Kristy King

 

Abstract

            This restoration project seeks to increase ecosystem connectivity in northern Manhattan slope forests. These communities are present in Inwood Hill Park, Isham Park, and Fort Tryon Park and these parks share a history of land-use that justifies their restoration as a single unit. The restoration of slope forests satellite to Inwood Hill Park will increase overall ecosystem resiliency and stability by providing a buffer against environmental stochasticity, invasive plant invasion, soil erosion, and the negative effects of isolation within an urban matrix. Invasive species removal at all three parks, and slope stabilization and native species planting at Isham and Fort Tryon Parks can be completed within a five-year period at a cost of $750,000. I propose that the New York City Parks’ Natural Resources Group forest restoration team will oversee the completion of this project, with assistance from volunteer groups and the Parks Opportunity Program. Rather than expensive yearly maintenance of the valuable ecosystem at Inwood Hill Park, and in the face of increased need for involvement at Isham and Fort Tryon Parks, this project will ensure long-term stability and will eventually lead to a decrease in administrative funds and management. This will allow valuable environmental grant funds to be diverted to new and alternative projects, broadening the horizons of ecological health and sustainability for New York City.

Introduction

Ecological restoration at the ecosystem level is crucial to the preservation of biodiversity, especially in fragmented urban landscapes. An opportunity for increasing connectivity and sustainability of native plant populations exists in northern Manhattan parks (Figure One). The slope forests of Inwood Hill Park have been largely restored by the New York City Parks’ Natural Resources Group over the past decade, and subsequent restoration of neighboring communities at nearby Isham Park and Fort Tryon Park will ensure the long-term sustainability of the entire slope forest ecosystem. These parks share a common land-use history, which justifies their restoration as a single ecological unit. Restoration activities at Inwood Hill Park have included invasive species removal, erosion mitigation, and native species planting. This restoration project proposal involves the application of these techniques to similar communities at Isham and Fort Tryon Parks in order to connect the forests into one corridor ecosystem. A project such as this comes at an important time in New York City restoration practice, as grant money to fund projects such as this is in decline and restoration for connectivity and sustainability in northern Manhattan will prevent the need for additional long-term monetary inputs on behalf of the Natural Resources Group. This restoration project draws from ecological theory applicable to restoration practice, including population and ecological genetics, ecophysiology, metapopulation theory, and community and evolutionary ecology. Successful restoration of the northern Manhattan slope forest ecosystem will increase the biological value of this urban landscape and ensure population persistence in the face of climate change and environmental stochasticity.

History

Inwood Hill Park comprises almost 200 acres on the northernmost tip of Manhattan and is the last “natural” forest on the island. The vegetation here is not controlled or planted in most of the park and the forest composition is representative of the native plant communities of historic Manhattan Island, including some of the oldest trees in the area (NYC Parks – Forever Wild). Although the forest at Inwood Hill has mostly reverted to a natural state, the park itself has been influenced by humans since long before the metropolis of New York City even existed. The Lenape Native American tribe once inhabited the rock shelters in Shorakapok Preserve (NYC Parks 2000). In the 17^th and 18^th centuries, the park was colonized by European farmers, and the forest was completely cleared of vegetation during the American Revolution (Loeb 1986). Following this, the area was primarily comprised of country homes for New York’s wealthy (including the Straus family of Macy’s fame and the Lords of Lord and Taylor) and philanthropic institutions such as the Dyckman Institute (Renner 2003). Inwood Hill was established as a park in 1916, and the buildings were demolished, allowing the forest to return to its natural state through the successional process. Because of this previous human influence on the landscape, many restoration and management projects are necessary to maintain the natural plant communities within the park while preventing the area from disturbances such as flooding, erosion, fire, and general human presence. The re-introduction of the American Bald Eagle to Inwood Hill is the most publicized instance of ongoing restoration projects, all of which are based out of the park’s Urban Ecology Center.

