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Methods for spatial and temporal land use and landcover assessment for urban ecosystems and application inthe greater Baltimore-Chesapeake region

TIMOTHY W. FORESMAN*Department of Geography, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA

STEWARD T. A. PICKETTInstitute of Ecosystem Studies, Box AB, Millbrook, NY 12545, USA

WAYNE C. ZIPPERERUSDA Forest Service, 1100 Irving Avenue, Syracuse, NY 13210, USA

Understanding contemporary urban landscapes requires multiple sets of spatially and temporally compatible data thatcan integrate historical land use patterns and disturbances to land cover. This paper presents three principal methods:(1) core analysis; (2) historic mapping; and (3) gradient analysis, to link spatial and temporal data for urbanecosystems and applies their use in the Baltimore-Chesapeake region. Paleoecological evidence derived from thegeochronology of sediment cores provides data on long-term as well as recent changes in vegetative land cover. Thisinformation, combined with contemporary vegetation maps, provides a baseline for conducting trend analyses toevaluate urbanization of the landscape. A 200-year historical land use database created from historical maps, censusdata, and remotely sensed data provides a spatial framework for investigating human impacts on the region. Ageographic information system (GIS) integrates core analyses with historic data on land use change to yield acomprehensive land use and land cover framework and rates of change. These data resources establish the regionalfoundation for investigating the ecological components of an urban ecosystem. Urban-rural gradient analyses andpatch analyses are proposed as the most appropriate methods for studying the urban ecosystem as they link ecologicaland social patterns and processes for varying degrees of urbanization.

Keywords:land use; land cover; urbanization; urban-rural gradient; paleoecology; GIS; historical mapping; Balti-more-Chesapeake region; history

Introduction

An understanding of processes that cause the shifting mosaic of land cover in regions should be basedon fundamental knowledge of the physical environment’s influence on vegetative communities as wellas human impact on the landscape. The incorporation of physical and human factors is especiallyimportant for environmental or ecosystem analysis in urbanizing and urban landscapes. Human impacthas become a major determinant for land cover through the various modifying activities associated withland use.

In attempting to understand these relationships, scientists and decision makers often rely upon clas-sification systems to document or map the spatial extent of land cover, land use, or a combination of land

* To whom correspondence should be addressed.

Urban Ecosystems,1997,1, 201–216

1083-8155 © 1997 Chapman & Hall

cover and land use. Comprehensive analyses of land cover and the influence of land use on environmentalservices for water, soil, air, and biodiversity, often require combining land cover and land use parameters.The United States Geological Survey (USGS) Andersonet al. (1976) classification is a prime exampleof a recognized national land use/land cover classification system. However, this combination is not assimple or straightforward as the existence and extensive use of land cover/land use maps and classifi-cation systems might lead an observer to believe (Lovelandet al.,1991; Westmoreland and Stow, 1992).

Separating land use and land cover data in initial stages of the analysis provides an improved basis forconnecting socioeconomic, physical, and ecological data on urban areas. This paper seeks to: (1) critiquethe combination of land use and land cover; (2) evaluate three methods that can be used to assess pastand on-going land transformations; and (3) show how these methods are used to construct a temporalmodel of land transformation in the Baltimore-Washington region. The three methods are core analysis,historic mapping, and gradient analysis.

Land cover distributions, the biophysical skin of the earth’s surface, are based on biogeographicvariations in the climate, topography, and edaphic factors. Land use distributions, the status of humanoperations on the earth’s surface, represent the arrangement of human activities based on land ownership,zoning policy, and management practices. Therefore, land cover/land use maps present an explicitcompromise with the inferred ability to conduct analyses on either land cover or land use. The incor-poration of these two distinct land surface phenomena for input into ecological processes studies ormodels requires appropriate scientific metrics. We offer the concept that landscape ecology studiesshould not rely on the aggregation of land cover and land use variables, especially for human dominatedlandscapes along urban-rural gradients (McDonnell and Pickett, 1990). Instead, we suggest that thesevariables should be collected and maintained as separate and distinct landscape parameters.

