Urbanization impacts on the structure and function of forested wetlands

Urban Ecosystems, 7: 89–106, 2004c© 2004 Kluwer Academic Publishers. Manufactured in The Netherlands.

Urbanization impacts on the structure and functionof forested wetlands

STEPHEN FAULKNER [email protected]. Geological Survey, National Wetlands Research Center, 700 Cajundome Blvd, Lafayette, LA 70506, USA

Abstract. The exponential increase in population has fueled a significant demographic shift: 60% of the Earth’spopulation will live in urban areas by 2030. While this population growth is significant in its magnitude, theecological footprint of natural resource consumption and use required to sustain urban populations is even greater.The land use and cover changes accompanying urbanization (increasing human habitation coupled with resourceconsumption and extensive landscape modification) impacts natural ecosystems at multiple spatial scales. Becausethey generally occupy lower landscape positions and are linked to other ecosystems through hydrologic connec-tions, the cascading effects of habitat alteration on watershed hydrology and nutrient cycling are particularlydetrimental to wetland ecosystems. I reviewed literature relevant to these effects of urbanization on the structureand function of forested wetlands. Hydrologic changes caused by habitat fragmentation generally reduce speciesrichness and abundance of plants, macroinvertebrates, amphibians, and birds with greater numbers of invasivesand exotics. Reduction in soil saturation and lowered water tables result in greater nitrogen mineralization andnitrification in urban wetlands with higher probability of NO−

3 export from the watershed. Depressional forestedwetlands in urban areas can function as important sinks for sediments, nutrients, and metals. As urban ecosystemsbecome the predominant human condition, there is a critical need for data specific to urban forested wetlands inorder to better understand the role of these ecosystems on the landscape.

Keywords: urban, forested wetlands, functions, water quality

Introduction

Prior to 1900, urban environments contained just a small percentage of the total globalpopulation. Urban populations worldwide have increased exponentially since that time. In1950, only 30% of the world’s population was found in urban areas; however, the projectedfive billion urban dwellers will represent 60% of the world’s population by 2030 (UnitedNations, 2002). At present growth rates, more people will live in urban than rural areas by2008.

Human effects on the environment are the product of population, resource consumption,and technology (Ehrlich and Holdren, 1974). An increasing population creates demandsfor energy, food, transportation, water treatment, and consumer goods all of which requireincreasing amounts of resources. The term “urbanization” describes an increase in humanhabitation linked with increased per capita energy and resource consumption, and extensivelandscape modification (McDonnell and Pickett, 1990). As the population becomes moreprosperous, demand for goods and services is often driven by more than just pure populationgrowth. Between 1975 and 1991, the number of U.S. survey respondents who thoughtthat a second television and an air-conditioned home were a necessity more than doubled(Harrison and Pearce, 2001). It is no surprise, then, that between 1980 and 1996 the number

90 FAULKNER

of television sets worldwide grew from 0.56 to 1.34 billion. Only 30 percent of this increasewas due to population growth while increased ownership levels accounted for the remaining70%. This is but one example of the widespread impact of population growth and theresources used to support the lifestyle of the population.

The resources consumed and wastes generated by a population can be quantified and the“ecological footprint” or area of land needed to support them can be calculated (Wackernageland Rees, 1996). Folke et al. (1997) concluded that the 29 largest cities of Baltic Europerequired an area of forest, agricultural, marine, and wetland ecosystems that was at least 565–1130 times larger than the area of the cities themselves in order to support their resourceconsumption and waste assimilation. These impacts are not only felt in the immediatevicinity of an urbanized area, but are often transported to distant locations via air and waterpollution. Nitrogen (N) export from the land to the coastal oceans is strongly influenced bypopulation density and anthropogenic N inputs (figure 1) and coastal eutrophication is linkedto a number of detrimental environmental impacts including harmful algal blooms, loss ofcoral reef and seagrass habitats, and hypoxia (National Research Council, 2000; Rabalais,2002; Rabalais et al., 2002). Transferring ecological impacts of population growth to distantareas dates back as far as the Roman empire, which depleted both the soils of North Africa(growing wheat) and the forests of the Levant (building ships to transport the wheat) in orderto feed the million or so inhabitants of Rome (Harrison and Pearce, 2001). The ecologicalfootprints of developed nations are much larger than those of undeveloped ones and theglobal average is 2.1 ha/person. Four planet Earths would be required to sustain all of theEarth’s population if they consumed resources at the current U.S. level (Wilson, 2002).

The effects of urbanization are not distributed equally across the landscape. Currently,75% of the Earth’s population lives on only 20% of the land (Harrison and Pearce, 2001) andit is increasingly concentrated in coastal regions. In 1950, New York City was the only city inthe world with a population of more than 10 million, which is the threshold for designationas a “megacity”. Currently, there are 14 megacities located in coastal zones worldwide, and40% of the world’s major cities with populations between 1 and 10 million are also locatednear coastlines (Tibbets, 2002). Although coastal areas represent only approximately 17%of the land area in the United States, they support 53% of the population (Culliton, 1998).The U.S. population is growing nearly three times faster in coastal areas than in interiorregions and, at the current growth rate, the coastal population density will reach 127 personskm−2 by 2015 (figure 2).