Geographically, Isham Park is located at the top of a large hill situated above the rest of the Inwood neighborhood. The area was historically valued for views of the Hudson and Harlem rivers, although apartment buildings now obstruct these views. The 20 acres of land was the previous residence of William Isham, a wealthy leather merchant (NYC Parks 1999). His mansion, stables, and greenhouse were all present at the top of the hill. Prior to this, it was an important site to the Battle of Fort Washington in the Revolutionary War and was also inhabited by Native Americans (Renner 1995). In these two ways, the historical land use of Isham Park is very much like that of Inwood Hill Park, which has important implications for the restoration of both parks as one unit. The descendants of William Isham donated the land to the city for preservation as a park in 1916 and additional acquisitions by the New York City Parks department in 1925 and 1927 physically connected Isham and Inwood Hill Parks (NYC Parks 1999). Rather than allowing the process of natural succession to occur, the park was landscaped to have more of an open, pastoral quality with many benches, shade trees, and an open lawn for recreation. The slopes on the west, south, and east sides of the park are comprised of natural vegetation forming secondary slope forest community leading to the streets below. Isham Park now serves as a town common and a green gateway to the larger Inwood Hill Park.

Fort Tryon Park comprises 66.6 acres and sits atop some of the highest public land in Manhattan (Renner 2003), providing valuable views of the Hudson River and the New Jersey palisades. Like Inwood Hill Park, the Lenape Native American tribe inhabited Fort Tryon Park until 1715. Fort Tryon Park was also an important site in the Battle of Fort Washington during the Revolutionary War and after the war was the location of mansions of wealthy New Yorkers, remnants of which can still be observed today. In 1917, John D. Rockefeller purchased the land and hired Frederick Law Olmsted Jr. to design a manicured landscape to preserve the valuable views as a city park for all to enjoy. The modern Fort Tryon Park opened in 1935, after years of transformation (NYC Parks 2001 [a]), and is the only northern Manhattan park with a design similar to that of Riverside or Central Park. The Cloisters, a branch of the New York Metropolitan Museum of Art, opened within the park in 1938, and houses the museum’s medieval collection. Although the majority of the park remains true to its Olmstedian design, the slopes leading up to the park are largely naturally occurring secondary forest communities.

 Restoration In Context

The striking physiognomy of the Northern Manhattan landscape, with its high ridges and deep valleys, was shaped by glaciers that receded approximately 10,000 years ago. Those same glaciers also influenced the shape of the Hudson River, the Finger Lakes in upstate New York, the Adirondack Mountains, and the Great Lakes (NYC Parks 2001 [b]). These geological features still influence the distribution and persistence of plant communities present today, with dry and steep areas with shallow soil on the slopes of the hills and moist valley forest in between. There are six different types of forest communities within Inwood Hill Park, described in Loeb (1986) as slope forest, north-facing forest, valley forest, successional forest, and successional field. This study will focus on the fragile slope forest communities, as they are present within all three northern Manhattan parks and are often the focus of restoration efforts because of their shallow, rocky soil and susceptibility to erosion. Slope forest is a dominant forest type in Inwood Hill Park due to the large change in elevation across a small area of land. In Isham Park, slope forest surrounds and leads up to the open pastoral hill. Similarly, in Fort Tryon Park, slope forest surrounds the entire park, which is at a markedly higher elevation than the streets below. Combined, they form one mostly fragmented ecosystem, but the parks are technically connected. Only one two-lane road divides Inwood Hill Park and Isham Park; Fort Tryon Park and Isham Park share a Hudson River coastline. Although the parks are still somewhat isolated, this level of connectivity should be sufficient to ensure plant population persistence, given this suite of species’ means of seed dispersal (more later on this topic; see table one).