Ambiguity of urban landscape terms and definitions for the gradient continue to plague both thescience and policy communities (Ratcliffe, 1997, pers. comm.). Our use of the term ‘‘urban to ruralgradient’’ (often simply urban-rural) encompasses the landscape zones recognized by such terms ascentral business district, suburban, urban extent, urban fringe, or city edge. We view this zone as agradient ranging spatially from the central city to the hinterlands of farms and forests. From an ecologicalperspective, the zone represents functional differences in transitional patches between city and country(McDonnell and Pickett, 1990; Pouyatet al.,1994). The gradient need not be a literal linear transect fromurban to rural, but is used to conceptually order the impacts of urbanization as one moves from urban torural. Our use of the term ‘‘framework’’ encompasses the conceptual setting of a study includingdatabase resources that provide spatial and/or temporal organization of a specific investigation’s context.

The ability to provide accurate and appropriately scaled land cover and land use data sets is essentialto investigate the many possible functional relationships and pathways for the human activities inter-acting with ecological processes (Pickettet al.,1997). Yet, a scheme for a universally accepted land coveror land use classification does not exist. Furthermore, the variety of schemes used to define globalvegetation are remarkably disparate (Estes and Mooneyhan, 1994). Likewise, plant ecologists tradition-ally have not incorporated human-induced factors as components for vegetation classification or mapping(Mueller-Dombois and Ellenberg, 1974).

Human factors, however, are increasingly being recognized as the driving force behind the rates andtrajectory of change upon the earth’s surface (Likens, 1991; Vitousek, 1994; Turner and Meyer, 1994).Global change researchers, however, have not been able to access appropriately scaled information toincorporate this complex mosaic of land cover and land use variables in their modeling schemes. Anunderstanding of the trajectories of these driving forces requires a temporal perspective. Both paleoeco-logical and historical land cover and land use maps are considered important to establishing the temporalperspective or framework of landscape dynamics for ecological studies (Brush, 1989). Defining historictrends along the conceptual urban-rural gradient is considered necessary for evaluating various impacts

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from past land transformations, testing temporal relationships discovered in space (Pickettet al.,1997),and attempting to predict potential future modifications in human dominated ecosystems.

Assessment and mapping methods

This paper explores three complementary methods—paleoecological, historical, and contemporary—being used by the authors to construct temporally and spatially explicit land cover and land use databasesin the context of long-term ecological studies within the Chesapeake Bay watershed. Whereas theobjective for using these complementary methods is to provide a quantitative framework within whichdetailed urban-rural ecological gradient studies can be performed, the methods are appropriate for anyenvironmental setting. Landscape processes are ineluctably connected to the functional relationships ofland ownership, economic prerogatives for resource exploitation, and human social or political influences(Forman, 1995). Studies directed at assessing environmental change or at delineating impacts fromnatural and human processes, depend upon a thorough foundation detailing the system’s long-termecological behavior as well as the specific causal agents of human-induced inputs. Fundamental knowl-edge of the processes and interrelationships of the physical, ecological, and social components on theurban-rural gradient are poorly understood (McDonnell and Pickett, 1990). As recognition of the im-portance of studying urban ecosystems increases, the concomitant requirement to provide historical andpaleoecological frameworks for these studies will also increase.

Urban-rural gradient.Ecosystem studies need to understand human influences on ecological processes(McDonnell and Pickett, 1990; Pickett and Cadenasso, 1995). Urban ecosystems represent the mostcomplex mosaic of vegetative land cover and multiple land uses of any landscape. Ecologists, seeking tounderstand the numerous pathways for successional states in land development relative to the vegetativestructures and impacts of these structures on biodiversity, recognize the need to better understand thiscomplex gradient (Pickettet al., 1987; Soule and Kohm, 1989; Zipperer, 1993). Recent investigationshave unveiled striking features of ecosystem processes along a gradient of urbanization (McDonnelletal., 1997). Functional differences include N-mineralization rates, decomposition rates, and carbon cy-cling (Pouyatet al.,1994; McDonnell and Pickett, 1990). These findings suggest that detailed analysesof land use histories are required to understand patterns of urban vegetation, i.e. forest patches, andecological processes along an urban-rural gradient (Sharpeet al., 1986; Zipperer, 1993).

An urban-rural gradient approach also improves our understanding of how human actions affecthydrologic processes and biodiversity. For example, hydrologic models of the urbanized areas thattraditionally relied on percent canopy cover to characterize urban landscapes. Because of increasedspatial resolution of the data, modelers are now investigating the influence of spatially explicit land coverand land use parameters on hydrologic processes (Band and Moore, 1995; Green and Cruise, 1995).Fauna also are being more intensely studied because of concerns for both biodiversity and assessment ofdisease vectors in urban landscapes, e.g. Lyme disease (Soule and Kohm, 1989; Stomset al., 1992).

