We propose evaluating tree cover for an entire urban area that is based on patch dynamics. ... Cambridge University Press and Clark University, New York. ..... of American metropolitan areas by ecoregion and potential natural vegetation.
Urban Ecosystems, 1997, 1, 229–246
Urban tree cover: an ecological perspective WAYNE C. ZIPPERER,* SUSAN M. SISINNI AND RICHARD V. POUYAT USDA Forest Service, 1100 Irving Avenue, Syracuse, NY 13210, USA
TIMOTHY W. FORESMAN University of Maryland Baltimore County, Department of Geography, 5401 Wilkens Avenue, Baltimore, MD 21228, USA
Analysis of urban tree cover is generally limited to inventories of tree structure and composition on public lands. This approach provided valuable information for resource management. However, it does not account for all tree cover within an urban landscape, thus providing insufficient information on ecological patterns and processes. We propose evaluating tree cover for an entire urban area that is based on patch dynamics. Treed patches are classified by their origin, structure, and management intensity. A patch approach enables ecologists to evaluate ecological patterns and processes for the entire urban landscape and to examine how social patterns influence these ecological patterns and processes. Keywords: tree cover; urban-to-rural gradient; patch dynamics; urban forest; ecological patterns and processes
Introduction Urbanization modifies the extent and type of vegetation on a landscape through losses from deforestation and fragmentation and gains from reforestation and afforestation (Godron and Forman, 1983; Sharpe et al., 1986; Zipperer et al., 1990). With deforestation and fragmentation there are reductions of native flora and fauna including keystone species (Murphy, 1988; Peter and Lovejoy, 1990), increases in non-native species, increases in stream sedimentation and water pollution (Douglas, 1990; Roger, 1994), and changes in air temperature, evapotranspiration, and albedo (Jager and Barry, 1990; Grimmond et al., 1994; Heisler et al., 1994). Despite these impacts, ecologists know relatively little about ecological processes of urban systems (Stearns and Montag, 1974; McDonnell and Pickett, 1990). This is borne out further by a recent search for ecological papers highlighting ecological patterns and processes studies conducted in the New York City metropolitan region (Flores et al., in review). The search included 2470 citations and identified only 145 citations on ecological processes. Our paper argues for a new approach to studying urban ecological patterns and processes: identification and evaluation of vegetation patterns through patch analysis. Little new data on vegetation are furnished; rather, existing research is reviewed to provide a direction for future studies of urban vegetation. Assessments of community structure, vegetation dynamics, and ecosystem processes within urban landscapes are discussed to support our approach. Patch analysis: an alternative approach Urban vegetation can be defined in two ways. First, it is defined as an assemblage of plant material above, on, and below the ground surface within an urban landscape (Sanders, 1984). This definition includes measurements of species structure, composition, and age; forest health; density; biomass; and leaf area.
* To whom correspondence should be addressed. 1083-8155 © 1997 Chapman & Hall
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The second definition focuses on process and identifies those plant assemblages that are regularly subjected to urban influences as urban vegetation (Sanders, 1984; McDonnell and Pickett, 1990). The process definition includes structural components and processes within urban areas as well as areas adjacent to or neighboring urban landscapes. Thus, a gradient of urban influences on vegetation exists and can be evaluated from the urban core to rural areas (McDonnell and Pickett, 1990). Analysis of vegetation in urban and urbanizing landscapes has been conducted at broad (Schmid, 1975; Dorney, 1977; Brady et al., 1979; Rowntree, 1984; Sukopp and Weiler, 1988; Nowak, 1994b) and fine (Lefkowitz and Greller, 1973; Stalter, 1981; Profous and Loeb, 1984; Rudnicky and McDonnell, 1989) scales. At each scale, sampling is done by political or management units rather than ecological units. Broad-scale analyses focus on either the land use or city-wide level and include descriptions and measurements of vegetation structure that relate to species composition, percent canopy cover, tree density, diameter distribution, and leaf surface area. Although this approach often assumes a homogeneity across a typology class (e.g. land use), it provides useful insights into tree management (e.g. Welch, 1994) or assessments of urban forest structure to improve air quality (Nowak, 1994a). Fine scale studies tend to focus on natural areas or management units with historic significance. These studies often include detailed analyses of site histories (Loeb, 1992, 1993) and community composition and structure (Rudnicky and McDonnell, 1989), and also providing a basis for site management (Sisinni and Anderson, 1993; Sisinni and Emmerich, 1995). These approaches are well suited to meet their individual objectives; however, because of their resolution (grain) and extent (Allen