Habitat structure: a fundamental concept and framework for urban soil ecology

Transcription

1 Roger Willims University Feinstein College of Arts & Sciences Fculty Ppers Feinstein College of Arts nd Sciences Hitt structure: fundmentl concept nd frmework for urn soil ecology Loren Byrne Roger Willims University, Follow this nd dditionl works t: Prt of the Arts nd Humnities Commons Recommended Cittion Byrne, L.B "Hitt structure: fundmentl concept nd frmework for urn soil ecology." Urn Ecosystems 10: This Article is rought to you for free nd open ccess y the Feinstein College of Arts nd Sciences t DOCS@RWU. It hs een ccepted for inclusion in Feinstein College of Arts & Sciences Fculty Ppers y n uthorized dministrtor of DOCS@RWU. For more informtion, plese contct mwu@rwu.edu.

2 Urn Ecosyst (2007) 10: DOI /s Hitt structure: A fundmentl concept nd frmework for urn soil ecology Loren B. Byrne Pulished online: 26 June 2007 # Springer Science + Business Medi, LLC 2007 Astrct Hitt structure is defined s the composition nd rrngement of physicl mtter t loction. Although hitt structure is the physicl templte underlying ecologicl ptterns nd processes, the concept is reltively unpprecited nd underdeveloped in ecology. However, it provides fundmentl concept for urn ecology ecuse humn ctivities in urn ecosystems re often trgeted towrd mngement of hitt structure. In ddition, the concept emphsizes the fine-scle, on-the-ground perspective needed in the study of urn soil ecology. To illustrte this, urn soil ecology reserch is summrized from the perspective of hitt structure effects. Among the key conclusions emerging from the literture review re: (1) hitt structure provides unifying theme for multivrite reserch out urn soil ecology; (2) heterogeneous urn hitt structures influence soil ecologicl vriles in different wys; (3) more reserch is needed to understnd reltionships mong sociologicl vriles, hitt structure ptterns nd urn soil ecology. To stimulte urn soil ecology reserch, conceptul frmework is presented to show the direct nd indirect reltionships mong hitt structure nd ecologicl vriles. Becuse hitt structure serves s physicl link etween socioculturl nd ecologicl systems, it cn e used s focus for interdisciplinry nd pplied reserch (e.g., pest mngement) out the multiple, interctive effects of urniztion on the ecology of soils. L. B. Byrne Intercollege Grdute Degree Progrm in Ecology, The Pennsylvni Stte University, 116 ASI Building,University Prk, PA 16802, USA L. B. Byrne Deprtment of Crop nd Soil Sciences, The Pennsylvni Stte University, 116 ASI Building, University Prk, PA 16802, USA L. B. Byrne Deprtment of Entomology, The Pennsylvni Stte University, 116 ASI Building, University Prk, PA 16802, USA Present ddress: L. B. Byrne (*) Deprtment of Biology, Roger Willims University, Bristol, RI 02809, USA e-mil: lyrne@rwu.edu

3 256 Urn Ecosyst (2007) 10: Keywords Hitt structure. Urn ecology. Urn soils. Lwn. Mulches. Microclimte Introduction Throughout history, descriing nd clssifying hitts (or iomes) hve een centrl endevors for ecologists. Whtever the clssifiction method (e.g., ptches or grdients) nd scle (e.g., locl to continentl), recognizing rod ptterns of ecosystem nd lndscpe structure is dependent upon differentiting vrious rrngements nd compositions of iotic nd iotic physicl mtter. However, s McCoy et l. (1990) suggested fter reviewing the literture, ecologists often tke the underlying physicl structure of nture for grnted. Few reserch frmeworks or gends hve een developed tht explicitly include it s centrl vrile of interest (McCoy nd Bell 1990, ut see Tews et l. 2004). Nonetheless, the orgniztion of physicl mteril cross spce nd time is importnt for ecologists to consider ecuse it serves s the stge of the ecologicl theter. The concept of hitt structure hs een dopted to encompss the study of the effects of the rrngement of ojects in spce on ecologicl vriles (Bell et l. 1990). Modified from Bell et l. (1990), hitt structure is defined s the mount, composition nd threedimensionl rrngement of physicl mtter (oth iotic nd iotic) t loction (Tle 1). Previous reserch hs shown tht hitt structure is n importnt direct nd/or indirect driver of mny ecologicl ptterns nd processes. It cn regulte community structure y providing resources (shelter, nutrients, nesting sites) nd mediting interctions (predtion, competition) for diverse rry of orgnisms in mny ecosystem types (see reviews in Bell et l nd Tews et l. 2004). Ecosystem processes re influenced y hitt structure through its modifiction of environmentl conditions nd resource vilility. Differences in hitt structures cross spce crete lndscpe ptterns, which in turn ffect communities nd ecosystem processes (Lovett et l. 2005). Becuse of its impct on vriles cross levels of ecologicl orgniztion, hitt structure provides useful multivrite concept tht cn help unify reserch nd theories mong ecologicl sudisciplines (Bell et l. 1990; Wrdle 2002; Lovett et l. 2005). In prticulr, the concept of hitt structure is highly relevnt to the emerging study of urn ecology. At its most fundmentl level, urniztion is process during which humns chnge the composition nd rrngement of physicl mtter in the lndscpe y, for Tle 1 Definitons of key terms relted to the study of hitt structure. Modified from McCoy nd Bell (1990) nd Beck (2000) Key tems Hitt structure Scle Complexity Heterogeneity Definitions The mount, composition nd three-dimensionl rrngement of iotic nd iotic physicl mtter within defined loction nd time; refers to complexity nd heterogenity of physicl mtter cross horizontl nd verticl physicl spce Extent of sptil re, volume nd/or timespn; refers to circumscription of loction nd time in which unit(s) of hitt structure is descried The solute mount of individul entities (components) of physicl mtter t defined scle; refers to mounts of mteril, its surfce re nd surfce re to volume rtio, i.e., the density of mtter within given volume Vrition of hitt structures within defined sptil or temporl scles; refers to numers of different structures; loction of chnge(s) in hitt structure is used to delinete oundries round ptches