Spatial analysis.Spatial analysis tools (e.g. geographic information systems and remote sensing) foracquiring, storing, and analyzing land cover and land use data can be successfully used to create regionalframeworks for interdisciplinary studies in urban ecosystems (Skole, 1994; Cowenet al.,1995). Knowl-edge of where land conversions occur and the spatial characteristics associated with these conversions arerequired to define the specific pathways and processes of change operating within urban ecosystems.Long-term ecological assessments need to build the groundwork for baseline, historical trends on landcover and land use patterns and provide insights to questions of rates of change for flora, fauna, nutrientfluxes, and human impacts. In addition, land cover changes must be assessed at different scales in orderto expose the different social, ecological, and physical drivers and constraints that may operate over therange of spatial scales.

Land use in the greater Baltimore-Chesapeake region 203

Paleoecological methods for land cover

Sediment cores from the Chesapeake Bay estuary and its tributaries have been essential in documentingthe land use effects of deforestation, agricultural expansion and intensification, and urban sprawl in thewatershed. Brush (1994) has compiled a substantial set of sediment cores, complete with biologicalindicators, to reconstruct the land cover conditions around the Chesapeake Bay estuary before Europeansettlement (Fig. 1). The sediment record portrays composite profiles of the areal extent of land cleared,along with sedimentation rates, ragweed dating profiles, planktonic diatom species, and submergedaquatic vegetation. These assemblages provide evidence of the land cover dynamics during the past fewcenturies, thereby providing an ecological trajectory for further assessment of contemporary urbanizationimpacts.

Careful analyses of the vertical increments of core samples were used to estimate estuarine sedimen-tation rates for the past 11 000 years (Brush, 1989; Cooper and Brush, 1991). The paleoecological recordpreserved in sediment cores provided critical evidence for comparing effects of human activities with theeffects of climate change in the past with an eye to the future. In the Chesapeake Bay region, ragweed(Ambrosiasp.), a prolific pollen producer, colonized areas of disturbed soil and, therefore, produced adistinct horizon related to large scale land clearance from agricultural activities. Another distinct horizonin this region was derived from the demise of the American chestnut in the late 1920s, when thedisappearance of chestnut pollen demarked the stratigraphic record. With adjusted sedimentation rates,actual calendar years were assigned to the samples (Brush, 1989). Reconstruction of the biologicalpopulations for land cover and estuarine ecosystems were calibrated with the dated samples. Changes infossilized indicators within cores were related to historically documented land use and compared to the

Figure 1. Paleoecological analysis of the upper Chesapeake Bay as determined from sediment core analysis (re-printed with permission from Brush, G. S., 1994).

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prehistoric chronology for a more complete understanding of conditions before and after human settle-ment impacts.

Integrating paleoecology and history: a picture of today and projections for tomorrow

Historical trends of land cover and land use impacts can be investigated using the convergent andcomplementary tools of paleoecological chronology, historical land use records, and modern vegetationmapping. For urban landscapes, these historical contexts, with the attendant data types for land andestuary biota present time-series data to better calibrate, with precision, the rates and patterns of urban-izing landscape parameters. Brush (1994) and Yuan (1995) have used cores to document the majorsettlement periods and land use trends impacting the ecological conditions for the Chesapeake Bay region(Table 1). Spruce and fir characterized the postglacial period dominating the Chesapeake Bay landscapefor approximately 3000 to 4000 years. This was followed by hemlock. As the climate warmed, the regionexperienced less precipitation, providing conditions for a closed-canopy forest of oak-hickory to domi-nate the landscape. The paleoecological record indicates that these conditions, which began about 5000years ago, continued up to European settlement in the 17th century when ragweed pollen providesevidence that approximately 20% was cleared for agriculture. Sedimentation rates display significantincreases when land clearance exceeded 20% (Brush, 1989). During the next 100 years, agriculturalexpansion on small farms for grain and tobacco resulted in additional land clearing of up to 40%.Deforestation for farming was augmented in many areas by timber cutting for energy demands of the ironfurnace industry (Travers, 1990; Brush, 1997). With the advent of guano-based fertilizers and theindustrial revolution, the last half of the 19th century experienced 60 to 80% land clearance for agri-culture. The 20th century saw continued land cover transformation, including drained wetlands and urbansprawl continuing in classic patterns of development (Von Eckardt and Gottman, 1964) around theBaltimore region. With farm abandonment, forests began to regenerate on previously cultivated land.Post World War II saw an expansion of urban land use significantly altering and impacting the region’s