and Hoekstra, 1991), they may be insufficient to assess ecological patterns or processes across an urban landscape. For example, understanding and managing hydrologic cycles in urban and urbanizing landscapes require not only knowing the percent of canopy cover, often the only vegetation parameter in hydrological models, but also specific information on location of tree cover, site orientation, and vertical structure of vegetation for the entire watershed (Band et al., 1993; Band and Moor, 1995; Neville, 1996). Similarly, location, distance between tree cover habitats, habitat type, shape and size, and adjacent land cover are critical components for conservation strategies (Pickett et al., 1992; Pickett and Parker, 1994; Forman, 1995). Since the collection of detailed data requires additional time, energy, and funds, study objectives should determine the type of data needs. An alternative approach is needed to investigate ecological patterns and processes within urban and urbanizing landscapes. This approach should link ecological patterns and processes at broad spatial and temporal scales and provide sufficient detailed information on community dynamics for fine scale site management and ecological analyses (Turner and Gardner, 1990; Allen and Hoekstra, 1992; Yaro and Hiss, 1996). In addition, because humans ultimately determine vegetation patterns, the approach should link ecological and social patterns and processes. A unit of measurement universal to all landscapes must be defined that effectively links, spatially and temporally, both ecological and social patterns and processes. We recommend that that unit be a patch. A patch is defined ‘‘as a relatively homogeneous area that differs from its surroundings’’ (Forman, 1995). Aerial photographs of urban landscapes reveal natural and anthropogenic patches. Because anthropogenic patches dominate the landscape, they are considered to be the background matrix. A background matrix is ‘‘characterized by extensive cover, high connectivity and has major control of landscape dynamics’’ (Forman, 1995). Vegetation patches exist within this matrix and include grass, shrub, and tree cover (treed patches). The term ‘‘treed’’ is used instead of forest because within urban landscapes tree cover occurs in patches with a woody understory and developed forest floor (e.g. natural areas) and with a grass or herbaceous understory (e.g. parks and yards; Fig. 1; Hobbs, 1988; Zipperer and Zipperer, 1992). A patch also can be defined by other attributes than just by physical structure. For example, census data are used to display social patterns spatially, and disparate areas can be viewed and classified (Grove and Burch, 1997).
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Figure 1. Treed patches represented at different scales. At the broad scale (aerial and remote sensing data), treed patches are presented as distinct areas of tree cover. At the fine scale, treed patches are represented by individual trees and understory structure. Information gathered at the fine scale can be aggregated to give broad scale representation; however, broad scale information can not be deaggregated to give fine scale information. Patches A and B represent different types of treed patch.
The patch, a fundamental unit of measurement for landscape analyses (Forman and Godron, 1981; Turner, 1989; Forman, 1995), is selected because it: (1) implies a relatively discrete spatial pattern; (2) implies a relationship of interactions and exchanges of one patch to another and to the surrounding matrix; and (3) allows for a unit of management (Risser et al., 1984; White and Pickett, 1985; Forman and Godron, 1986; Turner, 1989; Forman, 1995). Patches, however, are not spatially or temporally static. Disturbances and biotic processes cause changes, thus creating a dynamic-heterogeneous mosaic of patches (Pickett and Thompson, 1978; White and Pickett, 1985). Patch dynamics (i.e. how patches change spatially, structurally, compositionally, and functionally; White and Pickett, 1985) influence internal processes of the patch as well as interactions between itself and the adjacent matrix (Forman and Godron, 1986). Understanding patch dynamics within urban and urbanizing landscapes will yield insights into factors influencing how these landscapes function (McDonnell and Pickett, 1990). For illustrative purposes, we focus only on treed patches. Similar characterizations can be made for other patches of urban vegetation. When considering ecological processes, important patch attributes include origin, configuration, vegetation structure, and management intensity. We classify patches into three origin types: remnant, emergent, and planted. A remnant patch is defined as an area that was not cleared during site development and existed before development. Because of their age, these patches often are dominated by large diameter trees. Management varies from intensive to none. An emergent patch resembles a remnant patch, but instead has regrown on a site previously cleared for development (e.g. vacant lots and sites, fence rows, property boundaries). Structurally, these patches are characterized by pioneer species and small diameter trees (100 ha) in New York City revealed seed banks similar to rural forests (Kostel-Hughes, 