ForestScience,Vol. 34, No. 4, pp. 960-979.
Copyright1988by the Societyof AmericanForesters
SpeciesCompositionand Diversity During
SecondarySuccessionof ConiferousForests
in the Western Cascade Mountains
of Oregon
PETER SCHOONMAKER
ARTHUR MCKEE
ASSTRACT. Speciesdiversityand communitycompositionwere studiedat 23 siteson
similar western hemlock/Douglas-firforest habitats, in undisturbedold-growthstands
and standsat 2, 5, 10, 15, 20, 30, and 40 yearsafter clearcutting,broadcastburning,and
plantingwith Douglas-fir.Vegetationwas sampledwith three 5 x 60 m transectsat each
site. Invading herbs, then invadingand residualshrubs,and finally conifersdominated
throughthe first 30 years. Late send species,which accountfor 99% of cover in oldgrowth stands,are nearlyeliminatedimmediatelyfollowingdisturbance,but accountfor
almost40% of vegetativecoverafter 5 years,66% after 10 years,83% after 20 years,and
97% at 40 years. After an initial drop following disturbance,speciesdiversity trends
weakly upward with heterogeneitypeakingat 15 years and richnessat 20 years. This
initially high diversity (higher than that of old-growthstands)is short-lived.After the
tree canopy closes, speciesdiversity declinesreachingits lowest values at 40 years.
Only two specieswere eradicatedafter disturbance,both mycotrophs.Pacific Northwest old-growthforestsare relativelypoor in species,but moderatelyhigh in heterogeneity values.FoR. ScI. 34(4):960-979.
ADDITIONALKEY WORDS. Old-growthforests,disturbance,PacificNorthwest.
THE THEORETICALAND PRACTICALIMPORTANCE
of secondarysuccession
has long been recognized by forest ecologistsand managers,and successional sequenceshave been documentedthroughoutNorth America. Sev-
eralinvestigators
havesuggested
and/orshownempirically
thatspecies
diversity fluctuates during secondarysuccession,but the patterns and relationshipsare less than certain. This uncertaintyhas causedconcernamong
land managerschargedwith a governmentalpolicy of maintainingdiversity
in forest ecosystems(Cooley and Cooley 1984). This paper documents
changesin plant speciesdiversityand compositionon a chronosequence
of
plots 2 to 40 years after logging,burning, and plantingwith Douglas-fir
(Pseudostugarnenziesii[Mirb.] Franco), reveals some unexpectedtrends,
and discussimplicationsfor PacificNorthwestforests.
The conceptandmeasurement
of diversitycanbe appliedto attributesof
PeterSchoonmaker
is a graduatestudent,HarvardForest,HarvardUniversity,Petersham,
MA 01366, and Arthur McKee is ResearchInstructor,Departmentof Forest Science,College
of Forestry,OregonStateUniversity,Corvallis,OR 97731.The authorsaregratefulto Tim Sipe
and Drs. Mark Wilsonand David Fosterfor reviewingearlierdraftsof this manuscript,to Mark
Klopsch and Dr. Bernhard Schmidtfor help with data entry and statisticalanalysis,and to
JoanneLattin for carefulediting.JudyBrennemantypedseveraldraftsof thispaper;we thank
her for her patience.This researchwas supportedby the National ScienceFoundationBiological ResearchResourcesProgram(GrantsDeB 7925939and BSR 8300370)whosegrantsprovided full facilitiessupportand equipmentwhich made this studypossible.This is paper 1898,
Forest ResearchLaboratory, Oregon State University, Corvallis, Oregon, USA. Manuscript
receivedFebruary 8, 1988.
960/FOREST SCIENCE
ecosystems such as processes, structure, or species (Franklin 1984).
Speciesdiversitycan be viewedat three spatialscales:(1) the landscapeor
cover type level (gamma diversity), (2) the between-standlevel, for instance,alongan environmentalgradient(beta diversity), and (3) the withinstandor habitatlevel (alphadiversity)(Whittaker 1975).This paperdeals
specificallywith plant speciesdiversity at the alpha level, that is, the
numberandrelativeabundance
of specieswithin a particularhabitattype.
Someecologistshave predictedthat diversitywill increasethroughsuccession (Odum 1969, Harger and Tustin 1973), and others have observed
such increases(Tagawa 1964, Monk 1967, Holland 1971, Brunig 1973, Nicholsonand Monk 1974). Still others have predictedthat diversity might
decreaseduring a successionalsequence(Whittaker 1965, Pielou 1966a).
Habeck (1968) found that diversity steadily decreasedafter initial postfire
establishmentin cedar-hemlockforestsin the Northern Rocky Mountains.
Margalef (1963, 1968),Whittaker (1965), and Horn (1974)suggesta pattern
of increaseand thendecreaseduringforestsuccession.
Much empiricalevidencesuggests
thisthird patternis common(Loucks1970,Auclair and Goff
1971, Whittaker 1972, Shaft and Yarranton 1973, Bazzaz 1975, Johnsonet
al. 1976, Pett 1978, Bormann and Likens 1979, Hibbs 1983). Peet (1978) reportsa declineandthenan increasein speciesdiversitywith standagealong
the Colorado Front Range, and suggeststhat there may be more than one
peak in diversityduring succession,dependingon site characteristics.Most
recently, Norse et al. (1986) have proposeda model for an "idealized
forest"in whichearlyandlatesuccessional
species
accountfor twoperiods
of relativelyhighdiversity,in youngandmature/old-growth
forests,respectively. Finally, Drury andNisbett(1973)suggestthat "diversityis an ecological phenomenonindependentof succession."
These differing views and observationssuggestthat the relationshipbetweendiversityandsuccession
dependson manyvariables,the mostimportant of which may be type of community(e.g., terrestrial,aquatic,plant,
animal),geographiclocation,and circumstances
of disturbanceand succession (e.g., type, intensityand frequencyof disturbance,patch size, propagule dispersal,seedbank viability). To understandthis relationship,even
locally,patternsof diversityduringsuccessionshouldbe studiedin a variety
of habitatsand locationsunder a variety of disturbanceregimes.