Table 1. Land use and land cover historical trends for the Chesapeake region

Time frane Period Land use/land cover characterization

10 000–5000 B.C. Pre-human Boreal type forest succeeded by hemlock intoenclosed canopy mixed conifers-deciduousforest.

5000 B.C.–1600 A.D. Pre-European Oak-hickory, closed canopy forest dominateslandscape except for tidal wetlands andserpentine barrens, frequent fires.

1600–1750 Early settlement 20–30% land cleared for tobacco farming.1750–1850 Colonial towns to rural

agrarian intensification40% of land cleared, grain and tobacco farming

on small farms, deforestation for ironfurnaces and construction.

1850–1900 Agricultural transition toindustrialization

60–80% of land cleared for large farms,introduction of deep plough and guano-basedfertilization.

1900–1950 Modern urbanism andindustrialization

Chemical-based fertilizer, extension of farmsand urban with wetland drainage.

1950–Present Modern transportation,population growth andurban sprawl

Decrease in cultivated land, forest, i.e.generating, urban expansion.

(Modified from Brush, 1994)


ecosystems. The effect of this urban sprawl is only recently being quantified and set into a conceptualframework for assessing the ongoing rates of change in ecological processes and the various trajectoriesfor the ecological system components (Pickettet al., 1997).

Historical land cover assessments

Assessing the trajectories of land cover in areas impacted by rapid urbanization requires a series ofaccurate land cover maps. A baseline or ‘‘natural’’ vegetation is needed to assess historical changes inspatial and structural composition and to project expected vegetation patterns into the future. Whereasnumerous vegetative maps have been generated for the Chesapeake region (Brushet al., 1980; Dobsonet al.,1995; USGS, 1996), the lack of ground sampling and verification protocols restricts the applicationof such vegetation maps for quantitative floristic determination. For example, most of the vegetationclassifications based on satellite images display only gross aggregates of vegetative land cover classes(Henderson-Sellerset al.,1986). As ecologists’ interest lies in how one species or plant association willsucceed another, given a pattern of disturbance that is either climatic, stochastic (i.e. fire), or anthropo-genic, these coarse scale categories of vegetation patterns may not satisfy ecosystem-level analyses inurban areas.

Within the Chesapeake region, landscape-level vegetation maps did not exist until recently. Besley(1916), Stetson (1956), and others provided descriptions of forests existing at the time of Europeansettlement. This information can be augmented by witness tree records listing species at known locationsfrom the late 1600s to the early 1700s. At the turn of the century, Forrest Shreve and associates (1910)provided an excellent survey of Maryland’s flora by identifying both herbaceous and tree species ondifferent soil types. Also, for certain urban parks, descriptions of the forest species can be found inhistorical documents.

With the Chesapeake region, however, a definitive delineation of the natural forests of Maryland wasnot performed until the mid 1970s (Brushet al.,1980). Brush and associates (1980) defined the Marylandnatural forests as those regenerating naturally under a variety of disturbance patterns. The forests weredifferentiated, without attention to age classes, into 18 associations and mapped at a scale of 1:250 000.The original map was produced with a minimal mapping unit of two hectares (Fig. 2). Maryland’s forestswere mapped by recording all species present, irrespective of size, and generating polygonal boundariesaround all areas of homogeneous species composition recognized in the field by indicator species. Thesemapped associations include different stages and origins of forest growth, e.g. recently cut-over areas,reforested abandoned farmland, remnant forests undisturbed for more than 100 years, and hedgerows.

