tree performance in response to biotic and abiotic land management

27 Pages·2015·2.04 MB·English

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U.S. Department of Agriculture: Agricultural
Publications from USDA-ARS / UNL Faculty
Research Service, Lincoln, Nebraska
2015
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Emily E. Oldfield
Yale University, [email protected]
Alexander J. Felson
Yale University
D.S. Novem Auyeung
NYC Department of Parks & Recreation, NYC Urban Field Station, Bayside, NY
Thomas W. Crowther
Yale University
Nancy F. Sonti
USDA Forest Service, Northern Research Station, NYC Urban Field Station, Bayside, NY
See next page for additional authors
Follow this and additional works at: https://digitalcommons.unl.edu/usdaarsfacpub
Oldfield, Emily E.; Felson, Alexander J.; Auyeung, D.S. Novem; Crowther, Thomas W.; Sonti, Nancy F.;
Harada, Yoshiki; Maynard, Daniel S.; Sokol, Noah W.; Ashton, Mark S.; Warren, Robert J. II; Hallett, Richard
A.; and Bradford, Mark A., "Growing the urban forest: tree performance in response to biotic and abiotic
land management" (2015). Publications from USDA-ARS / UNL Faculty. 1536.
https://digitalcommons.unl.edu/usdaarsfacpub/1536
This Article is brought to you for free and open access by the U.S. Department of Agriculture: Agricultural Research
Service, Lincoln, Nebraska at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in
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Nebraska - Lincoln.
AAuutthhoorrss
Emily E. Oldfield, Alexander J. Felson, D.S. Novem Auyeung, Thomas W. Crowther, Nancy F. Sonti, Yoshiki
Harada, Daniel S. Maynard, Noah W. Sokol, Mark S. Ashton, Robert J. Warren II, Richard A. Hallett, and
Mark A. Bradford
This article is available at DigitalCommons@University of Nebraska - Lincoln: https://digitalcommons.unl.edu/
usdaarsfacpub/1536
RESEARCH ARTICLE
Growing the urban forest: tree performance in
response to biotic and abiotic land management
Emily E. Oldfield1,2, Alexander J. Felson1,3, D. S. Novem Auyeung4, Thomas W. Crowther1,
Nancy F. Sonti5,6, Yoshiki Harada1,7, Daniel S. Maynard1, Noah W. Sokol1, Mark S. Ashton1,
Robert J. Warren II8, Richard A. Hallett9, Mark A. Bradford1
Forestsarevitalcomponentsoftheurbanlandscapebecausetheyprovideecosystemservicessuchascarbonsequestration,
storm-water mitigation, and air-quality improvement. To enhance these services, cities are investing in programs to create
urban forests. A major unknown, however, is whether planted trees will grow into the mature, closed-canopied forest on
whichecosystemserviceprovisiondepends.Weassessedtheinfluenceofbioticandabioticlandmanagementonplantedtree
performance as part of urban forest restoration in New York City, U.S.A. Biotic treatments were designed to improve tree
growth,withtheexpectationthathighertreespeciescomposition(sixvs.two)andgreaterstandcomplexity(withshrubsvs.
without)wouldfacilitatetreeperformance.Similarly,theabiotictreatment(compostamendmentvs.without)wasexpected
toincreasetreeperformancebyimprovingsoilconditions.Growthandsurvivalwasmeasuredforapproximately1,300native
saplingsacrossthreegrowingseasons.Thebioticandabiotictreatmentssignificantlyimprovedtreeperformance,whereshrub
presenceincreasedtreeheightforfiveofthesixtreespecies,andcompostincreasedbasalareaandstemvolumeofallspecies.
Species-specificresponses,however,highlightedthedifficultyofachievingrapidgrowthwithlimitedmortality.Pioneerspecies
hadthehighestgrowthinstemvolumeover3years(upto3,500%),butalsothehighestmortality(upto40%).Mid-successional
species had lower mortality (<16%), but also the slowest growth in volume (approximately 500% in volume). Our results
suggestthattherewillbetrade-offsbetweenoptimizingtreegrowthversussurvivalwhenimplementingurbantreeplanting
initiatives.
Keywords:afforestation,compost,ecosystemservices,greeninfrastructure,nativespecies,restoration,urbanforestry
heatislandeffect,sequesteringcarbon,increasingstorm-water
ImplicationsforPractice
infiltration,andpromotingwildlifehabitat(Nowak2006;Pataki
• Speciesgenerallyconformtotheirecologicalgrowthrate etal.2011;Gaffinetal.2012).Plantingtreesisoneofthepri-
classifications for non-urban systems, highlighting the mary means to achieve these services, and the benefits of the
utilityoftheseprinciplesinpredictinggrowthpatternsand urbantreecanopyareincreasinglyrecognized(McPhersonetal.
speciesinteractionsinurbanafforestationinitiatives.
• Pioneer species had the highest incremental growth in
Authorcontributions:AJF,MAB,EEO,RAH,MSAconceivedanddesignedthe
basal area, height, and stem volume. They also suffered
research;allauthorsperformedfieldwork;EEOanalyzeddata;DSM,RJWprovided
thehighestmortality,highlightingthatlandmanagerswill statisticalguidance;EEOwrotethefirstdraftofthemanuscript;allauthors
need to balance trade-offs in achieving survival versus contributedtodatainterpretationandpaperwriting.
growthgoals. 1SchoolofForestryandEnvironmentalStudies,YaleUniversity,370ProspectStreet,
• Plantingshrubsalongsidetreesathighstockingdensities NewHaven,CT06511,U.S.A.
2AddresscorrespondencetoE.Oldfield,[email protected]
canleadtogreatergrowthintheheightofplantedtreesto 3SchoolofArchitecture,YaleUniversity,180YorkStreet,NewHaven,CT06511,
achievefastercanopyclosure. U.S.A.
4NYCDepartmentofParks&Recreation,NYCUrbanFieldStation,431WalterReed
• Theuseofcompostcanincreasetreegrowth;however,it
Road,Bayside,NY11359,U.S.A.
maytakemultipleyearsforcomposteffectstomanifest. 5USDAForestService,NorthernResearchStation,NYCUrbanFieldStation,431
WalterReedRoad,Bayside,NY11359,U.S.A.
6USDA Forest Service, Northern Research Station, Baltimore Field Station, 5523
ResearchParkDrive,Baltimore,MD21228,U.S.A.
7DepartmentofHorticulture,CornellUniversity,134PlantSciencesBuilding,Ithaca,
Introduction NY14853,U.S.A.
8DepartmentofBiology,SUNYBuffaloState,1300ElmwoodAvenue,Buffalo,NY
Asurbanpopulationscontinuetogrow,investingingreeninfra- 14222,U.S.A.
9USDAForestService,NorthernResearchStation,Durham,NH03824,U.S.A.
structurehasbecomeapolicyimperativeforlargecitiesacross
the globe. In the pursuit of sustainability and resilience, cities ©2015SocietyforEcologicalRestoration
areinvestinginprojectsintendedtosupportarangeofecosys- doi:10.1111/rec.12230
Supportinginformationat:
temservicessuchasimprovingairquality,reducingtheurban http://onlinelibrary.wiley.com/doi/10.1111/rec.12230/suppinfo
September2015 RestorationEcology Vol.23,No.5,pp.707–718 707
Urbanafforestationandlandmanagement
1997;Nowaketal.2001;McPhersonetal.2005).Researchers demonstrateimprovedperformancecomparedwithtreeswithin
havequantifiedtheeconomicvalueoftheurbanforestcanopy, two species plots. The addition of shrubs tested the impact of
finding that trees provide millions of U.S. dollars worth of competitiveandfacilitativeinteractionsbetweenplantedshrubs
ecosystem service benefits (Brack 2002; Nowak etal. 2002). andtrees,whichcouldeitherleadtoreducedorenhancedtree
To bolster urban tree canopy, cities are now investing equally performance when compared to plots without shrubs (Van-
large sums of money in street tree planting programs and the dermeer 1989). The addition of compost tested whether plots
creationofnewurbanforests(Patakietal.2011;Nowak2012). receiving amendment promoted vigorous growth and biomass
Indeed,anumberofafforestationeffortsincitiessuchasNew accumulationforcarbonstorage,givenpositiveeffectsofcom-
York City, London, Los Angeles, and Auckland are investing postonwateravailability(Ryals&Silver2013).Weassessed
inthecreationofclosed-canopyurbanforests,plantingnative the performance of approximately 1,300 individuals of six
treesaplingswiththeexpectationthattheywilldevelopintoa nativespeciesacross54experimentalplotsin2011,2012,and
matureforestcapableofprovidingahostofecosystemservices 2013,followingplantingofthe3-to5-year-oldseedlingsinthe
totheurbanlandscape(Oldfieldetal.2013). autumnof2010.
Toconstruct urbanforests, land managers generallychoose
treesnativetotheregionwithnoknownsusceptibilitytopests
andpathogens.Theplantingchoicesforthesenewurbanforests Methods
typically contain assemblages of native species growing in an
environmentwithoutanaturalanalogue.Assuch,itisdifficult SiteDescriptionandExperimentalDesign
toprojectwithcertaintyhowwelltheplantedtreeswillperform The experiment, the New York City Afforestation Project
in the urban environment. Yet the success of these programs (NY-CAP), is located in Kissena Corridor Park (40∘44′N,
relies on the creation of closed-canopy forests that will even- 73∘49′W; 114cm MAP, 13∘C MAT), a 40-ha urban park in
tually attain maturity, as mature trees provide the majority of eastern Queens, New York, U.S.A. The project was devel-
environmentalbenefits(Nowaketal.2002;Troxeletal.2013). oped as a “designed experiment,” which incorporates ecolog-
Research in non-urban systems shows that mixed species ical research with urban design (Felson etal. 2013). The park
plantings can provide ecological benefits over monocultures, contains 54 research plots (Fig. 1A). The Natural Resources
includingincreasedbiodiversity,improvednutrientcyclingand ConservationService(NRCS)classifiedsoilsacrossKissenaas
soilfertility,andincreasedcarbonsequestration(Forresteretal. Laguardia-Ebbetscomplex,meaningthesoilsarewelldrained,
2006; Piotto 2008). These benefits, however, depend on the loamy-skeletal to coarse-loamy fill soils with more than 10%
speciesplanted,thewaytheyinteractwitheachother,andlocal human-derived artifacts. Our research plots fall in the Ebbets
sitecontingencies(Piotto2008). soilseries,characterizedbylessthan35%ofcoarsefragments
Usingknownecologicalgrowthrateandshadetoleranceclas- (NRCS2009).
sificationscanhelpguideexpectationsforforestgrowth.How- Theresearchplotswereseparatedintoeightdifferenttreat-
ever,ahostofstressorsintheurbanenvironmentcanthreaten ments, consisting of a crossed arrangement of tree species
theviabilityofnativeplantings.Theseincludedegradedsoils, composition (six species vs. two species), stand complexity
drought,andaggressiveexoticsthatcanoutcompetethenatives (with shrubs vs. without), and soil amendment (with compost
(Robinson&Handel2000;Rawlinsonetal.2004;Sullivanetal. vs. without). We refer to these treatments as composition,
2009;Overdyck&Clarkson2012).Theseissueshighlightthe shrub,andcompost.Notethatthisexperimentwasnotdesigned
fact that there is no prior knowledge that the planted vegeta- totestmechanismsofdiversityonproductivity(e.g.sampling
tionissuitabletothesite(Robinson&Handel2000;Kowarik effect or complementarity), but rather to evaluate if species
2011).Whatisneededarestudiesthatredressthepaucityofdata composition in plots had an effect on performance of the
assessingthesuccessofnative,urbanplantingefforts(Oldfield planted trees. Replication is uneven and is organized as fol-
etal.2013).Withoutsuchdata,urbanafforestationprojectsare lows:twospecies/noshrubs/nocompost,n=9;twospecies/no
investmentsthatlacksufficientinformationinformingthelike- shrubs/compost, n=5; two species/shrubs/no compost, n=9;
lihood of success. A key question is then: will planted native two species/shrubs/compost, n=5; six species/no shrubs/no
treesgrowandcreateforestscapableofprovidingtheintended compost, n=5; six species/no shrubs/compost, n=8; six
ecosystem services? Understanding how species will interact species/shrubs/no compost, n=5; six species/shrubs/compost,
witheachotherandwiththeirenvironmentiskeytodesigning n=8. Each plot is 15×15m (225m2) and includes 56 trees
successful afforestation programs (Piotto 2008), in non-urban planted 2.1m apart from each other. The two species plots
andurbansystemsalike. include 28 Tilia americana and 28 Quercus rubra (Fig. 1B).
This study—a research component of New York City’s The six species plots are composed of 8 individuals of
MillionTreesNYC Initiative—explored the performance of an T. americana and Q. rubra, plus 10 individuals each of
assemblage of six native species under various treatments Celtis occidentalis, Carya spp., Prunus serotina, and Quercus
known to impact tree growth. Species composition explored alba (Fig. 1C). As the NY-CAP exists within a heavily used
the effect of the number of planted species (two vs. six) on urban park, the plot layout was designed as an offset grid
tree performance.Given researchhighlighting greater produc- (Fig. 1B & 1C) to both facilitate research and serve as an
tivity of trees planted in mixture (Forrester etal. 2006; Piotto esthetic landscape amenity (Felson etal. 2013). Tree species
2008), we expected that trees within six species plots would selected for this project had to meet the following criteria:
708 RestorationEcology September2015
Urbanafforestationandlandmanagement
(A)
(B)
(C)
Figure1. (A)AnaerialviewoftheafforestationresearchplotsinKissenaCorridorParkinQueens,NewYork,U.S.A.;(B)plotlayoutoftheplantedtreesin
twospeciesplots;and(C)layoutoftheplantedtreesinsixspeciesplots.Measurementsofdiameter,height,andstemvolumeweretakenforthecentral24
treesineachplotlocatedinthesubplotslabeleda–f.
September2015 RestorationEcology 709
Urbanafforestationandlandmanagement
all species are native to the region, and have no known sus- s ee ee
cterereapettei,bmialoinrtytdaltisotlyop.weSstegslreaocnwtdeedrpsatrtdehieofsgfeecrnionsngktaniionnwatnhemtiorixcsahouafsdeerawptioiddlee,srapmnreocaedds- percentage ShadeTolerance ntermediatntermediatIntolerantntermediatntermediatTolerant
h II II
(Table1). wt bn
cTurerTeerdseefwrsoewmreePr3ei-npteollaan5nt-deysdeNairnu-orOsledcrtyosabipnelriCn2og0ls1u0mm.bePaulssau,nrNtienedgwtarJepeeprssreowyx,eiUmre.aSpt.erAoly-. mentalgro GrowthRateassificatio RapidSlowRapidSlowModerateModerate
e Cl
0.6–1.2minheight,withrootmassescontainedineither1or2 cr
gTgaerelrleoisnnwh(oaelpreepsrpoolaxfniamtpeapdtrewolpyitrhi3a.at7eh9saionzrde7-th.o5e8lhdoLmu,sereecsthhpaeenctitrzieevedelrpyoo)ostctso-h(naotpaleipnrdeoirxgs--. vals,andin Mortality(%) 40122016128
iostmhhferatuhtcbeeolsym5(542p5ospspcltemocatsimdersieea,ncm4de1emivtepeernldatanscn.tosdSmphapeprorupsbprtosloxaatni)nmddianhtheaaellyfrcbrw2os5sewsrceemedrpedldaeepnseltiapegn)dn.tewwHdiiattalhhft confidenceinter 3me(cm) MeanStemVolumeaIncrementYears1–3)(%) 355654611894575321022
the same time as the trees, and were planted in holes large % u (
easAinnpsdo,ouHchgyeahnrmubtomsamawcceicatlohinsminnvamiebroagincdinuhaimtapen,lroaoAt,osiatcnnmlcedlpauVsidasieebssuss.rhyTnrruhuiabemcsmadS,oeasnamttnaabdbtuuucPmnaad;snaaicncnatudnsmahhdrevueribbnrss-- edin2013,95 StemVol 95%ConfidenceInterval 370–994346–4931762–2707358–500518–6201980–2321
gHyaeeatrurbmaoc.feApoluafsnuptlillnasgnp,teiancngiedssssuluifsbftseeriseqduinesncigltuncdiofielcdoanniintzadFtiieeolbsnaocnokfeattfhtaeelr.pt(lh2oe0ts1fi3rbs)yt. suresrecord MeanSE± 682±159419±372234±241429±36569±262150±87
pocspSFfisSiaaTA2twmtwWdsitwwtarnrnhhimrhpmofl0iioueeevokhpheetageaceeriprl1aeileeaoeaepenepenmoedallrrdts1frlsnrdntdstrtmemPtosolehMiee)sebeodi.ddefyifx,rtcnrwdettdifelfexetoeeeeeeinAotereoprwrsntmierddeparosangarmrmofeetptalrttaledaesmoiod.lraohei.ecrwhlpenaheruletftsesechaharl(aetdctnero1ilwpyoiseet2hrteteoslehrnrnhfo5oiahdylsnr0uibiseeooyagiewuseorrbiwdnc1rsaeansldasca1toeeemutlmdcue4elt.ohorr1s5lttiaiecnrdet)mceewoneom0arert.cotdciivhahiidicnedinaueogumemntenpBhaademgmelalnhsglpiomtltlpr.onnsnpitiatlaesaiaetv-vtptnodTuelrotsehtprnrnaeoelafitetflielimhde.dldths,agtcctnrrtaeyedevitsohahhefddTnala,srroarasdtteeatteemtcihpaonttrtebaevmabrc“ohoter-stcmfsdiroaeiideseritmeuotanooeoptieohei,pmtsacelrstvginvoarptrleov,palnooroeeoausitotrfeldlmlnmrwaddtcluhfbotaaieysrtlidohsenot”fvntmhemmeatitoiaslehotfeooodedenlt2bifiiemiaelrfsnnnylesggaz.es;ter1ar5dsg-rnntuheccealnr,iphciewecea2dairtgamtntdoaelea,-afidamamus9eageictldalrn3drucacasei6nr2oeenoiipdlm-eiernafcct4apnmeatsduteeebfiendoshrsaitletedrbgdeetcepclpdmanosrtasrhpeumlereliath,btihatasetftsp1lntrlhscetoesooyor.t.n.cgro0ttetrsuerttueroeeWthraIts0mtobsHreabsoroiennrkelttvvsdloeamesueadsewblp.ee(splicyimeanirr5wdllAlnd2sdgeoa(e,easadi,psAlerechnrfnuescnsnooaetesolapmitrtsoracwtuacnnlsexsereiitOlnrhtnougansuh)ctwoothdcoetitttttnchuoglrwooneeeheheeanoodueheisrsdddnddogeesss---f-f-tt.,. Table1.Growthmeasuresandgeneralspeciescharacterizationsforplantedtreespecies.Means±SEforgrowthmeafrom2011to2013. 2BasalArea(cm)Height(m) MeanMeanBasalArea95%Heightaa95%ConfidenceIncrementConfidenceIncrementSpeciesMeanSEInterval(Years1–3)(%)MeanSEInterval(Years1–3)(%)±± Celtisoccidentalis7.66±1.145.44–9.898061.72±0.131.46–1.98240Caryaspp.6.49±0.425.67–7.312351.73±0.061.60–1.8536Prunusserotina21.60±1.8218.03–25.175522.55±0.112.34–2.7772Quercusalba8.16±0.517.16–9.162871.46±0.051.37–1.5541Quercusrubra7.98±0.277.45–8.523111.88±0.031.81–1.9437Tiliaamericana23.05±0.7121.65–24.453702.54±0.042.46–2.6291 aCalculatedastheproportionincreaseintreesizeduringyears1–3relativizedtoinitialmeasurestakeninyear1.bGrowthrateclassificationsandshadetolerancearealsoincluded,whichwereobtainedfromtheUSDAPlantdatabase(USDANRCS2015).
710 RestorationEcology September2015
Urbanafforestationandlandmanagement
on some species prevented taking diameter measures 10cm Results
above ground level. Given the differing morphologies of the
TreeSurvival
trees,rootcollardiameterprovidedaconsistentmeasureacross
speciestotestfordifferencesingrowth,anditisoftenusedas After 3years, a total of 180 of the 1,296 assessed trees were
ameasuretotestforstatisticaldifferencesinthegrowthoftree scored as either dead or missing. By year 3, more trees were
seedlings (Halter etal. 1993; Page-Dumroese etal. 1998). We recordedasmissingversusdead(109vs.71).Whentreeswere
repeated these assessments of height and diameter in August visiblydead,wescoredthemassuch.Incaseswhenwecould
2012andAugust2013.Weusedrootcollardiametertocalculate notlocatetrees,wescoredthemasmissing.Missingtreeslikely
basalareaonanindividualtreebasis,whichwasthensummed resultedfrommultiplefactors,includingvandalism,theft,fire,
acrosstreestodeterminetotalbasalareaperplot.Toestimate andsevereherbivory.Sixspeciesplotshad1.6-timesmoredead
stembiomassvolume,weusedtheformulaforacone,usingthe andmissingtreesthantwospeciesplots(p<0.001).Composted
heightandrootcollardiametermeasures(Magnussen&Reed plots also had a higher degree of mortality (17±1%) than
2004).Likebasalarea,stemvolumewasrecordedattheindivid- non-compostedplots(11±1%)(p<0.01).Mortalityincreased
ualtreelevelandthensummedtodeterminetotalstemvolume acrossthe3yearsfrom1%inyear1,to6%inyear2,toalmost
attheplotlevel. 14% in year 3. Mortality varied among species, with Celtis
occidentalis having the highest percent mortality after 3years
at40%andTiliaamericanahavingthelowestat8%(Table1).
As tree size varied between species at the time of planting,
StatisticalAnalyses
wetestedtoseewhetherourfirstheightmeasurementtakenin
Weanalyzedhowplottreatments(composition,shrub,compost)
2011 was a significant predictor of tree mortality in 2012 and
affectedsurvivalandgrowthintreediameter,height,andstem
2013.Treeheightin2011didhaveasignificanteffecton2012
volume, both across research plots as a whole and also by survival (p<0.05) for C. occidentalis, Carya spp., Quercus
species. For the mortality analyses, we used a generalized rubra,andT.americana;andasignificanteffect(p<0.05)on
linearmodelwithabinomialerrordistribution.Forthegrowth
2013 mortality for C. occidentalis, Carya spp., and Quercus
analyses, we used a linear mixed model (LMM) approach to
alba.Largertreessufferedlessmortalitythansmallertreesfor
account for the unbalanced replication (see Site description
each of those species; however, it is difficult to determine the
andexperimentaldesign)andthenon-independentspatialand ultimatecauseofmortality.
temporal associations in our data. By specifying individual
tree nested in plot as a random effect, our LMMs account
for the possibility that plots paired by the shrub treatment Plot-LevelResponse
(see Fig. 1B & 1C) are more similar to each other than to For each research plot, we calculated total plot basal area,
other plots, and also account for the fact we had repeated the mean height, and total plot stem volume to determine
measurements on the same units (i.e. the same tree in 2011, howtreatmentsaffectedtreeperformanceattheplotlevel.All
2012,and2013).Ourapproachisthenconceptuallyequivalent metricsrespondeddifferentlytoplottreatments,althoughthey
tousingnestingandrepeatedmeasuresinANOVA,andisnow consistentlyincreasedfromyear1toyear3,demonstratingthat,
widely adopted in environmental science (Bolker etal. 2009; overall,thetreesweregrowingasopposedtodyingorindecline
Bradfordetal.2012;Krameretal.2012),giventheadvantages (Table2).
itprovidesinmodelpowerandevaluation(seenextparagraph). By year 3, total tree basal area per plot was 23% higher in
Plottreatments(composition,shrub,compost)andyear(1,2,3) composted versus non-composted plots (p=0.031). Total tree
wereidentifiedasfixedeffects.Wealsotestedforinteractions basal area was also 13% higher in plots planted with shrubs
betweentreatmentsandbetweentreatmentsandyear.Wetested (p=0.01)and7%higherintwospeciesplotscomparedwithsix
for treatment effects on the relative growth in basal area and species plots (p=0.05; see Fig. 2A). Relative growth in basal
stem volume (calculated as the proportion increase in growth areaincrement(BAI)wascalculatedasthepercentincreasein
during years 1–3 relativized to initial measures in year 1) at growthfromyear1toyear3.Compostwastheonlytreatment
the species and plot level. We specified relative growth from tohaveasignificanteffectonBAI(p=0.003),withcomposted
year 1 to year 3 as the response variable with plot treatments plots growing 69% faster compared with non-composted
identified as fixed effects and tree nested within plot as a plots.
randomeffect. Species composition had a significant effect on tree height
Toselectthebestmodelsfortreegrowth,weusedthemodel (Table 2). As with basal area, height was greater in two
withthelowestAkaikeinformationcriterion(AIC)score(Burn- species plots with trees 10% taller than those in six species
ham etal. 2011). Variance inflation factors less than 5 indi- plots (Fig. 2B). The best-fit statistical model retained both a
cated that collinearity was sufficiently low among predictor shrub-by-year interaction and a compost-by-year interaction,
variables. The LMMs were all fit assuming a Gaussian error withthosetreatmentshavingpronouncedeffectsinyear3but
distribution. We used the “nlme” package in the “R” statis- not years 1 and 2 (Tables 2 & S1, Supporting Information).
tical program for our LMMs, and the summary function in In year 3, the measured height of trees was 8% higher in
the “LMERtest” package to estimate model coefficients and composted plots and 12% higher in plots planted with shrubs
pvalues. (Fig.2B).
September2015 RestorationEcology 711
Urbanafforestationandlandmanagement
Table 2. Coefficientsandpvaluesforplottreatmenteffectsonplanted (A) Two Species Six Species
treegrowthasmeasuredastotalbasalareaperplot,meanheight,andtotal
stemvolumeperplotofindividualtreesacross2011–2013.Thenegative
coefficientsforthemaineffectofcompostfortotalbasalarea,height,and 400 No Shrub
total stem volume are due to their interaction with year and cannot be Shrub
ionvteerrptirmetee.dCaoloenffiec,ihenigtshlaignhdtipngvatlhuaetstahreepporessiteinvetedefffoerctesaochf cpoamrapmoestterg.rAowll )−2m
tsrteaatitsmtiecnaltlsyrseitganinifiedcawntit(hpin<t0h.e05b)etset-rfimtssataretissthicoawlnminodbeolldarteexptr.esented,and (2*otm 300
pl
Coefficient pValue per
(A)Totalbasalarea(cm2/m2) al Area 200
Intercept 16.61 0.44 as
B
Composition −33.55 0.053 al
ot 0
Shrub 24.16 0.010 T 0
1
Compost −61.68 0.031
Year 82.76 <0.001
Compost×Year 44.43 <0.001
(B)Height(m) 0
Intercept 1.18 <0.001 No Compost Compost No Compost Compost
Composition −0.21 0.01
Shrub −0.16 0.081 Two Species Six Species
Compost −0.19 0.12 (B) 3.0
Year 0.27 <0.001 No Shrub
Shrub
Shrub×Compost 0.13 0.06
5
Shrub×Year 0.11 <0.01 2.
Compost×Year 0.11 <0.01
(C)Totalstemvolume(cm3/m2) 0
2.
Intercept −3748.3 0.13
Shrub −4412.6 0.098 m)
CYeoamrpost −76790339..41 <00..002061 eight ( 1.5
H
Shrub×Compost 4069.6 0.032
0
Shrub×Year 2653 0.023 1.
Compost×Year 4183.4 <0.001
5
0.
Total tree stem volume per plot and growth rate were sig-
0
nificantly affected by compost with an interaction with year 0.
No Compost Compost No Compost Compost
(p<0.001). Stem volume growth rate was 73% greater in
composted plots over the course of 3years. Consequently, Figure2. Treatmenteffectsonaperplotbasisfor(A)basalareaand(B)
total stem volume on composted plots was 27% higher than heightofmainstem.Wecalculatedthetotalbasalareaperplotandthe
non-composted plots. The best-fit statistical model for total meantreeheightacrosstheresearchplotstoassesswhichplottreatments
weredrivingdifferencesinplot-levelgrowth.Errorbarsrepresent±SE.
stem volume per plot also retained shrubs as a main effect,
Thenumberofreplicatesareasfollows:twospecies/noshrubs/no
a compost-by-shrub interaction, and a shrub-by-year interac-
compost,n=9;twospecies/noshrubs/compost,n=5;two
tion. The presence of shrubs led to significantly more tree
species/shrubs/nocompost,n=9;twospecies/shrubs/compost,n=5;six
stem volume per plot (p=0.02), with shrub treatment plots species/noshrubs/nocompost,n=5;sixspecies/noshrubs/compost,n=8;
containing24%morestemvolume(ofthetrees)thanplotswith- sixspecies/shrubs/nocompost,n=5;sixspecies/shrubs/compost,n=8.
out(Fig.3). Thegraphsincludemeasuresonlyfromyear3,andincludeallplot
treatmentsaseachwasretainedinthebest-fitstatisticalmodels.See
Table2fortreatmentcoefficientsandpvalues.
SpeciesResponse
Allsixoftheplantedtreespeciesgrewmarkedlyinallrecorded the species) for trees in composted plots. Models for all six
measuresacrosseachsuccessivegrowingseason(seeTable3& speciesretainedasignificantcompost-by-yearinteraction,with
Tables S2 & S4). We could assess the influence of only two diametersinyear3foreachspeciesmarkedlygreaterincompost
treatments, shrubs and compost, for four of the tree species, versus no-compost plots, with minimal differences in years 1
whereasfortheothertwospecies(Q.rubraandT.americana), and2(seeTablesS2&S4).TiliaamericanabutnotQ.rubra
we could also assess the effects of the species composition retained a significant species composition by year interaction,
treatmentbecausetheywerepresentinboththetwoandthesix withT.americanainsixspeciesplotsshowinggreatergirthin
speciesplots(seeFig.1B&1C). rootcollardiameterthanthoseintwospeciesplots,particularly
Compost had a positive effect on root collar growth with inyear3(6.11±0.17cmvs.4.86±0.085cm,respectively;see
measures of diameter being 15–30% greater (depending on Fig.4A).
712 RestorationEcology September2015
Urbanafforestationandlandmanagement
Table3. Means±SEforeachmeasureofgrowthforspeciesinsixspeciescompositionplotsfor2013(for2011and2012values,seeTableS4).Valuesfor
2013capturetheeffectthatplottreatmentsarehavingonindividualspecies’growthforthelatestmeasurementyear.Significanttreatmenteffects(p<0.05)are
showninboldtextforthedifferencesbetweenplotswithandwithouteithershrubsorcompost.SeeFigures4–6andTableS2forsizemeasuresforQuercus
rubraandTiliaamericana.
SixSpeciesCompositionPlots
Shrubs Compost
Difference Difference
Variable (−) (+) (With–Without) (−) (+) (With–Without)
Celtisoccidentalis Diameter(cm) 2.81±0.24 2.58±0.33 −0.23 2.28±0.20 3.03±0.31 0.75
Height(m) 1.74±0.15 1.68±0.23 −0.06 1.44±0.15 1.93±0.15 0.49
Volume(cm3) 594±130 781±309 187 309±75 969±268 660
Caryaspp. Diameter(cm) 2.72±0.13 2.78±0.12 0.06 2.52±0.11 2.92±0.13 0.4
Height(m) 1.61±0.08 1.84±0.09 0.23 1.57±0.10 1.84±0.08 0.27
Volume(cm3) 379±49 456±56 77 316±45 497±54 181
Prunusserotina Diameter(cm) 4.78±0.35 4.86±0.30 0.08 4.33±0.32 5.18±0.31 0.85
Height(m) 2.29±0.15 2.70±0.16 0.41 2.42±0.17 2.65±0.14 0.23
Volume(cm3) 2148±363 2315±323 167 1754±335 2585±331 831
Quercusalba Diameter(cm) 3.16±0.15 3.03±0.12 −0.13 2.77±0.13 3.30±0.12 0.53
Height(m) 1.39±0.06 1.54±0.07 0.15 1.29±0.06 1.56±0.06 0.27
Volume(cm3) 440±61 417±39 −23 285±31 515±52 230
0 Heightresponsestoplottreatmentswerealsofairlyconsis-
0
00 tent in the sign of the effect across species, with significant
4
No Shrub ormarginallysignificantshrub-by-yearinteractionsretainedin
)2 Shrub modelsforfiveofthesixspecies(Caryaspp.,Prunusserotina,
−
m
0 Q.alba,Q.rubra,andT.americana).Allofthesespeciesexhib-
* 0
3 0
m 0 ited greater height in plots planted with shrubs versus plots
c 3
( without, with differences most pronounced in year 3 (Tables
ot
pl S2 & S4). For instance, the height of Q. rubra was 14%
me per 20000 gTrileiaatearminersicharunba vaelrssousrentaoinsehdruba pcloomtspobsyt-ybeya-rsh3ru(bFiign.te5rAac)-.
u
ol tion, indicating that trees were tallest in compost-amended
V
m 0 plots planted with shrubs and that this response would not
al Ste 1000 hmaevnetsbaeleonnepr(eFdigic.t4edB)f.rNomotaebitlhy,ercotmhepocsotmepffoescttsoornshtrreuebhteriegaht-t
Tot were less consistent across species than the effects of com-
post on root collar diameter and basal area, where there were
0 always compost-by-year interactions. For example, for three
No Compost Compost species(C.occidentalis,Q.rubra,andP.serotina),therewere
no significant compost or compost-by-year effects on height
Figure3. Treatmenteffectsperplotfortotalstemvolume.Theeffectsof
(TablesS3&S5).
theinteractionbetweentheshrubandcomposttreatmentareclearly
Stem volume, like basal area, responded fairly consistently
apparent,withcompostedplotscontainingshrubshavingsignificantly
morestemvolumecomparedwiththeothertreatments.Errorbars and positively to compost among species, with five of the six
represent±SE.Thenumberofreplicatesareasfollows:noshrub/no speciesshowingasignificantcompost-by-yearinteraction(for
compost,n=14;noshrub/compost,n=13;shrub/nocompost,n=14; allfivespecies,p<0.05).Quercusrubrawastheonlyspecies
shrub/compost,n=13.SeeTable2fortreatmentcoefficientsandpvalues.
that did not retain a compost-by-year interaction in its best-fit
statistical model for growth in stem volume. However, the
Basalarearesponsesmappedontothoseforrootcollardiam- bestmodeldidretainacompost-by-shrubinteraction,withthe
eter growth (as it was calculated from each tree’s root collar positive effect of shrubs on stem volume manifesting only in
diameter).YetonlycomposteffectswereapparentforBAIfor composted plots (Fig. 5B). Tilia americana retained a signif-
eachspeciesfromyear1toyear3.Compostedplotshadhigher icant year interaction for each of the three treatments (Table
growthinbasalareathannon-compostedplotsforallspecies, S3), where treatment effects on stem volume were most pro-
withsignificantdifferences(p<.05)forC.occidentalis,Carya nouncedinyear3(TableS2),withsixspecies,compostamend-
spp., and T. americana; compost had a marginally significant ments, and shrub presence all equating to greater stem vol-
effectonQ.alba(p=0.06)(Tables4&5). ume in 2013 (Fig. 6). The best-fit models for relative growth
September2015 RestorationEcology 713
Urbanafforestationandlandmanagement
(A) Two Species Six Species survivalandgrowtharesparse.Weshowthatsixnativespecies
7 respondrelativelyconsistentlytoplottreatments,yetindividual
No Shrub speciesperformancevariesmarkedly.Treespeciescomposition
6 Shrub explaineddifferencesinthebasalareaandheightoftreesatthe
plot level, with two species plots having taller trees on aver-
5 ageandgreaterbasalarea.Thisresultappearscounter-intuitive
given expected positive effects of plant diversity on ecosys-
m) temfunctioning(Isbelletal.2011),butinourstudythiseffect
c 4
er ( appears to be driven by a single high-performing species.
et
m Specifically,halfofthetreesintwospeciesplotswereT.amer-
a 3
Di icana. This species’ mean diameter and height in year 3 were
substantially greater than those of the other planted species,
2
except Prunus serotina, which is of similar size (Table 1).
Differences in individual species performance also appear to
1
account for the fact that six species plots had greater rates of
mortality, owing to the high incidence of mortality (40% by
0
No Compost Compost No Compost Compost year3)ofCeltisoccidentalis(Table1).Therefore,treespecies
compositioneffects(i.e.2vs.6speciesplots)attheplotlevel
(B) 5 appeardrivenmorebythecharacteristicsofindividualspeciesin
3. No Shrub
thedifferenttreatmentlevels,ratherthanthecompositiontreat-
Shrub
3.0 mentinfluencingthestrengthofpositiveornegativeinteractions
amongtreeindividuals.
5
2. There was evidence of species composition treatment
0 effects on the strength of interactions when individual species
2.
responses were considered. For example, despite explaining
m)
ht ( 1.5 the greater growth of trees within the two species treatment
g
Hei attheplotlevel,T.americanahadgreaterindividualdiameter
0
1. and volume in the six species plots by year 3. In contrast,
5 Q. rubra, the second species in the two species plots, had
0.
greater volume in the two versus six species plots, but this
0.0 effect was not statistically significant despite being retained
NoCompost Compost
in the best-fit model. The effects of the species composition
Figure4. TreatmenteffectsforTiliaamericanaon(A)diameterand(B) treatmentonthesetwospeciespointtoinfluencesofcompeti-
height.Forbothrootcollardiameterandheight,thebest-fitstatistical tionand/orfacilitationbetweenthesetwospecies.Itisdifficult
modelsretainedtreatment-by-yearinteractions,andsotheresultsforyear to disentangle these effects to determine direct causation, but
3onlyarepresentedinthesegraphs,asthatistheonlyyearthatexhibits such interactions are considered key in shaping how mixtures
statisticallydifferentresultsinmeasuredparametersbetweentreatments.
perform(Forresteretal.2006;Piotto2008).Forexample,itis
Errorbarsrepresent±SE.For(A)thenumberofreplicatesareasfollows:
possible that T. americana grew less in the two species plots
twospecies/noshrubs/nocompost,n=108;twospecies/no
shrubs/compost,n=60;twospecies/shrubs/nocompost,n=108;two because the large individual size of this species generated
species/shrubs/compost,n=60;sixspecies/noshrubs/nocompost,n=20; intra-specific competition that was more pronounced than the
sixspecies/noshrubs/compost,n=32sixspecies/shrubs/nocompost, inter-specificcompetitionwiththesmallerstaturespeciesinthe
n=20;sixspecies/shrubs/compost,n=32.For(B)thenumberof sixspeciesplots.Notmutuallyexclusiveisthepossibilitythat
replicatesareasfollows:noshrub/nocompost,n=128;noshrub/compost,
Q. rubra could have a more suppressive (or less facilitative)
n=92;shrub/nocompost,n=128;shrub/compost,n=92.SeeTableS3
effect on the growth of T. americana than other species in
fortreatmentcoefficientsandpvalues.
the experiment, although the mechanism for this is unclear.
Ourexperimentisnotdesignedtoteaseoutthesemechanisms
in stem volume from year 1 to year 3 all retained compost,
as we lack an experimental control that would help elucidate
withithavingasignificanteffect(p≤0.05)onC.occidentalis,
these possible effects. Regardless, our results suggest that
Caryaspp.,andQ.alba.Composthadamarginallysignificant
planting T. americana in more diverse mixtures improves its
effect (p=0.06) on T. americana’s relative growth in volume
performance.
(Table4).
Thepresenceofshrubsinfluencedtheperformanceofthetree
species,againsuggestingthatthetreatmentsalteredinteractions
amongplantedindividuals.Shrubpresencewasassociatedwith
Discussion
greaterheightoftreespecies,exceptforC.occidentalis.Forthe
Growth performance of juvenile trees is a critical determinant otherfivespecies,ashrub-by-yearinteractionwasretainedinall
oftheviabilityandsustainabilityofurbanafforestationinitia- ofthebest-fitstatisticalmodels(seeTablesS3&S5),reflecting
tives (Ruiz-Jaen & Aide 2006), yet data on planted juvenile the fact that the influence of shrubs was most pronounced by
714 RestorationEcology September2015