4 Urn Ecosyst (2007) 10: exmple, trnsforming forests nd deserts into shopping mlls nd lwns. Becuse mny types of hitt structure creted in urnized ecosystems do not exist in non-urn ecosystems, trditionl ecologicl reserch frmeworks nd methods my not e fully relevnt for pproching the study of urn ecology (Kye et l. 2006). However, the concept of hitt structure is inherently rod nd therefore fcilittes comprison of the ecologicl chrcteristics of heterogeneous types of urn hitt structure with ech other nd those of non-urn ecosystems (Byrne 2006). In ddition, the concept encourges finescle (i.e., cm to m), on-the-ground description of chrcteristics of the physicl mteril t study sites rther thn simply ctegorizing loctions into generic types of lnd cover (e.g., urn green spce) s is often done in corse-scle, lnd cover mpping projects. For these resons, mong others, incorportion of hitt structure into urn ecology s fundmentl concept cn help guide mechnistic reserch (sensu Shocht et l. 2006) out the multiple, intercting effects of urniztion on ecologicl vriles. Hitt structure is n especilly useful concept for investigting reltionships etween the ove- nd elowground components of urnized ecosystems. In generl, very little is known out the effects of urniztion on the ecology of soils. However, s is true for ll terrestril ecosystems, soils in urnized ecosystems provide numer of criticl ecosystem services tht should e conserved for oth ecologicl nd economic resons (Wll 2004). Thus, mjor chllenge for urn ecologists is to generte sic dt out the ecology of urn soils needed to help guide the mngement of urn ecosystem services (Kremen 2005). Hitt structure emphsizes the multivrite, ground-level, fine-scle perspective needed for developing questions nd hypotheses out how humn ctivities nd socioculturl systems tht dictte ptterns of oveground urn hitt structures ffect elowground ecologicl vriles. The primry ojective of this pper is to illustrte how hitt structure provides fundmentl concept for the study of urn soil ecology. This will e ccomplished in three wys. First, pulished reserch out urn soils will e reviewed from the viewpoint of hitt structure to provide n overview of the current knowledge se. This review is divided into four sections (iotic conditions, orgnisms, ecosystem processes nd lndscpe ptterns) which reflect the min foci of previous reserch. In ddition to pulished studies, results from recent field experiment tht compred the ove- nd elowground ecologicl chrcteristics of four types of urn hitt structure (unmowed vegettion, lwn, rk nd grvel mulches) re presented s cse study (Byrne 2006). Second, reltionships etween socioculturl vriles nd urn soil ecology will e discussed s they re linked vi humn mngement of oveground hitt structure. Third, synthetic conceptul frmework with hitt structure s its conceptul core is presented s tool to help guide the development of reserch questions out urn soil ecology. Overll, it is hoped tht the review nd frmework stimulte interdisciplinry interest in, nd reserch out, reltionships mong humn cretion nd mngement of hitt structure nd soil ecology in urnized ecosystems. Aiotic soil properties At sic physicochemicl level, hitt structure dicttes the iotic environmentl conditions t loction, including resource vilility. In this section, the effects of hitt structure on iotic soil properties will e considered in terms of soil temperture, physicl nd chemicl properties nd resource pools.

5 258 Urn Ecosyst (2007) 10: Altertion of mcroclimte conditions y urniztion hs long een recognized s the urn het islnd effect in which densely urnized environments hve higher ir tempertures thn their surroundings (e.g., Bornstein 1968). Likewise, oveground urn hitt structures medite soil tempertures s determined y their interception nd sorption of solr rdition nd ility to trnsfer het energy into the soil (Geiger et l. 2003). Thus, soils shded from sunlight y trees nd shrus cn generlly e expected to remin cooler on verge thn those in loctions without cnopies such s lwns (Avondet et l. 2003; Geiger et l. 2003). Severl studies hve shown tht elowground het islnds were creted in soils directly eneth nd surrounding pvement nd grvel mulch lyers, oth of which trnsfer het into the soil more effectively thn orgnic detritus (Hlverson nd Heisler 1981; Celestin nd Mrtin 2004; Montgue nd Kjelgren 2004; Mueller nd Dy 2005). For exmple, Byrne (2006) found tht, during mid-dy hours, grvel-covered soils were 8 20 C wrmer thn soils under rk mulch, lwns nd unmowed old fields nd tht lwn nd rk-covered soils ecme wrmer thn those under old fields (Fig. 1). At night however, soil tempertures converged nd ecme similr mong the four types of hitt structure. Thus, soil tempertures cross urnized ecosystems my e chrcterized y high temporl vriility nd exhiit fine-scle sptil heterogeneity tht reflects sptil ptterns of oveground hitt structure. Shocht et l. (2004) suggested tht modified microclimte ptterns within urnized ecosystems might temporlly shift, or even eliminte, sesonl dynmics of ecologicl ptterns nd processes s compred to those seen in non-urnized environments. Testing this hypothesis remins frontier in urn ecology ecuse so few studies hve exmined the roder consequences of ltered soil Fig. 1 Aiotic microhitt conditions in four types of urn hitt structure. Men hourly ground tempertures collected with dtloggers on June 1, Dt re significntly different (P<0.05) t times of temperture divergence. From L.B. Byrne, unpulished dt. Men grvimetric soil wter content from the sme hitt plots in June Brk mulch dt differ significntly from ll other hitts (P<0.05). From Byrne (2006). Dt points for oth vriles re mens from four replicted plots. SE rs nd sttisticl differences mong dt points re not shown for visul clrity o C grvimetric wter content :00 02:30 04:00 6/1 6/3 6/5 6/7 6/9 6/11 6/13 06:00 08:00 10:00 12:00 14:00 time of dy (hr) 6/15 Julin dte 16:00 18:00 6/17 6/19 6/21 6/23 6/25 6/27 6/29 lw n old field rk mulch grvel mulch 20:00 22:00