The consumption of land resources needed to support this growth is increasing at a fasterrate than the population itself. While the U.S. population grew 19% from 1982 to 2000,developed land area increased by 42%, and this gap is projected to widen (figure 3). Growthin urban land area will be greatest in the southeastern United States, with a projected increaseof 24.3 million ha from 1992 to 2040, resulting in a net loss of 4.8 million ha of forestland(Wear, 2002). This growth will differentially impact forested wetlands in the southeasternUnited States because of the high density of wetlands in this region. While the southeasternstates (LA, MS, AL, GA, FL, SC, and NC) represent only 25% of the land area in theconterminous U.S., they contain 47% of all wetlands in the conterminous U.S., of which 74percent are forested wetlands (Hefner et al., 1994). Approximately 1.2 million ha of forestedwetlands were lost from 1975 to 1997 (Hefner et al., 1994; U.S. Department of Agriculture,

URBANIZATION IMPACTS ON FORESTED WETLANDS 91

Figure 1. Nitrogen exports from land to coastal waters as a function of population density (A) and net anthro-pogenic inputs (B). (Adapted from Howarth et al., 1996, 2002)

2000). While historical wetland conversions were dominated by agriculture, developmentnow accounts for the majority of forested wetland loss in the southeastern United States(U.S. Department of Agriculture, 2000). Brady and Flather (1994) also concluded thatwetland loss due to development was three times greater in coastal states than inland states.

In this paper, I review the impacts of urbanization on the structure and function offorested wetlands. There are few direct studies on forested wetlands in urban settings.Of the approximately 2,300 citations in a recent bibliography of forested wetlands in thesouthern United States (Conner et al., 2001), only two have “urban” in their title. Therefore,most of the literature cited here is used to illustrate the impacts of urbanization on forestand wetland ecosystems that are applicable to urban forested wetlands. The first step in

92 FAULKNER

Figure 2. U.S. population density growth in coastal and noncoastal areas from 1960–2015. (Adapted fromCulliton, 1998)

Figure 3. Comparison of U.S. population growth and land development from 1982–2025. [projected from USDANational Resources Inventory (U.S. Department of Agriculture, 2003) and U.S. Census Bureau, 2000]


minimizing our ecological footprint is to document the impact of landscape alteration towetlands and develop policies that will minimize loss of wetlands and the ecosystem servicesthey provide. As urban ecosystems become the predominant human condition, minimizingwetland loss is critical to retaining ecological functions in urban environments.

Ecosystem services

Forested wetlands provide a variety of ecosystem services including biodiversity, recreation,flood storage, and wildlife habitat (Messina and Conner, 1998) and their biogeochemicalfunctions transform and retain pollutants (Faulkner, 2004). The pollutant retention and trans-formation processes are effective mechanisms for water quality improvement, an importantecosystem service in urban watersheds. The anaerobic conditions in the wetland drive themicrobial conversion of nitrate (NO−

3 ) to N2 or N2O, effectively removing NO−3 from the

system (Jordan et al., 1998; Hunter and Faulkner, 2001). Phosphorus and metals are gener-ally attached to suspended particles and retained through wetland sedimentation processes(Faulkner and Richardson, 1989; Lockaby and Walbridge, 1998). At the watershed scale,these biogeochemical functions link wetlands to surrounding ecosystems through hydro-logic pathways that extend beyond the wetland perimeter.

The ecosystem services provided by forested wetlands in urban and urbanizing watershedsare not well quantified. The ecology of urban environments has only recently been recog-nized as a gap in our understanding of landscapes (McDonnel and Pickett, 1990; McDonnelet al., 1997; Zipperer et al., 2000). Ehrenfeld (2000) noted that the unique attributes ofurban wetlands required different metrics for functional assessment and restoration thanthose in rural areas. Because of their fragmented nature and the high value of land in urbanareas, urban wetlands are often more susceptible to conversion and loss than their ruralcounterparts. Despite heightened awareness and policies such as no net loss, the currentregulatory framework encourages mitigation of unavoidable impacts to wetlands throughmitigation banks that are often in watersheds far removed from where the impact(s) oc-curred (National Research Council, 2001). This misplaced mitigation results in a net lossof wetland functions in urban settings.

Habitat fragmentation

Land use and cover change are the most pernicious impacts of humans on natural ecosys-tems, and they have cascading effects on atmospheric, hydrologic, and biogeochemicalcycles at local, regional, and global scales (National Research Council, 1993; Vitousek,1994). Habitat fragmentation is the alteration of previously continuous habitat into spatiallyseparated and smaller patches (Dale et al., 2000). Over half of the temperate broadleaf andmixed forest biome has been fragmented or removed by human activity worldwide (Wadeet al., 2003). Habitat conversion that is due to urbanization has a greater impact on naturalenvironments than forestry and agriculture (Marzluff and Ewing, 2001). This differentialimpact is a function of the dissimilarity between urban environments and natural areas andthe low probability that urbanized land will revert back to a natural state (high persistence

94 FAULKNER

Figure 4. The relative impact of land use changes reflecting the greatest effects result from the combination ofhigh persistence of change and low similarity to natural habitat in urban areas. (Adapted from Marzluff and Ewing,2001)

to change) once the conversion has taken place (figure 4). This combination of dissimilarityand persistence is particularly detrimental to wetlands because the core structural and func-tional attributes are inextricably linked to the hydrologic regime of the wetland. When thathydrologic regime is significantly altered by urban land use, the alteration is almost alwayspermanent, moving further away from the undisturbed condition. Since the main purpose ofurban drainage networks is to move water away from inhabited areas as quickly as possibleand prevent flooding, the hydrology is rarely restored even if the habitat is rehabilitated.

Fragmentation of natural habitats has a detrimental impact on both native flora andfauna, causing changes in species’ abundance and distribution, community composition,and ecosystem function (Saunders et al., 1991; Yarie et al., 1998). These impacts are theresult of changes in patch size and shape caused by habitat loss, as well as increased edgehabitat and edge effects (Faaborg et al., 1995). The increased area of forest edge relative toforest interior creates greater opportunity for invasive and exotic plants to infiltrate an area.Ehrenfeld and Schneider (1991) found increased loss of indigenous plant species and moreupland and exotic species in Atlantic white cedar (Chamaecyparis thyoides [L.] B.S.P)swamps subjected to disturbance from development. They reported significantly greaternumbers of invasive plant species in wetlands located in housing developments and thosesubjected to direct stormwater runoff (figure 5).