In the PacificNorthwest,therehasbeenconcernthat loggingof late-successional,old-growthforestecosystems
may decreaseplant speciesdiversity on a local and regional scale and adverselyaffect rare, threatened,or
endangeredspecies(Franklin 1984,Norse et al. 1986).Someplant ecologists in the region have noticed, however, a net increasein numbersof
speciesafter clearcuttingand burning(Dyrness1973,Zamora 1982,Franklin
1984).This increaseand accompanyingchangesin communitycomposition
in seralforestecosystems
havebeenacceptedintuitively,but lack adequate
documentationbeyond one to two decadesof forest succession.
Previousstudiesof secondarysuccession
followingclearcutloggingand
burningwest of the CascadeMountainshave describedonly broad successional stages(Kienholtz 1929, Ingram 1931, Issac 1940)or have not gone
beyond early successionalstages(Morris 1958, Yerkes 1960, MuellerDombois 1965, Steen 1966, West and Chilcote 1968, Dyrness 1973). All
thesestudieshave focusedon compositionalchanges.
Although plant speciesdiversity and communitycompositionhave been
tabulated for a variety of coniferoushabitats in the Northwest (del Moral
1972,delMoral andFleming1979,Zobelet al. 1976),fewforestecologists
in
northwesternNorth America have studieddiversity and compositionfor
DECEMBER 1988/961
chronosequences
of a decadeor longer, and only one of these studieswas
made after clearcutloggingand burningin coniferousforests(Zamora 1982).
Reiners et al. (1971) measuredspeciesdiversity and communitycomposition in a primary successionalsequenceon sites previously occupiedby
glacialice at Glacier Bay, Alaska. Alaback (1982)documentedchangesover
700 years in vegetativestructureand biomassin a wide variety of environmental conditionsin coastalspruce-hemlockforestsin southeasternAlaska.
Zamora (1981) determinedunderstorystructureand speciesrichnessup to
27 years after clearcutting and burning in the Abies grandis/Pachistima
myrsiniteshabitat type of north central Idaho. No studiesof succession
west of the CascadeMountainsof Oregonhave examinedboth richnessand
evennesscomponentsof plant speciesdiversitybeyondinitial stagesof secondary successionwhile holdingvariation in elevation, aspect,slope,community type and treatmentto a minimum.
The objectiveof our studywas to documentthesechangesin diversityas
well as in compositionover a 40-year chronosequence
of similar sitesthat
had been clearcut logged, broadcastburned, and planted with Douglas-fir
seedlings.These first 40 years encompassa period of rapid structuraland
compositionalchange,from bare groundthroughherb, shrub, and early
closedtree canopy stages.Althoughthis periodis relatively shortwith respectto a full successionalsequenceof 600-1000 years in theseconiferous
forests,it may be the most dynamicphase(seeBormannand Likens 1979),
and henceof much ecologicalinterest.
On a more practicallevel, 40 years represents40%-60% of the rotation
time of a managedforest in the PacificNorthwest. Hence, knowledgeof the
effectsof clearcuttingandbroadcastburningon the plantcommunityshould
help thoseconcernedwith plant speciesdiversityto understandthe consequencesof the mostcommonsilviculturaltreatmentin this region.
STUDY
SITES
Data were collectedfrom JunethroughAugust 1980in the westernCascade
Mountains of Oregon on the H. J. Andrews Experimental Forest and on
surroundingUSDA Forest Servicelandsand privately ownedtimber lands.
This regionof the westernCascadesof Oregonis characterizedby deeply
dissectedterrain with generallywell-developedsoilson Tertiaryvolcanic
substrate.From 200 to 400 cm of precipitationfall annuallyin this area,
mostof it betweenOctoberand April. Summersare relativelydry (Franklin
and Dyrness 1973).
Coniferousforestsof two age classes,400+ and 125 years, predominate
over this area (Teensma1986,Morrison and Swansonin press).At lower to
middle elevations, the older stands are dominated by western hemlock
(Tsuga heterophylla [Raf.] Sarg.) and Douglas-fir, and the younger by
Douglas-fir.Pacificsilverfir (AbiesamabilisDougl. ex Forbes)and noblefir
(Abies procera Rehd.) dominate in both age classesat higher elevations.
This studywas carried out at low to middle elevationsin the Tsugaheterophylla/Rhododendronmacrophylum/Gaultheriashallon habitat type (Dyrhesset al. 1974).This relativelyabundanttype occursin warm and moderately dry environments(Zobel et al. 1976) on approximately20% of the
landscapeof AndrewsExperimentalForest.
Vegetationwas sampledin three 450-year-oldstandsand on 20 sitesthat
had beenclearcut,broadcastburned,andplantedwith Douglas-firseedlings
approximately2, 5, 10, 15, 20, 30 and 40 yearsbefore sampling.Forty years
was the samplinglimitationbecausethere were no older clearcutson the
•2/FOREST
SCIENCE
TABLE 1. Site characteristicsof selectedstudyareas on Tsugaheterophylla/RhododendronmacrophyllundGaultheriashallonhabitat types in and around the H. J.
Andrews Experimental Forest.
Age
(yr)
Site
no.
Old-growth
(400+)
I
2
2
3
I
2
5
10
15
20
30
40
Year of treatment
Clearcut Burned Planted
Aspect
(o)
Slope
(%)
Elev.
(m)
Area
(ha)
................. uncut .................
................. uncut .................
334
312
15
15
610
730
---
.................
76
77
314
300
20
15
35
530
610
970
-8.1
7.3
uncut .................
78
79
78
82
820
4.4
340
35
30
30
1030
15.4
81
76
n.a. •
n.a.