1995). Other studies of urban remnant forest patches have indicated a higher density of seedlings of non-native species than native species (Rudnicky and McDonnell, 1989). Information on seed banks in emergent patches is lacking. Additional studies are necessary to understand the dynamics of site colonization by both native and non-native species. Urban landscapes may have higher population densities of seed predators than rural landscapes (Nilon, 1996). Current studies in natural areas within New York City are being conducted to assess the impact of seed predators on forest regeneration and to determine whether there is a preference for native or non-native seeds (Nilon, 1996). Replicated studies need to be conducted in natural areas in other cities. Seed predation, however, cannot be examined independent of seed dispersal (van der Valk, 1992). Seed predators may cache their seeds in adjacent locations, thus assisting dispersal and germination away from the parent plant. With the successful dispersal to an available site, germination depends on environmental and biotic conditions (Pickett et al., 1987a). Seed germination is just one aspect of species performance (Table 1). Other considerations include: how does the urban environment (heat island effects, air pollution, human activities, non-native species) affect native species performance? A 10-year study along an urban-to-rural gradient from New York City to Litchfield County, Connecticut has identified changes in soil conditions, nutrient cycling, and decomposition when comparing urban and rural stands (for a review, see McDonnell et al., 1997). These changes may have significant influences on native species performance and rehabilitation efforts in natural areas in urban landscapes (McDonnell, 1988; Pouyat and Zipperer, 1992; Pouyat et al., 1995; Zipperer and Pouyat, 1995). In addition, changes in the disturbance regime and community structure, high densities of non-native species, and human activities may create conditions better suited to non-native species in remnant and emergent patches (McDonnell and Roy, 1997). Nutrient and carbon cycling Extensive literature exists on nutrient and carbon budgets for forest ecosystems in nonhuman dominated watersheds. Nutrient inputs include the atmosphere, nitrogen fixation, rock weathering, and hydrologic. Nutrient losses include hydrologic, gaseous, and particulate losses to the atmosphere, fire, and forest harvesting. In the urban landscape, fertilizers and high levels of NOx and SOx supplement nutrient inputs, whereas removal of trees, branches, leaves, shrubs, fruits, and grass clippings increase nutrient losses. Unfortunately, only a limited number of studies have assessed nutrient cycling within the urban landscape and these studies have focused on individual components of the landscape (i.e. lawns and remnant forest tracts) rather than the entire urban watershed. In a recent paper, McDonnell et al. (1997) discussed how litter decomposition and nitrogen cycling within remnant oak patches varied along an urban-to-rural gradient extending from New York City to Litchfield County, Connecticut. They found that urban forests exhibited faster litter decomposition and nitrification rates than rural forests even though the urban forest had reduced fungal and microarthropod populations and poorer leaf litter quality than rural forests. Decomposition and nitrification differences were attributed to two anthropogenic causes: increased average temperatures caused by the urban heat island effect and the successful colonization of earthworms (Pouyat et al., 1995; McDonnell et al., 1997). Using the same oak stands, Groffman et al. (1995) evaluated carbon dynamics and separated soil carbon into four pools: (1) readily mineralizable carbon with a turnover time of days to weeks; (2) labile carbon with a turnover time of weeks to months; (3) potentially mineralizable carbon with a turnover time of
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months to years; and (4) passive carbon which is very recalcitrant with a turnover time of years to decades and possibly centuries. Analyses revealed that urban forests had lower labile carbon and higher total passive carbon than rural forests (Groffman et al., 1995). These studies need to be replicated in other urban landscapes and extended to other types of treed patch. An important element with respect to nutrient cycling is the effect of vegetation structure on the movement of nutrients within and through a treed patch. Structural features such as canopy size and leaf area have not been measured from an ecological context (e.g. patch type), but have been measured from a management context (i.e. land use; Nowak, 1994b). For the City of Chicago, average leaf area index (excluding grass) for tree-covered areas was 6.0; a value toward the lower end of the range (5–8) for deciduous forests (Barbour et al., 1980). This low value was attributed to an understory of grass and impervious surfaces. The highest percent of total leaf area for Chicago was observed on vacant lots (8.4%), institutional lands (e.g. parks, cemeteries, and golf courses; 37.8%), and residential areas (49.7%; Nowak, 1994b). For the different treed patches, remnant, with its understory of saplings, shrubs and herbaceous