Fragmentation and structural changes to forested wetlands also impact the fauna that de-pend on these ecosystems for habitat and food, particularly those with specific habitat needs(Schiller and Horn, 1997). Species richness and abundance of neotropical migratory birds


Figure 5. Effect of increasing development on an index of species change (A) and total number of invader species(B) in Atlantic white cedar (Chamaecyparis thyoides) swamps. Control—undeveloped, Near—proximate to un-paved roads, Developed—within housing developments, Run-off—within housing developments and subjected todirect stormwater runoff. Significant differences (P < 0.05) among mean values for different wetland types areindicated by different letters. (Adapted from Eherenfeld and Schneider, 1991)

increases with increasing forested wetland area and additional adjacent wetlands (Freemarket al., 1995). Croonquist and Brooks (1993) reported a 50% decrease in the number ofneotropical migratory birds in riparian wetland habitats located in disturbed watershedscompared with those in undisturbed landscapes. Observed changes in community structureof lake shore bird assemblages in the northeastern United States are consistent with declinesin forest interior species relative to edge species in response to forest fragmentation fromlake shore residential and urban development (Allen and O’Connor, 2000).

Similar fragmentation effects from urbanization have been observed on wetland amphib-ian populations. Amphibians are dependent on wetlands for shelter and breeding habitatand are readily affected by wetland alteration. Knutson et al. (1999) found that lower frogand toad abundances were significantly correlated with the presence of urban land and

96 FAULKNER

higher abundances with the amount of wetland forests. They concluded that roads andpollutants were important contributors to frog and toad mortality in urban environments.Lehtinen et al. (1999) also reported lower amphibian species richness with greater wetlandisolation and road density in both urban and agricultural ecoregions, the two factors mostimportant in predicting species richness. Wetland isolation from habitat fragmentation isespecially detrimental to amphibians as local subpopulations periodically go extinct andmust be reestablished from neighboring habitats (Lehtinen et al., 1999).

The coastal wetland forests along the U.S. Gulf of Mexico are critical to the survivalof neotropical migratory birds returning to the United States from wintering grounds inCentral and South America (Moore, 2000). Disturbance has altered the understory andsubcanopy composition of forested wetlands in the Chenier Plain in Louisiana, and thisalteration affects the number of birds found in the disturbed stands (figure 6(A)). Barrow etal. (2000) reported significantly fewer recaptures of migratory birds in the near-ground andsubcanopy habitats of disturbed coastal forest stands compared with undisturbed controls(figure 6(B)). Bird species that have specialized food foraging requirements can be affectedby alterations to vegetation in their habitat, while changes to understory structure andcomposition negatively affect frugivores and nectarivores (Knopf et al., 1988). Migratorybird species are also negatively impacted when habitat alterations change food resourcephenology and abundance (Barrow et al., 2000). Changes in forest species composition inboth the canopy and understory/shrub layer are a common result of urbanization (Ehrenfeldand Schneider, 1991; Rudis, 1995; Guntenspergen and Levinson, 1997).

The direct and indirect effects of habitat alteration on the structure and function of urbanforested wetlands are substantial. The results of Barrow et al. (2000) also suggest that inaddition to the complete loss of habitat from conversion to urban environments, significanteffects on faunal use of forested wetlands can occur with more subtle disturbances to foreststructure.

Hydrologic and biogeochemical changes

Since unavoidable impacts to forested wetlands require mitigation under state and federalstatutes, and draining wetlands to make them suitable for urban development is both difficultand expensive, the drier forest habitats are more often fragmented by land use change(Rudis, 1995). This disturbance to upland habitats in the watershed has a direct effect onboth the hydrologic regime and water quality of aquatic and wetland systems (Allan andFlecker, 1993; Hunsaker and Levine, 1995; Carpenter et al., 1998; Azous and Horner, 2000).Urbanization converts natural habitats to land uses with impervious surfaces that block theinfiltration of precipitation and contribute to changes in hydrology that degrade downstreamecosystems. Impervious surfaces also serve as a transport system that channels pollutantsdirectly into drainage networks and aquatic resources as storm runoff (Arnold and Gibbons,1996). This results in higher peak flows, reduced time to peak flow, increased runoff volume,and diminished baseflow all of which compromise stream habitat quality (Schueler, 1994;Finkenbine et al., 2000). As urban land use stabilizes in the watershed, the large amountof impervious surface and low sediment production results in incised stream channels andlowered water tables in the riparian zone (Groffman et al., 2003).


Figure 6. Neotropical migratory bird use of coastal forests of the Chenier Plain, LA showing less frequent use (A)and fewer numbers of recaptures (B) in disturbed versus undisturbed forested wetland stands. Asterisks indicatesignificant differences between control and disturbed (∗ = P < 0.05,∗∗ = P < 0.01). (Modified from Barrowet al., 2000)

The impact of urbanization on runoff and discharge has been well documented (Dunneand Leopold, 1978; Arnold and Gibbons, 1996; Burges et al., 1998; Booth et al., 2002)and the greater surface flow from increased impervious surfaces carries pollutants intoadjacent wetland and aquatic ecosystems, degrading water quality and habitat. Althoughurban land use comprises only about 5% of the Lake Champlain watershed, it contributes37% of the total phosphorus (P) load (Hegman and Borer, 1999). Both N and P concentra-tions are elevated in urban runoff (Zampella, 1994; U.S. Geological Survey, 1999). Smithet al. (1992) concluded that urban streams have the second highest levels of nitrates andphosphorus, exceeded only by waters adjacent to row-crop agriculture. Point discharges

98 FAULKNER

and nonpoint source runoff from urban areas are the top two sources of water pollution forestuaries while urban runoff ranks third for lakes and fourth for rivers (U.S. EnvironmentalProtection Agency, 2000). Paul and Meyer (2001) have recently reviewed the effects ofurban development on stream geomorphology, biology, and chemistry.