66/67
68
73
61/63
60/63
328
24
322
330
340
330
50
342
350
30
35
30
30
25
30
35
25
30
730
670
880
970
760
850
790
820
670
10.9
6.5
12.1
15.8
28.3
25.1
15.8
16.2
11.3
3
76
78
81
324
I
75
75
76
2
3
I
2
I
2
3
I
2
75
75
71
68
65
67
65
59
59
76
75
71
69
65
67
65
60
59
3
59
60
61/64
276
30
610
11.3
I
49
n.a.
n.a.
300
10
820
16.2
2
3
51/52
51
52
51
53
56
330
34
25
20
640
820
19.4
I
40/41
43
0
10
460
247
2
3
36/39
36/39
36/39
36/39
310
310
25
25
550
500
151
150
n.a.
n.p.2
n.p.
8.9
• n.a. = not available.
• n.p. = not planted,naturalregeneration
desiredhabitat type with reliabletreatmenthistoriesor documentation.For
all but one of the age classes,three siteswere selectedfor uniformityof
aspect, slope, elevation, habitat type and treatment (Table 1). Criteria of
uniformitycouldbe met on only two sitesof the 10-yearageclass.
We acknowledgelimitationsof studyingsuccessionwith multiple-site
spatialchronosequences
rather than single-sitetemporalchronosequences.
As Oliver (1982)states:"For a groupof standsof differentagesto be a true
chronosequence,
they shouldbe on similarsoilsand sites;have similarclimatesand climatichistories,originateafter similarpreviousdisturbances,
ßand have similar speciesand spatialand age distributions."We addressed
this problemby reviewingthousandsof sitescatalogedon the USDA Forest
Service'sTotalResourceInventory(TRI) systemfor the BlueRiver Ranger
District in additionto standmapsfor clearcutson the AndrewsExperimental Forest. We found 104 sitesthat appearedto meet our criteria for
uniformity:NW-N-NE facing;midslope;15-30% slope;500-1000 m elevation; similarlogging(clearcut,high lead) and burning(mediumintensity,
duringautumn)treatments;and Tsugaheterophylla/Rhododendron
macrophyllum/Gaultheriashallonhabitattype. Most siteshad inadequatedocumentation of planting density and stocking levels. These 104 sites were
'field-checked,and 20 siteswere selectedfrom them.The 40-yearsiteson
privatelyownedland were identifiedfrom standmapsprovidedby the land
owners, similarlyfield-checkedand selectedto best meet our criteria.
DECEMBER 1988/963
6O
5O
• 4o
Iz
,_1
Q.
u. 30
o
Li.I
rn
•THIRTY
AFTER
•OLD
_1
YEARS
TREATMENT
GROWTH
0
1--
10
102550
Ioo2oo 3•304(•05(•0600
'
CUMULATIVE
AREA(mz)
700
'
8•)0 900
FIOUR• 1. Species/areacurvesfor richestand poorestsitesin two age classes.Transectsfrom
all sitesfell within the shadedand/or hatchedrangesshown.
METHODS
Three sitesof each ageclass(only 2 for the 10-yearage class)were sampled
with three belt transects60 m x 5 m per site. The transectswere placedin
clearcuts50 m above or below loggingroads,20 m from the edgeof the cut,
and markedwith a woodenstakeand flagging.All transectsrun perpendicular to the slope.Speciespresentwere notedin six 5 m x 10m subplotsper
transect,and cover valueswere estimatedvisuallyto the nearestpercentage
up to 10%and to the nearest5% thereafteron 3 larger 5 m x 20 m subplots.
This techniquewas practicedfor 1 month to ensureconsistency.Sampling
excluded grasses, mosses, and lichens. Nomenclature follows Hitchcock
and Cronquist(1973).
Cover values of the three 5 m x 20 m subplotswere averagedto give a
valuefor eachtransect.Mean covervaluesfor eachspecieswere calculated
from the nine transectsin each age class.Species/areacurvesfor transects
rise rapidly until about 100m2, after which their rate of increasesteadily
declinesup to 900 m2 (Figure 1). It is likely that a few morespecieswouldbe
addedafter 900 m2, but it is unlikelythis wouldgreatlychangethe observed
patternsor our interpretations.
Plant species were separated into six classificationsestablishedby
Franklin and Dyrness (1973) and Dyrnesset al. (1974)accordingto growthform ,and their occurrence in late-successional and seral communities: co-
nifers, deciduoustrees•residualshrubs,invadingshrubs,residualherbs,
and invadingherbs (Table 2). Residualherbs and shrubsare those species
that havepersistedon the sitefrom the originalunloggedstand.Changesin
964/FOREST SCIENCE
DECEMBEg 1988/965
FOREST SCIENCE
6
DECEMBER 1988/967
968/FOREST SCIENCE
absolutecover and relative cover amongthese structuralclasseswere analyzed for each age class.
Diversity was calculated with two indices: richness,the total number of
species(S) talliedper 300 m2 transect,per 900 m2 site,andper 2700m2 age
class, and heterogeneity(H'), estimatedby the Shannon-Wienerformula
(Shannon and Weaver 1949),
H'= - • ptlogc
Pl
i•l
where Pi is the relative percentageof cover of individualspecieson each
transect(Figure 4) or in each age class(Figure 5). The latter index is applicable when a population is too large for all its members to be counted
(Pielou 1966b).It considersboth richnessand evennesscomponentsof diversity; that is, the numberof speciesand the evennessof their abundance.
For example, ten specieseach occupying10% of an area would have a
higherH' than if oneof the speciesoccupied91% andthe otherninedivided
the remaining 9%. H' is also more sensitiveto rare speciesthan S (Peet
1974).
Many indicesof diversityhavebeenproposed(Peet 1974),yet thesetwo
remain among the most commonly used even thoughthey have their drawbacks (Pielou 1966b, Peet 1974, Christensenand Peet 1984). Comparison
with other studiesis possiblebecauseof their wide use (e.g., Monk 1967,
Loucks 1970, Auclair and Goff 1971, Bazzaz 1975, Zamora 1982, Hibbs
1983).