6 Urn Ecosyst (2007) 10: (nd ir) temperture ptterns for ecologicl vriles in urnized ecosystems nd their soils. Although humn ctivities ssocited with urniztion often trget mnipultion of oveground hitt structure, they usully result in concomitnt ltertion of the structure of soils nd their chemicl properties. Most drmticlly, ntive soil profiles re distured through, e.g., removl, compction or uril (Crul 1985; Lorenz nd Kndeler 2005). Hlverson nd Heisler (1981) found tht construction ctivities reduced the ph nd incresed the snd content in soils under sphlt. In ddition, the ulk density, nitrogen (N) nd orgnic mtter content of urn soils cn e ltered y humn ctivities, especilly mngement of vegettion structure (Green nd Oleksyszyn 2002; Pouyt et l. 2002; Hope et l. 2005; Kye et l. 2005; Lorenz nd Kndeler 2005). Schrenroch et l. (2005) nd Goluiewski (2006) found tht ge of lndscpes (i.e., time since initil urniztion) ws significnt predictor of mny soil properties including orgnic mtter content which incresed with lndscpe ge. However, s shown y Byrne (2006), soil properties cn chnge quickly (e.g., within 16 months) fter ltertion of oveground hitt structure (Figs. 1 nd 2). Thus, the physicochemicl chrcteristics of urn soils often exhiit higher Fig. 2 Soil properties under four types of urn hitt structure. Men (± SE) ph vlues of two smpling dtes in Men (± SE) soil ulk density of two smpling dtes in N=4 for ech vrile nd hitt type. For oth vriles, mens with different letters differ significntly (P<0.05) s nlyzed with ANOVA. From Byrne (2006) ph c ulk density (g cm -3 ) Lwn Old field Brk mulch Grvel mulch

7 260 Urn Ecosyst (2007) 10: levels of sptil nd temporl heterogeneity thn ntive soils (Crul 1985; Pouyt et l. 2002; Hope et l. 2005; Lorenz nd Kndeler 2005) due to quickly chnging humn mngement regimes cross time nd spce (e.g., in terms of disturnce nd resource inputs). Therefore, it is criticl for ecologists to mesure s mny soil vriles s possile t ll sites in urn studies to ensure tht ccurte conclusions re mde out reltionships mong humn ctivities, oveground hitt structures nd the structure nd ecology of urn soils. Hitt structure ffects soil resource pools t loction in two wys: (1) directly when the hitt structure provides the resource (e.g., plnts produce roots nd litter) nd (2) indirectly when it medites resource vilility through the iotic community (e.g., reduced litter vilility due to consumption y undnt detritivores) or environmentl conditions (e.g., higher soil tempertures increse wter evportion). In severl studies, soil cron pools were found to e greter in lwns thn ntive (desert nd shortgrss steppe) or other urn hitt types (old fields) due to the presence of vegettion (i.e., hitt structure) tht hd higher ove- nd elowground net primry productivity (NPP) (Green nd Oleksyszen 2002; Byrne nd Bruns 2004; Shocht et l. 2004; Kye et l. 2005; Goluiewski 2006). In ddition to mngement of plnts, humns impct resource inputs to urn soils vi removl or ddition of mterils (e.g., lwn clippings, leves, mulches) on the soil surfce. The type (orgnic or inorgnic), quntity (i.e., density, Fig. 3) nd qulity (N content) of mnged surfce mterils cn impct wide rnge of soil vriles. For exmple, soil wter content cn e incresed y the plcement of dense surfce lyers of mteril (i.e., mulches) which reduce evportion rtes (Fig. 1; Byrne 2006). Soil moisture ptterns cross urnized ecosystems re certinly lso ffected y the identities nd density of plnts which interct to drive locl evpotrnspirtion rtes (e.g., Eviner 2004). However, this topic remins lmost entirely unexmined in urnized ecosystems. The vilility of other resources (e.g., oxygen nd N) in urnized soils cn lso e ffected y humn mngement of oveground hitt structure tht lters soil wter (Fig. 1) nd orgnic mtter vilility (Fig. 1; Hope et l. 2005; Kye et l. 2005; Byrne 2006). For exmple, when orgnic mtter inputs re of low qulity, s with rk mulch tht hs high C-to-N-rtio (Fig. 3), inorgnic N my e removed from the soil solution y soil microes (immoiliztion) to meet their N requirements s they decompose the mulch (Fig. 4; Byrne 2006). This cn reduce the vilility of soil N for plnts in res covered with rk mulch, n indirect effect of hitt structure. In generl, however, very little is known out reltionships nd feedck mechnisms mong soil resource pools, hitt structure nd ecosystems processes nd services in urnized ecosystems ecuse so few studies hve een conducted. Soil orgnisms As is the cse for iotic conditions of urn soils, precious few studies hve een conducted out the effects of urniztion on soil iot, especilly studies compring communities elow different types of oveground hitt structure. (More studies hve compred the iot of urn versus rurl forests ut they fll outside the scope of this review.) Arthropods hve een the focus of most reserch conducted to dte nd re therefore, y necessity, the min focus of this section nd the lndscpe ptterns section elow. In generl, it hs widely een shown tht ground-dwelling nd soil rthropods re strongly influenced y hitt structure (Bell et l. 1990; Lngellotto nd Denno 2004). In

8 Urn Ecosyst (2007) 10: Fig. 3 Men quntity of ded orgnic mtter in the litter lyer of four types of urn hitt structure collected in My Men (± SE) C-to-N rtios of ded orgnic mtter from the litter lyer in three types of urn hitt structure collected in My For oth vriles, N=4 per hitt nd mens with different letters differ significntly (P<0.05) s nlyzed with ANOVA. From Byrne (2006) Quntity of detritus inputs (g N m -2) c d Lwn Old field B rk mulch Grvel mulch c Qulity of detritus inputs (C-to-N rtio) Lwn Old field Brk mulch n erly urn ecology study, Nuhn nd Wright (1979) concluded tht the ctivity nd undnce of nts cross heterogeneous urnized lndscpe were determined y ptterns of vegettion structure nd microclimte. In ddition, they found tht soils eneth sidewlks were common loction for nests of certin species. Ntuhr et l. (1994) reported tht differences in the structure nd composition of detritus lyers mong lwns, fields nd forests in n urn prk yielded differences in the species richness nd undnce of soil mites nd collemolns mong the hitt types. More recent studies hve lso concluded tht severl hitt structure-relted vriles, rther thn ny one fctor lone, intercted to drive ptterns of ground nd soil rthropod undnces nd community structure cross heterogeneous urnized environments (Fig. 5; McIntyre et l. 2001; Shocht et l. 2004; Byrne 2006). Hitt structure cn lso influence urn rthropod communities through top-down nd ottom-up trophic mechnisms (Brmen et l. 2002; Shocht et l. 2004; Feth et l. 2005).