Beginning in 1994, the Natural Resources Group’s (NRG) forest restoration team has played an active role in forest restoration in northern Manhattan, focusing their efforts within Inwood Hill Park on invasive species removal, native species planting, and erosion mitigation. Invasive species treated have ranged from common invasive plants establishing themselves after a disturbance (Lonicera japonica, Acer plantanoides) to remnants of former lawns and gardens (Wisteria floribunda, Rosa multiflora). Ailanthus altissima, the “tree that grew in Brooklyn,” Phellodendron amurense, and Acer pseudoplantanus are other tree species that “escaped” from cultivation to colonize forested areas and endanger native species establishment and success. Invasive species present in Inwood Hill Park have historically been those that do not allow establishment of native species due to the rapid growth of vines that tend to choke and kill large trees while also preventing seedlings to establish and mature. Former lawn species such as those present in Inwood Hill are known to inhibit successional change and prevent the natural progression of plant communities within a forest (McLachlan and Basely 2003). Acer plantanoides, a common invasive tree, has historically been present in Inwood Hill Park and is more currently causing a problem in Fort Tryon Park (Wenskus 2004). Although Acer plantanoides is also present within Isham Park, invasive species here are mainly of the weedy herbaceous variety, as the park’s small size and proximity to major thoroughfares create a frequent disturbance regime that makes conditions favorable for these generalist species.

The Natural Resources Group has invested much time and money to removal of these plants and associated planting of native species (Wenskus and Kortebein 2002). This planting serves as habitat enhancement for native birds and small mammals that also assist in seed dispersal of indigenous plant species. Their presence and dispersal facilitation leads to increased ecosystem sustainability and the ability to support a diverse assemblage of plant and animal species, which is an additional goal of the NRG.  Planting of native species has also been performed in order to reduce erosion on the steep slope forests within Inwood Hill Park. Slopes are physically stabilized using fabric and fiber products that provide immediate erosion control, but the plant material imbedded within these matrices are what ensure long-term stability. Coir logs are the main form of slope stabilization used in Inwood Hill. They consist of coir fiber rolled into a long tube, secured with wooden stakes, and prevegetated with Parthenocissus quinquefolia, a native vine species (Wenskus and Kortebein 2002). Less steep slopes are often stabilized using a retention wall, creating a terraced effect. Over time as the slope becomes more stable and the forest naturally regenerates, downed woody material will naturally prevent erosion. This is already occurring in some areas of Inwood Hill Park and is characteristic of a mature, sustainable forest ecosystem.  This slope stabilization also serves to reduce sediment pollution into the Hudson River, which is immediately adjacent to Inwood Hill Park on its west edge. More recently, a small restoration endeavor has begun in Isham Park, enlisting volunteers from local schools to assist Urban Park Rangers in slope stabilization and native species planting (NYC Parks 1999). Restoration in Fort Tryon Park has primarily involved reconstruction and renovation of park structures due to overuse and has not focused on ecological restoration per se until recently, when a NYS Environmental Protection Fund grant made slope stabilization a priority of the NRG forest restoration team (Wenskus 2004). Restoration projects in Fort Tryon Park are rare, with a focus primarily on renovation and reconstruction of the park due to years of overuse and inadequate design (Quennel Rothschild Associates 1984).  

Over the six year period from 1998 to 2003, NRG forest restoration has invested millions of dollars in projects taking place in northern Manhattan. From 2001 to 2003, grant funding declined (Wenskus 2004), and it has become necessary to manage the park for sustainability to prevent the need for extensive monetary inputs primarily supporting the removal of invasive species and their replacement with native plant communities. Currently, the NRG forest restoration team intensively sweeps an area for non-native species removal annually for the first two years and then subsequently at 18-month intervals until the newly planted tree canopy closes, which can take decades. As funding does not often support this type of maintenance, the restoration team can only do so much in order to ensure persistence of native plant communities as a crucial component of the Inwood Hill Park ecosystem (Wenskus 2004).