The 18 forest associations were found to be related closely to patterns of available water, regardlessof the topographic and geologic differences separating the physiographic provinces (Brushet al.,1980).Lithologically different substrates are often similar hydrologically. A good example is the chestnutoak-post oak-blackjack oak association that occupies several different substrates with common hydro-logic characteristics. This same association was identified on fragipan, gravel, and serpentine, whichdiffer texturally and chemically but are hydrologically similar (Brushet al., 1980). Where human-induced hydrological alterations have occurred, shifts in association distribution have resulted. Accordingto Brushet al. (1980), current land use has had little influence on existing forest patterns. Except foractivities such as road building and construction that can change drainage patterns, correlations betweenthe forest associations and substrate units were found to remain generally consistent. These findings andthe forest map provide a spatial framework or ecological reference point to investigate historical landcover trajectories.

Contemporary land cover assessments and mappingAdvanced tools for spatial analysis, i.e. remote sensing and GIS, represent important technologies forproviding local, regional, and global histories (Acevedoet al.,1996; Skole and Tucker, 1993; Brondizio

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et al., 1996). First, remote sensing enables quantitative measurement of the earth’s surface biophysicalproperties, with a legacy of more than 50 years of aerial photography for many portions of the globe andabout 25 years for global satellite coverage. Remote sensing offers plant ecologists and soil scientists, aswell as others tasked with mapping the earth’s surface phenomena, both a medium for recording thespatial distribution of features of interest and data for analyzing the reasons for these distributions.Limitations of remote sensing, that is, the inherent resolution for discriminating cover types relative tothe minimum mapping unit for the investigation, can be addressed primarily by field verification. Fieldverification, however, imposes a labor cost that invariably restricts the geographic extent of detailedmapping surveys. In addition, national standards do not exist for performing verifications, but ratherconventions exist for vegetation or soil sampling that are derived from ‘‘schools’’ of experience. Asurvey of field technique texts or manuals will substantiate this finding (Loundsbury and Aldrich, 1986;Csuros, 1994). Lack of verification standards results in significant disparity in the accuracy of land covermaps from remotely sensed data. A rigorous approach to verification issues was demonstrated by CoastalChange Analysis Program (CCAP) along the east coast (Dobsonet al., 1995). This project relied uponboth labor-intensive methods and the use of regional experts to generate highly accurate, and tested, landcover classifications from Landsat Thematic Mapper and aerial photography. Although the classificationscheme adopted the use of a combined land use/land cover approach, the methods for verification arescientifically defensible. In addition, CCAP classification provides a GIS format that is optimal formanaging land cover data in a digital form (Dobsonet al., 1995).

Historical land use mapping

Paleoecological analysis that generates land cover chronologies presents extremely valuable data forlocal and regional ecosystem studies. By itself, however, the paleoecological analysis does not providethe spatial precision necessary to reconstruct a farm-by-farm, neighborhood-by-neighborhood assessmentof past and present land use (Brondizioet al.,1996). Quantitative, historical land use information can be

Figure 2. Maryland’s Natural Forests Map Displaying Thirteen of Eighteen Forest Associations (recreated fromBrush et al 1980).


effectively reconstructed through the careful compilation of complementary sets of tabular, census, andmapping data sources. This approach was used in the San Francisco Bay area (Kirtlandet al.,1994) andthe Baltimore-Washington area (Acevedoet al., 1996; Foresmanet al., 1996).

The Baltimore-Washington project compiled historic maps, demographic data, environmental param-eters, and satellite images to map human-induced land transformations from 1792 to 1992 for theBaltimore-Washington region (Fig. 3). All data were reformatted to a common spatial reference using aGIS. This figure portrays the dramatic increase in urban expansion and population growth experiencedduring the past two centuries. With more than 7 million people in the greater Baltimore-Washingtonregion, the study site captures one of the nation’s fastest growing examples of urbanization spreading intowetland, forested, and agricultural ecosystems (Foresmanet al.,1996). The increasing occurrence of themegalopolis formation has been highlighted by urban geographers concerned about the relationships ofboth social and environmental ills associated with urban decay (Von Eckardt and Gottman, 1964).

Land use compilation for the urban extent and the transportation categories was accomplished with acombination of techniques including manual map transcriptions, table digitizing, and digital imageprocessing of the satellite data. The exact compilation procedure depended upon the condition and arealextent of the source materials. A set of criteria for urban extent was established based on comparingsettlement patterns from map or satellite sources and cross referencing the locational information withcensus data. An urban center, or town, was registered when a population of 500 persons was reached.Census data were used to track the numbers and characteristics of the population back to the 1790 censusnationally, and before 1790, depending on the county or incorporated area records. Using standardizeddata allows the use of this technique for much of the American urbanized landscapes.