9 262 Urn Ecosyst (2007) 10: Fig. 4 Net N minerliztion nd N 2 O flux from four urn hitt structures. Men (± SE) of net N minerliztion rtes from two field incution periods in 2004 (June Septemer). Men (± SE) N 2 O flux from soil collected in sttic closed chmers in June N=4 for ech hitt type. Mens with different letters differ t P<0.05 s nlyzed with ANOVA. From Byrne (2006) ug N g s oil -1 dy c c ug N2O- N m -2 hr Lwn Old field Brk mulch Grvel mulch For exmple, Byrne nd Bruns (2004) found tht certin collemolns were more undnt in lwns (mnged without chemicls) thn unmowed fields perhps due to greter vilility of mowed- clipping detritus in lwns, ottom-up provision of resources. Alterntively, microclimte conditions medited y hitt structure could promote topdown control of certin rthropods when predtor numers (e.g., spiders, eetles) increse in preferred hitts (Shocht et l. 2004; Feth et l. 2005). Incresed understnding of how ove- nd elowground food-wes (nd linkges etween them) re ffected y urn hitt structures is needed to inform the design nd mngement of urnized lndscpes in which eneficil predtors re conserved nd provide the ecosystem service of consuming pests (e.g., Brmen et l. 2002; see lndscpe ptterns section elow). Another importnt issue relted to rthropods, soils nd hitt structure in urnized ecosystems is the distriution nd undnce of humn disese vectors. For exmple, mny studies in urnized lndscpes hve reported tht locl tick undnce nd therefore proility of exposure to lyme disese is ffected y soil microclimte (especilly humidity) which is lrgely determined y interctions mong vegettion, detritus nd soil

10 Urn Ecosyst (2007) 10: Fig. 5 Men (± SE) spider undnces in four types of urn hitt structures. Spiders were collected y hnd from 25 cm 2 qudrts in Septemer From L.B. Byrne, unpulished dt. Men (± SE) erthworm undnces in four types of urn hitt structures verged over six smpling dtes Erthworms were hnd sorted from 25 cm 3 soil smples. From Byrne (2006). For oth vriles, N=4 per hitt type nd mens with different letters differ significntly (P<0.05) s nlyzed with ANOVA Spider undnce (# m -2 ) c Erthworm undnce (# m -2 to 25 cm depth) Lwn Old field Brk Grvel mulch mulch structure (e.g., Ostfeld et l. 1996; Guerr et l. 2003). Such results cn e communicted to the pulic using hitt structure s focus ecuse this concept provides rod frmework within which lndscpe mngement guidelines tht im to reduce the proility of disese exposure in urnized ecosystems cn e developed. During the process of urniztion, humns often remove ntive vegettion nd replce it with wholly new comintions of plnt species (including mny non-ntive ones) tht might not otherwise co-exist (Whitney nd Adms 1980; Hope et l. 2003; Thompson et l. 2003; Mrtin et l. 2004). A mjor frontier for urn soil ecology reserch lies in compring the direct nd indirect effects of heterogeneous urn plnt communities (e.g., lwns, grdens) on soil orgnisms. Recent reserch out the influence of plnt species identity nd richness on soil iot (e.g., Korthls et l. 2001; Wrdle 2002; Wolfe nd Kilronomos 2005) suggests tht humn-designed plnt communities my hve unique nd perhps unexpected effects on urn soil iodiversity nd, in turn, ecosystem processes nd services. However, to dte, reltionships mong humn-designed urn flors, the ptterns of hitt structure they crete nd soil orgnisms hve not een widely investigted. Likewise, the responses of most groups of soil iot to non-vegettion types of humncreted urn hitt structure hve not een studied. Thus, next to nothing is known out the diversity of life (or lck thereof) inhiting soils eneth, e.g., mulched grdens,

11 264 Urn Ecosyst (2007) 10: uildings, rods nd prking lots. Byrne (2006) found tht erthworm undnces decresed nd incresed in soils covered with grvel or rk mulch (ut tht lcked plnts), respectively, s compred to their undnces in soils under lwns nd unmowed old field vegettion (Fig. 5). These results, s well s the ones summrized ove for rthropods, indicte tht humn-mnipultion of oveground (plnt nd non-plnt) hitt structure cn strongly influence soil iot. Unfortuntely, lmost no other studies were found in the literture tht reported comprisons of soil iot mong different urn nd non-urn hitt types. Thus, we remin long wy from understnding how the design nd mngement of urnized lndscpes impct soil iodiversity. Ecosystem processes Although ecosystem processes hve rrely een exmined in urnized soils, foundtionl studies hve ppered in recent yers. As for soil iotic conditions nd orgnisms, rtes of mtter nd nutrient trnsformtions vry widely mong urn nd non-urn hitt types (Green nd Oleksyszyn 2002; Milesi et l. 2003; Groffmn et l. 2004; Schrenroch et l. 2005; Kye et l. 2004, 2005; Byrne 2006). Studies in rid iomes hve shown tht irrigted lwns hve up to 2.5 nd 10 times greter CO 2 nd N 2 O flux from soils, respectively, thn xeriscped nd ntive lndscpes due to incresed wter inputs, NPP nd microil ctivity (Green nd Oleksyszyn 2002; Kye et l. 2004, 2005). Although certin urn hitt types my e highly productive, urniztion my reduce regionl NPP rtes s ws oserved in forested lndscpes of the Southestern United Sttes (Milesi et l. 2003). Vriility in NPP rtes mong different types of urn hitt structure (from zero in pved res to very high in fertilized lwns) my in turn give rise to high sptil heterogeneity in the C pools nd fluxes of urn soils (Pouyt et l. 2002; Byrne 2006). Higher N inputs (e.g., from tmospheric pollution nd lwn fertilizer) my lso contriute to greter rtes of NPP, C nd N turnover nd net N losses in urnized ecosystems s compred to surrounding ntive ones (Bker et l. 2001; Groffmn et l. 2004; Lw et l. 2004; Hope et l. 2005). Yet, urnized ecosystems hve lso een oserved to retin lrge mounts of their N inputs (possily in soils), upwrds of 75% s oserved in Bltimore, MD (Groffmn et l. 2004). Becuse so few studies hve een conducted to dte, mny opportunities exist for generting fundmentlly new dt out how urniztion ffects rtes of N turnover, ccumultion nd loss in urn soils t locl scles nd, in turn, contriutes to ltered ptterns of N cycling t regionl nd glol scles (Bker et l. 2001; Kye et l. 2004, 2006). The studies discussed ove compred ecosystem processes etween urnized nd nonurnized ecosystems. Even fewer studies hve compred them mong soils eneth the different types of hitt structure tht comprise urn lndscpes. Schrenroch et l. (2005) mesured key ecosystem processes in urn soils from rnge of hitts nd ges nd concluded tht ge of the urn environment gretly influenced soil C nd N pools nd fluxes. They oserved tht older urn soils hd lower CO 2 flux nd greter rtes of N minerliztion thn more recently distured soils (Schrenroch et l. 2005). As nother exmple, Byrne (2006) oserved significnt differences in N minerliztion rtes nd N 2 O flux mong soils under lwns, old fields, nd lyers of shredded rk mulch nd grvel mulch (Fig. 4). In this study, it ws hypothesized tht differences in N cycling mong the four hitt types were driven y differences mong them in soil iotic conditions (Fig. 2), litter quntity (Fig. 3) nd qulity (Fig. 3) nd erthworm undnces (Fig. 5) (see Byrne (2006) for dditionl dt out plnt communities, C nd N cycling). Significnt