I propose that restoration of slope forests in nearby Isham Park and Fort Tryon Park for removal of invasive species, erosion mitigation, and planting of native species will increase the connectivity of these three native plant populations and will ensure the long-term persistence of the entire ecosystem. This project should be a future priority of the NRG forest restoration team if they wish to decrease Inwood Hill’s dependence on their efforts. I hypothesize that Inwood Hill Park’s connectivity with these adjacent natural areas will also help to make the forest more resilient against drought, insect outbreaks, storms, and disease due to an increase in genetic variation both among and within native plant populations at these three parks. In addition to the benefit of increased ecological resiliency, this project will also attempt to ameliorate effects of the isolation of Inwood Hill Park (and northern Manhattan in general) and the impending evolutionary dead-end present here due to the potential influence of inbreeding depression and limited gene flow. Inwood Hill is surrounded by water on two sides (Hudson River, Harlem Ship Canal) and residential/urban areas on the other, both of which can potentially limit the influence of adjacent plant populations on the genetic variation within the park. Although the active component of this proposal is the restoration and management of nearby parks, the ultimate goal is to preserve the valuable forest ecosystem at Inwood Hill Park already established and maintained by NRG’s forest restoration team.

Invasive species removal

            Invasive species removal is the primary step in this restoration project and will begin with an absolute removal of all garlic mustard (Allaria petiolata) from Inwood Hill Park. Although this infestation is not on the slope forests that are the focus of this project, the removal of this invasive species is in dire need of completion and I have thus decided to include it in the budgetary requirements for this proposal. The garlic mustard in Inwood Hill Park is dominant in the portion of the forest that it occupies, preventing other native species from establishing, even though extant populations are directly adjacent. This one-year eradication of garlic mustard will further ensure the long-term stability of Inwood Hill Park and support the years of effort on behalf of the Natural Resources Group.

            Invasive species removal in Isham and Fort Tryon Parks will be a much more intensive project, likely to be completed within five years. Japanese barberry (Berberis thunbergii), pearlbush (Exochorda spp.), and Norway maple (Acer plantanoides) are three invasive plants that need to be eradicated from both parks. Japanese barberry and pearlbush are both remnants of former lawns that tend to crowd the forest understory, preventing the establishment and persistence of native plant species. The land use of all three of these parks reflects the invasive species present in these communities and makes it easier to remove them from the entire northern Manhattan ecosystem. The theoretical basis for invasive species removal is grounded in community and evolutionary ecology (discussed below). Intensive removal of invasive species in Isham and Fort Tryon parks will occur every six months for two years, followed by continued monitoring and intermittent removal for the next three years. This plan is indicative of previous efforts of the Natural Resources Group’s forest restoration team, and this group’s experience with invasive removal will be crucial to the effectiveness of this project.

 Erosion Mitigation

            Eroded and eroding slope forests in Isham and Fort Tryon Parks will be rebuilt using the techniques described above, previously used by the Natural Resources Group in Inwood Hill Park. Slopes are eroding if there is soil compaction, an obvious deposition of soil on trails and roads below the slope, and/or a lack of living plants and trees on the slope. Slope forest is more of an issue in Isham Park than in Fort Tryon Park, but Fort Tryon Park is large enough that more money is required to mitigate erosion here (see table two). Coir logs and jute mats will be used on steeper slopes, and will be vegetated with native herbaceous species such as Parthenocissus quinquefolia and Carex pennsylvanica to provide long-term support. Broad slopes such as those present in Isham Park (figure three) will be fortified with soil from lower altitudes that have previously eroded and held together with retention walls, creating a terraced landscape that will be vegetated with native trees and shrubs. As discussed above, as restoration generates a healthy and self-sustaining forest, the presence of downed woody material will naturally stabilize slopes in Isham and Fort Tryon Parks, as it presently does within Inwood Hill Park.

The process of erosion mitigation will occur over a two year period in Isham and Fort Tryon Parks. During late summer of year one, jute logs, coir mats, and retention walls will be installed along eroding slope forests. Soil fortification will occur when necessary on broad slopes. Logs and mats will be vegetated at this time with native herbaceous species. During the following fall and winter, the performance of slope reinforcements will be monitored and reinforced where necessary. The true test of these structures will be assessed in spring, after the thaw comes and the rainy season begins. The following summer and fall will be spent reinforcing, rebuilding, and fortifying the slopes where necessary, and winter and spring will bring a final assessment.