To comparethe floristic similarityof seralstandsto the undisturbedoldgrowth age classwe calculatedcoefficientsof communitysimilarity using
Sorenson's index (S/):
SI=
2c
-A+B
x 100
where c is the number of speciesin commonbetweentwo age classes
havingA and B number of speciesrespectively(Mueller-Domboisand Ellenberg1974).SI valuesrangefrom 100for age classeshavingexactly the
same speciesas the old-growth sites, to 0 for those having no speciesin
common.SI valuesalso reflect floristic changesbetweenseral stages.
We also calculatedspeciesturnover (TO), a more direct measureof floristic changefrom one age classto the next:
L+G
TO=•
A+B
where L is the number of specieslost and G is the number of speciesgained
betweenage classes,and A and B are the sameas in Sorenson'sidex.
All variableswere analyzedas a functionof time sincemanipulation,and
their patternsthroughtime were examinedfor trendstoward or away from
original old-growth conditions.
RESULTS
A total of 132plant specieswas found on the 23 sites,55 in the 3 old-growth
standswhere late seral speciesaccountfor 99% of total plant cover. Cover
valuesof residualspeciesduringthe first few growingseasonsafter clearcut
DECEMBER 1988/969
200
180
•
160
-ß ------
140
TOTAL VEGETATIVE
CONIFERS
RESIDUAL SHRUBS
INVADING SHRUBS
RESIDUAL HERBS
INVADING HERBS
COVER
120
I00
80
6O
4O
2O
0
OLD
2
GROWTH
5
I0
YEARS
15
SINCE
20
50
40
TREATMENT
FIGURE 2. Percentage cover for growth-forms versus years since logging and burning. The
values show early dominanceby herbs, then by shrubsand finally by conifers. Total cover
increasesrapidly for 20 years; thereafter the rate of increase slows. Invading herbs and
shrubsare nearly eliminated40 years after treatment.
loggingand broadcastburningare at first low but rapidly increasein importance. Residual species account for almost 40% of total cover at 5 years,
66% at 10 years, 73% at 15 years, 83% at 20 years, 90% at 30 years, and 97%
at 40 years.
The general pattern of recovery on our sites during the first 40 years after
logging (Figure 2) is similar to that observed elsewhere (Kienholtz 1929,
Ingram 1931, Issac 1940, Zamora 1982):
1. Percentage cover of all growth-forms except invading herbs decreasessharply
immediately after loggingand burning.
2. Invading herbs. including some exotic species. increase rapidly. and such
speciesas fireweed (Epiiobium angustifolium L.), woodland groundsel(Senecio
syivaticus L.), common St. Johnswort (Hypericum perforatum L.) and common
thistle (Circium vuigare [Savi] Airy-Shaw) dominate briefly. Total herb cover
risesto almost 70% at 5 years.
3. An increase in residual herb cover lags slightly behind the dramatic rise of invading herbs.
4. Invading and residual shrubsincrease concurrently with a decreasein invading
and then residual herb cover.
5. Total percentageof cover increasesrapidly up to 15 years;thereafterthe rate of
increase
970/FOREST
slackens.
SCIENCE
I00'
RESIDUAL
HERBS
90
80
RESIDUAL
o
SHRUBS
II•IVADIN•
L•
Q.
Ld
50
L•
CONIFERS
20
IO
o
OLD2
5
GROWTH
I0
YEARS
15
SINCE
20
30
40
TREATMENT
FIGURE3. Relative cover values for growth-formsversusyears since loggingand burning.
The patterns reveal early dominanceby herbs after loggingand burning of old-growth and
subsequentdominance of shrubsand conifers. Deciduoustrees account for little of the total
cover in this habitat type.
6.
Conifers and residualshrubsclose the canopy at 20 to 30 years. Invading shrub
and herb cover continues
7.
to decline.
Forty years after loggingand burning, conifer cover is 82%, shrub cover is 57%,
and herb cover is 53%. Cover by invading herbs and shrubshas become very
small.
Residual herb cover is especiallyhigh on two of the 40-year sites. Much
of this herb cover is Oregon oxalis (Oxalis oregana Nutt. ex T. & G.), indicating wetter conditions than typical for the Tsuga heterophylla/Rhododendron macrophyllum/Gaultheriashallon habitat type. If these two siteswere
excluded, herb cover at 30 and 40 years would be identical.
Relative percent cover reveals the pattern of dominance of various
growth-forms in each age class (Figure 3). The old-growth forest is dominated by conifers (49% tel. cover) with lesser amounts of shrubs (33% tel.
cover) and herbs (18% rel. cover). After loggingand burning, we see first
herb, then shrub, and finally conifer dominancewith canopy closure at 20 to
30 years. At 2 years, 53% of relative cover is accountedfor by invading
herbs and 72% by invading and residual herbs combined. By 15 years,
shrubs account for 46%, conifers for 28%, and herbs for 24% of relative
DECEMBER 1988/971
65
,-6oA
hi
I-
ra
o,.
50-
'"4540Q.
• 3o-'
"'-,...,
900m
SITES
Z 25-
---- 3OOm
aTRANSECTS
3.0-
x2.5a• i._s
hi
1:3
O
,
2.O- 1'
",
1.5-
oT,
oLD
,b
io
3b
4o
GROWTH
YEARS SINCE TREATMENT
FIOu•J•4. (a) SpeciesHchness($) versusyearssince1ol•nS andburnin
and 20 yearsandis relativelylow for old-srowthstands.Co)I-IeterogeneRy
(/-/') versusyears
since1ol•ng and bunfizZ. The peaksin/-/' (Shannon-Wiener
index)at 2 and ]5 yearsprecedethosefor Hchness.Old-srowthvaluesfor heterogcneity
are moderatelyhigh.The trends
in diversityremainthe samewhen calculatedby transect,site, and age class.
cover. At 20 years, conifers increase to 43%, shrubsdecline to 38%, and
herbs decline to 15%.