12 Urn Ecosyst (2007) 10: differences in the soil properties nd iogeochemicl cycles mong the experimentl hitt structure plots studied y Byrne (2006) were oserved within 16 months fter their cretion in previously unmnged old field. This suggests, in greement with Schrenroch et l. (2005), tht urniztion nd humn mngement inputs cn quickly chnge soil communities, resource pools nd ecosystem processes. Given the high sptil nd temporl heterogeneity of humn ctivities in urnized ecosystems, the serch for generl reltionships etween ptterns of urn hitt structure nd ecosystem processes my prove enormously chllenging. Nonetheless, generting dditionl dt out how vrious types of urn hitt structure ffect ecosystem processes is criticlly needed to inform the development of lndscpe mngement methods tht seek to conserve ecosystem services tht, e.g., promote soil fertility in urnized ecosystems (Wll 2004; Kremen 2005). Lndscpe ptterns In this section, two reserch topics relted to lndscpe ptterns nd soil ecology tht hve een exmined in urn ecosystems re discussed: lndscpe context nd frgmenttion. The urn-rurl (U-R) grdient pproch hs een used to investigte how different levels of urniztion (i.e., urn, suurn, rurl) surrounding focl study site (i.e., its lndscpe context) influence its ecology. Most U-R grdient reserch hs focused on forests without compring them to other hitt types. Nonetheless, discussion of these studies is included in this review ecuse they illustrte the importnce of including lndscpe context s key vrile of interest in urn soil ecology reserch. The most well chrcterized U-R grdient to dte consists of ok forests locted in New York City, NY nd suurn nd rurl Connecticut (McDonnell et l. 1997). Numerous iotic nd iotic vriles, nd ecosystem processes in the soil nd lef litter were found to differ long this U-R grdient. For exmple, urn forests exhiited 2 3 C higher verge monthly soil tempertures (from ), higher soil hevy metl concentrtions nd lower lef litter iomss thn rurl sites (McDonnell et l. 1997). Aundnces of soil mites nd collemolns nd fungl growth rtes on lef litter were lower in the urn sites during certin sesons nd were negtively correlted with soil hevy metl concentrtions (McDonnell et l. 1997). In contrst, non-ntive erthworms were found to e more undnt in the urn forests (McDonnell et l. 1997). Differences in the iot, iot nd lef litter chemistry long the U-R grdient pper to influence vriility in C nd N pools nd fluxes mong the forest ptches (McDonnell et l. 1997; Crreiro et l. 1999; Zhu nd Crreiro 2004). The mechnistic reltionships etween these oservtions nd urn hitt structure ptterns in the forests lndscpe contexts re not well understood. However, the lesson to e lerned from this nd other grdient studies (e.g., Avondet et l. 2003; Pvo- Zuckermn nd Colemn 2005) is tht sptil ptterns of hitt structure in study site s lndscpe context cn strongly impct the ecology of its soil, possily to greter degree thn the chrcteristics of its own hitt structure. The ecologicl effects of lndscpe context hve lso een studied in urn ecosystems in reltion to the conservtion of eneficil ground-dwelling nd soil rthropods. Although urn lndscpes generlly hve high sptil heterogeneity, the diversity of plnt species nd their hitt structurl complexity cn e reduced loclly resulting in lrge ptches of homogeneous microclimte nd resource vilility (e.g., in expnsive lwns). If numers of predtory rthropods re reduced in structurlly simple lndscpes, outreks of lwn nd grden pests (e.g., soil grus) cn occur (Shrewsury et l. 2004). Mnipulting ptterns of hitt structure provides wy to conserve nturl enemies tht cn help keep