 

Native species planting

            The suite of native species to be planted in Isham and Fort Tryon Parks (table one) have been chosen for several reasons. These species are already present within Inwood Hill Park and are physiologically adapted to the sandy loam and rocky soils present in slope forest landscapes. The New York City Native Species Planting Guide (Luttenberg et al. 1993) suggests these plants for projects such as this one, and the plants are all commercially available at local and regional nurseries that use genetic stocks native to the New York area. This suite of species represents a broad variety of functional groups, ensuring that all niches are occupied and potential ecosystem roles filled. Since Isham and Fort Tryon Parks are both more suited to recreational use and human presence than the forest at Inwood Hill Park, these plants were also selected to serve an aesthetic value similar to that of ornamental plants commonly used to vegetate park landscapes. Seasonal dynamics, showy flowers, and attractive form are all represented in this group of plants. Plants, rather than seeds, will be planted in all cases. This process will occur over a two year period. Jute logs and coir mats will be vegetated in year one, and newly stabilized slopes and terraces will be vegetated in year two.

Budget

            Table two presents the budgetary requirements for this restoration project, to be carried out over a five-year period. Although $750,000 seems like a high price for restoration of one ecosystem, it is important to note that establishing this connectivity of native plant populations will theoretically reduce the long-term inputs required on behalf of the Natural Resources Group. This project can be seen as an investment for the future of northern Manhattan slope forests, and will allow future grant monies to be allocated to new projects in the area and elsewhere in New York City. The 2003 forest restoration budget for the Natural Resources Group was $2.2 million dollars for all projects in the five boroughs. Their budget is comprised primarily of grants from governmental organizations such as the New York State Environmental Protection Fund and the Clean Air/Water Bond Act. These grants are often renewable for a series of years (Wenskus 2004). The yearly budget of $150,000 for this restoration project will represent only a fraction of the total grant money used on forest restoration projects, yet will provide long-term benefits that far outweigh the costs.

Restoration Staff and Assistance

            The Natural Resources Group forest restoration team will be heavily involved in the on-the-ground planning and action for this restoration project. This group has been involved in fifteen years of forest restoration in the New York area and is the best and most appropriate for following through with this project. Due to their historical involvement in northern Manhattan, they are invested in the success of slope forest restoration and will undoubtedly carry the project through to its completion. Of course, this group will need to be supplemented by a slew of volunteer helpers for the actual physical labor. Volunteers also have a history of involvement in forest restoration, and have worked alongside the Natural Resources Group for years. I envision local groups that have a personal interest in northern Manhattan parks to be heavily involved in this project, including the Friends of Inwood Hill Park, Friends of Fort Tryon Park, Inwoof (a local organization for Inwood dog owners and their pets), and the Inwood Community Supported Agriculture group that meets weekly in Isham Park. Residents of Inwood and Washington Heights who use the parks for running, walking, sports, and picnics will also likely help with native species planting and/or invasive species removal. Larger city-wide organizations such as Partnerships for Parks, the City Parks Foundation, and Parks Conservation Corps will be enlisted to help as well. One organization that I would specifically like to involve with this restoration project is the Parks Opportunities Project (POP), a relatively new division of New York City Parks and Recreation that provides seasonal horticultural employment for NYC residents on public assistance. This organization provides training, job stability, and a living wage for those who may not possess marketable skills required to obtain a job. Judging by the number of organizations who have an interest in the health of their local parks as well as the number of neighborhood residents who are in dire need of employment in general, manpower should not be an issue in the successful completion of this restoration project

Although the technical details and historical context discussed above comprise the bulk of the actual restoration project, there is an important theoretical foundation that has inspired the nuts and bolts and will influence the outcome of the project, making this system a landmark study with potentially far reaching applications for urban forestry worldwide. This restoration project will be accomplished through careful attention to population and ecological genetics, ecophysiological constraints, metapopulation theory, community dynamics, and evolutionary restoration ecology.