Heterogeneityand speciesrichnessincreaseslightlyto peak at 15 and 20
years respectively after logging and burning. Then they decline to low
valuesat 40 years(Figure 4). In comparisonwith early-successional
stands,
old-growthforestshave relatively high heterogeneitybut low speciesrichness. Only the 40-year age class has fewer species.After loggingand
burning,speciesrichnessrisesirregularly(with a dip at 10years)to a highof
39 speciesper 300 m2 at 20 years,thendeclinesto a low of 28 speciesper
300 m2 at 40 years(Figure4a). Heterogeneityshowsa similartrend with
972/FOREST
SCIENCE
somenotableexceptions:the peakis at 15insteadof 20 years;the dip is at 5
years; and the old-growthvalue is higher than that of four other seral age
classes(Figure 4b). The pattern of high diversityat shruband herb stages,
lower diversity in old-growth, and lowest diversity in youngconifer stands
occurs regardlessof the area for which it is calculated(i.e., 300 m2 transects,900m2 sites,or 3 sitescombinedfor 2700m• ageclasses).An analysis
of variance showsthat there is no significantvariation between transects
(Table3), and that variationbetweenagesis significant.Variationdue to age
is substantially
greater(Fs = 16.51,FH, = 22.59)thanamongsitevariation
(Fs = 5.33, FH, = 5.01). If a quadraticterm is addedto the analysis,it
accountsfor a significant(P < 0.001) amount of the variation due to age,
indicatingthat the rise and fall of both indiciesaround15to 20 yearsis a real
trend. Least significantdifferencesbetween age classesfor S and H' are
2.87 and 0.16 (P < 0.05) and 3.73 and 0.21 (P < 0.01).
Speciesrichnessand heterogeneityfor variousgrowth-formsfollow similar but morevariabletrajectoriesthanthe compositepatternfor all species
(Figure 5). The number of tree species remains essentially the same
throughoutthe chronosequence
whereasheterogeneitydeclines,reflecting
the increaseddominanceof Douglas-fir. Heterogeneity and richnessof
shrubsand herbs peaks at 2 to 5 years and again at 15 to 20 years, and
thereafter declines. Variation between age classesis much higher for invadingshrubsand herbsthan for residualspeciesin thesegrowthforms.
Coefficient of communityvalues (relative to old-growthconditions)decreasefrom 2 to 15 years, then increasefrom 20 to 40 years as the canopy
closes(Figure 6a). The downwardtrend in similarityto the old-growthcommunity is due to an abundanceof invadingspeciesmore than to a loss of
residualspecies.The subsequent
increasein similarityis dueto a declinein
invadingspecies(especiallyherbs)and a slightincreasein residualspecies.
Speciesturnoverdeclinesfrom 2 through30 yearswith a slightincreaseat
20 years (Figure 6b) beforebeginningto climb againafter canopyclosure.
Again, the initial high valuesare due more to invadingspeciesgainedthan
to a loss of residuals.From 5 through40 years, numberof specieslost is
aboutequalto numberof speciesgained.
DISCUSSION
The pattern of rapid changefrom herb, to shrub, and finally conifer dominanceupon canopyclosureat 20-30 yearswas expected.The high species
turnover (Figure 6b) immediatelyafter treatmentfollowed by a levelingoff
at 30-40 yearsindicatessuddenchangeandpartialrecovery.The steepdrop
and subsequent
increasein coefficientof communityvalues(Figure6a) also
indicaterapidinitialcompositional
change,
whichappears
to slackenWith
canopy closure. Higher communitycoefficientsat 30 and 40 years suggest
TABLE 3. Analysisof variancefor transect,site, and age (generallinear model
procedure).
Sourceof variation
DF
Fs
P
Fn,
P
Site (age)
2
7
15
0.82
16.51
5.33
n.s.
0.001
0.001
0.30
22.59
5.01
n.s.
0.001
0.001
Error
Total
44
68
Transect
Age
DECEMBER 1988/973
RESIDUAL
SHRUBS 8• HERBS
TREES
INVADING
SHRUBS 8•HERBS
S ©©©ß ß ß Se•
I
I
40q
S I
I
30 J
I
le
20Jee
0/* ß ß ß
el
I
?
SHRUBS
RESIDUAL SHRUBS
INVADING
SHRUBS
I
40•
30 J
w
Q.
(/3
w
ß ß
20•eee ß
I0•
O'
H'
w
Q.
H'
ß
o
ee
Q.
h
HERBS
40 J e e
RESIDUAL
ß ß
S
,
i
HERBS
INVADING
HERBS
S
3011e ß
IOJ
'_%
0
OG
O•
OG
3•) 40
YEARS SINCE TREATMENT
FIGURE 5. Species richness(S) and heterogeneity(H') values for selectedgrowth-forms
versusyears sinceloggingandburning.Invadingherbsand shrubscontributemostto overall
diversity patternsfrom 2 to 5 years and shrubscontributemost from 15 to 20 years.
that speciescompositionis mostdynamicduringthe first 30 yearsof succession, approachingbut by no means reaching that of an old-growth forest;
further changesunder more uniformconditionsare due to slowerprocesses
of mortality and speciesreplacement,which may take hundredsof years in
the Pacific Northwest.
After clearcuttingand burning,speciesdiversityis initially low, peaksat
15 to 20 years, then declinesafter canopyclosure.The high diversityup to
20 years is brief, lastingless than one-thirdof the rotation period in intensively managed forests in this region, and is boosted slightly by exotic
species. An initial increase in speciesdiversity followed by a decrease
during secondaryforest successionhas been documentedfor areas other
than the Pacific Northwest by many authors(Loucks 1970,Auclair and Goff
974/FOREST SCIENCE
>-
7o
A
hi
B
0.4
0.2
D
2
5
I0
15
20
.'.'50
40
GROWTH
YEARS
SINCE TREATMENT
F•OURE6. (a) Coefficientof communityversusyearssinceloggingand burning.The coefticient declinesthrough15 yearsafter treatment,then risesas residualspeciesincreaseand
invadersare lost. (b) Speciesturnoverversusyearssinceloggingandburning.The highvalue
at 2 years in speciesturnoveris due to initial invaders.Valuesdeclinefrom 2 to 30 years.