plants, is projected to have the highest leaf area index and is predicted to have an index similar to that of a deciduous forest. Even though emergent patches are dominated by small diameter trees, they also have an understory and are predicted to have a leaf area equal to or slightly less than planted patches on a per area basis. Planted trees often are characterized by large diameter trees (Nowak, 1994b). Street trees, for example, because of their size (50% of the street tree population was $31 cm dbh), accounted for 24.0% of the total leaf area in Chicago and 43.7% of the leaf area in residential areas (Nowak, 1994b). Although canopy size and leaf area measurements are used to assess ecological function such as transpiration and carbon sequestration, another important consideration is tree physiology (Waring and Schlesinger, 1985). Within an urban environment, ambient temperatures are 5–10°C higher than rural temperatures (Ackerman, 1985). During summer months, ambient temperatures may exceed the optimum temperature range for photosynthesis, thus reducing CO2 uptake and transpiration. Consequently, urban trees may have lower CO2 uptake and transpiration rates than rural trees during those months; however, this difference may be offset during the spring and fall when warmer temperatures may enhance photosynthesis and transpiration rates (Waring and Schlesinger, 1985). Microclimates created by the size, shape, and type of treed patch may differ among treed patches and subsequently influence CO2 uptake and transpiration rates. For example, the microclimate created by remnant and emergent patches may be cooler and more humid than that created by a planted patch of similar size and shape. This difference may influence the duration individual trees, within a patch, can transpire and uptake CO2 for photosynthesis. Obviously, additional research is needed to determine a carbon budget for an urban landscape, how different treed patches influence that budget, and how it differs from carbon budgets in rural landscapes. The interception of pollutants also may differ among patch types. Deposition of pollutants occurs laterally, from the side, and vertically, from above the canopy (Pouyat et al., 1995). In patches with an understory, lateral deposition generally occurs within 30 m from the patch edge, a zone spatially defined as ‘‘road side environment’’ (Airola, 1984). Depending on patch width and edge type, remnant and emergent patches, with their understory, should intercept more lateral deposition than a planted patch. In contrast, a planted patch initially may intercept more vertical deposition because of its large canopy and leaf area (Nowak, 1994b); however, more vertical deposition actually may be filtered out by remnant and emergent patches because the pollutant passes through the canopy, sapling, and shrub strata. In fact, optimum configurations involving spacing, patch shape, and vertical structure probably exist that maximize evapotranspiration rates, rain, and particulate matter interception, and carbon sequestration for different urban landscapes. More research is needed to determine how landscape configuration of patch types influences these processes.
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Eventually, particulate matter will reach the ground surface through rain fall and litter. Within remnant patches, particulate matter, such as heavy metals, accumulate in the forest floor (Pouyat and McDonnell 1991). This accumulation affects both nutrient cycling, carbon storage, and soil fauna (Pouyat et al., 1994; Groffman et al., 1995; Pouyat et al., 1995). Because of a developed forest floor, accumulation of pollutants is hypothesized to be the greatest in remnant patches and the least in planted patches. Overall, because of their ability to possibly intercept and accumulate more pollutants, remnant and emergent patches are hypothesized to have a significant effect on air and water quality within urban landscape. The extent of the effect depends on the areal coverage of these patch types. This is not to say planted patches are unimportant within an urban landscape, but rather, from an ecological perspective, remnant and emergent patches seem to significantly influence ecological processes. Conclusion Urban areas are a mosaic of physical, ecological, and social patches. How these patches interact to control fluxes of water, nutrients and carbon, and biodiversity is one of the principal goals for the long term ecological research program in Baltimore, Maryland (Fig. 6). As a step toward achieving this goal, we propose a scheme of identifying and describing treed patches that is different from current vegetation
Figure 6. A composite model of the effects of urbanization on ecological phenomena modified from McDonnell and Picket (1990) and McDonnell et al. (1997). The arrows indicate causal linkages and feedbacks between the components of urban areas and ecological phenomena. Social-economic policies and decisions are the principal drives of the system; however, a feedback from ecosystem effects influences these decisions and subsequently urban biota and urban structural features. The italic text represents further stratification of a component based on experimental design and results (see McDonnell et al., 1997).