Population and Ecological Genetics

Important theoretical aspects from the fields of population and ecological genetics that are represented in this restoration project include the preservation of genetic diversity among populations (all three parks), the adequate distribution of genetic variation as linked to life-history traits, and the introduction of appropriate genetic diversity to the slope forests of Isham and Fort Tryon Parks,. The increase in native species population size induced by the restoration of Isham and Fort Tryon Parks will reduce the potential effects of isolation on the previous efforts at Inwood Hill Park. The facilitation of connectivity inherent in this proposal will also ensure that adequate gene flow prevents genetic divergence and isolation of the populations (Falk et al. 2006). The life-history traits of the suite of species discussed above will facilitate adequate dispersal between slope forest communities because of their dispersal methods (table one). Most of the selected plants are dispersed via birds and small mammals, which can easily travel between the three parks due to their geographic proximity and newly established ecological connectivity. Wind dispersal will also be possible between these slope forest communities due to the hilly nature of northern Manhattan. Changes in elevation between the three parks will facilitate wind patterns that lead to downhill propagule dispersal. Geography is also crucial in the location of source material for restoration sites and also involves the use of population genetics; for this project, I will not sample for the three parks to be restored from within Inwood Hill Park because doing so would decrease genetic diversity among populations. Also, Inwood Hill may have a limited seed surplus due to the recent nature of restoration projects here. Source material (seeds and seedlings) will be purchased from regional plant nurseries endorsed by the New York City native species planting guide (Luttenberg et al. 1993).   Their stocks are derived from other neighboring populations and will therefore provide an accurate suite of genetic material for introduction to Isham and Fort Tryon Parks This will ensure that the restoration sites utilize “sufficient diversity to allow adaptation to new circumstances while avoiding the adverse effects of introducing genotypes that are poorly adapted to the environment” (Falk et al. 2006).

Ecophysiological constraints

Slope forests are notoriously difficult places for plant growth due to rocky soils, erosive tendencies, and lack of water retention. The valley forest in Inwood Hill Park is actually the richest portion of the forest, species-wise (Loeb 1986), but I chose to not focus on this ecosystem because it is inherently less prone to disturbance and is a more rare forest community that could not easily be linked with neighboring parks. The suite of species chosen for introduction to Fort Tryon Park and Isham Park (table one) already exist within the slope forests of Inwood Hill Park and are suited to well-drained sites on ridgetops or slopes with shallow loam or sandy loam soils (Luttenberg et al. 1993). The root systems of these plants are shallow enough to effectively extract water from the well-drained and often dry soils present in slope forest communities. The persistence of this suite of species in Inwood Hill Park implies that they are tolerant to the often shady nature of mature forest communities, ensuring that these species will persist in the restoration sites as time passes and the forests become more stable and mature.

Metapopulation theory

Metapopulation theory will be used to link the population ecology ideas discussed above with the biogeography of northern Manhattan. In order for a metapopulation to persist, there should be a threshold number of 15-20 suitable patches, but there will likely be more than this throughout the three parks, which will decrease the overall probability of extinction (Maschinski 2006). A relatively uniform stand of canopy species will be planted in each of the slope forest communities identified in figure two. Three to five patches of shrub species will be included in each patch and five to ten patches of herbaceous species will be planted. Figure two also shows relative size and isolation of slope forest patches in northern Manhattan. The patches will be populated with native species in proportion to their size and will likely persist in the same fashion. Since these parks are so close together, it is likely that no one patch is more isolated than another. The most isolated patches are likely in Inwood Hill and Fort Tryon Parks, but these patches are large enough to counteract the potentially damaging effects of isolation. Corridor quality is also influential to metapopulation size; this project attempts to overcome the difficult nature of the urban residential matrix present in Inwood by making more efficient use of the narrow corridors present through inducing similarity between slope forest communities. The similar plant populations in these three parks will facilitate movement of birds and small mammals by providing a greater variety of habitat and foraging locations.

In addition to distance between patches, the spatial arrangement of patches was selected to enhance dispersal opportunities. Slope forest communities are located in the general direction of the next “stepping stone” of the ecological corridor (figure two). Maschinski (2006) states that “the closer restored populations are to intact habitats and populations, the greater the opportunity for dispersal leading to colonization and persistence.” Treating Inwood Hill as an intact population dictates that the establishment of new populations in the neighboring parks may be dependent on their distance to Inwood Hill Park. In fact, this park may serve as a source population. As it is a central location to Fort Tryon and Isham Parks, migration will occur to Inwood Hill Park as well. This is important, as the entire motivation of the project is to establish satellite populations of native plants in order to stabilize the populations at Inwood Hill.