1971, Shaft and Yarranton 1973, Bormann and Likens 1979, and Hibbs
1983).
The trendsfor thetwo indicesof diversityareweak,withrelativelylarge
variancesthat tendto be minimalin old-growthsites.The weak trendsand
high variation indicatethe uncertaintyof predictingspeciesdiversity
throughtheseandsubsequent
stagesof secondary
succession.
An unexpectedobservationis that highvaluesfor heterogeneity
immediatelyprecedepeaksfor speciesrichness(Figure4). A possibleexplanation
is that periodsof peakheterogeneity
in secondarysuccession
coincidewith
periodsof unresolvedcompetitionin both early herb-dominated
and later
shrub-dominated
stages.As a few speciesbecomemoresuccessful,
many
more declinein abundancewithoutbeingeliminated.This resultsin a decline in heterogeneitybut an increasein speciesrichnessbecausenew
speciesare continuallybeingadded.Anotherview is that high diversity
occursduringtransitionfrom oneassemblage
to anotherandmay reflectan
overlap of assemblages.
A reviewof thefirst 20 yearsof the chronosequence
clarifiesthispattern.
An earlypeakin heterogeneity
2 yearsafterclearcutting
andburningis due
DECEMBER 1988/975
to high numbersof all growth-forms,invadingherbsbeingdominant.At 5
yearsafter treatment,speciesrichnesshasbeenincreasedby invadingherbs
and shrubs,and heterogeneityhas been decreasedby preemptionby a few
dominantinvadingspecieswithouteliminationof many species.Epilobium
speciesaccountfor only 23% of cover at 2 years, but at 5 yearsthey account
for 39% of cover, and the five most dominantspeciesaccountfor 63% of
cover. Invading species,herbsand invadingherbsare the only vegetation
classesin which heterogeneityprecedesrichnessat 2 to 5 years(Figure5),
and thus they may be largely responsiblefor the compositepattern at this
early stage.
The secondpeakfor heterogeneityat 15yearsprecedesthe highvaluefor
speciesrichnessat 20 years,possiblybecause20-year-oldsites,like 5-yearold sites,have severaldominantspeciesas well as holdoverspeciesthat are
rapidly declining.The five most dominantspeciesaccountfor 56% of total
cover in the 20-year age class comparedto only 44% in the 15-year age
class. High heterogeneityvaluesat 15 years precedepeak richnessat 20
yearsfor residualshrubsandresidualherbs(Figure5), suggesting
that these
vegetationclassesare responsiblefor the compositepattern at the shrub
stage.
Old-growth forests in the study area are species-poorrelative to early
seralcommunities,but they have moderatelyhigh heterogeneitybecauseof
the equitabilityof species.Whittaker(1965)notedthe low speciesrichness
of coniferous forests in the Pacific Northwest; however, the evenness of
abundancethat producesmoderatelyhigh heterogeneityhas not been previouslyshown.That late-successional
ecosystems
mighthavelower species
diversity than earlier successional
stageshas been predictedby Margalef
(1958, 1968), Whittaker (1975) and Horn (1974), and is confffmedhere for
this foresttype.
Old-growth standswere slightlyhigher in richnessthan 40-year stands
and higher in heterogeneitythan 30- and 40-year stands. These young
forests may be less diverse becausethe denseclosed canopy of a 30- to
40-year Douglas-firstandcreatesa relatively uniformlow-lightenvironment
which fewer understoryspeciescan tolerate.The growthof shade-tolerant
speciesintothe canopyandeventualfragmentation
of the canopy,creatinga
more heterogeneous
light regime, permits establishmentand growth of a
wider variety of species.
The overallpatternof diversityduringsecondarysuccession
on our sites
followsthat proposedby Norse et a1.(1986)with a few exceptions.Instead
of onepeak, two periodsof peakdiversitycorresponding
to herb-dominated
and shrub-dominatedstages,occur in early succession.In addition, oldgrowth forests appear to have lower speciesdiversity values than some
early-successionalstageswhereas Norse et al. (1986) depict old-growth
forests as the most diverse.
While someearly-successional
stagesare more diversethan old-growth
stages,clearcuttingdoesnot necessarilygeneratethe highestspeciesdiversity on a largerlandscapescale.BrownandCurtis(1985)suggest
that smallto moderate-sizedclearcutsfollowed by planting and even-agedmanagement increasesbetween-standdiversity while decreasingwithin-stand diversity.This may be true in many cases,especiallywith respectto wildlife
habitat,but in regardto plant speciesdiversityit is an oversimplification.
Our resultsshowthat changesin plant speciesdiversityduringsecondary
succession
are complexand difficultto predict.Diversitywithin a clearcut
fluctuatesbetweenhigh and low valuesseveraltimes duringthe courseof
succession.
The richnessandevennesscomponents
of diversitymaypeakat
976/FOREST SCIENCE
differenttimes,anddiversityvaluesfor differentgrowth-forms
andsuitesof
species(early-vs. late-successional)
maybe differentyet interdependent.
The implications for managers are manifold. If diversity changes
throughouta sere,especiallyin early stages,how is plant speciesdiversity
estimatedfor a forestof many even-agedpatches9.
One methodmightentail
using data such as ours to determine the relative amount of each succes-
sionalstagethat producesa givenspeciesdiversityoverthe landscape.The
managershouldbear in mind, however,that there are manyindicesof diversity,eachsensitiveto differentcomponents
of diversity(Christensenand
Peet1984).Speciesdiversityshouldbe evaluatedwith caretYlllly
selectedindicesin conjunctionwith other information,and in light of site specific
factorsandthe spatialscaleunderconsideration.