The increased surface runoff and pollutant loadings resulting from urbanization ultimatelyflows into wetlands and streams in the watershed. Huryn et al. (2002) reported significantlyhigher NO−

3 concentrations in streams in urban watersheds than those dominated by forestsand wetlands. Urban forested wetlands in King County, WA had higher NO−

3 and total Pconcentrations in surface water than control sites in nonurbanized watersheds (Horner et al.,2000b). Increased urban land use and associated vehicle miles correspond with increasedconcentrations of polycyclic aromatic hydrocarbons (PAHs) in sediments of urban lakes andreservoirs (Van Metre et al., 2000). Paul et al. (2002) found that the percentage of urbanland in mid-Atlantic and southern New England watersheds was one of the most importantlandscape metrics explaining the variation in metal and PAH concentrations in estuarinesediments.

Land use patterns within a watershed also have significant impacts on ecosystem pro-cesses. Tree leaves are the primary energy base for streams in forested watersheds in easternNorth America (Fisher and Likens, 1973), and a specific functional group of macroinver-tebrates known as “shredders” is responsible for processing leaves into smaller organicparticles (Cummins and Merritt, 1996). Huryn et al. (2002) found significant differencesin macroinvertebrate attributes among watersheds dominated by forests, wetlands, agricul-ture, and urban land uses. The EPT (Ephemeroptera + Plecoptera + Trichoptera) indexis a bioassessment metric that reflects changes in litter processing rate in streams, and itwas significantly higher in watersheds dominated by wetlands than by urban land uses(figure 7). Macroinvertebrate assemblage structure was also radically different among landuse categories, with only one taxon found in urban land use environments compared to fivefor the wetlands (figure 8). These changes corresponded to differences in stream nitrate

Figure 7. Mean EPT (Ephemeroptera + Plecoptera + Trichoptera) index among land-use categories. Error barsare +1 SE. Significant differences (P < 0.05) among mean values for different land-use categories are indicatedby different letters. (Adapted from Huryn et al., 2002)


Figure 8. Mean biomass of leaf-litter shredding macroinvertebrate taxa in wetland and urban streams. Error barsare +1 SE. (Adapted from Huryn et al., 2002)

concentrations (Huryn et al., 2002). Similar decreases in EPT species have been reportedwith increasing forest fragmentation (Stewart et al., 2001) and increased watershed im-perviousness (Wang and Kanehl, 2003). Runoff from urban land uses, like golf courses,also creates significantly different macroinvertebrate community structure compared withforested reference streams (Winter et al., 2002).

Nutrient cycling is also affected by urbanization primarily through changes to hydrologyand nutrient loadings. Urban watersheds have highly altered hydrologic flow paths andincised stream channels which can lower groundwater levels and soil moisture (Groffmanet al., 2002; Ehrenfeld et al., 2003). Atlantic white cedar swamps in developed watershedshad significantly higher N mineralization and nitrification rates than swamps in undisturbedwatersheds, which was attributed to changes in soil pH, ash content, and microbial pop-ulations in the developed sites (Zhu and Ehrenfeld, 1999). Pouyat and Turechek (2001)(nonwetland sites) and Groffman et al. (2002) also found significantly higher soil NO−

3 lev-els and net nitrification in urban sites compared to forested reference sites along urban-ruralgradients in New York and Maryland. This increase in NO−

3 availability can result in greaterNO−

3 exports from urban watersheds; however, there were no differences in denitrificationpotential or microbial biomass between the rural and urban sites in Maryland (Groffman

100 FAULKNER

Figure 9. Aerial photographs of the Bluebonnet Swamp watershed in Baton Rouge, LA showing the 54%increase in urban land area from 1941 (A) to 2001 (B).


and Crawford, 2003). The absence of any significant differences was primarily due to awide range of denitrification potentials among the urban sites with the highest rate comingfrom an urban herbaceous wetland that developed near the outlet of a broken storm drain,resulting in a constantly saturated soil. This high denitrification rate indicates that forestedwetlands in urban areas do not necessarily have inherently low denitrification potentials, butthat the hydrologic controls over denitrification are the dominant factor. Therefore, whenappropriately protected and managed, urban forested wetlands can function as important Nsinks protecting downstream aquatic resources.

While NO−3 circulates as a dissolved constituent in runoff and groundwater, P is exported

from disturbed systems primarily in the particulate form bound to sediment particles (Baker,1992; National Research Council, 2000). Metals in urban runoff are also primarily in theparticulate form (Breault and Granato, 2000; Backstrom et al., 2003) and their accumulationin wetlands increases with increasing urban land use in the watershed (Paul et al., 2002).Erosion rates for urbanizing watersheds can reach 50,000 Mg kg−1 yr−1 compared to 4,000Mg kg−1 yr−1 for agriculture and <100 Mg kg−1 yr−1 for undisturbed areas (Novotnyand Olem, 1994). Forested wetlands in agricultural watersheds are effective at retainingsediments and phosphorus (Cooper et al., 1987; Gilliam, 1994; Craft and Casey, 2000).While metal concentrations are generally low in runoff from rural areas, metals are stilltransported with sediment particles and retained in forested wetlands (Hupp et al., 1993).