Finally, metapopulation theory dictates that the highest priority sites for introduction are those with the largest area closest to the intact population because they experience the highest probability of natural dispersal and recolonization. Given this, Fort Tryon Park seems to be the priority restoration site for native species introduction. Although this is true, the relative health of the slope forest community at Isham Park also calls for great attention. I propose that no one site is more important than another, but that both parks are equally important to ensure long-term ecosystem stability in northern Manhattan.

Community and evolutionary ecology

Community ecological theory will also be incorporated into this restoration plan, primarily to prevent the persistence of non-native species in Inwood Hill Park and the ongoing need for their removal by the Natural Resources Group. Theory dictates that diverse communities are more resilient to perturbations, so the restoration to increase genetic variation (discussed above) will ultimately contribute to this resiliency. Spatial heterogeneity introduced by the addition of three new slope forests to native plant populations will alleviate competitive pressure between native and non-native species. Successful restoration of slope forests in Isham Park and Fort Tryon Park will likely result in increased gene flow in the northern Manhattan slope forest ecosystem (including Inwood Hill Park); high gene flow among locally adapted populations of invasive species may lead to their extirpation (Boulding and Hay 2001). Although this is a risky assumption, it may be a possible side effect of overall increased ecosystem resiliency.  It is also important to consider and prevent the migration of invasive plants present in Isham and Fort Tryon Parks to Inwood Hill Park. The intensive invasive species removal occurring at the beginning of this project and persisting for five years of restoration will be initially targeted at those plants that have previously been problematic in Inwood Hill because it is likely that conditions are still appropriate for those plants to return.

Although species will be established in equal proportion at both of the restoration sites, the core-satellite species pool model will likely function in this ecosystem of northern Manhattan slope forests. This model dictates that some core species are found at all sites and satellite species are less likely to colonize and are more likely to face local extinction. Core species will likely be those that are dispersed by birds and small mammals (see table one), as they will most easily travel between the three communities. Wind-dispersed species will likely face greater challenges in traveling between sites, as logic dictates that uphill dispersal is more difficult than dispersal downhill. Wind-dispersed species are also not physically affected by increased habitat connectivity, as wind patterns act independently of biotic and ecological gradients and are an effect of abiotic large-scale processes manipulated by local geographic patterns.

This project also takes into account evolutionary restoration ecology in its ultimate aim to manage gene flow to and from Inwood Hill Park by establishing neighboring populations in order to decrease genetic diversity both within and among populations. Natural colonization is preferable (Stockwell et al. 2006), which is one reason why the satellite areas are being restored – to encourage sustainability and continued evolution of plants within Inwood Hill Park’s ecosystem through natural dispersal between the four parks in northern Manhattan. The literal increase in population size for these native slope forest plant species will also facilitate long-term evolution in all three parks. Small populations like the ones currently present at Inwood Hill Park may not have sufficient phenotypic variation for selection to occur (Stockwell et al. 2006), and increasing the amount of genetic material available to this area will prevent local extinction.

 Measuring success/adaptive management

The success of this project will be determined through a thorough assessment of the effects of restoration on both the newly restored slope forests as well as the stability of Inwood Hill Park. Slope forests in Isham and Fort Tryon Parks can be assessed by comparing them with more stable communities in Inwood Hill Park. The stability of the entire ecosystem post-restoration can be measured through determining the need for further restoration and maintenance within Inwood Hill. If erosion is persisting and invasive species are still an issue, it is likely that some changes in the management strategy need to occur before the completion of the five-year project. The progress will be assessed yearly with respect to several variables: effective invasive species removal, adequate erosion mitigation, and native species establishment and persistence (see timeline, above). Determination of whether the results of this project are sufficient can be determined by how much money and effort on behalf of the Natural Resources Group is still required within Inwood Hill Park, and comparing this value to how much was spent prior to restoration of Isham and Fort Tryon Parks. Ideally, no additional management will be required within Inwood Hill Park after the five year restoration period, and the three slope forest communities will support each other through newly established ecosystem connectivity.