Figure 3. Two-hundred years of urban growth for the Baltimore-Washington region.

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Two hundred years of urbanized growthThe eight images displayed in Figure 3 illustrate urban growth in the greater Baltimore-Washington areafrom 1792 to 1992. This region covers a 2-degree latitude by 2-degree longitude area for much ofMaryland, the northern portion of Virginia, parts of West Virginia and Pennsylvania, and the District ofColumbia.

After a century and half of colonial settlement, an urban pattern began to occur around the ports suchas Annapolis, Baltimore, and Alexandria as the primary focal points for development. Located on the fallline between the Coastal Plain and the Piedmont Plateau, the towns became important trade centers forthe plantation-based economy, driven by the cultivation of tobacco, grain, and livestock.

By the 1850s, Baltimore became an expanding urban and industrial center with the bulk of the 1million people in the region residing in Baltimore City. The Baltimore and Ohio Railroad connectedBaltimore and the Midwest. In contrast, Washington remained a relatively small city. A network of ruralcities began to develop in the hinterlands of Baltimore, whereas many colonial era ports throughoutMaryland and Virginia became inaccessible for shipping because of heavy siltation, a result of defor-estation and overuse of agricultural fields.

At the turn of the 20th century, two million people in the region began to form the suburbanenvironment with the advent of street cars and extensions of rail lines. Half of Maryland’s populationresided in Baltimore with western towns growing along the rail line. Suburban sites continued to growand by 1925, the ‘‘inter-urban’’ rail lines allowed commuter villages to become towns. This growthresulted from the economic prosperity of an increasingly industrialized Baltimore City.

At the half century mark, significant filling of suburban areas adjacent to the central cities wasaugmented by the additional growth of urban areas made possible by the advent of the automobile andimproved transportation infrastructure. Growth of the federal government in the 1930s and 1940s spurredgrowth around Washington and the adjacent counties. By 1972 and continuing to the last two frames of1982 and 1992, the urban growth connected the corridor between Baltimore and Washington. Rampantgrowth and urban flight continue to change the land use in the area from agriculture and forest to urban.By 1992, more than 8 million people resided in the study area at an average density of 466 persons persquare mile (Ratcliffe, 1997, pers. comm.).

Visualization methodsOnce a historical land use database is compiled, the data can be applied to other investigations. As digitaldata, the historical land use structure can be used to evaluate the impact of the land use on land coverdynamics. One approach is to visualize data from different perspectives. Masuoka and associates (1995)generated an oak-hickory forest closed canopy image for the Baltimore region by draping the spectralsignature for forest canopy, as derived from Landsat TM data, over a digital elevation model for theregion (Fig. 4). The resulting depiction provided a perspective of Baltimore much as it would have lookedbefore European settlement. The classification of urban land use was then projected over this historicallyaccurate view of the landscape for the 200-year period. Through visualization, it is apparent that theagriculture land use classifications were missing from the image. These projections provided an artificial,bimodal perspective for forested and urban categories that represent a mix of land cover and land use.The combination of these classification conventions visualized for the landscape represents land coverand land use classification’s dual nature as a foundation for ecological studies. Significant enhancementsthrough the creation of additional data to the database are needed to correct the duality, and provide amore spatially accurate definition of historical land transformations and conversions for the land surfacecharacteristics.

The methods for creating historical land use databases are appropriate for most of the North Americancontinent in terms of defining processes of land transition and urban landscape dynamics, as well as otherenvironments, for example, the tropics (Brondizioet al., 1996; Skole and Tucker, 1993; Tuckeret al.,


1985). By using national data assets that are relatively easy to combine with digital databases from theNational Spatial Data Infrastructure (NSDI), we envisage the construction of a national set of historicaldatabases (NSDI, 1994). The potential for producing this set of databases over the next decade is quitereasonable considering the interest by federal agencies to build upon San Francisco and Baltimore-Washington historical mapping experiences. The potential for cooperative development of one kilometerland cover and land use databases for whole continents is also feasible by adopting similar NSDIstructures for the globe (Tuckeret al., 1985; Lovelandet al., 1991; Earthmap, 1995).

Figure 4. Baltimore area prospective of two-hundred years of urban growth on forested background.