At the standlevel, postlogging
treatmentmaybe especiallyinfluentialon
subsequent
changesin diversity.A hardburn may eliminate,at leasttemporarily, some residual speciesthat would otherwise return quickly via
sprouting.Timing of treatmentmay alsobe important.For example,the
effet of herbicideson speciesdiversitymay dependon when (both duringa
growingseasonandduringa rotation)the chemicalsare applied.Giventhat
the goalof herbicideuseis to releasecropspeciesandthushastenthe successionalsequence,the period of highestplant speciesdiversity (from
15-20 years) may be greatly reducedor bypassedaltogether.In addition,
densestockingwith Douglas-firpreemptsspacefrom otherspeciesand resultsin early canopyclosure,furtherreducingspecies-rich
herb and shrub
stages.
Yet maximizingspeciesdiversitymay not be an appropriategoalunder
certain circumstances.Many plants in the most diverseherb and shrub
stagesare commonweedyspeciesunlikelyto becomethreatenedor endangered, especiallyas more forestsare convertedto youngerage classes.
Rather, maintainingspeciesdiversityof particularecosystemsor habitats
may be paramountwhere the total area of the habitatin questionhasbeen
reducedto the point where speciescouldbecomelocallyextinct.
Few old-growthspecieswere eradicatedduringthe first 40 years after
loggingand burning.Only two disappeared,Merten's coralroot(Corallorhiza rnertensianaBong.) and pinedrops(PterosporaandromedeaNutt.).
The loss of these speciesshouldbe causefor concernfor land managers
who are chargedwith maintainingspeciesdiversitybecauseboth are mycotrophs, indicatorspeciesfor an organic-richforest floor. Once eradicated,
their return may dependin part on recolonizationof the siteby mycorrhizal
fungidispersedby animals(Maseret al. 1978).Thustheseplantspeciesmay
not readilyreturnor be easilyreintroduced,especiallyunderconditionsof
intensivemanagementwhere most stemsare removedand remainingdebris
burnedin 60- to 90-year rotations.
Plant speciesdiversity should be documentedbeyond 40 years after
clearcutting.Studiescurrently underwayat the H. J. Andrews Experimental Forest are gatheringsuchdata and are beingcarded out on permanent study sites in a variety of habitatsand geographiclocations.More
studiesare needed,however,with larger samplesincludingrarer species
that may be more sensititveto clearcuttingand proneto eradication.
LITERATURE
CITED
AL•ACK, P. B. 1982.Dynamicsof understorybiomassin Sitka Spruce-Western
Hemlock
forestsof Southeast
Alaska.Ecology63:1932-1948.
AucI•d•, A. N., andE G. GOFF.1971.Diversityrelationsof uplandforestsin the western
great lakes area. Am. Nat. 105:499-528.
DECEMBER 1988/977
Bazz_•z, E A. 1975. Plant speciesdiversityin old-field successional
ecosystemsin southern
Illinois. Ecology56:485-488.
BoPaoaq•, E H., and G. E. Lmmqs. 1979. Pattern and processin a forested ecosystem.
Springer-Verlag,New York. 253 p.
BRow•, E. R., andA. B. CURTIS.1985.P. 1-15 in Managementof Wildlifeandfishhabitatsin
forestsof westernOregonand Washington.Part 1. E. R. Brown, ed. USDA For. Serv.
332 p.
BRIJNIG,E. E 1973.Speciesrichnessand standdiversityin relationto site and succession
of
forestsin Sarawakand Brunei (Borneo).Amazonia3:293-320.
CI-ImSTE•qSEN,
N. L., and R. K. PEET.1984.Measuresof naturaldiversity.P.43-58 in Natural
diversityin forest ecosystems.J. L. Cooley and J. H. Cooley,eds. Inst. of Ecol., Univ. of
Georgia, Athens.
COOLEY,K. L., and J. H. Cooley(eds.). 1984.Naturaldiversityin forestecosystems.
Inst. of
Ecol., Univ. of Georgia,Athens.
DEL MORAL, R. 1972. Diversity patternsin forest vegetationof the WenatcheeMountains,
Washington.Bull. Torrey Bot. Club 99:57-64.
DEE MOPO,
L, R., and R. S. FLEblING. 1979. Structure of coniferousforest communitiesin
westernWashington:Diversity and ecotypeproperties.Vegetatio41(3):143-154.
DR•JR¾,W. H., and I. T. C. NISBET. 1973. Succession.J. Arnold Arboretum 54:331-368.
DYRNESS,C. T. 1973. Early stagesof plant succession
followingloggingand burningin the
WesternCascadesof Oregon.Ecology54:57-69.
DvmaEss,C. T., J. E FRm•KLI•, and W. H. More. 1974.A preliminaryclassificationof forest
communitiesin the central portion of the western Cascadesof Oregon. U.S.I.B.P. (Int.
Biol. Prog.) Conif. For. Biome Bull. 4. Univ. of Washington,Seattle. 123p.
FPO•mma, J. E, and C. T. DvmaEss. 1973. Natural vegetationof Oregon and Washington.
USDA For. Serv. Gert. Tech. Rep. PNW-8. 417 p.
FR.a•KLI•, J. F. 1984.Commentson Natural Diversity.P. 31-34 in Natural diversityin forest
ecosystems.J. L. Cooley and J. H. Cooley,eds. Inst. of Ecol., Univ. of Georgia,Athens.
HABECK, J. R. 1968. Forest successionin the Glacier Peak cedar-hemlockforests. Ecology
49:872-880.
HARGER,J. R. E., and K. T•Js?m. 1973. Successionand stability in biologicalcommunities.
Part 1: Diversity. Internat. J. Environ. Stud. 5:117-130.
HIBBS, D. E. 1983. Forty years of forest successionin central New England. Ecology
64:1394-1401.
H1TCI-ICOCK,C. L., and A. CRONQIJIST.1973. Flora of the Pacific Northwest. Univ. of Washington Press, Seattle. 730 p.
HOLLAND, P. G. 1971. Seasonalchangein the shootflora diversityof hardwoodforest stands
on M. St. Hilaire, Quebec. Can. J. Bot. 49:1713-1720.
HoRn, H. S. 1974.The ecologyof secondarysuccession.Ann. Rev. Ecol. Syst. 5:25-37.