Urban growth in Baton Rouge, LA, has increased developed land by over 50% from1941 to 2001 in the 396 ha watershed surrounding Bluebonnet Swamp (figure 9), a 16 hadepressional forested wetland dominated by baldcypress (Taxodium distichum [L.] L.C.Rich.) and water tupelo (Nyssa aquatica L.) (Sanders, 2002). Since urban forested wetlandsare subjected to increased runoff and higher contaminant loadings than their more ruralcounterparts, the sediment retention function of these wetlands is the primary mechanismfor retaining sediments, P, and metals. A comparison of sedimentation rates in Bluebon-net Swamp indicates a significant increase from the historical (137Ce dating) rate of 0.5cm yr−1 to the recent (feldspar marker horizon) rate of over 2 cm yr−1 (figure 10). This

Figure 10. Comparison of historic (Cesium-137 dating) and current (feldspar marker horizon) sedimentationrates (mean + SE) in an urban forested wetland (Bluebonnet Swamp, Baton Rouge, LA). Asterisks indicatesignificant differences between the two methods (∗∗ = P < 0.01). (Adapted from Sanders, 2002)

102 FAULKNER

Table 1. Comparison of historic and current soil metal con-centrations (mean ± se) in an urban forested wetland (Blue-bonnet Swamp, Baton Rouge, LA,). Means followed by thesame letter are not significantly different in pre- vs. post-1963 sediments, P < 0.05. (From Sanders, 2002)

Core data

Analyte Pre-1963 sediment Post-1963 sediment

Pb 32 ± 45 (a) 45 ± 20 (a)

Cd 0.32 ± 0.51 (a) 2.00 ± 1.40 (b)

Cr 11.0 ± 3.8 (a) 10.7 ± 5.0 (a)

Cu 5.32 ± 5.6 (a) 22.7 ± 6.8 (b)

Zn 56.2 ± 7.5 (a) 95 ± 35 (b)

Ni 11.1 ± 2.2 (a) 17.0 ± 4.1 (b)

P 389 ± 106 (a) 555 ± 184 (b)

increase in sedimentation rate corresponds with a significant increase in concentrationsof cadmium (Cd), copper (Cu), zinc (Zn), nickel (Ni), and P in sediments deposited after1963 (Table 1). Although there are few comparable studies in urban forested wetlands,several of those do compare soil and sediment metal concentrations in urban environments.Forested wetlands in highly urbanized watersheds in King County, WA had higher lev-els of Pb, Zn, and As compared with those in rural watersheds (Horner et al., 2000a).Similar results were reported by Parker et al. (1978) for soil Cd Cu, Pb, and Zn levels inurban and rural forested wetlands in Indiana. Detention pond and wetland sediments incentral Florida accumulated Pb, Zn, and Cr from highway runoff (Schiffer, 1989). Murrayet al. (2004) found significantly higher concentrations of Cd, Cr, Cu, Pb, Ni, and Zn in thesoils of watersheds dominated by industrial land uses compared with residential uses insoutheastern Michigan. Increased area of human land use was significantly correlated withincreased sediment Cu, Ni, and Zn concentrations in Swedish lake sediments (Lindstrom,2001).

Conclusion

It is clear from this review that urbanization has both direct and indirect effects on forestedwetland structure and function. Changes in hydrologic regime and increased contaminantand nutrient loadings can change plant species composition and nutrient cycling. These im-pacts can also alter the species richness and abundance of avian, amphibian, and macroin-vertebrate populations. Urban forested wetlands are sinks for nutrients, sediments, andmetals, thereby protecting downstream ecosystems. The important ecological services ofhabitat, flood storage, and water quality maintenance provided by these areas are perma-nently lost from the watershed when they are replaced by mitigation wetlands outside thewatershed. There is a critical need for data specific to urban forested wetlands in order tobetter understand the role of these ecosystems on the landscape.


References

Allan, J.D. and Flecker, A.S. (1993) Biodiversity conservation in running waters. Bioscience 43, 32–43.Allen, A.P. and O’Connor, R.J. (2000) Hierarchical correlates of bird assemblage structure on northeastern U.S.A.

lakes. Environ. Monit. Assess. 62, 15–37.Arnold Jr., L.C. and Gibbons, C.J. (1996) Impervious surface coverage: The emergence of a key environmental

indicator. J. Am. Planning Assoc. 6, 243–259.Azous, A.L. and Horner, R.R. eds. (2000) Wetlands and Urbanization. CRC Press, Boca Raton, FL.Backstrom, M., Nilsson, U., Hakansson, K., Allard, B. and Karlsson, S. (2003) Speciation of heavy metals in road

runoff and roadside total deposition. Water Air Soil Poll. 147, 343–366.Baker, L.A. 1992. Introduction to nonpoint source pollution in the United States and prospects for wetland use.

Ecol. Eng. 1, 1–26.Barrow, W.C., Chien, C.C., Hamilton, R.B., Ouchley, K. and Spengler, T.J. (2000) Disruption and restoration of en

route habitat, a case study: The Chenier Plain. In Stopover Ecology of Nearctic-Neotropical Landbird Migrants:Habitat Relations and Conservation Implications (F.R. Moore, ed.), pp. 71–87. Studies in Avian Biology No.20. Cooper Ornithological Society, Camarillo, CA.

Booth, D.B., Hartley, D. and Jackson, R. (2002) Forest cover, impervious-surface area, and the mitigation ofstormwater impacts. J. Am. Wat. Resour. Assoc. 38, 835–845.

Brady, S.J. and Flather, C.H. (1994) Changes in wetlands on nonfederal rural land of the conterminous UnitedStates from 1982 to 1987. Environ. Man. 18, 693–705.

Breault, R.F. and Granato, G.E. (2000) A synopsis of technical issues of concern for monitoring trace elements inhighway and urban runoff. USGS Open File Report 00–422. U.S. Geological Survey, Denver, CO.

Burges, S.J., Wigmosta, M.S. and Meena, J.M. (1998) Hydrological effects of land-use change in a zero-ordercatchment. ASCE J. Hydrolog. Engin. 3, 86–97.

Carpenter, S.R., Caraco, N.F., Correll, D.L., Howarth, R.W., Sharpley, A.N. and Smith, V.H. (1998) Nonpointpollution of surface waters with phosphorus and nitrogen. Ecol. Appl. 8, 559–568.