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Contemporary land use assessment and mapping

Contemporary land use assessment and mapping can be accomplished in part with the same tools andmethods described in the ‘‘Contemporary Land Cover’’ section. However, the critical difference lies inthe inability to determine from remotely sensed resources (aircraft or satellite) the ownership of the land,the ownership practices on the land, and policies controlling land management activities. Obtaining thesekey land management variables for extensive areas of the landscape requires significant investigativeresources. For urban areas, obtaining land use information requires access to parcel-level records of deedsand zoning typically located at county recorder’s and assessor’s offices. Only recently has the existenceof digital files from these offices become a viable data source for investigators. Previous research effortsrequired painstaking data collection from county offices and then conversion to digital formats foranalysis. Outside the urban areas, access to land use data for agriculture can be obtained through theNatural Resource Inventory of the Natural Resource Conservation Service (NRCS, 1996). However,some level of confidentiality is retained, thus limiting data access. Other records are culled from variousfederal and state offices. With federal, state, and local agencies modernizing their land managementrecords in GIS compatible formats, the ability to generate accurate and verifiable land use maps andconduct assessments on land use impacts will improve dramatically.

Discussion: generalizing the approach

The urban-rural gradient is a conceptual tool to permit the comparison of pattern and process of systemsover wide areas and has been recommended to the ecological community as a means of addressingsignificant gaps in our knowledge of the urban landscape (McDonnell and Pickett, 1990; Allen andHoekstra, 1992). An approach based on differences along a gradient can be applied to a wide range ofgeographically disparate landscapes experiencing urbanization. A principal benefit of a gradient approachderives from the advanced level of understanding of gradients from different active disciplines, i.e.ecological (McDonnell and Pickett, 1990), social (Grove, 1996), and hydrological (Band and Moore,1995). A second key approach to urban ecosystems is the use of patch analyses to define vegetation andsocial patterns. Hydrological or physical processes can also be linked to changes in patch configuration.

The patch approach complements the gradient approach because it is spatially explicit and can be usedhierarchically to discover the scales at which certain patterns and processes operate. Ecological, social,and hydrological fluxes can occur among patches. Integrative modeling of ecological, social, and physi-cal variables when linked or defined as patches, can be used to identify the functional interactions of thesevariables (Pickettet al., 1994).

The suggestion that both gradient analysis and patch analysis are effective approaches toward inte-grating ecological, social, and physical variables of an urban landscape is based on the depth of theo-retical understanding for these approaches in the different disciplines. From the ecosystems perspective,we are concerned with the relationships between structure, species composition, and the arrangement ofland cover patches, from the urban core to the rural hinterlands, plus how these relationships influencethe function of hydrological processes. To reach an advanced state of integration amongst ecological,social, and physical variables requires keeping the land cover and land use parameters separate.

Beyond our concern for spatial arrangements of the urban ecosystem in our field studies lies a concernfor its temporal arrangements. Whereas alternative modeling designs can incorporate temporal data setsinto ecosystem models (Farmer and Rycoft, 1991), these efforts are limited to time slices of land coverand land use data. Peuquet (1994) has offered a conceptual framework for pursuing various avenues fortime series analysis. However, little progress has been made outside the inherent approach of using timeslices for modeling input and analysis (Foresmanet al., 1996).


Paleoecological records of indicator organisms and materials derived from estuarine sediment haveproven valuable for reconstructing the history and prehistory of the urban watersheds. Analysis ofsediment cores for pollen, preserved diatoms, and remains of other organisms, when calibrated by carbondating, generates important evidence of land cover patterns and relationships between land use andestuarine ecosystems. This evidence provides a chronology for defining regional vegetation communitydynamics. Although there are relatively few local or regional sediment analyses, the importance of theexisting data points for temporal and spatial calibration, and assessment of human-induced land trans-formations cannot be overstated. Investigations of the role humans have played in the Chesapeake Baywatershed and estuary have demonstrated the significance of human-induced alteration from the states ofpostglacial ecosystems to the present configuration. With paleoecological studies, Brush (1994, 1997) hasdocumented radical landscape transformations of land cover and land use in the watershed. Thesetransformations not only had profound negative impacts on the diversity of estuary biota but alsoinfluenced water quality and quantity. Although historic land cover and land use analysis can provideimportant understanding regarding the rates of change for major landscape elements, paleoecologicalrecords are necessary to establish the long-term performance of ecosystems in response to disturbance(Clark, 1990).