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inventorying techniques used in urban landscapes. This work, coupled with patch analysis of social and physical patterns (Grove and Burch, 1997; Foresman et al., 1997), will allow us to produce high resolution data that are necessary for simulation models to estimate ecological and socioeconomic fluxes for an entire watershed and city. Likewise, the proposed scheme is applicable to both human and nonhuman dominated landscapes, thus enabling the evaluation of urban effects on ecological processes across a gradient of urbanization from urban to rural. Adding data from historical records and aerial photographs will enable us to test hypotheses about how social and ecological patterns have changed and how they may change in the future. Acknowledgments We would like to thank Dave Nowak, Mark Walbridge, and two anonymous reviewers for their help in improving the manuscript. Funding support was provided by USDA Forest Service Research Unit NE-4952. References Ackerman, B. (1985) Temporal march of the Chicago heat island. J. Clim. Appl. Meteorol. 24, 547–554. Airola, T. M. (1984) Species structure and soil characteristics of five urban sites along the New Jersey Palisades. Urban Ecol. 8, 149–164. Allen, T. F. H. and Hoekstra, T. W. (1991) Role of heterogeneity in scaling of ecological systems under analysis. In Ecological heterogeneity (J. Kolasa and S. T. A. Pickett, eds.), pp. 47–68. Springer-Verlag, New York. Allen, T. F. H. and Hoekstra, T. W. (1992) Toward a unified ecology. Columbia University, New York. Auclair, A. N. and Goff, F. G. (1971) Diversity relations of upland forests in the western Great Lake region. Am. Nat. 105, 499–528. Bailey, R. G. (1983) Delineation of ecosystem regions. Environ. Manage. 7, 365–373. Band, L. E. and Moor, I. D. (1995) Scale: landscape attributes and geographical information systems. Hydrol. Proc. 9, 401–422. Band, L. E., Patterson, P., Nemani, R. and Running, S. W. (1993) Forest ecosystem processes at the watershed scale: incorporating hillslope hydrology. Forest Meteorol. 63, 93–126. Barbour, M. G., Burk, J. H. and Pitts, W. D. (1980) Terrestrial plant ecology. The Benjamin/Cummings Publishing Company, Inc., Menlo Park, CA. Barlow, E. R., Cramer, M., Heintz, J. L., Kelly, B. and Winslow, P. N. (1987) Rebuilding Central Park: a management and restoration plan. Central Park Conservancy, New York. Birdsey, R. A. (1990) Inventory of carbon storage and accumulation in U.S. forest ecosystems. In Proceeding of the IUFRO world congress (H. E. Burkhart, G. M. Bonnor and J. J. Lowe, eds.). School of Forestry, Virginia Polytechnic Institute and State University, Blacksburg, VA. Publication FWS-3-90. Bormann, F. H. and Likens, G. E. (1979) Pattern and process in a forested ecosystem. Springer-Verlag, New York. Brady, R. F., Tobias, T., Eagles, P. F. J., Ohrner, R., Micak, J., Veale, B. and Dorney, R. S. (1979) A typology for the urban ecosystem and its relationship to larger biogeographical landscape units. Urban Ecol. 4, 11–28. Cramer, M. (1993) Urban renewal: restoring the vision of Olmsted and Vaux in Central Park’s woodland. Restor. Manage. Notes 11, 106–116. Dorney, R. S. (1977) Biophysical and cultural-historic land classification and mapping for Canadian urban and urbanizing landscapes. In Ecological/biophysical land classification in urban areas (E.B. Wiken and G.R. Ironside, eds.), pp. 57–71. Environment Canada, Ottawa. Ecological Land Classification Series, No. 3. Dorney, R. S., Guntenspergen, G. R., Deough, J. R. and Stearns, F. (1984) Composition and structure of an urban woody plant community. Urban Ecol. 8, 69–90. Douglas, I. (1990) Sediment transfer and siltation. In The earth as transformed by human action (B. L. Turner, W. C. Clark, R. W. Kates, J. F. Richards, J. T. Mathews and W. B. Meyers, eds.), pp. 215–234. Cambridge University Press and Clark University, New York.
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