13 266 Urn Ecosyst (2007) 10: pest numers in check (i.e., the gol of conservtion iologicl control). For exmple, incresing the diversity of flowering plnts in urnized lndscpes might provide fvorle hitts nd resources for predtors nd prsitoids nd increse their undnces (Brmen et l. 2002; Shrewsury et l. 2004; Reek et l. 2005). Alterntively, smll ptches (e.g., 3 3 m) of unmowed vegettion or mulch tht hve dense nd complex detritus lyers cn provide refugi for predtory rthropods (e.g., spiders, nts) in highly mnged lwns nd, in turn, increse their overll ctivity levels cross urnized lndscpes (Byrne 2006). These perspectives reflect the utility of using hitt structure s focus for discussing pplied ecologicl prolems nd their solutions in urnized ecosystems. Another lndscpe-scle topic tht hs een investigted in urnized ecosystems is the effects of hitt frgmenttion on ground rthropod communities. Severl studies found tht rthropod communities were ffected y the ptch sizes of urn hitts in conjunction with shifts in plnt species composition t ptch edges (Feth nd Kne 1978; Miyshit et l. 1998; Bolger et l. 2000; Gi nd Hochuli 2002). In generl, islnd iogeogrphy theories hve een supported in urn ecology studies with positive reltionships oserved etween species richness nd ptch size (Feth nd Kne 1978; Miyshit et l. 1998; Bolger et l. 2000). However, txon specific responses hve een oserved to e more vrile with some species persisting in smll frgments ut not lrger ones nd vice vers (Gi nd Hochuli 2002). Miyshit et l. (1998) oserved tht spiders ody sizes influenced, in prt, which species persisted in urn forest ptches of different sizes, e.g., lrger species were sent from smll ptches. Bolger et l. (2000) emphsized tht time since initil frgmenttion ws n importnt fctor tht influenced rthropod communities in the ptches they exmined. Results from these studies suggest tht mny fctors lone, or in comintion, cn ffect rthropod communities in highly frgmented urnized lndscpes. Hitt structure provides centrl concept for pproching the description of urn lndscpe ptterns nd investigting their effects on the distriution nd movement of soil orgnisms cross urnized ecosystems. Socioculturl vriles nd urn soil ecology The multivrite literture review presented ove illustrtes tht hitt structure provides useful perspective from which to exmine the effects of humn mngement of urnized lndscpes on urn soil ecology. In ddition, hitt structure is highly relevnt to the study of urn ecology in generl ecuse cretion nd mintennce of desired hitt structures is often key ojective of humn urn lndscpe mngement ctivities. Following this, indirect reltionships etween vriles ssocited with humn socioculturl systems (e.g., welth, politics, vlues) nd urn soil ecology cn e considered ecuse socioculturl vriles influence humn lndscpe mngement ctivities tht drive ptterns of urn hitt structure nd, consequently, elowground vriles. Although exmintion of reltionships etween socioculturl nd ecologicl vriles is centrl gol of urn ecology reserch (e.g., Pickett et l. 1997; Grimm et l. 2000; Hope et l. 2003), few, if ny, of the studies discussed ove ttempted to link socioculturl vriles with ecologicl ptterns in urn soils. However, smll ut growing ody of oveground-centered reserch hs reveled tht the structure of urn plnt communities is significntly relted to socioculturl vriles such s household income nd lifestyle ehvior (Hope et l. 2003; Mrtin et l. 2004; Grove et l. 2006). In other words, s Whitney nd Adms (1980) stted, Urn plnt communities re s much product of the culturl environment s they re prt of the physicl lndscpe (p. 446). For exmple, in

14 Urn Ecosyst (2007) 10: Phoenix, AZ, Mrtin et l. (2004) found positive reltionships etween fmily incomes nd plnt species richness in residentil lndscpes, suggesting tht welth might enle people to crete more structurlly diverse grdens. Often, urnized lndscpes re mnged to visully convey informtion out the welth nd esthetic sensiilities of the property owners, s when lwns re mnged with chemicl inputs nd rked free of leves (Nssuer 1995; Lw et l. 2004; Byrne 2005). An exciting interdisciplinry opportunity for urn ecologists is to exmine the degree to which socioculturl vriles tht drive humn mngement of urnized lndscpes might explin some of the fine-scle sptiotemporl heterogeneity of ecologicl ptterns nd processes in urn soils. Hitt structure will certinly help in this endevor ecuse it provides the physicl nd conceptul link needed to concurrently exmine the socioculturl nd ecologicl vriles ssocited with loction, s illustrted in the conceptul frmework introduced elow. Hitt structure conceptul frmework The primry premise of this rticle is tht hitt structure provides useful concept for orgnizing the multivrite study of the direct nd indirect effects of urn hitt structure on the ecology of urn soils. As such, hitt structure ws used s strting point from which to crete conceptul frmework (Fig. 6) tht cn fcilitte the development of questions nd hypotheses for urn soil ecology reserch. Although severl conceptul frmeworks hve previously een proposed to guide urn ecologists (Pickett et l. 1997; More heterogeneity nd complexity Sptil extent c Vriles of the socioculturl system ptterns nd processes in humn demogrphics, resources, nd institutions Humn ctivities (urniztion, lndscpe mngement) removl, cretion nd mintennce of physicl mteril Hitt structure composition nd rrngement of physicl mtter (oth iotic nd iotic) t loction Lndscpe ptterns sptil rrngement of hitt structure ptches, oundries Less heterogeneity nd complexity Orgnisms community structure, food wes Ecosystem processes nd services trnsformtions of mtter, energy nd nutrients Aiotic conditions, resources soil structure, microclimte, wter, C, N d Less proility of chnge in hitt structure Temporl extent More proility of chnge in hitt structure Fig. 6 A conceptul frmework using hitt structure s centrl concept to illustrte reltionships mong socioculturl vriles, humn ctivities nd c ecologicl vriles. d Exmintion of the vriles nd their reltionships cn e exmined cross sptil nd temporl scles. See text for further discussion. Modified from Byrne (2006)

