant adaptations as they are discovered in the populations. 

The cleanup, removal of invasives, and introduction of species phases of the restoration plan for Highbridge Park will take place over a period of 5 years.  In the first year, the comprehensive site survey and cleanup will be started in the park. Seed collection and starting of 1500 trees (approximately 10 trees of each of the three time capsule species per acre, as well as native niche species) and understory plants will begin at nurseries and greenhouses.  Volunteer efforts will be coordinated with the help of the New York Restoration Project and the Highbridge Coalition, but the restoration efforts are likely to require two full-time employees.  The budget for year one is $250,000 to cover salaries, greenhouse costs, and surveying equipment.  In the second year removal of invasives onsite will begin, using volunteers and paid employees.  Again, a budget of $250,000 will cover salaries, greenhouse costs, disposal costs, and equipment/pesticides necessary in the removal process.  In years 3-5, introduction of the sprouted trees will begin, in addition to ongoing invasive removal with a budget of $500,000, $600,000, and $750,000, respectively, as maintenance and monitoring costs ramp up.  After the initial five years, ongoing maintenance and monitoring costs will require a yearly budget of $100,000 to cover salary and equipment, so an endowment of $1.25 million will be required.  The total budget for the project will be $3.6 million.          





Haugen, Linda. 1998.  "How to Identify and Manage Dutch Elm Disease."  USDA Forest Service, Northeastern Area.   Accessed October 10, 2006.


Wikipedia. 2006. "American Elm." The Wikimedia Foundation.   Accessed October 10, 2006.

Wikipedia. 2006. "American Chestnut." The Wikimedia Foundation.   Accessed October 17, 2006.

American Chestnut Cooperators’ Foundation. (Date Unknown). Website. Virginia Tech: Department of Plant Pathology.  Accessed October 17, 2006.

D’Antonio,C.M. and Chambers, J.C. 2006. Chapter 12: Using Ecological Theory to Manage or Restore Ecosystems Affected by Invasive Plant Species. in Foundations of Restoration Ecology. Falk, Palmer, and Zedler, eds. Island Press: Washington, D.C. 260 – 279.

Naeem, Shahid. 2006. Chapter 10: Biodiversity and Ecosystem Functioning in Restored Ecosystems: Extracting Principles for a Synthetic Perspective. in Foundations of Restoration Ecology. Falk, Palmer, and Zedler, eds. Island Press: Washington, D.C. 219 - 237.

Williams, A. 2005. Parks Even the Parks Dept. Won't Claim. New York Times, 7/6/2005.    

New York State Department of Environmental Conservation. New York State’s Most Threatened and Endangered Plants.  Accessed October 17, 2006. 

Save Our Hemlocks. (Date Unknown). Saving Our Hemlocks from the Hemlock Wooly Adelgid.  Accessed October 17, 2006. 

Webb, S. L.,  Pendergast, T. H. IV,  Dwyer, M. E. Response of native and exotic maple seedling banks to removal of the exotic, invasive Norway maple (Acer platanoides). Journal of the Torrey Botanical Society,  Apr-Jun 2001.



Last Updated by James Danoff-Burg, 20 Dec 06