Ecological modeling needs human land use activities and how these changes influence spatial patternsand movement of energy, species, and materials in urban environments. The historical trends in land useand land cover provide not only an important perspective but a fundamental set of inputs for assessingcumulative impacts at multiple temporal and spatial scales. However, as illustrated in Figure 4, land useand land cover should be kept as separate categories. Models for integrated assessment, and integratedregional models will certainly benefit from a richer set of temporal data resources for the landscape ifthese data resources contain land cover structural and functional descriptions with accurate spatialreferencing (Pickettet al.,1994). The effects of land use practices and policies and their relationship tovegetation can then be investigated with greater precision.

GIS provides the critical capacity for integrating historical documents with maps and remotely senseddata to construct high quality regional environmental data repositories. The Baltimore-WashingtonRegional Collaboratory (http://www.umbc.edu/bwrdc) represents an example of a regional data reposi-tory. The complex spatial nature of land cover and land use interactions requires GIS for managing thesedatasets as inputs to comprehensive ecological assessments (Cowenet al.,1995; Stomset al.,1992). Thespecialization required to master this technology and to harness these information systems for ecologicalassessments has, until recently, been a major obstacle for most multidisciplinary science teams. Spatialstatistics are increasingly being linked to remote sensing and GIS technology, enabling scientists to testhypotheses and develop specialized models (Schlagel and Newton, 1996). With the advent of effectivemodeling and visualization mechanisms to document, analyze, and present historical trends in land coverand land use, a pronounced improvement in technical and interpretive capabilities exists for urbanecosystem studies. Environmental modelers may wish to access land use and land cover data from a GISstructure for model input or to calibrate their models (Oreskeset al.,1994; Schlagel and Newton, 1996;Stomset al., 1992). Modelers are clearly interested in integrating ecosystem components from GISdatabases to study and predict human influences on physical processes (Meyer, 1996; Bormannet al.,1993; Grove, 1996). Ecological models use GIS object-oriented databases with relational databasemanagement architectures to provide time sequential or time differential data analyses (Cowenet al.,1995; Green and Cruise, 1995; Peuquet, 1994; Foresmanet al.,1996). The greatest challenge facing themodeling communities is integrating environmental, human, and physical models into a single set ofregional models (Blood, 1994; NSTC, 1995; Skole, 1994). Database managers, ecologists, and themodeling community need to continue working together to creatively address issues of calibration,

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uncertainty and error propagation, simplification or aggregation, resolution, and scale. These are espe-cially important for the development of regional models (Pacala, 1994).

Summary

The earth’s population has doubled in less than 50 years and within the past two decades the extent ofurbanization for the world’s major metropolitan areas has increased exponentially in both developed andless-developed nations (Meyer, 1996). Urban land increased by 54 362 000 hectares in the United Statesbetween 1960 and 1980 (Frey, 1984) and by 3 963 600 hectares in the Chesapeake watershed between1980 and 1990 (NCRI, 1996). For the Chesapeake watershed, this decadal increase of approximately 40to 60% in urban land use occupied nearly 10% of the watershed’s total acreage (NCRI, 1996; NRCS,1996). By 1995, 21 world cities had populations of more than 6 million living in urbanized areas thatformerly supported agricultural or forest land cover and land use (World Almanac, 1996). The resultingurbanization has drastically altered ecological patterns and processes. Using standard tools of GIS,historical mapping, census data, and satellite imagery, studies on the east and west coasts have designedprocedures to spatially assess land cover and land use transitions and modifications (Kirtlandet al.,1994;Acevedoet al.,1996). Higher spectral and spatial resolution of historic land cover and land use databasesin combination with paleoecological core data may yield better analyses, but the analyses will depend onthe local resources and expertise available for specific geographic regions. Regardless, researchersthroughout the nation can benefit, at a minimum, from landscape level data by using an urban-to-ruralgradient approach to investigate urbanization impacts on ecological processes.

Acknowledgments

This research was supported in part by NASA Mission To Planet Earth Grants NAGW 5040 and NAGW5070. We wish to thank Professor Grace Brush of Johns Hopkins University and Mike Ratcliffe of theBureau of the Census for sharing their research and providing review comments. We would also like tothank Shawn Dalton for her review comments.

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