Conner, W.H., Hill, N.L., Whitehead, E.M., Busbee, W.S., Ratard, M.A., Olzap, M., Smith, D.L. and Marshall,J.P. (2001) Forested wetlands of the southern United States: A bibliography. Gen. Tech. Rep. SRS–43. U.S.Department of Agriculture, Forest Service, Southern Research Station, Asheville, NC.

Cooper, J.R., Gilliam, J.W., Daniels, R.B. and Robarge, W.P. (1987) Riparian areas as filters for agriculturalsediment. Soil Sci. Soc. Am. J. 51, 416–420.

Craft, C. and Casey, W.P. (2000) Sediment and nutrient accumulation in floodplain and depressional freshwaterwetlands of Georgia, USA. Wetlands 20, 323–332.

Croonquist, M.J. and Brooks, R.P. (1993) Effects of habitat disturbance on bird communities in riparian corridors.J. Soil Water Conserv. 48, 65–70.

Culliton, T.J. (1998) Population: Distribution, Density and Growth. National Oceanic and Atmo-spheric Administration (NOAA) State of the Coast Report. Silver Spring, MD: [online] URL:.http://oceanservice.noaa.gov/websites/retiredsites/supp sotc retired.html

Cummins, K.W. and Merritt, R.W. (1996) Ecology and distribution of aquatic insects. In An Introduction to theAquatic Insects of North America (R.W. Merritt and K.W. Cummins, eds.), pp. 74–86. Kendall/Hunt, Inc.Dubuque, IA.

Dale, V.H., Brown, S., Haueber, R.A., Hobbs, N.T., Huntly, N., Naiman, R.J., Riebsame, W.E., Turner, M.G. andValone, T.J. (2000) Ecological principles and guidelines for managing the use of land. Ecol. Appl. 10, 639–670.

Dunne T. and Leopold, L.B. (1978) Water in Environmental Planning. Freeman, New York.Ehrenfeld, J.G. (2000) Evaluating wetlands in an urban context. Urban Ecosys. 4, 69–85.Ehrenfeld, J.G. and Schneider, J.P. (1991) Chamaecyparis thyoides wetlands and suburbanization: Effects on

hydrology, water quality and plant community composition. J. Applied Ecol. 28, 467–490.Ehrenfeld, J.G., Cutway, H.B., Hamilton IV, R. and Stander, E. (2003) Hydrologic description of forested wetlands

in northeastern New Jersey, USA- an urban/suburban region. Wetlands 23, 685–700.Ehrlich, P.R. and Holdren, J.P. (1974) Impact of population growth. Science. 171, 1212–1217.Faaborg, J., Brittingham, M., Donovan, T. and Blake, J. (1995) Habitat fragmentation in the temperate zone. In

Ecology and Management of Neotropical Migratory Birds (T.E. Martin and D.M. Finch, eds.), pp. 357–380.Oxford Univ. Press, New York.

104 FAULKNER

Faulkner, S.P. (2004) Soils and Sediment: Understanding Wetland Biogeochemistry. In Wetlands (S. Spray and K.McGlothlin, eds.), pp. 25–46. Rowman Littlefield, Inc., Lanham, MD, 216 p.

Faulkner, S.P. and Richardson, C.J. (1989) Physical and chemical characteristics of freshwater wetland soils.In Constructed Wetlands for Wastewater Treatment (D.A. Hammer, ed.), pp. 41–72. Lewis Publishers, Inc.,Chelsea, MI.

Finkenbine, J.K., Atwater, J.W. and Mavinic, D.S. (2000) Stream health after urbanization. J. Am. Water Resour.Assoc. 36, 1149–1160.

Fisher, S.G. and Likens, G.E. (1973) Energy flow in Bear Brook, New Hampshire: An intergrative approach tostream ecosystem metabolism. Ecol. Mono. 43, 421–439.

Folke, C, Jansson, A., Larsson, J. and Costanza, R. (1997) Ecosystem appropriation by cities. Ambio 26, 167–172.

Freemark, K.E., Dunning, J.B., Hejl, S.J. and Probst, J.R. (1995) A landscape ecology perspective for research,conservation, and management. In Ecology and Management of Neotropical Migratory Birds (T.E. Martin andD.M. Finch, eds.), pp. 381–421. Oxford Univ. Press, New York.

Gilliam, J.W. (1994) Riparian wetlands and water quality. J. Environ. Qual. 23, 896–900.Groffman, P.M., Boulware, N.J., Zipperer, W.C., Pouyat, R.V., Band, L.E. and Colosimo, M.F. (2002) Soil nitrogen

cycle processes in urban riparian zones. Environ. Sci. Technol. 36, 4547–4552.Groffman, P.M. and Crawford, M.K. (2003) Denitrification potential in urban riparian zones. J. Environ. Qual. 32,

1144–1149.Groffman, P.M., Bain, D.J., Band, L.E., Belt, K.T., Brush, G.S., Grove, J.M., Pouyat, R.V., Yesilonis, I.C. and

Zipperer, W.C. (2003) Down by the riverside: Urban riparian ecology. Frontiers in Ecology and the Environment1, 315–321.

Guntenspergen, G.R. and Levenson, J.B. (1997) Understory plant species composition in remnant stands along anurban-to-rural land-use gradient. Urban Ecosys. 1, 155–169.

Harrison, P. and Pearce, F. (2001) AAAS Atlas of Population and Environment. American Association for theAdvancement of Science. University of California Press, Berkeley, CA.

Hefner, J.M., Wilen, B.O., Dahl, T.E. and Frayer, W.E. (1994) Southeast wetlands status and trends, mid-1970’sto mid-1980’s. U.S. Fish and Wildlife Service, Atlanta, GA

Hegman, W. and Borer, C. (1999) Estimation of Lake Champlain basinwide nonpoint source phosphorus export.Tech. Rep. 31. UVM Water Resources & Lake Study Center, Burlington, VT.