15 268 Urn Ecosyst (2007) 10: Grimm et l. 2000; Alerti et l. 2003), none hve explicitly incorported hitt structure s fundmentl ecologicl concept. The frmework presented here is not ment to replce the others. Rther, dopting hitt structure s n orgnizing focus genertes n lterntive frmework tht is more suitle for fine-scle (i.e., cm to m), on-the-ground pproches to soil ecology reserch. Such pproches re needed to help revel soil ecologicl ptterns within heterogeneous urnized lndscpes tht might e missed using roder-scle frmeworks (e.g., tht emphsize ptch dynmics) nd methodologies (e.g., mpping of lnd cover t corse scles). However, the frmework ws designed to e flexile nd rod such tht it cn e used in ny study where hitt structure is vrile of interest (including those of non-urn ecosystems). The orgnizing ckone of the frmework is socioculturl vriles (Fig. 6) driving humn ctivities (e.g., construction, grdening; Fig. 6) tht modify hitt structures which influence other ecologicl vriles (Fig. 6c). Socioculturl vriles tht influence humn ctivities hve een discussed previously (Pickett et l. 1997; Grimm et l. 2000; Lw et l. 2004; Byrne 2005) nd could e redily dpted into the frmework; they re not discussed further ecuse they re not primry focus of this rticle. However, the frmework does highlight the importnce of humn ctivities s mechnistic link connecting socioculturl nd ecologicl vriles, point often ignored or underemphsized in previous discussions of urn ecology (ut see Grimm et l. 2000). Although sutle, explicit recognition of this linkge is necessry to more fully understnd how socioculturl vriles re trnslted through humn ctivities into ptterns of hitt structure nd, in turn, other ecologicl vriles. Hitt structure is n pproprite vntge point from which to exmine other ecologicl vriles ecuse it is the physicl stge on which they interct. In the hitt structure frmework (Fig. 6), four min components of ecologicl systems re included following the topics used to orgnize the urn soil ecology literture review: iotic conditions nd resources, orgnisms, ecosystem processes nd lndscpe ptterns. A numer of sic questions cn e sked out the reltionship etween ech of these nd hitt structure, exemplifying its power s n integrting concept for generl ecologicl reserch in ddition to urn soil ecology studies. Wht re the iotic conditions nd resource pools generted y the hitt structure? How does the hitt structure ffect the undnce nd interctions of orgnisms? How re ecosystem processes ffected y hitt-structure medited vriles? How do the lndscpe ptterns creted y heterogeneity of hitt structures cross spce ffect the movement of orgnisms nd nutrients? For soil ecologists, these questions cn e rephrsed to help exmine reltionships etween oveground hitt structure nd elowground ecology. For such questions, it is criticl to er in mind (s shown in the literture review sections) tht hitt structure my hve oth direct nd indirect effects on mny soil vriles. This cn e illustrted in the frmework y connecting hitt structure to ny of the four other ecologicl components directly or indirectly through nother component. For exmple, hitt structure cn influence locl undnces of soil rthropods (e.g., collemolns) directly y providing surfce res over which they cn move nd hide from predtors. Indirect effects of hitt structure on soil rthropods would occur when the structure cretes fvorle microclimtes or provides food resources. Countless such reltionships could e descried for ny comintion of vriles while using hitt structure s common focl point. In most urn ecology studies, hitt structure is n essentil priori considertion ecuse study loctions (e.g., forests, lwns, strems) re chosen (or creted) sed on physicl structures nd/or the sptil context of structures surrounding them (e.g., s in riprin zones nd U-R grdients). A generl description of the structures within nd round

16 Urn Ecosyst (2007) 10: study sites is usully included in most reserch rticles (s exemplified y McIntyre et l. 2001) nd my e sufficient for mny reserch ojectives. However, more rigorous nd detiled exmintion of the effects of hitt structure on ecologicl vriles requires mking the hitt structure concept nd frmework more opertionl. Specificlly, three key dimensions of hitt structure should e considered when dopting it for studies of soil ecology in urn (nd non-urn) ecosystems: heterogeneity, complexity nd scle (McCoy nd Bell 1990). Heterogeneity refers to kinds of structures (McCoy nd Bell 1990, p 18) nd is generted y differences in the composition (i.e., physicochemicl structure) nd/or rrngement (i.e., distriution in spce) of mtter mong loctions (Tle 1). Thus, two discrete loctions will e heterogeneous in hitt structure if 1) the rrngement of mtter hving the sme composition differs etween them or 2) one loction contins mtter of different composition thn the other. In contrst, complexity refers to mounts of structure (McCoy nd Bell 1990, p 18) nd is generted y the numers, or volume, of distinct, individul entities (e.g., ldes of grss, wlls) t loction (Tle 1). Surfce re of mteril is key feture of hitt structure complexity tht dicttes the spce over which orgnisms, nutrients nd energy cn interct (Beck 2000). Generlly, loction with greter numer of distinct structures cn e expected to hve more surfce re nd higher complexity. Note, however, tht two hitts with similr mounts of mtter my differ in complexity if the numer of distinct structures comprising one hitt is lrger thn in the other nd therefore the hitts differ in their surfce res. For this reson, the surfce reto-volume rtio cn e n pproprite description of hitt complexity. In mesurements (or generl descriptions) of hitt structure, cution must e tken to not confuse complexity with heterogeneity (McCoy nd Bell 1990; Beck 2000). These properties cn vry independently of ech other nd differentilly ffect ecologicl vriles. In mny studies, complexity nd heterogeneity hve een confounded, thus lessening their vlue for providing insight into how different dimensions of hitt structure influence orgnisms nd ecosystem processes (McCoy nd Bell 1990; Beck 2000). (Reviewing methods for mesurement of complexity nd heterogeneity is outside the scope of this pper; see Bell et l. 1990; Beck 2000; nd Tews et l for dditionl discussion.) As is true for the ecologicl literture t lrge (McCoy nd Bell 1990), the urn soil ecology ppers reviewed ove hve not discussed the reltive influence of the complexity versus heterogeneity of hitt structure on ecologicl vriles. A mjor chllenge for future hitt structure reserch will e to opertionlize these concepts further so tht their reltive effects on urn soil ecology cn e discerned. As emphsized y mny of the studies discussed in the literture review, nother importnt considertion for ll hitt-structure studies is scle, oth temporl nd sptil. A key dvntge of the hitt structure concept is its flexiility for use t wide-rge of scles (e.g., meters nd ptches to kilometers nd iomes). The definition dopted in this pper (Tle 1) is intentionlly silent out pproprite scles of study ut emphsizes tht the sptil nd temporl extents must e defined for ech unit (or type) of hitt structure exmined. It is importnt to explicitly descrie the extent of ech study site ecuse the complexity nd heterogeneity of hitt structures will chnge with incresing sptil nd temporl scles. As in ll ecologicl studies, the extent of re nd time encompssed y reserch on urn hitt structures should e relevnt to specific ojectives nd the orgnisms nd processes under investigtion (McCoy nd Bell 1990). However, s discussed in the lndscpe ptterns section, the sptil scle of most studies should e lrge enough to include some considertion of the lndscpe context round study sites ecuse of potentil influences of the surrounding hitt structures on the ecology of the study sites of interest.