Conclusion

The ecological theories presented above, as well as the history of land use and restoration in northern Manhattan justify the restoration of Isham Park and Fort Tryon Park for connectivity with Inwood Hill Park of native plant populations on slope forests. This proposal explains how these parks, although somewhat fragmented by an urban residential matrix, can be restored as one ecosystem unit. Their restoration will increase large-scale ecological resiliency by inducing gene flow between populations and preventing the effects of isolation that reduce genetic diversity within an isolated ecosystem. Migration of native plant propagules between slope forest communities will be facilitated by dispersal by birds, small mammals, and wind that will also benefit from increase ecosystem connectivity. This ecosystem stability will also prevent the future migration of invasive species into all three of these parks, with additional protection provided to the valuable natural forest at Inwood Hill Park. The slope stabilization aspect of this project will mitigate the damaging effects of erosion on slope forests through initial mechanical means as well as eventual native plant persistence and support. I believe that it is worth the investment of the Natural Resources Group to undertake this restoration project due to its implications for long-term ecosystem health and function.     

Figures and Tables:

Table 1: native species to be planted in Isham and Fort Tryon Parks, along with their dispersal methods.

 

 

park	restoration activity	yearly price	number of years	total
Inwood Hill Park	invasive species removal	$100,000	1	$100,000
Isham Park	invasive species removal	$10,000	5	$50,000
	erosion mitigation	$50,000	2	$100,000
	native species planting	$50,000	2	$100,000
Fort Tryon Park	invasive species removal	$20,000	5	$100,000
	erosion mitigation	$100,000	2	$200,000
	native species planting	$50,000	2	$100,000
			Grand Total	$750,000

Table 2: budgetary requirements for slope forest restoration in northern Manhattan.

Figure 2: location of slope forest patches within northern Manhattan parks.

Figure 3: Broad slopes such as this one in Isham Park are best stabilized using a retention wall.

References:

Falk, Donald et. al. “Population and ecological genetics in restoration ecology,” in Foundations of Restoration Ecology. Eds. Donald A. Falk, Margaret A. Palmer, and Joy B. Zedler. 2006. Island Press: Washington D.C.

Boulding, E.G. and T. Hay. 2001. Genetic and demographic parameters determining population persistence. Heredity 86: 313-324.

Loeb, Robert. 1986. Plant communities of Inwood Hill Park, New York County, New York. Bulletin of the Torrey Botanical Club 113 (1): 46-52.

Luttenberg, Danielle et al. 1993. Native Species Planting Guide for New York City and vicinity. City of New York Parks and Recreation: New York.

Maschinski, Joyce. “Implications of population dynamic and metapopulation theory for restoration,” in Foundations in Restoration Ecology. Eds. Donald A. Falk, Margaret A. Palmer, and Joy B. Zedler. 2006. Island Press: Washington D.C..

McLachlan S.M. and Basely, D.R. 2003. Outcomes of longterm deciduous forest restoration in southwestern Ontario, Canada. Biological Conservation 113: 159-169.

New York City Department of Parks and Recreation. Forever Wild: Inwood Hill Park
Shorakapok Preserve. http://nycgovparks.org/sub_about/parks_divisions/nrg/forever_wild/site.php?FWID=38 

New York City Department of Parks and Recreation. 1999. Isham Park historical sign. http://www.nycgovparks.org/sub_your_park/historical_signs/hs_historical_sign.php?id=6462

New York City Deparment of Parks and Recreation. 2000. Inwood Hill Park historical sign. http://www.nycgovparks.org/sub_your_park/historical_signs/hs_historical_sign.php?id=7730

New York City Department of Parks and Recreation 2001 (a). Fort Tryon Park historical sign. http://www.nycgovparks.org/sub_your_park/historical_signs/hs_historical_sign.php?id=12315

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