Horner, R.R., Cooke, S.S., Reinelt, L.E., Ludwa, K.A. and Chin, N.T. (2000a) Water quality and soils. In Wetlandsand Urbanization (A.L. Azous and R.R. Horner, eds.), pp. 47–67. CRC Press, Boca Raton, FL.

Horner, R.R., Cooke, S.S., Reinelt, L.E., Ludwa, K.A., Chin, N.T. and Valentine, M. (2000b) In Wetlands andUrbanization (A.L. Azous and R.R. Horner, eds.), pp. 213–244. CRC Press, Boca Raton, FL.

Howarth, R.W., Billen, G., Swaney, D., Townsend, A., Jaworski, N., Lajtha, K., Downing, J.A., Elmgren, R.,Caraco, N., Jordan, T., Berendse, F., Freney, J., Kudeyarov, V., Murdoch, P. and Zhao-Liang, Z. (1996) Regionalnitrogen budgets and riverine N and P fluxes for the drainages to the North Atlantic Ocean: Natural and humaninfluences. Biogeochemistry 35, 75–139.

Howarth, R.W., Sharpley, A. and Walker, D. (2002) Sources of nutrient pollution to coastal waters in the UnitedStates: Implications for achieving coastal water quality goals. Estuaries 25, 656–676.

Hunsaker, C.T. and Levine, D.A. (1995) Hierarchal approaches to the study of water quality in rivers. Bioscience45, 193–203.

Hunter, R.G. and Faulkner, S.P. (2001) Denitrification potentials in restored and natural bottomland hardwoodwetlands. Soil Sci. Soc. Am. J. 65, 1865–1872.

Hupp, C.R., Woodside, M.D. and Yanosky, T.M. (1993) Sediment and trace element trapping in a forested wetland,Chickahominy River, Virginia. Wetlands 13, 95–104.

Huryn, A.D., Huryn, V.B.N., Arbuckle, C.J. and Tsomides, L. (2002) Catchment land-use, macroinvertebratesand detritus processing in headwater streams: Taxonomic richness versus function. Fresh. Biol. 47, 401–415.

Jordan, T.E., Weller, D.E. and Correll, D.L. (1998) Denitrification in surface soils of a riparian forest: Effects ofwater, nitrate, and sucrose additions. Soil Biol. Biochem. 30, 833–843.

Knopf, F.L., Sedgwick, J.A. and Cannon, R.W. (1988) Guild structure of a riparian avifauna relative to seasonalcattle grazing. J. Wildl. Manage. 52, 280–290.


Knutson, M.G., Sauer, J.R., Olsen, D.A., Mossman, M.J., Hemesath, L.M. and Lannoo, M.J. (1999) Effects oflandscape composition and wetland fragmentation on frog and toad abundance and species richness in Iowaand Wisconsin, U.S.A. Conserv. Biol. 13, 1437–1446.

Lehtinen, R.M., Galatowitsch, S.M. and Tester, J.R. (1999) Consequences of habitat loss and fragmentation forwetland amphibian assemblages. Wetlands 19, 1–12.

Lindstrom, M. (2001) Urban land use influences on heavy metal fluxes and surface sediment concentrations ofsmall lakes. Water Air Soil Poll. 126, 363–383, 2001.

Lockaby, B.G. and Walbridge, M.R. (1998). Biogeochemistry. In Southern Forested Wetlands: Ecology and Man-agement (M.G. Messina and W.H. Conner eds.), pp. 149–172. CRC Press, Boca Raton, FL.

Marzluff, J.M. and Ewing, K. (2001) Restoration of fragmented landscapes for the conservation of birds: A generalframework and specific recommendations for urbanizing landscapes. Rest. Ecol. 9, 280–292.

McDonnell, M.J. and Pickett, S.T.A. (1990) Ecosystem structure and function along urban- rural gradients: Anunexploited opportunity for ecology. Ecology 71, 1232–1236.

McDonnell, M.J., Pickett, S.T.A., Groffman, P., Bohlen, P. and others. (1997) Ecosystem processes along anurban-to-rural gradient. Urban Ecosys. 1, 21–36.

Messina, M.G. and Conner, W.H., eds. (1998) Southern Forested Wetlands: Ecology and Management. CRC Press,Boca Raton, FL.

Moore, F.R., ed. (2000) Stopover Ecology of Nearctic-Neotropical Landbird Migrants: Habitat Relations andConservation Implications. Studies in Avian Biology No. 20. Cooper Ornithological Society, Camarillo,CA.

Murray, K.S., Rogers, D.T. and Kaufman, M.M. (2004) Heavy metals in an urban watershed in southeasternMichigan. J. Environ. Qual. 33, 163–172.

National Research Council (1993) The Role of Terrestrial Ecosystems in Global Change. National Academy Press,Washington, D.C.

National Research Council (2000) Clean Coastal Waters: Understanding and Reducing the Effects of NutrientPollution. National Academy Press, Washington, D.C.

National Research Council (2001) Compensating for Wetland Losses Under the Clean Water Act. NationalAcademy Press, Washington, D.C.

Novotny, V. and Olem, H. (1994) Water Quality: Prevention, Identification, and Management of Diffuse Pollution.Van Nostrand Rheinhold, New York, NY.

Parker, G.R., McFee, W.W. and Kelly. J.M. (1978) Metal distribution in forested ecosystems in urban and ruralnorthwestern Indiana. J. Env. Qual. 7, 337–342.

Paul, J.F., Comeleo, R.L. and Copeland, J. (2002) Landscape metrics and estuarine sediment contamination in themid-Atlantic and southern New England regions. J. Environ. Qual. 31, 836–845.

Paul, M.J. and Meyer, J.L. (2001) Streams in the urban landscape. Annu. Rev. Ecol. Syst. 32, 333–365Pouyat, R.V. and Turechek, W.W. (2001) Short- and long-term effects of site factors on net N-mineralization and

nitrification rates along an urban-rural gradient. Urban Ecosys. 5, 159–178.Rabalais, N.N. (2002) Nitrogen in aquatic ecosystems. Ambio 31, 102–112.Rabalais, N.N., Turner, R.E. and Wiseman, W.J. Jr. (2002) Hypoxia in the Gulf of Mexico, a.k.a. “The Dead Zone.”

Annual Review of Ecology and Systematics 33, 235–263.Rudis, V.A. (1995) Regional forest fragmentation effects on bottomland hardwood community types and resource

values. Landscape Ecol. 10, 291–307.Sanders, R.L. (2002) Sedimentation rates and metal retention in an urban Louisiana swamp. Master’s thesis.

Louisiana State University, Baton Rouge, LA. 80 p.Saunders, D.A., Hobbs, R.H. and Margules, C.R. (1991) Biological consequences of ecosystem fragmentation: A

review. Conserv. Biol. 9, 18–32.Schiffer, D.M. (1989) Effects of highway runoff on the quality of water and bed sediments of two wetlands in

central Florida USGS Water-Resources Investigations Report 88–4200 U.S. Geological Survey, Denver, CO.Schiller, A. and Horn, S.P. (1997) Wildlife conservation in urban greenways of the mid-southeastern United States.

Urban Ecosys. 1, 106–113.Schueler, T. (1994) The importance of imperviousness. Watershed Prot. Tech. 1, 100–111.Schueler, T. and Holland, H.K. 2000. The Practice of Watershed Protection. Center for Watershed Protection,

Ellicott City, Maryland.

106 FAULKNER

Smith, R., Alexander, R.B. and Lanfear, K.J. (1992) Stream water quality in the conterminous United States—Status and trends of selected indicators during the 1980’s. In National Water Summary 1990–1991: StreamWater Quality. Water-Supply Paper 2400. U.S. Geological Survey, Reston, VA.

Stewart, J.S., Wang, L., Lyons, J., Horwatich, J.A. and Bannerman, R. (2001) Influences of watershed, riparian-corridor, and reach-scale characteristics on aquatic biota in agricultural watersheds. J. Am. Wat. Resour. Assoc.37, 1475–1487.

Tibbets, J. (2002) Coastal cities: Living on the edge. Environ. Health Perspect. 110, A674–A678.United Nations (2002) World Urbanization Prospects—The 2001 Revision. United Nations Population Division,

New York, NY.U.S. Department of Agriculture. (2000) Summary Report: 1997 National Resources Inventory (revised Decem-

ber 2000), Natural Resources Conservation Service, Washington, DC, and Statistical Laboratory, Iowa StateUniversity, Ames, Iowa.

U.S. Department of Agriculture (2003) National Resources Inventory 2001 Annual NRI Urbanization andDevelopment of Rural Land. Natural Resources Conservation Service, Washington, DC [online] URL:http://www.nrcs.usda.gov/technical/land/nri01/urban.pdf

U.S. Environmental Protection Agency (2000) National Water Quality Inventory 2000 Report. Washington, DC.U.S. Geological Survey (1999) The quality of our nation’s waters—Nutrients and pesticides. USGS Circular 1225.U.S. Census Bureau (2000) United States Census 2000. [online] URL: http://www.census.gov/population/

projections/nation/summary/np-t1.txt.Van Metre, P.C., Mahler, B.J. and Furlong, E.T. (2000) Urban sprawl leaves its PAH signature. Environ. Sci.

Technol. 34, 4064–4070.Vitousek, P.M. 1994. Beyond global warming: Ecology and global change. Ecology 75, 1861–1876.Wackernagel, M. and Rees, W.E. 1996. Our Ecological Footprint: Reducing Human Impact on the Earth. New

Society Publishers, Inc. Gabriola Island, BC.Wade, T.G., Riitters, K.H., Wickham, J.D. and Jones, K.B. (2003) Distribution and causes of global forest frag-

mentation. Conserv. Ecol. 7(2), 7. [online] URL: http://www.consecol.org/vol7/iss2/art7Wang, L. and Kanehl, P. (2003) Influences of watershed urbanization and instream habitat on macroinvertebrates

in cold water streams. J. Am. Water Resour. Assoc. 39, 1181–1196.Wear, D.N. (2002) Land use. In Southern Forest Resource Assessment. (D.N. Wear and J.G. Greis, eds.), pp.

153–173. Gen. Tech. Rep. SRS-53. Asheville, NC: U.S. Department of Agriculture, Forest Service, SouthernResearch Station.

Wilson, E.O. (2002) The Future of Life. Alfred A. Knopf, Inc. New York, NY.Winter, J.G., Somers, K.M., Dillon, P.J., Paterson, C. and Reid, R.A. (2002) Impacts of golf courses on macroin-

vertebrate community structure in Precambrian Shield streams. J. Env. Qual. 31, 2015–2025.Yarie, J.K., Viereck, L., Van Cleve, K. and Adams, P. (1998) Flooding and ecosystem dynamics along the Tanana

River. BioScience 48, 690–695.Zampella, R.A. (1994) Characterization of surface water quality along a watershed disturbance gradient. Water

Resour. Bull. 30, 605–611.Zhu, W.X. and Ehrenfeld, J.G. (1999) Nitrogen mineralization and nitrification in suburban and undeveloped

Atlantic white cedar wetlands. J. Environ. Qual. 28, 523–529.Zipperer, W.C., Wu, J., Pouyat, R.V. and Pickett, S.T.A. (2000) The application of ecological principles to urban

and urbanizing landscapes. Ecol. Appl. 10, 685–688.

Documents

Urbanization impacts on the structure and function of forested wetlands