INGRAM, D.C. 1931. Vegetation changesand grazing use of Douglaas-fircutover land. J.
Agric.Res.43:87-417.
Iss.•½, L. A. 1940.Vegetationsuccession
followingloggingin the Douglas-firregionwith special reference to fire. J. For. 38:716-721.
JOI-INSON,W. C., R. L. BURGESS,and W. R. KEAMI•!ERER.1976. Forest overstory vegetation
and environmenton the MissouriRiver floodplainin North Dakota. Ecol. Monogr.46:5984.
KIElqI-IOLTZ,R. 1929. Revegetation after logging and burning in the Douglas-fir region of
Western Washington.Illinois State Acad. Sci. Trans. 21:94-108.
LOIJCKS,O. L. 1970. Evolution of diversity,efficiencyand communitystability.Am. Zool.
10:17-25.
M_ARG,•LEF,
D. R. 1958.Informationtheory in ecology.Gert. Syst. 3:36-71.
MARGALEF,D. R. 1963.On certainunifyingprinciplesin ecology.Am. Nat. 97:357-374.
Mmm•a.EF, D. R. 1968.Perspectives
in ecological
theory.Univ. ChicagoPress.Chicago,I11.
111 p.
978/FOREST SCIENCE
MASER,C., J. M. TRAPPE,and R. A. NUSSBAUM.1978. Fungal-small mammal inter-relationshipswith emphasisOregonconiferousforests.Ecology59:799-809.
MONK, C. D. 1967.Treespeciesdiversityin the easterndeciduousforestwith particularreference to North Central Florida. Am. Nat. 101:173-187.
MORRIS,W. G. 1958.Influenceof slashburningon rcgeneratinn,other plant cover, and fire
hazzardin the Douglas-firregion.USDA For. Serv. Res. Pap. PNW-29.
MORRISON,P. H., and F. J. SWANSON.Fire history in two forest ecosystemsof the central
westernCascaderange,Oregon.USDA For. Serv. Gen. Tech. Rep. PNW No.--. In press.
MUELLER-DOMBOIS,
D. 1965.Initial stagesof secondarysuccession
in the CoastalDouglas-fir
and WesternHemlock zones.P. 39-41 in Ecologyof WesternNorth America. Volume 1. V.
Krajina, ed. Univ. of BritishColumbia,Vancouver,Canada.
MUELLER-DOMBOIS,
D., and H. ELLENBERG.1974.Aims and methodsof vegetationecology.
Wiley, New York. 547 p.
NICHOLSON,S. A., and C. D. MONK. 1974. Plant speciesdiversity in old field successionon
the Georgia Piedmont. Ecology 55:1075-1085.
NORSE,E. A., et al. 1986.Conservingbiologicaldiversityin our nationalforests.The WildernessSociety,Washington,DC. 116p.
OocrM, E. P. 1969. The strategyof ecosystemdevelopment.Science 164:262-270.
OLIVER,C. D. 1982.Standdevelopment--itsusesandmethodsof study.P. 100-- 112in Forest
successionand standdevelopmentresearchin the Northwest, J. E. Means, ed. For. Res.
Lab., OregonState Univ., Corvallis.
PEET, R. K. 1974. The measurementof speciesdiversity. Ann. Rev. Ecol. Syst. 5:285-307.
PEET,R. K. 1978.Forestvegetationof the ColoradoFront Range:Patternsof speciesdiversity.
Vegetatio37(2):65-78.
PIELOG, E. C. 1966a. Speciesdiversity and pattern diversity in the studyof ecologicalsuccession. J. Theor. Biol. 10:370-383.
PmLOV, E. C. 1966b.Shannon'sformulaas a measureof specificdiversity:Its useand misuse.
Am. Nat. 100:463-465.
•s,
W. A., WORLEY,I. A., and D. B. LAWRENCE.1971. Plant diversity in a chronosequenceat Glacier Bay, Alaska. Ecology52:55-59.
SHAFI, M. I., and G. A. YAmoa,rroN. 1973.Diversity,tiGristicrichnessand speciesevenness
during a secondary(post-fire) succession.Ecology 54:897-902.
SI-IA•NON, C. E., and WEAVER,W. 1949.The mathematicaltheory of communications.Univ.
of Illinois Press, Urbana.
STEEN,H. K. 1966.Vegetationfollowingslashfiresin onewesternOregonlocality.Northwest
Sci. 40:113-120.
TAGAWA,H. 1964.A studyof the volcanicvegetationin Sakurajima,Southwest
Japan.I. Dynamicsof vegetation.Memoirs of the Faculty of Science, Kyushu University. Series E:
Biology 3:165-228.
TEENSM-%P. D. A. 1986.Fire history and regimesof the central westernCascadesof Oregon.
Ph.D. diss, Univ. of Oregon,Eugene. 186p.
WRST, N. E., and W. W. CI-IIL½OTE.1968.Seneciosylvaticusin relationto Douglas-firclearcut
successionin the OregonCoastRange.Ecology49:1101-1107.
WI-trrrtd•.ER,R. H. 1965. Dominanceand diversity in plant communities.Science 147:250260.
WHrrrAK•R, R. H. 1972.Evolution and measurementof speciesdiversity.Taxonomy21:213251.
WHrrrAKER, R. H. 1975. Communitiesand ecosystems.MacMillan, New York. 385 p.
YEaKeZ, V. P. 1960. Occurrenceof shrubsand herbaciousvegetationafter clearcuttingoldgrowthDouglas-firin the OregonCascades.USDA For. Serv. Res. Pap. PNW-34.
ZAMORA,B. A. 1982. Undrstory developmentin forest succession:An examplefrom the inland Northwest. P. 63-69 in Forest successionand stand development research in the
Northwest, J. Means, ed. For. Res. Lab., OregonState Univ., Corvallis.
ZOBEL,D. B., et al. 1976.Relationshipsof environmentto composition,structureand diversity of forest communitiesof the Central Western Cascadesof Oregon. Ecol. Monogr.
46:135-156.
DECEMBER 1988/979