CharlesB. Halpern,ShelleyA. Evans,Cara R. Nelson, Don McKenzie,and DeniseA. Liguori, D vls on of Ecosystern
ScienceCo ege oi ForestResoufcesBox352100 Un versityol Washngton,SeatlleWashngton9B1952100
of ForestScence, OregonStateUnivefstyCorva s, Oregon
DavidE. Hibbs and MeloraG. Halal, Departrnenl
973315705
Responseof ForestVegetationtoVaryingLevelsand Patternsof
Retention:An Overviewof a Long-termExperiment
Green-tree
Abstract
Timbcr harvestwiih fetentid oflive ( grccn') trees.snags,and krg\ is now r shndard praclice on fedeml lllan ix" hnds $ ithir
the rangc of lhe northernspottedowl. Although specillc guideI ineshave been adoplcdfor thc leveI s and spatial conllgufations ol
retainedstructures.neithef the ecologic.rl:rssumptionsihat undeflie theserecommendationsno. lhe outcomesof thesepfactices
hrlc bccn rigorously tested. The Demonnralion of Ecosy\ren ManagementOplion\ (DEM() stud,vexrmines the responsesof
in the Paciiic Norrh$cs! to varying levels(pefcentageoi basalarca)alrd patterns(dispefsedvefsusrsgresttlcd)
lbrcsr ccos_r'srems
ol green-treeferenrion. ln rhis papcr $edescribe vegetalionstudiesthat lorm the ibundatioo ofthis l(xlg-term. iDlerdi\ciplinar)
cxperiment. \\re fe!ielr the rcsulls ofretrospectile anallses and simulation models lr'hich suggeslthat even mirinal lc\cl! of
rclc tion m ay have inpo|lant ellccB on ibrest de!e]opment. \!'e descibc thc characlerisiics of ihe DEIUO sites.lhe erpcrimcnlal
dcsign. ard the pfincifal lrlrirbles of inlcrcs! (stand structufe.tree regeneralionand growth. and undeNtor) conposilion rnd
di!ersir,"-).\!c speculaterbout the silviculrural and ecologicalfesponsesof lores6 to \aryjng levels and paftemsof retentionand
tbcus ir panicula. on the d! namicsof the ibrcsl undcrnory. \\'e anticiprte strolg c|llrtras|sanrong lreannentsin $e e\tabl i\hnent
of e|rly sclal. opcn site sfecies. aDd in the pcrsislcnceof shxde-toleranlplants associalcd$it| older forests or fore\t-interiof
eD!ironmL'nts.Short-rermfesponsesarc likcl) to be driven by \'.rfiation iD the distribulion and intensit! of harvestdistufbance.
Longcr lcfln rends afe erpecled lo rcllcct the elltcrs of contfastingpaltem\ of canop,vretention. Wt conclude b) discussins
\omc ofdre scientific challengesiaccd in designing rnd inplenertins largc scalc.interdisciplinar),experiments.
lntroduction
Traditionally. managementof fcderal forests in
westernOregon and Washingtonhas fbcusedon
thc production. utilization. and regenemtionof
timberresourccs.Until fairlv recently(late 1980s),
clearcutlogging andbroadcastbuming werc standard practices. Clearcut units were dispersedin
spaceand timc, loggingslashwas burnedto reduce fire hazard and/or competilion from noncrop species,andtrccsrvereplantedwith unifirrm
s p r c i n ga t h i g hd e n . i t i e . .l t h r r .b c c o r n ien e r c r . ingly apparent,ho$'ever,that theseharvestpracticesdo not producethe samecomplexity of pattern as arisesthrou-qhnatural disturbance-a
complexitythat is thoughtto underliethc recoY
ery of forestecosystemstirllowing large scale
(e.9..Franklinet al. 1995.Perryand
disturbanca
Amaranthus1997). For examplc,historicwildlires in the wcstemCascadeRange\\'erecpisodic,
varying in size,tiequency.and iDtensity$'ith elevation,topography,and local climate (Tecnsma
1987.MorrisonandSrvansonl990,Agee 1993.).
Even where naturaltires wcre catastrophic.individual treesor -troupsof treesoften survivedand
largeamountsof $,oodydebrisremainedaslogs
and snags. It has also becomeapparcntthat the
structural and conpositional attribules thirt de
prilctice\
r eloplhrnuph.,'nr cnri,'nalnrJnapcrncnt
difl'er markcdly liom those shaped b)' natural
(Spieset al. 1988.Hansen
processes
successional
et al. 1991.SpiesandFranklin1991.Halpemand
Spies1995,Tappeineret al. 1997).
policicsandprlcFederalforestmanagement
ticeshave changedsignilicantlyin the last de
cadein responseto glowing concemsthat tradihaveled to widespreadlossand
tional approaches
fragnentation of old-growth ecosystcnlsrnd 1o
attendantdeclinesin biologicaldiversit) (FEMAI
1993.Thomaset al. l993,Tuchmanncral.1996).
A new framewortriis emerging,characterizedby
a shift in ernphasisf'r1)mnranagementof a single
commodityresourccto a broaderconsideration
of ecosystem
conpoDents
andtunctions.L(x)scly
'ecosystem
nranagemcnl.'thislpproach
detlnedas
seeksto incorporateecologicalknowledgeinto
forest nanagementto sustainbiologicaldiver
sity.as well as tinrberproduction(Kessleret al.
Christensen
et al. 1996).
l992. Grurnbine199:1,
Vol. 73, SpecialIssue.1999
NorthwestScience,
i
l 9 9 L rb l n r c \ o n h \ e n
Scre
,\li .i!hn,.\.r1.d
21
Consequently.f()restmanagershave bcgun to
implement silvicultural prescriptionsthat are intendedto more closelymimic the plocesses
and
outcoltlcs of natur_aldisturbanceand successt0n
by retainingstlucturalcharacteristics
ofthe fomcr
tbrest-in particul![,largeli\€ trecs("greentrees"),
snags,andlogs (Franklin l989, Hopwood1991,
Franklinet al. 1997).
Green-ffeeol stuctural retentionis now a standard componentof harvestpfescriptionsii)r fedcral "matrix 'landswithin thc rangeofthe nofthern spotted owl (S/fl-r ot:cidetrtulistaurina).
Although specilic guidelineshave been adopted
for thc levelsandspatialconfigurationsofretained
structures(USDA rnd USDI 1994).neitherthe
ccoJogicalassumptions
that underlietheserecommendationsnor the outcones of thcse practices have been tested. Thc Demonstrationof
Ecosystem
Management
Options(DEMO) study
is amongthetirstin thePacificNorthwestto systenaticallycxaminetheresponses
ofdiversegroups
of tbrest organismsand processesto erperimental variationin the amountandpatternoflive trees
retainedthroughharvest(Aubry et al. 1998). In
this papcr \\ e provide an overview of vegetation
studiesthat folm a loundationof this long-term.
interdisciplinarvresearch.Our studieshave two
tundanental objcctives: ( 1) to elucidatethc ef'fccts of the level and pattern ol retentionon key
elementsoffi)rest structureandcomposition,and
to suggest
possiblemcchanisms
lbr theseelfects;
and (2) to provide intbrmation on changesin fbreststructureandconpositi()Irnecessary
to interpret the responsesofthe organismsand processes
under investigationin companionstudies(sce
Aubry et al. 1999and other papcrsin this vol
ume). A goalofthis paper indeeda majorgoal
ofthis entirevolume is to encourage
additional
researchparticipationin the DEMO study. There
are numcrousoppot'tul'lities1otake advantageof
the cun-entexperimcntaldesignand the comprehensi\csetofbaselinevegelationdata.
We begin rvith a review ofthe litemture on thc
responses
of forestcommunitiesto retentionof
p r e e nr r c e \ :l h c r c s u l t r r l ' n r eiro u . e \ p e r i t n c n t : .
retospectivc analyses,ald modcling effbns have
becnusefulin designingthc currentwork andhavc
stimulatedsomeofthe hypotheses
u,epose.Second. wc describethe ran-tcof envirolments. forest tvpes.and disturbancehistoriesrepresented
by the DEMO sitcs. Third, we proviclca brief
overvic\\"
ofthe experimental
design.discussthc
28
H r L l p e rent r l .
primary variablesofinterest,andoutlineourlongterm sampllngapproaches.Fourth. we presenta
seriesof hypothesesthat focus on the responses
of forestunderstorycomnunities to varying levels and patternsof retention.We discussthe eco
logical basesof our hypothesesand describcthe
analyticalapproachesusedto testthem. We concludeby discussingsomeof the scientiticchallengestacedin designingand implementingla-rgescale,interdisciplinarystudiesthat must balance
ecologicaJinterests,expedmentalrigor, and relevancefor forest managemeot.
VegetationResponseto Green-tree
Betention:A Reviewof the Literature
The conceptualbasisfor green-trceretentionlies
in the strongfunctional links amongtbrest structure.biologicaldiversity.andecologicalprocess
observedin natural forcst ecosystems(Franklin
et al. l981, 1987;Harmonet al. 1986;Maseret
al. 1988:Schowalter1989;Ruggieroet al. 1991;
Spiesand Franklin1991;North lgg3). Thereis
strong circumstantialevidence that the maintenance and/or recovery of forcst organismsand
processesfitlowing disturbanceare enhancedby
thepcrsistence
oflennant fi)reststructures
(c.g..
Perryet al. 1989.Franklinet al. 1995,Perryand
Amaranthus1997).Franklinet al. (1997)review
thehistoryofdevelopment.
diversityofgoalsand
approaches,and resulting expeclationsof silvi
cultural systemsthat enploy structuralrctention.
Emphasizingthe maintenance
of biologicaldivelsity.they attributean array ofecological funclions to retainedsffuctures.Forexample.residual
trees may ameliorateextremesin microclimate
(Chcnet al. 1993.1995),provideenergysubsidiestbr heterotrophic
organisms(Perry1994).sene
as habitat fbl arborealspecies(Berg et al. 1994.
CareyandJohnsonl995. PcckandMcCune 1997).
serveas sourcesfor recolonization
of organisms
associatedwith odgillal stilndstructures(Busing
et al. l995),or enhanceconnecrivity
by tacilitaring movementof organismswithin the matrix of
managedforest(Franklinet al. 1997).
Retentionof green trees should contribute in
similar fashion to the persistenceancl recovery
of tbrest underslory communities. yct. to date,
tew studieshave explicitly addrcssedthis question. We trre aware of only rt single. shofi-term
studythathasevaluated
the consequcnces
ofdis
p e r \ e dr c l ( n l i , ' nl b r l b r c ' t u n J e r s t , ' rc1o m t T t u n i tics. Nulh et al. (1996)fbund that, l6 rno alier
harvest,the speciescomposition of a dispersed
retentionunit (27 trees/ha)rcscmbledthat of an
adjacelt c]earcutto a greaterdegreethan that of
an adjacent,intact forest (reflectingabundantestablishment
of earlysuccessionai
species).How
ever.shade-tolerantforest specieswcre more diverseand abundantin the dispersedunit than in
theclcarcut,suggestingthatfbrestunderstorytaxa
may benefit from the increasedshading and/or
reduceddisturbanceaftbrded by retained trees.
Untbrtunately,these short term, unrcplicated
observationsaffbrd only a limited view of the
possibleellects of green tree retention.
ment measurements
in the DEMO studyu'ill pro
vide a basisfor separatingthe potentialeffectsof
retainedtreestiom initial speciescompositionand
other sitelevel vadation.
Considerablymore atlentionhasbeendevoted
to thc effectsof rcsidual treeson forsst standdevelopmentandtreegrowth. Forexample,through
analysisof annualgrowth rings North et al. (1996)
observcdthat 1br 6 yr alter dispersedretention
harvest,the basalareaincrementofretainedDou
glas-fn (.Pseudot.sugq
melr..iesii')was 157r lower
than that in an adjacent.uncut forest. This is not
thercsponsetypicallyobsen'edin youngerthinned
stands(reviewsin BanettIt980], Bailey [l9961.
North et al. u9961); it may ret'lect a 'thinning
shock" in which resourcesare differentially allocated to roots (Oliver and Larson 1990). Long
term [leasurementsof tree growth among replicate DEMO blocks may suggestwhether these
arelocal orgeneralphenomena,andwhethcrthey
representshort-termor more persistentefTects.
Studiesofunderstoryresponses
to thinningin
voungertbrestsmay otler additional insights. It
hasbeenfound that thinninggenerallyleadsto
greaterplant speciesdiversitl' (in large pan due
to the establishmentofinvasive or exotic species)
andto increasedplantcover(Witler l975.Alaback
and Herman 1988. Bailey 1996).althoughthe
natureof theseresponsesmay vary with the vegetation,environment.or intensityofthinning. For
example,rvherethinningleadsto increaseddomi
nanceby clona)shrubs(e.g..Tappeiner
et al. 1991.
Huffman et al. 1994). thcrc nray be a resultant
dcclinein speciesdiversity(AlabackandHennan
1988).Therehasbeenlittlc consideration
of the
eff'ectsofthinning on plant speciesthat arc ass(F
ciated with late-seral environmentso[ that are
(but seeBaiJey
sensitiveto loggingdisLurbance
tree6l).
lnsightsinto the potentiallong-tefmconse
quencesof green-treeretentioDfor understory
connunitics may be aidedby retrospectiveanaly
sesofnarural.two tiered(two-agcd)stands.These
young-to-maturetorests support an emergcnt
canopyofolder, scatteredresidualtreesandhave
been describedas the natural analoguesof fbreslsthalmay dcvelopfi)llowingretentionharvest
(e.g.,RoseandMuir l996, PeckandMcCune 1997,
Zenneret al. 1998). Traut(199.1)describeddif
terencesin understoryspeciescompositionin plots
\\ ith and without natwal, residual trees arrd attribuled thesc differences.in part, to the effects
o l r e r i d u atlr e e \o n l h ed e n . i t )a n de o n r p o s i t i , ' n
ofthe rcgcnerating
treelayer Unfbfunately.causal
mechanisms
cannotbe deducedthroughretrospcctive analysis,nor is it possibleto distinguishdirect effects(e.g., shadingby residualtrecs)from
indirectones (e.g.,speciesinteractionsor local
variationin disturbance
history.initial conposi
tion. or other site features).Replicated,prc{reat-
Retrospectivestudies of natural, two tiered
standsalso provide insights into the long-terrn
efftcts of dispersedretentionon forest development and tree growth. Regionalcomparisonsof
70- to 1lo-yr-old standswith and without older
(>200yr) residualtreessuggestthataboveathrcshold density (ca. l5 trees/ha),residualsctrn aft'ect
the composition of younger cohorts and reduce
their basal area growth (Rose and Muir 1996).
Negativeeflects on basalarea growth have also
b e e nr e p o r t e d
l r o m p r i r ( d - p l o r. l u d i e . i n u e \ r ern Oregon. Here, however,the strongesteffect
per unit basal area of residualtrces occurredat
thelowestlevelsofresidualtreedensity(5 trees/
ha:Zenneret al. 1998)orbasalarea(5-10m']/ha;
Acker et al. l99li).
S t u n dt e r p o n . e .t o B r c c r - l r e er e r e n L i om
na v
alsoincludeincreasedmortality dueto windthrow
of isolatedresidualtrees. Removalof 30 607cof
the initial volume of rnatureand old growth forests in nofihwestem British Colunrbia resulted
in ratesof windthrowof0.3 to 8.27cof trceswithin
2 yr after harvest(Coatesl997). However,nortality can vary narkedly from site te site.rcflect
ing variationin the lcvel ofretention, initial stand
densityandcomposition.standage,local weather
conditions,and landfbrrn. For example,among
4.1green-treeretention sites in westernOregon,
ratesof windthrow-relatedmortalityrangedftom
0 to nearly 607o(meanof 16%).with the greatest
Iossesobservedsoon atier harvest(Adler u99z1l
VegetationResponseto Green-trccRetention
29
andHunterI 1995Icitedin Franklinet al. | 1997l).
to representa diversity of foresttypes in westem
By comparison,annual ratgs of mortalit)' in
Oregon(UmpquaN. F) andWashington(Gifford
unmanagedmature and old growth folests of
PinchotN. F. andWashingtonDNR) (seeAubry
rvestemOregonand Washingtonaverage<17c
et al. 1999).Blocksencompass
markedlydiffer(Franklinet al. 1987). The randomizedblock
ent physicalenvironnents,standages,and forest
J c . r p n, ' i t h . D t \ 4 O . r u d ) u i l l p e r n r i sr e p u r . r structures(Table l). Siteswere chosen1()minitioD of treatnenl effects (level or pattcrn of rcmize the variationin environmentand vegetation
tentioD)on mofiality from effectsassociatedwitl'l
amongtreatmentunitswithin eachblock,within
sitc-spccifi
c t'rctors(e.9..bpography.soils).
the constraintsimposedby currentlandscapeconfigurationsandpastmanagement
activities(Aubry
Forest gap and stand-levclgrowth and yicld
ct
al.
1999).
To
varying
degrees
this goal was
modelshavealso beenusedto explorethe posachieved(butseea discussion
of this issuein the
sible effectsof varying levelsofdispersed retenChallengessection.belor').
tion on fbrest standdevelopmentand wood pro
duction.Someofthe predictions
ofthesemodels
Acrossblocks,elevations
rangeti-omca.200
are intuitive (indeed some are predetemlinedby
1700m. slopesvrry trom gentleto steep,andnearly
growth equations,McKenzie 199,1).For example.
(Tablel). Most sites
rll aspectsarerepresented
the degreeto \\,hich models simulatc thc dcvcllie $'ithin the westeln hemlock (fsir.qri
(e.g.,
opmentof old-grorvthstandcharacteristics
hetetopltllla) forest zone (Franklin and Dyrness
tree size classstructure)improveswith the nurr1973).Two blocks,LittleWhite Salnon andDog
ber of treesretainedthrough harvest(Hansenet
Prairie are locatedin the more eastrly grand t'ir
l l . l Q Q 5 r n. J I c l r t l \ .1 , j\ l e r r . ' ullr r g g i n g . r . u n r u - (Abies grandis) and whitc t\r (Alties
concoLor)
lative basalalea and volune growth are reduced
zones.respcctively.
and a third, ParadiseHills,
when live trees are retuined(Birch and Johlson
occupiesthe colder, $etter, silver fir (ADies
1992.Hansenet al. 1995). Othersimulationreornabi lis) ane. Pse uclot
sugamenie.!il doninates
sults,althoughless intuitive.are supportedby
all sitesbut associated
treespeciesvary fiom block
r-effospective
analysesof natural stands. In par
to block (Table1).
licular. rcductionsin standbasalareagrou,thper
Foreststfuctwe variesanong blocks. reflectresidual tree are predictedto be strongestat the
ing diffelencesin standage(c.g..Little Rivervs.
lowestlevelsof retention(5 trees/ha:Hansenet
Layng Creek).regenerationand disturbancehisal. 1995). Still olhsr simulationresultsarequite
tory, and physicaleDvironment(Table 1). Forest
surprisilg and seen to coutradict empirical obunderstory
compositionalsovalies:sone blocks
servations.Rrr cxamplc.underlow levelsofdisgroundlayers(e.g.,Dog Plai
havedepauperatc
persedretention(asfeu'as 5 trees/ha)
and given
rie)
while
others
suppo
denseherb and shrub
sufficienttime (240 yr). naturallyestablishing.
(e.g..
communities
Layng
Creek. Little White
shade-tolerantspeciessuch as westem hemlock
Salmon). Despitethis heterogeneity,
many blocks
l.TsugaheterophtlLa)andwesternredcedar(Tftfn
sharethe samedominant taxa (e.g., vinc maple
plicatal can oulcompetc plantcd Douglas-fir
(Hansenet al. 1995). To what extent the spatial
lAcer circinatuml, Oregongrape lBerberi.t
aervosaf.
salal lGaultheria.r/rdfurnl.
and huck
configLuation
ofresidual|rccsmay influcnccstand
leberty fVat:cittiurn spp.I ; Table 1.1.Ditlerences
development
hasnot beenexaminedempirically
in stand sffuctureand compositionamong treator through simulation. despite the fact that agment units within each block are generallysmall
gregatedretentionis now a staldard practice on
(USDA
compared
to difterencesamongblocks (Table 1;
fcdcralmatrixlandsin theregion
andUSDT
but seeChallengessectionand Table ,l).
199.1).Conparisonsof aggregatedand dispcrscd
retentionin the DEMO designexplicitlytestfor
511.5h,s hadvaryinghistoies of rccentdissuch effects.
(Aubryet al. 1999).Forexample,
turbance
Layng
CreekandCapitolForestaresecond-rotation
stands
Methods
(i.e.,clearcutbut regenenlednaturally)rnd all
tbur blocksonthe UmpquaN. F.havebcenthinned
StudyAreas
or salvagelogged. In contrast,thc three blocks
To establisha broad geographicand ccolo-qical
on the Gifibrd PinchotN. F. supporl mature,undisturbedforest.
baseofinterence.eight studyblocksrvereselected
-10
Hrlpern et rl.
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A
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VegetationResponseto Green-trceRetention
3l
Experimental
andSampng Designs
Aubry et al. (1999)describethe DEMO experi
mentaldesignand harvcstprcscriptionsrn an rntroductory paper in this volume. To rcview. at
six
each of the eight study blocks (replicates).
harvesttreatmentshave beenrandomly assigned
to l3-ha experimental(treatment)units. Trcatmentsdift-erin the level(percentageofbasal area)
and/or spatialpattern (dispersedvs. aggregated)
ofretainedtreesas follows: (l) 100%rctention
(control): (2) 75%,retention (haryestedas three
"gap" trcatcircular. 1-ha gaps-hencetbfth the
ment);(3) 407. dispersed
retention:(4) 40% aggregatedretention (as five. cilcular l-ha retention patches);(5) 15'ladispersedretention;and
(6) 157. aggregatedretention(as two, circular I
ha retentionpatches)(seeFigure 1). By design,
thesetreatmentsproducedistinctly differentpatternsof vadation in standstructureand environment: relativclyhomogeneous
conditionsin the
control and dispersedretention treatments,and
t\\,ocontrasting
conditions(intacttbrestandharvestedarea)in thc aggregatedretentionand "gap"
treatments.We empl0ytvo sanrplingdesignsto
. : r p t u r cl h ( c o n l r J . l \a n dg r l d i e n t .i n r c c e t r t i o n
a. Disoersed
retention
treatment
rcsponsethatffe expectedto arisefiom thesetreat
nlenls.
Effects
SamplingDesignfor Treatment
The first sampling design,which perrnitsanalysis of treatmenteft'ects(e.g., Hypothesesl-3,
below). usesa set of permanentvegetationplots
anayed acrossa grid system at each site (63 or
64 gdd points$,ith40-m spacing,FigureI ). The
numberand spatialdistributionsof sampleplots
vary by treatnent, however. ln the dispersedretention and control treatments,where standcon
ditions will be relativelyhomogenous,32plots
are placed systematicallyat altemategrid points
(Figure la). In the aggregatedretentionand gap
treahnents,where we expect two distinct posthar-vestenvironments.plots are placedat all grid
points within the aggregates(or gaps), and at a
subsetofpoints in the surroundingmatrix (yield
ing 32 or 37 plotsper treatment,Figure1b).
Strongcontrastsin harvesttreatmentswill prcd u c em a r k e d i l f e r e n c ei n
: l h ec o r n p o : i r i o \ni ./ e
structure,andspatialdistributionofoverstorytrees.
We arcpafiicularlyinterestedin theelfectsofthesc
trcatmentson volumeandbasalareagrowth: stem
b. Aggregated
retention
treaknent
Figure L Plot locations(open circle\) lbf srmplimg(a) relati\el) homogeneousconditions iD the dispersedfeteDtiontfeatment\
"gap' (757.
and (b) contr.rstingconditi(msin the aggregatedretentiontreatnents. Control units are sanpled as in (a)i
retentior) units as in (b). Spatialgrrdients \i illin aggregatedretentiL'nunits are sanpled using ca. 100n long traDsects
(d.rk iires ir b. seealso figurc l).
32
Halpemet al.
TABI-E 2. Variablcsofi ei: n and derivedstatisticsfbr the two pfnnaD samplingaffroaches (seeNiethodsand Figurc 2 ). X =
variable sampled;blank = variablc no! samplcd.
Treatnent-level Within-treatmenr
gradienrs
eiiecb
Physical Sitc Charact€ristics
Elclation. aspecl.slope
Topogr.rphicposition, slope coDfiguratxnr
Ground lurfacc condidon\ (c.g.. mineral
s o i l .s l o n e .l r t t e r ,l o g )
Coare woody debris
X
X
X
Deri\'ed statistics
X
X
X
rolunre and dcnsity (b,,-speciesand decay
Post-harvestdisturbance
(e.g.. vegetariondamage,soil disturbance)
Ovefitory Vcgetation
Canop) co!er
Tree dia clcrs (>5 cm dbh)
X
X
densit). basalarea.\,olurre. and diame|er
distributions(by species)
Trcc characteristics(e.9., carop) class,vigor.
cro\'"n ralio. damagc)
'liee
mortality: phvsical conditions and
fates and causesof nrortalit! (b)'.spccics)
Tree heighrs: rotal. canop! depth
heighi and canopy depth distributions(by
Snags: diameGr. height. decay class
density (b,vspecics.djamercr.deca). aDd
h e i g h tc 1 a \ s )
Understory Veg€tation
Lichcnsi prcscnce/ilbsence
Bryophytcs: prcscnce/absence
Herbsi co\ er. hcighl
X
X
X
Tall shrubs: cover. height
Undcrslor) fccs
S e e d l i n g(s< 1 0 c m t a l l ) : n o . n c m s
S a p l i n g s( > 1 0 c m t . l . < 5 c n d b h ) :
coler, no. slens, height class,ofigilr
(nalural !s. plunred)
X
X
X
X
speclesliequencv and divcrsil)
speciesfrequenc! and di!cr\ill
speciesfrequenc!. diversiry.and hcighl
dislfibutidr\
speciesliequenc). dilersity. and height
distribulions
fiequency xnd density (b,vspecies)
ircqucncy.densill. size structure,and rates
ofnoriaLrt) (b) speciesand size cl.rs\)
"Conduclcdin the.10% uggregatedretentiur treatmentsat Bufte and PafadiseHills blocks only.
density:diameter,height.andcanopy-depth
dis
tributionsof canopyand sub-canopy
trees:rates
andcausesof morlality; and recruitmentandfates
of snags(Table 2). A seriesof nestedcircular
plots (Figure 2a) will be used to sample these
overstoryrcsponsesat 3 5 yr inteNals. Pefmanently tagged trees will be measuledfor diameter, canopy position, structuml attributes,and
moftality. Tree height and crown depth will be
measurcdtirr a subsetof sterns. Snagswill be
measuredfor diametetheight,decayclass,and
angleof lean.
As with the overstory,the forest undersk)ryis
likely to respondto strongcontrastsin harvest-
rclated disturbance(and resultingcanopy conditions) among treatments.We are paticularly interestcdin treatmente1l'ects
on plant speciesdir'erritl rrichne:sanJerenne\.r:c!,mmunit)compo:ition
and structure(the proponionalrepresentationof
different plant groupsor 'functional' types); and
growth,and nrertalityof regen
the establishment,
eratingtrees(Table2). We will alsoconsiderunderstoryattributesthat areknown to influencethc
abundanceand distributionof u'ildlile (e.g.,plant
cover and vefiical stratification.recruitrnentand
dyramics of coruscwoody debris) (Table 2). A
seriesof belttrmsects,intcrceptiines,andnicroplots
nestedwithin eachtreeplot will be usedto sanple
VegetationResponseto Creen-treeRetention
33
Fi:ure:.
Plol r.d rrrnscct designsfof sampling rfe.Ltmentefiects (a, b) and spatial gradicnl\ (c, d) $llhin aggregatedretention
olefstory chltracterisrics(a) arc mcasurcdwitli. nestedcircular plots: 0.01 ha for trees
unils. For lreannent efi_ects.
5 . 0 1 1 . 9c m d b h ( T , ) . 0 . 0 . 1h a l b r t r e e s> 1 5 . 0 c m d b h ( l ) . a n d 0 . 0 8 h a f o r s n a g s( S ) . U n d e N b r y \ , e g e l . r r i o(rbr ) i s
s a m p l e dr \ i r h : b e l t r f r n s e c t s( B ) l b r d c n s i l yo l \ a p l i n g s ( t r e e s > 1 0 c J nt a l l . < 5 c m d b h ) r i n t e r c e plti n e s( L ) f b r c o v e t
hr'ighl oftall shrubsand r.rplings.dislurbanceasscssmcnts.
and volume ofcoarse woody debri\l and lllicr'oflds ($. 0.1
x 0.5 nr) ibr g.ound su|facech.mctefistics,presenceofbryophycs and macrolichens.coler and heighl olherbs xnd loll
s h r u b s . a n d d c n s i r ) o f r e e s e e d l i n g , t ( < l 0 c m t a l lO
) .v e r s t L i rc)a n o p , " ' c o ! e r icsn i n a r c d $ i t h a s p h c r i c a l d e n s i o m c lac!r
rhe end poinls (c) ol cich inrerceptIine. For spatialgfadientsin uggregaledrercndon|rcarlncnls(c). undcrnory \ cgcta
t i o n i s s r m p l c dw r r h l 3 b i n d s p e r t r a n s e c t .W i t h i n e a c hb r n d ( d ) t t r e : s u b p l o t s( s p . 1 r I n ) i o r c o l c r , 4 1 c j g holl a l l
Dricfopbts (lrp, 0.2 r 0.5 nl) lor prcsencc/abscncc
of brtophlt!'st and
speciesand dcn\i!icr of tree seedlings/saplingsr
cnd poi.rs (c) lor o\'erslor] crnopy cover. SeeTable 2 ibr a compl.lc lin ol \ariablcs.
lhe responses
of undcrstoryspecies.changesin
ground surt'acccharacteristics,and the types and
(seeFigurc
lcvclsof hawestrelateddisturbance
2 captiontbr details). The ncstcdplot designwill
also pcnnil direct conelationof overstoryand
understorychilracteristics.
Sampling Design for Spatial Gradients
Within Aggregated Betention Treatments
A scconddesignwill beusedto samplefine-scale
gradientsin undersk)rvrcsponsewithin the ag
3,1
Halpern et irl.
grcgatedretention treatments(e.9., Hypotheses
4-7.below). This approachis linited to the,10%
at Buttc and ParadiscHills.
retentiontreatments
Hereour tbcusis on the extentto which the I -ha
rgtainconrponents
of the originalunaggregates
derstoryand/oramelioratcmicroclimaticconditions in thc surroundingmatrix. Bandsof salrple
plots arc arrayedalong four o hogonaltransects
ecotonal
ofca. 100rn in lcngththatspanharvested.
(edge).andintacttbrestenvir()nments
(Figurc2c).
T h <p e r y e n d i c u lpulra L e t n ern' lt- l r r n . et ,s l e r n r i t '
examination of the effects of aspecton spatial
coverfbllowing large-scaledisturbance(Halpern
and Spies1995). The secondgroupconsistsof
"early-sera1"or "open-site" species. These include annual,biennial, and perennialherbs (native and exotic) that typically dominate recently
communities(Il
disturbedandetuly successional
14 speciesof Halpem [1989];seealso Dyrness
andMcKee [l988], Halpem
[9731, Schoonmaker
and Franklin [1990]). Thesetaxa are character
izedby long-distanceseeddispersal,rapidgrowth
rates,and high fecundities. The third group of
interest,"forest dominants,"are woody and herbaceousspeciesthat dominate mature and oldgrowth forestunderstodes.Thesetaxa are tolerant of disturbance(R3 speciesof Halpern| 19891;
e.g.,Acer circinatum,Berberisnervosa,Gaulth
eria shallon, andPolystichummunitum) awl can
occupy a broad range of canopy conditions and
seralstages(Spies1991).
gradientsin vegetation. Bandsare spacedmore
closely (every 5 m) near the edges(ca. fiIst 15wherewe anticipate
25 m) ofthe 1-haaggregates
sharyergadients in vegetationresponse,and morc
widely (every l0 m) in the harvestedmatrix and
forest interior (Figure 2c). A seriesof sub-plots
andmicroplotswithin eachband(Figure2d) will
be usedto samplechangesin understorycomposition and structure (Table2; seeFigure 2 caption for details).
Mechanisms,
and
Hypotheses,
AnalyticalApproaches
It is not possiblein this paperto describethe full
rangeof expectedecological or silvicultural responsesto varying levels or patternsof greentreeretention. However.to stimulatefuture discussionandto guideongoinginquiry andanalysis,
we have summarizedour expectationsin a table
of predicted,qualitativeresponscsto the six ex
perimentaltreatments(Table 3). Thesepredictions draw upon the resultsof ernpirical studies
of naturaldisturbanceand succcssion.plant en
plant Iite historystratevironmentrelationships,
gies, and vegetationrcsponsesto more convenloggillg.
tionalmethodsofsilviculture(i.e..clea-rcut
commercialthinning). Othersare more speculative and rcflect generaltheory that is untestedin
theseforestecosystems.
Hypothes zed Responses Among
Treatments
In the lbllowing sections,\\,eexplorea setof
predictionsin depthas examplesof the typesof
questionsthat we haveposedin this study. We
which
focuson theresponse
ofthe forestunderstory
containsthe virstmajority olplant spcciesin these
tbrests. and in which we anticipate short terrn
changesto be nost dynamic. Our hypotheses
considerthree groups of taxa for which we anticipatedistinctpattemsof response.The lirst
"fbrest-interior" specics: subordinateforest
are
hcrbsthat arenot restdctedto. but arefbund nrost
liequently and with greatestabundancein, for
environments
est-interiorand late-successional
(e.g.,coral rcotlCordllorhizqspp.l,princes pine
lChimap hikt utnb el I q tal. rattlcsnake-plantain
lGootlyeraobkrtgifolial, twayblade[lstera spp.].
pyro\afPtrola pictal; Spics 1991.Halpernand
S p i e sl o 0 5 r . l \ 4 r n )o l t h c . eh e r h \i r r e\ e n . i l i r e
to disturbances
associated
with loggingandbroadcast burning (R5 speciesof Halpern u9891; see
alsoHalpernet al. [1992])andarethoughtto rcq u i r el , ' n ep c r r L ' dr.r ft i m e f o r p o p u l a t i o nt .o r e
An underlyingpremiseof green-treeretentionis
that the suNival of organismsassociatedwith
tbrest-interierenvironmentswill increasewith the
propofiion of live treesretainedthrough harvest
(Franklinet al. 1997). Conversely,
invtrsionol
or open-sitespeciesshould
early-successional
declinervith increasedlevelsofretention. To date.
havenot beentestedempiritheseassumptions
cally. nor have $e fomrs of theserelationships
(e.g.,aslinearor non-linear
funcbeenafiiculated
l i o n s .o r a s t h r e . h o l dr e . p o n s e . )T. h e r ' <i s e r e n
greaterunceftainty regardingpossiblcdiffcrcnccs
in responscwithin aggregated
and dispersed
re
tention (Franklin et al. 1997). We prcdict that
l h e - e! r u u p .u i 1 1r e . p r, n , lI ' l i ' l l ,' u r :
Hyoothesis 1.-Tlre qbtotdenceand dirersit)
of forcst interior specieswill hcrease v,ith the
percentageoJ green trees retained (Figure 3a).
This prediction rests on two likely outcomesof
levelsof retention:(l) reducedlevels
increasing
ofdisturbanceto soils and unde$tory vegetation.
and (2) greateramountsofshading liom retained
sludies
trees.The resultsof long{cm, succcssional
following clearcut logging and burning suggest
that both conditions should pronote survival of
fbrest-interior
species(Halpern1989.Halpernet
a1.1992.HalpemandSpics1995).In thcscstudics.
speciesextinctionsinducedby harvestwere cor
relatedwith the intensitv of soil disturbance. In
VesetationResponseto Green-ffeeRetention
35
2i
=
e 1
v.a
1
E i i
:
Y. f"
:
P-H
7
z
? t z
i
l
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P E
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z t a !
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c
:
=
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3
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;
.
:
}
E
g
-t
:
i s
i..
i
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a
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"
+.E 2 '-
l
i a =
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'u
/.t^
/:-i i tZ
<
s
=
.
=-' - ; = 1=; -
9,1
i
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l
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!
a
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t =""
. ;.;.i
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7 9
,
=-2E - r :
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i 2 +- 4 a E=
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9 ;
a-e
a t ;
i l -
t
;= :
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a i = '-
;
= r :
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r i E 4 ,
2
-.
-
-
v :
- ! ' - . = -
1 t
i
-a . : aa i L : ; ^
7 ; -- i= E
y-!
,
! = _ _
:
t = ' - ' 2 - :
- | - . v u a
d
E
1 . - 11 " ' r =
i 1 ! 4 F E r
.= ?/ - ' .a t : f l 9. .t a .
:
z
2
= e " ; , 1i E
2;=2
.Li L" . -- Ei =
' i > : =i z: . t i, !>y !n 1, . 2a. =:- .Pei =; '
-t t-
Y 2 Y ' , ! - - 2 - = : i - : : l s : !
E
? = = : = = = = = : = - y . ! . = y
f
; ii
i
c ! , :
= i i=
a e E
.a
; ' ,
j t . ; i . * 2 " . ! = = i
. 22
i,.
F 2 ; i e E2 =
t ; i =-4 i ; i t, = - d 7 r ==j ,' at! ,at , : ,
4 : a ) E: ; - i : : = !r :2 ; =; =t =. =- zL : i z= :
F : 2
a z a
36
E
Z
?
=
^
.i i :E == €i + . :
? = Z 2 r Y - = = Z = ? Zi
i t s t t i - t E 5 P - r & - i Id
Halpem et al.
'
=
1 7 4 = ! i
a i ' z - Zr = 4I
species
a. Forest-intenor
I
o
o
c)
o
(d
.9.
c
c
o
c)
c)
so
(5
o)
(r
E
100
15
75
40
PercentRetention
100
15
40
75
PercentRetention
Figure 3. Prcdictedshort-tenn.qualitativeresponscs(meanand lafiance) to rarying levelsandpattcrnsofgreen-treerelenlion fi)l
tlvo groups ofunderslory iaxa: (a) lbren inlcrior speciesand (b) early-seralor op!n 'ile .pe.:re(.
addition,even where forest soils remainedundisturbed,removal of the overstorycanopy was
sulficient to cause mortality of somc sensitive
species.
Hypothesis2.-Ilre abundancednd diversitt
of earlr- seru| open-siteqtecies u,ill decreasewith
thepercentdgeof greentees ret.lined(Figure3b).
This hypothesishas a strongbasisin life history
theory (Grime 1977.Grace 1990). As light, potential establishmentsites,and soil resourcesbecome more limiting with greaterlevels of retention. early-seralspeciesare at a competitlve
disadvantage.In studiesof post haryestsuccession,thesetaxatypically showpositivcresponses
to disturbancaintensity and negativeresponses
to canopyclosure(Dyrness1973;Halpem 1988.
and McKee 1988;Halpern
1989;Schoonmaker
andSpies1995).
Hypothesis 3. For both forest-interior dnd
earl-t-seral,open-sitespecies,the mean and the
yaiation in abtndancettruldiversitt will begreater
in dggregdtedrete tion treatmentstlnn in dispersed retentio tre1tmefits(Figure 3). \Ne as
sumethat in dispersedretentionunits. uniformly
high levelsoflogging disturbanceand associated
microclimatic stresswill lead to signiticant loss
of forest-interior speciesand to widespreades
tablishmentof early-seralspecies.In conlrast.in
aggregatedretentionunits we anticipatespatially
distinct pattens of survivaland invasion: in har
vestedportions of theseunits, survival of forest
interior specieswill be low and establishmentof
early-seralspecieswill be high; conversely,in the
Ietentionpatches,survival of forestinterior specieswill be high, andestablishmcntof open sites
specieswill be low.
Analytical Approachesfor HvoothesesI -3.lo te.t lor the el'fect.ol trealments,'n species
abundanceand diversity,we will use analysisof
variance(ANOVA) (or repeatedmeasuresANOVA
tbr long-termeffects)coupledwith plannedcomparisonsof means(SokalandRohlf I981). Due
VesetationResDonseto Green-treeRetention
TABLE :1. Varirtion in sland struclureand speciesabundancepfror to harvestamong lhe six lrcatmen|sunits at Pafrdise Hills.
Washington. Valuesare | 996 r.carmcnl ncans. Tokls repre\ent the summed cover of al I specieswilhin a stratuDr
r(nrh selectedsDeciesafe \ho$,n).
Treatmentl-rnit
Disp.
Olerstory Structure
'Iiee
(no.,4ra)
denslt,v
Basalarea(Drr/hal
r005
10.9
512
16.1
Percent Coyer of Understor) Strata and Sclccted Speies
Herb srraturnltoral)
12.7
16.8
0.3
Cotnlts tutltdLn\t\
Cauhhetia shdllon
Xenph\llLtL tenar
Tall shrub slratum (olal)
Uutl innor n(nbruntk?un
\/. onliPliunt/r: uhskutnse
Undcrstory trcc \lralum (told)
7.\ugah(k'rophrlla
0.9
2.',l
1.',l
6.5
0.1
:1.2
2.5
t.5
t.]
o.ri
t.9
2.2
9.3
10.7
23..1
1.7
5.1
l.-l
4.2
1.9
to the patchynatureof plant speciesdistributions,
we havefound high levelsof variation in ftequency.
cover,anddiversityamongpre-harvest
treatment
units within sone blocks (e.g..Table4). Thus,
to irnprove our ability to detect ffeatmentlevel
eff'ectswe will analvze1brc/ratgesin speciestiequency,cover,or diversity relativeto initial (preharvest)conditions. Analysis of covarrance
(ANCOVA) may be usedto testfor effectsofother
potcntiallyimpoftantvariables(e.g..levelof disturbance,coyer ol coarse woody debris). Spccies abundancewill be measuredasfiequencyof
occurrence
rt varyingspatialscales(micropiots,
transects.and plols) andasmeanplotJcvcl cover.
Speciesdiversitywill be expressedas richness
and heterogeneity(Hill 1973)at the salre spatial
scales.To examinethe effectsof treatments
on
the r'.r,'i.rlrilil ofresponses(Hrpolhesls3). simiIar analyses*ill be conductedusing the coefficientsof variation of theseparameters. Significance levels will be adjusted for multiple
comparisonsusing the sequentialBonferroni
method as describedby Rice ( 1989)
Additional correlation and regressionanaly
seswill be conductedat smallerspatialscales(e.9.,
transectsor plots) to explorc finer-scalerelationships with tbrest canopy cover. shrub layer de
velopment.groundsurtaceconditions,and har38
Halpcrnct al.
AgC.
Disp.
81',7
67.1
612
58.7
6lJ,l
86.7
19..1
t.5
33.5
0.2
6.1
1.4
1.2
0.6
tJ.l
2.1
1.2
10.2
1.0
1.1
1.8
8.8
2.1
6.0
u.5
28.6
10.7
11.7
t.9
lt.6
10..1
2.2
o.2
2.3
t.2
0.9
0.2
132
79.1
0.5
\.1
5.8
1.0
0.3
3.5
3.1
0.1
vest-relateddisturbance.Theserclationshipsmay
vary with, or independentlyof. changesin the
level and pattem of retention.
Hypothes
zedGradients
- Within
Agg'egaled
Relello. eaLme'lrs
Aggregates(retention patches)are predicted to
serveas refugia fbr plant speciesthat are sensitive k) disturbanceor to open-canopyconditions,
andassourcesofpropagulesfor sunoundingharvestedareas(e.g.,Busing et al. 1995,Halpern
andSpies1995,Franklinet al. 1997).Giventhe
slow ratesof dispersalof sotne understorJspecies(JuJes1997),decades
ofobservationnraybe
requiredto fullyjudge the potentialof aggregales
to serveas sourcestbr recolonizationof late seral species. However,the short-tenn effectivenessof aggregatesas local refugia can be tested
by monitoring species'persistencefollowing harvest. Thc tbllowinghypotheses
addressthe ex
tent to which the tloristic composition of forest
aggregates
is maintainedthroughharvest:
H v p , t t h e . i .r t . - l V l t h i t u q q e g u t t , , : 1 ' r ' i,, t
associatedw,ithlbrest-i terior or lute seral enyiromnantsvill decreusein abntdance and diversil with proxinih to the aggregctteedge(FigLtre
4r. This prediction assumesthat in ftagmented
Q'
o
(U
Forestdominants
o
_g
o
E
Early-seral,
open-site
species
Forest-interior
species
Center
Harvest
Area
ForestAggregate
Figufe'1. H)pothesized sho 'terln, qualitati!e responsesof three groups of undcrstor) uxa rs a iunction |)1 disunce lioln the
(radius of ca. 56 m). ResponsesareexpecGdto diller berwecnnonh (N ) and sourh (S)
edgeof r I +r ibfe\t rLggregate
tircing edges.
habitats,the distibutions of fbrest-interior species arc shapcd by edge efl'ects.manifestedas
gladientsin microclimate,herbivory.or the abundanceofcompetingspecies.Evcniftbrest sfucture
is retained.forest fragments experiencegreater
variabilityin air andsoil temperuture,
higherlevels
of light.greaterwind speed.andlowcr hunidity
than do larger intact stands(Chen et al. 1992.
Matlack 1993). [n low-elevationforestsof the
Pacific Northwest.alterednicroclimates can extend morethan 200 rn inwardflom thc forcstedge
(Chcn et al. 1995). (By comparison,the I ha
aggregates
in the currentexperimentaldesignhave
radii of ca. 56 m). To what cxtent understory
p l a n t se r h i b i t p r r c l i e l! r ' r d i e n r .i n r e . p o n . ei s
unclear(Walcs
1972;Matlack1993,1994a,1994bt
Fraver 1994). Empiricalsuppertlbr the etlects
of edgeson lbrest understoriesis scarcein the
PacificNorthrvest. In the Klamath Mountainsof
northern Calitbrnia, Frost ( 1992) fbund that the
abundancet-'fsomc fbrcst hcrbs dcclincd fbr dis-
tancesofupto 60 m fiom thetbrest-clearcut
beundary. However,some of the more sensitiveherbs
obsen'edin thesenixed coniferousforests those
with affinitiesf or cooler.moisterenvironments
lie neartheir southemdistributional
limits. Our
gradientanalyseswill indicatewhethersimilar
pfllcm: areioundin thcmorcrnc:i(en\ironmcnl:
of westernOregon and Washington.
ln addition to microclimatic effects,forest edges
may alsobc associatcd
with greaterratesef herbivory (Alvenon et al. 1988)or regeneration/releaseofwoody plantsthat canoutcompetesmaller
herbaceousspecies(Flost 1992)(seeHypothesis
6 ) . A I l h o u B h\ o m c I i \ r c : l - r n l c r i , .' rp e e i e .r n r l
maintaintheir currentdistributions,$e expect
shot-tenn declines in abundalce and divelsity
towardthe forestedge. As edgecontrasKdcclinc
with time (as regeneratingstandsdevelop). we
expcct the gradualrccovcrl of some of these
species.
VegetationResponseto Grccn-trccRetcntion
39
HJpothesis5. Eurlr--seral,open-site.\pecies
will declhe in abundance(ud dfiersih with dis
latrceinto theforestaggregqte;gradientsm abLlndance and diversin )rill be steeperand shorter
than those of lbrestinterior species(Figure 1).
It has been observedin other forest ecosystems
thatchangesin micrcclimateandlocaldisturbance
regime along the lbrest edge may promote the
invasion of exotic or ruderal taxa (Ranneyet al.
1981.Laurance1991,BrothersandSpingam1992,
Frost 1992). The distancesto which early-seral
speciesinvademay be more closely linked to the
depth of edge-relateddisturbancethan to gradients in microclimate, thus we expect relatively
short, steepgradientsin response.As the effects
ofdisturbancediminishwith time,we expectthese
gradientsin seralspecjesdistributionsto become
lessapparent.
Hyoothesis6. ThedominantJbre.tt
understory
specieswill shovtltositive responseslo the cre
ation of edges,pettking in abtntlance within a
short distanceof the forest qg,grcgate-haryested
nntrix bounclar,v( Figirre 4). Many of our domispp.,Gaultnantunderstorytaxa(e.g.,Vaccb'rium
heria shalLon,andAce,..ift ir..rtrrr) possessmorp h o l o g ia. l o r p h y s i o l o g i c tar lr irt . re . 9 . . ' r t c n s ier
rhizome systems.variableleaf morphology,and/
or potentialfor rapid vegetativespread)that confer resistanceto disturbanceand enablevigorous
e r p a n s i o no n ( e r e \ o u r ( c c o n d i t i o n .i m p r o r e
(AlabackandHennan1988;Halpern1988,1989i
Huffman et al. 1994;O'Dea et al. 1995). It is
likely that increasesin Iight along the boundaies
will inducerapid
ofnewly createdforestaggregates
lateral and vefiical growth of thesetaxa.
Hvpothesis7. tdSe effects,as tlelineclby the
distancesto v,hit:hearl.\,-seralspeciespenetrate
or forest-interior species are lost (and bt the
nognitudeoftlrcsechanges),will be greateru,here
environmentalconditions(te more stressJul(i.e.,
akng south awl west Jacfug edgesas opposed
to north- qn(l eastfacing edges) (Figure 1). Thrs
prediction is basedon the assumptionthat environmental conditions that inhibit fbrest-interior
speciesand enhancecstablishmentof open-site
speciesare accentuatedalong south- and wcstg aspects(WalesI 972,RanneyI 977,Matlack
t-acin
1993.Chen et al. 1995,Murcia 1995). In the
ell-ectsmay
PacificNorthwest.theseaspect-related
be magnifiedby greaterratesofteetall or canopy
damagealong south- and west-lacingedgesdue
to the stronglydirectionalnatureofwinter storms.
40
Halpemet al.
Anal),ticalApproachesfor Hl'potheses,+7.
Analyses of spatial gradientswithin aggregated
retentiontreatmentswill be usedto elucidatehow
speciesabundance(frequencyand cover) and di
versity vary with distancefrom the forest-aggregateedge(Hypotheses4-6, Figure,l). Abundance
or diversity measureswill be ht to linear or nonlinear regressionmodels(Sokal and Rohlf 1981,
Zar 1981). For speciesthat exhibit a signiticant
correlationbetweenabundanceanddistancefrom
edge,the form of the mostappropriatemodel(e.g.,
linear, logarithmic, or exponential)may provide
insight into the mechanism(s)of decline or advance(e.g.,invasionor die-backasisolatedindividualsorin a wave-tiont;sers, Matlack 1994a).
To examinepossibleaspect-related
differencesin
these effects (Hypothesis7, Figure 4), we will
testfor the significanceof an aspectterm in multiple regressionsof abundanceon distanceand
will alsobe usedin
aspect.Multiple regression
exploratoryfashion to test for relationshipswith
otherbiotjc anryorabioticfactorsthat vtlly at finer
spatialscalesand that may influence,or be influphenomena(e.9.,treecover.
encedby,edge-related
ground surfaceconditions,local disturbance.or
The sequentialBonfenoni
associatedvegetation).
rnethodwill be usedto adjust significancelevels
(Rice 19891.
for multiplecomparisons
The l-ha sized aggregatesin the current design are relatively large with respectto the range
ofprescribedpatchsizes(0.2 to >l.0 ha.)forgrcentree rctention units on federal matrix lands (USDA
andUSDI 1994).Howevet they aremuch smaller
thanarcnecessaryto escapemicroclimaticeffects
associated
with edges(Chenet al. 1995).Although
our studiesdo not explicitly testfor the effectsof
aggregate
size,theresultsof thesegradientanalyses
may suggestwhether smaller aggregatesmight
seNeasplant rcfugia,or whetheredgeeffectsare
such that larger retention units are necessaryto
maintain pxrticular speciesor groupsof species.
The ScientiticChallenges
of LargeExperiments
scale,Interdisciplinary
Numerouschallengesarisein designingandimple
menringlffge \cale.inlerdi.ciplinury
e\ferimenls.
particularly when thesestudiesare motivatedby
concernsover natural resourcemanagementor
policy. The DEMO experimentaldesign and its
componentstudiesrepresenttheproductsofa long
anditerativeprocessin whichwe haveincorporated
a rigorousexperimental
diverseecologicdinterests,
design,and the needsand concemsof land managersand the public (Abbott et al. 1999,Aubry
et al. 1999,Franklin et aJ. 1999). Many of our
decisionsrepresenttradeoffs: they resolve par
ticular problemsbut in turn may limit statistical
power,ecologicalor silviculturalinf'erence,
or
harvest elliciency. In this section. we provide
severalexamplesof the conceptualand methodthathaveshapedour studies.
ologicalchailenges
ments and resulting ecological responses. For
example,becausethe total basalarearetainedin
forest aggregatesdictatesthe total basalarearetainedin dispersedunits.pre-treatment
di11'erences
will skew the true propofiion of basal area re
taired.
Samplearea antl sanple size: inherent trudeoffs
in intenlisciplinary studie.\. If the spatialscales
of sampling in this experimentwerenot constrained
by interdisciplinarylinkages,vegetationstudies
could employ greaterreplicationof smaller-sized
treatmentunits, thus cnhancingstatisticalpower
However, vegetationmeasurementsare integral
to interpretinglaunal responses(Lehmkuhl et al.
mustbe exrminedat
1999)which,by necessity,
largerspatialscales( l3 ha). As a consequence.
our ability to detect teatment ellects tbr some
componentsofthe fbrestcommunitymay be compromisedbytherelativclysmallnumbersof sample
units.
Experimental rigor versussite-speciJicmanagementqpproache,s.To minimize the potential
confoundingof post-harvestactivitieswith reten
tion treatments,our silviculturalprescriptions
require that site-preparation
and tree-planting
specifications
areconsistentamongall harrestunits
within a block (Aubry et al. 1999). However,where
enrir,'nment:.
erperimenlal
unit' r.ri.upyJiFlerent
suppoft different plant communities,or generate
different densitiesof slash.this consistencymay
lead to a prescdptionthat is lessthan optimal for
rrlr iculturrlobjectire'.Ecologicalinterpretation..
as well as the applicability of pafiicular results
for managers,must be temperedby theseconsiderahons.
Naturul variation in vegetationcomposition
o n d s t r u c l u r e . D e s p i t e. r t t e m p l sl i , I n i n m , / e
within-block variability,at sqne sitestopographic
landdesignations,
and pastmanconfigurations,
agementactivities(e.g.,adjacentharvestunitsor
roads)precludedthe establishnentof six, 13-ha
experimentalunitswithin a forestofhomogeneous
topography,vegetation,and standhistory. Consequently,standage,tbreststructure,and species
varyamongexperimental
unitsprior
composition
to treatment (Table 4). This heterogeneityhas
implicationsfor interprctingtheexperimentalfeat-
Variation in vegetationstructureand compo
sition prior to treatment(Table 4) must also be
consideredin interpretingpost har.,'est
responses.
To minimizetheseeffccts,we will fbcuson cfirrge.t
in abundance/diversitybetween pre- and post
harvestcommunities.Nonetheless,
wherecomp e l i l i \ ei n l e r r c l i o na. r e i r n p o n a ndt e t e r r n i n a n t .
of speciesrcsponses,and where there are large
differencesin the initial abundance
of competitors. effectsof treatmentsmay be difficult to detect.
Separttthgeffectsofbgging disturbancefrom
ellectsof retention: short-tenn versuslong-term
considerations. A primary goal of our research
is to elucidate the effects of varying levels and
pattemsofgreen-treeretentionon lbrest commu
nity development.Although therehasbeenconcertedeffbrt to mjnimize eftectsof confounding
variation (Aubry et al. 1999).operationally,levels ofdisturbancewill changemarkedly with the
proportion of basal area removed and with the
pattemin which it is removed.Thus,it is likely
that short-termresponsesof vegetationwill re
flect differentialpatternsof loggingdisturbanca
as much as, or more than. pattems of retention.
Sampling of post-harvestdisturbancecharacteristics will permit an analysisof theseeffects. As
disturbanceefttcts diminish with time, canopy
effects may dominate understoryresponses,although legaciesof difterential disturbancemay
(Halpern
continueto shapesuccessional
trajectories
1988). Thus,green-tree
reientionstrategies
incorporatemorethanvariationin thelevelandspatial
arangementofretained structwes they encompassan arrayofco-varyingpaftemsandprocesses.
To the extentthat we can distinguishdisturbance
ellects from those associatedwith canopy structures,we may better understandthe mechanisms
that shapevegetationresponseand more succcssfully guide new strategiestbr fbrestmanagement.
Acknowledgements
D. Berg andJ. Franklin conceivedofthe original
designtbr a large-scaleexpedmenton green-tree
retentionin the Pacific Nonhwest. R. Cunis and
VecetationResDonseto Green tree Retention
4l
D. Minore contributed to the initial vegetation
samplingdesign.G. Spycherhasbeeninsh'unental
in developingthe databasenanagementsysten.
The goalsand approachespresentedin this paper
reflect the ideasand suggestionofmany individu
als: R. Abbott, M. Amaranthus.K. Aubry, L.
Bednar,D. Berg, R. Bigley.N. Christensen,
D.
DeBell, N. Diaz. D. Ford, J. Franklin. M.
Hemstrom, A. Hofion, D. Neeley.L. Noris, M.
Raphael.J. Reynolds,G. Smith,F. Swanson,J.
White. and B. Woodard. We thank S. Acker, K.
Aubry P Burton,D. Luoma,A. Olson,M. Raphael,
J. White, and an anonymousreviewer for com
mcntson carlicrvcrsionsof thismanuscript.For
their tirelesseffbrtswe gratetullyacknowledge
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