17 270 Urn Ecosyst (2007) 10: Becuse defining sptil nd temporl scles of study re criticl to the working definition of hitt structure, they re incorported into the conceptul frmework s xes long which ecologicl vriles cn e plced to fcilitte genertion of hypotheses out them (Fig. 6d). Although the study of lndscpe ptterns nd ecosystem processes inherently include time nd spce dimensions, plcing these nd other vriles within mtrix of smller or lrger scles suggests rnge of lterntive hypotheses tht cn e developed out how dynmics of socioculturl nd ecologicl systems chnge together or independently through time nd spce (Pickett et l. 1997; Grimm et l. 2000). Investigting the vriility in reltionships etween ecologicl nd sociologicl systems cross wide-rnge of temporl nd sptil scles is frontier for urn ecology in generl nd specificlly for understnding the ecology of urn soils (Kye et l. 2004). A finl issue relted to urn hitt structure tht hs not yet een ddressed concerns the wys in which humn ctivities my indirectly ffect hitt structure without direct modifiction of physicl mtter. In prticulr, such ctivities include those tht involve inputs of chemicls (e.g., pollution or lwn fertiliztion) or orgnisms into the environment (e.g., introduction of non-ntive or iologicl control species). In the conceptul frmework, these re illustrted y rrows connecting humn ctivities (Fig. 6) to the orgnisms nd iotic conditions (Fig. 6c). Chemicl nd orgnism inputs y humns my or my not e intentionl or intended to ffect hitt structure. For exmple, inputs of hevy metl into soils from industril or trnsporttion ctivities nd invsions of nonntive erthworms re not intentionl ut my lter ptterns of hitt structure, perhps in undesirle wys (e.g., incresing or decresing litter decomposition rtes; McDonnell et l. 1997). In contrst, fertilizer nd pesticide inputs into lwns re intentionl nd directed towrd mintining hitt structure (i.e., grsses) in desired forms (i.e., green, one species; Lw et l. 2004; Byrne 2005). In turn, such mngement prctices indirectly influence soil orgnisms nd iotic vriles through their direct effects on the hitt structure. Although such reltionships pper complex t first, the hitt structure conceptul frmework (Fig. 6) provides n effective rodmp for guiding discussion nd interdisciplinry nlyses out reltionships mong socioculturl vriles, humn mngement of hitt structure nd wide rnge of ove- nd elowground ecologicl vriles. Conclusions A key chllenge for urn ecologists is to tese prt the reltive contriutions of socioculturl versus iophysicl fctors s drivers of ecologicl ptterns nd processes (Grimm et l. 2000; Hope et l. 2003). Hitt structure is useful concept for ddressing this chllenge ecuse it focuses ttention on vrile (i.e., physicl mtter) tht is simultneously relted to oth socioculturl nd ecologicl systems. The ojective of mny humn lndscpe mngement ctivities in urnized ecosystems, which re influenced y socioculturl vriles, is to crete nd mintin desired types of hitt structure (e.g., lwns, rods). In turn, ptterns of hitt structure ffect mny other ecologicl vriles (Bell et l. 1990). Thus, hitt structure provides physicl nd conceptul link etween socioculturl nd ecologicl vriles tht cn e exploited s fundmentl theme for urn ecology reserch (Byrne 2006). Specificlly, the study of urn soil ecology cn e fcilitted y the fine-scle, on-theground multivrite perspective emphsized y the hitt structure concept. The review of urn soil ecology reserch presented in this pper illustrtes the multivrite wys in which heterogeneous hitt structures influence the iotic conditions, resource pools,

18 Urn Ecosyst (2007) 10: orgnisms nd ecosystem process in the soils of urnized ecosystems. Key insights emerging from the review of this reserch suggest the following opportunities for studying urn soil ecology: & & & & & Becuse very little reserch hs een conducted out urn soil ecology, endless opportunities exist to develop oservtionl nd experimentl (especilly fctoril) studies tht tese prt the direct nd indirect effects of urn hitt structures on soil ecologicl vriles. Hitt structure provides link etween socioculturl nd ecologicl vriles nd etween ove- nd elowground hitts. Thus, it cn e used s focus for exploring reltionships etween socioculturl vriles nd ptterns of urn soil ecology. However, this reserch opportunity is chllenged y the high level heterogeneity in types of urn hitt structure nd resons why humns mnge urnized lndscpes which my mke discovery of generl reltionships difficult. Ecologicl chrcteristics of urn soils cn chnge quickly fter ltertion of oveground hitt structure (Byrne 2006) nd my differ widely mong soils of different ges (e.g., Schrenroch et l. 2005). Investigting detils out the temporl ptterns of chnge in urn soils is n open frontier for future reserch. Reserch out urn soil ecology is needed t the fine scles relevnt to humn mngement of ckyrds in order to ddress pplied questions relted to mnging pest popultions (e.g., herivores, disese vectors) nd soil processes tht provide urn ecosystem services. Hitt structure provides frmework for developing esy-tounderstnd lndscpe mngement guidelines. Reltionships etween soil ecologicl vriles nd humn-creted urn plnt communities s well s non-vegettion types of urn hitt structure remin lmost wholly unexmined ut my yield unexpected nd importnt insights. A min conclusion from the review of the urn soil ecology literture is tht there is currently derth of knowledge out how soils re influenced y urniztion nd humn mngement of urnized lndscpes. Although hitt structure is lrgely underpprecited nd underutilized in the roder ecologicl community, it provides useful concept for elucidting direct nd indirect reltionships mong socioculturl nd ecologicl vriles tht interct to determine the ecologicl chrcteristics of soils t given loction. A conceptul frmework sed on the concept of hitt structure hs een presented to fcilitte the development of interdisciplinry questions nd hypotheses out the ecology of urn soils. It is hoped tht this rticle hs succeeded in illustrting tht hitt structure provides fundmentl concept nd frmework for the study of urn soil ecology nd, thus, hs inspired others to egin the importnt tsk of improving our understnding of how humn cretion nd mngement of different types of urn hitt structure impct the ecology of soils. Such reserch is criticlly needed to inform the design nd mngement of urnized lndscpes in which fvorle levels of soil iodiversity nd ecosystem services re conserved. References Alerti M, Mrzluff JM, Shulenerger E, Brdley G, Ryn C, Zumrunnen C (2003) Integrting humns into ecology: opportunities nd chllenges for studying urn ecosystems. Bioscience 53: Avondet JL, Blir RB, Berg DJ, Eert MA (2003) Drosophil (Dipter: Drosophilide) responses to chnges in ecologicl prmeters cross n urn grdient. Environ Ent 32: Bker LA, Hope D, Xu Y, Edmonds J, Luver L (2001) Nitrogen lnce for the Centrl-Phoenix (CAP) Ecosystem. Ecosystems 4: