Society for Conservation Biology
Implications of Global Climate Change for Biogeographic Patterns in the Greater Yellowstone
Ecosystem
Author(s): William H. Romme and Monica G. Turner
Source: Conservation Biology, Vol. 5, No. 3 (Sep., 1991), pp. 373-386
Published by: Blackwell Publishing for Society for Conservation Biology
Stable URL: http://www.jstor.org/stable/2385909 .
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ofGlobalClimate
Implications
ChangeforBiogeographic
in theGreater
Patterns
Ecosystem
Yellowstone
WILLIAM H. ROMME
BiologyDepartment
FortLewis College
Durango,CO 81301, U.S.A.
MONICA G. TURNER
Environmental
Sciences Division
Oak Ridge NationalLaboratory
Oak Ridge,TN 37831-6038, U.S.A.
Abstract: Projectedchanges in global climatehave substantial ramificationsfor biological diversityand the management of natural areas. We explored thepotential implications ofglobal climate changefor biogeographicpatternsin
the Greater YellowstoneEcosystemby using a conceptual
model to compare threelikelyclimatescenarios:(1) warmer
and drierthan thepresent;(2) warmerand drier,but witha
and (3)
compensatingincreaseinplant wateruse efficiency;
warmer and wetter than the present. The logical consequences of each scenario areprojectedforseveralspeciesand
communitytypeschosen to representa range of local climate conditionsand biotic responsesin theGreaterYellowstone Ecosystem.The upper and lower timberlineappear to
be particularlysensitiveto climate change. The upper timberline is likelyto migrateupward in elevation in response
to temperaturechanges,whereas the lower treelinemay retreatunderdrierconditionsor move down slope underwetterconditions.In all scenarios, the extentof alpine vegetation in theecosystemdecreased.Climate-inducedchanges in
thefire regimein theGreaterYellowstoneEcosystemwould
probably have substantial consequencesfor the extentand
age-class distributionofforestcommunities.Alterationsin
thedistributionand extentofgrassland communitieswould
affectthepopulations of large ungulates.Our analyses suggest directionsfor establishinglong-termmeasurementsfor
theearly detectionof responsesto climate change.
Paper submittedJuly
1, 1990; revisedmanuscriptacceptedDecember
I11,1990.
Resumen: Los proyectados cambios globales del clima
tienenramificacionessubstancialespara la diversidadbiol6gica y el manejo de las areas naturales. Exploramos las
implicacionespotenciales de los cambios globales del clima
en lospatrones biogeogrdficosdel ecosistemade GreaterYellowstone utilizando un modelo conceptualpara comparar
tresposibles escenarios climaticos: (1) mas caliente y seco
que el actual; (2) mciscalienteysecopero con un incremento
compensatorio en la eficiencia del uso de las plantas de
agua; y (3) mas calientey mas humedo que el actual. Las
consecuencias l6gicas de cada caso son proyectadaspara
varias especiesy tipos de comunidad escogidospara representar un rango de condicionesclimaticas locales y de respuestas bi6ticas en el ecosistemade GreaterYellowstone.El
limitesuperiore inferiorde presencia de arboles parece ser
particularmente sensitivo a los cambios climaticos. Es
posible que el limitesuperiormigrehacia altas elevaciones
en respuestaa los cambios de temperaturamientrasque el
inferiorpuede retractarseen condiciones mcissecas o moEn todos los
verseladera abajo en condicionesmas ht*medas.
escenariosclimaticosanalizados, se disminuy6la extensi6n
de la vegetacionalpina del ecosistema de GreaterYellowstone.Los cambios climaticos inducidos con regimende incendios en el ecosistema de Greater Yellowstone seguramentetendrdnconsecuencias substanciales en la extensi6n
y la distribuci6n de edad-clase de las comunidades de
bosque. Alteraciones en la distribuci6ny la extensi6n de
comunidades de pastizales afectarda la poblacion de los
para
grandes ungulados. Nuestroandlisis sugieredirectrices
el establecimientode medidas a largo plazo para detectar
tempranamentelas respuestasa los cambios climaticos.
373
ConservationBiology
Volume 5, No. 3, September1991
374
Change
ofClimate
Implications
Introduction
presentschallengesto theconA changingenvironment
servationof biologicaldiversityand the managementof
naturalareas. The challengesarise in partfromthe tremendous uncertaintyabout the directionand magnichanges.For example,clitude offutureenvironmental
mateis a centralcontrollingfactorin all ecosystemsand
in some regionsduris projectedto changedramatically
ingthenextcentury.Global climatechangewould have
ramifications
for naturalareas (Peters &
far-reaching
Darling1985; Tangley1988; Davis 1989a; Grahamet al.
1990; Davis & Zabinski,in press), but the magnitude,
rate, and spatiotemporalcharacteristicsof potential
changes in temperatureand precipitationregimesare
not well understood. In the face of environmental
change,effectivenaturereservemanagementmust be
of the major biotic and
based on (1) an understanding
abioticcontrolson the ecological componentsand processes ofthe area,(2) an expectationofhow thesecontrolsmay change,eitherthroughnaturaleventsor human actions, and (3) an evaluation of the probable
effectsofthesechangeson thebiotain thereserve(Golley 1984). In thispaper,we explore potentialimplicationsofglobal climatechangeforbiologicaldiversityin
YellowstoneNationalPark(YNP) and the GreaterYellowstoneEcosystem(GYE), whichincludesYNP, Grand
Teton National Park,and portionsof surroundingnationalforestsand otherlands (Clark & Harvey1988).
in the GYE,we
Frompaleoecological reconstructions
know thatmajor changesin regionalclimateand vegetationhave occurredin thepast.Duringthemostrecent
glacialperiod (ca. 20,000 to 16,000 yrsago), the upper
timberlinein thispart of the RockyMountainsapparentlywas 600 to 1200 m lower thantoday,and mostof
the YellowstonePlateau was glaciated(Barnoskyet al.
1987:312). As global temperaturesincreased and glaciers retreatedat the end ofthe Pleistocene(ca. 14,000
to 13,000 yrs ago in Yellowstone; Barnosky et al.
1987:299), the upper timberlineshiftedupward,and
coniferousforestsbecame establishedon the Yellowstone Plateau (Baker 1970, 1976, 1986; Waddington&
Wright1974). The earlyHolocene (ca. 10,000 to 4,000
yrsago) was a period of maximumwarmthin the Yellowstone region,and some species such as Douglas-fir
(Pseudotsuga menziesii) apparentlygrew at higherelevationsthantoday(Barnoskyet al. 1987:302). The climate became somewhatcooler and possiblywetterin
the mid-Holocene,and the lower timberlinein the easternportionoftheGYE moveddownwardabout 5400 to
4400 yrsago (Reider et al. 1988).
As a resultof anthropogenicincreasesin carbon dioxide and othergreenhousegases, anotherepisode of
global climate change is expected in the coming century(Bolin et al. 1986). The climatechange maybe of
a magnitudesimilarto thatat theend ofthePleistocene,
ConservationBiology
Volume 5, No. 3, September1991
& Turner
Romme
but theprojectedrateis likelyto be an orderof magnitude greaterthan changes that occurred in the past.
Currentsimulationsusing the generalcirculationmodels (GCMs) (e.g., Hansenet al. 1988; Schlesinger& Zhao
1988; Wetherald& Manabe 1988) project an average
rise in global temperaturerangingfrom1.5 to 4.5 C,
with greaterwarmingin winterthan summerand increased warmingwith increased latitude (Dickinson
1986). Increasedprecipitationat highlatitudesand decreased summerprecipitationand soil moistureat middle latitudesof the northernhemisphereare also predicted. These projections are at the global scale; the
magnitudeand even the directionsof climate change
will probablydifferregionally.
ProjectedClimateScenariosin theGYE
In the RockyMountains,regionalprojectionsofclimate
changebased on GCM simulationssuggestelevatedtemperatures,reduced winterprecipitation,earlierspring
runoff,
and elevationalshiftsin vegetationzones (Neilson et al. 1989). In Figure1,we summarizethepotential
effectsof these changes on the GYE. All of the general
circulationmodels project increased temperaturesin
response to elevated concentrationsof atmospheric
CO2 and othergreenhousegases, and warmertemperatureswill lead to higherpotentialevapotranspiration
(PET). Thus,the GreaterYellowstoneEcosystemwould
be subjected to highertemperaturesand greaterevaporativedemandsthanat present.There is considerable
about othereffectsofclimatechange(Lamb
uncertainty
1987; Cushman& Spring,in press). The GCMs cannot
reliablypredict regional precipitationpatterns;thus,
rainfallmay increase,decrease, or remainthe same in
the GYE (Cushman et al. 1988; Neilson et al. 1989). In
addition,elevatedatmosphericCO2 mayhave directeffectson vegetation.For example,water use efficiency
(W[UE) may increasewith elevated C02, but the magnitude and duration of this increase are unknown
(Norby & O'Neill 1989). Thus, the warmer temperawould
turesand therise in potentialevapotranspiration
increaseplant water stressunless compensatedforby
increased precipitationor enhanced water use efficiency. Finally,atmospheric CO2 enrichmentcould
stimulateincreasedprimaryproductivity,
althoughthis
potentialincrease may not be realized because other
constraints(e.g., droughtstress or nutrients)may be
morelimiting(Strain& Bazzaz 1983; Oechel & Reichers
1986).
We explore the possible implications of these
changes and theiruncertaintiesby creatingthree climate scenariosthatencompassthe rangeof conditions
likelyto exist by the end of the next century:
1. A warm,dry scenario, in which temperatureand
PET increase, precipitationdecreases or remains
Romme
& Turner
ofClimate
Implications
Change 375
andtrace gases
g
p~InreiiasedInrese
Decreased
-or-
precipitation
Iag ncreased
i
U
SmallincreaseinWUE
Figure 1. Potential effectsofprojectedclimate change in theGreaterYellowstoneEcosystem.PET
evapotranspiration;WUE = plant water use efficiency.
unchanged,and WUE increasesonlyslightly;
plants
are subjected to elevated temperatures,
C02, and
droughtstress.
2. Anintermediate
scenario,inwhichtemperature
and
PET increase, precipitationdecreases or remains
unchanged,and WUE increasessufficiently
to compensate for elevated PET; plants are subjected to
elevated temperaturesand CO2 but there is no
change in droughtstress.
3. A warm,wet scenario,in which temperaturesand
CO2 increase, precipitationincreases, and WUE
increasessignificantly;
plants are subjected to elevated temperatures and C02, but to reduced
droughtstress.
For each scenario,we projected the probable effects
on severalspecies and communitytypeschosen to represent a range of local climate conditionsand biotic
responsesin theGYE. Communitytypesincludedalpine
communitiesabove the upper timberline,nonforest
communitiesbelow the lower timberline,and the forested zone. Species includedwhitebarkpine (Pinus albicaulis), an importanttree near upper timberline;
Douglas-fir
(Pseudotsuga menziesii),whichgrowsnear
lower timberline;and elk (Cervis elaphus) and bison
(Bison bison), whichgenerallylive at higherelevations
duringsummerbut migrateto lower elevationsin winter.
We emphasize that the ecological changes we de-
=
potential
scribed in this paper are projections,not predictions.
Our present understandingof the impendingclimate
changesand of the biotic and abiotic constraintson the
distribution
and abundanceofspecies and communities
in the GreaterYellowstone Ecosystemare still too rudimentaryto permitconfidentpredictions.Nevertheless, we thinkthatthis exercise has heuristicvalue in
thatithelpsdelimittherangeand magnitudeofpossible
changesin the system.It also maysuggestareas where
researchand monitoring
programsmaybe mosturgent
if managersare to deal effectively
with the potential
climatechangesthatwe face in the next century.
(1) TheWarm,DryScenario
The possible consequences ofwarmer,drierconditions,
with minimalphysiologicalcompensation,are summarized in Figure2. An importantdirecteffectof elevated
summertemperatureswould be a lengtheningof the
growingseason at highelevations.Because low temperatures and a shortgrowingseason apparentlyare the
primaryconstraintson treegrowthat theuppertimberline in the RockyMountains(Daubenmire 1954; Krebs
1985), it is likelythatthe upper timberlinewould shift
to a higher elevation. Interpretationsof the generalcirculation-model
predictionssuggestan elevationaldisplacement of temperaturezones of 462 to 1155 m
(Neilson et al. 1989). This would be comparableto the
ConservationBiology
Volume 5, No. 3, September1991
& Turner
Romme
ofClimate
Change
Implications
376
WARM,DRY SCENARIO (No. 1)
Increased
temperature
No change or
reduced precipitation
Increased
PET
Increaseddroughtstress
at lowertimberline
Increased growing
season at upper
timberline
[
Upwardshiftoflower
ofue]timberline
Upwardshiftof uppei
timberlineil1
t
Small increase
in WUE
Te
l
flt
of
Contraction
alpine communities
Decreased area and
increased habitat
forobligate
fragmentation
alpinespecies (e. g., rosy
finch,waterpipit,arctic
gentian,alpine chaenactis); some local extinctions
fl LPt
e intotalforested
~~~~~Decreas
area; increasedfire
frequency;
shiftto
younger
predominantly
age classes.
Decreased area and increased
forobligate
habitatfragmentation
forestspecies (e.g.,
old-growth
marten,goshawk,orchids,twinflower);some local extinctions;
decrease in whitebark
pine;
increase in Douglas-fir.
inarea of
~~~~~~~~~Increase
elevationnonforest
~~~~~low
communities;upward
shiftofsagebrushgrassland communities;
semi-desertvegetation
morecommonat lower
elevations
Increased ungulatewinter
rangewithinParks and
Forests; increasedarea and
of habitatfor
connectivity
nonforest
species; increased
componentof semi-desert
flora,fauna,and communities.
Figure2. Potential effectson theGreaterYellowstoneEcosystemof a climate that is warmerand drierthan the
presentclimate (scenario 1).
of 400 to 1000 m
upward shiftin species distributions
thatmayhave occurredin Yellowstoneduringthe Sangamonian interglacialperiod some 127,000 yrs ago
(Baker 1986:728). For theprojectionsin thispaper,we
use a conservativeestimateof 460 m. The upper timberlinein the GYE at presentis around2900 m (Patten
1963; Waddington& Wright 1974; Despain, 1991);
thus,global warmingcould move the upper timberline
to around 3360 m or higher.
A higheruppertimberline
would mean a substantially
smalleralpine zone. Indeed, the alpine zone could disappear completelywithinthe borders of Yellowstone
National Park,where the highestpoint (Eagle Peak)
reaches only 3440 m. There are higher mountains
nearby,in theAbsaroka,Teton,and Wind Riverranges,
where an alpine zone would persist.Nevertheless,the
habitatarea for obligate alpine species would be reduced and fragmented
throughoutthe GreaterYellowstone Ecosystem(Figs. 5 and 6). This in turncould lead
ConservationBiology
Volume 5, No. 3, September1991
to local extinctionsof some alpine species and communities(MacArthur& Wilson 1967; Diamond 1975; Soule
et al. 1979; Wilcox 1980). Examples of species restrictedto the alpine zone include the arctic gentian
(Gentiana algida), alpinechaenactis(Chaenactis alpinus), rosy finches(Leucosticte spp.), and water pipit
(Anthusspinoletta).
The lower timberlinein theRockyMountainsappears
to be controlled mainly by summer drought stress
(Daubenmire 1943; Krebs 1985). Increased potential
withoutcompensatingincreases in
evapotranspiration
precipitationor water use efficiencywould result in
accentuateddroughtstressat lower elevationsand an
upwardshiftin the lower timberlineof 460 m or more
(Neilson et al. 1989).
With a comparable upward shiftin both the upper
and lower timberlines,
the totalforestedarea would become smallerthanat present,because thereis less land
area availableat progressively
higherelevations(Figs. 5
Romme
& Turner
ofClimate
Implications
Change 377
INTERMEDIATESCENARIO (No. 2)
[ Increased
Increased
PET
[temperature
[ Increased growing
season at upper
timberline
No change or
reduced precipitation
Large increase
inWUE
No change in drought
stress at lowertimberline
I
No change in lower
timberline
Upwardshiftof uppe
timberline
Increase intotalforested
Conracionof
alpine communities
area; increaseinfirelo-evtn
shiftto prefrequency;
dominantly
younger
agenofrscmuite
No change in area of
|
nlow-elevationml
classes.
Decreased area and
increased habitat
forobligate
fragmentation
alpinespecies (e. g., rosy
finch,waterpipit,arctic
gentian,alpine chaenac-
tis);somelocalextinctions.
No change or reducedarea of
habitatforobligateold-growth
forestspecies (e.g., marten,
goshawk,orchids,twinflower);
some local extinctions;
decrease
in whitebark
pine; increase in
Douglas-fir.
No change in area ofwinter
ungulate range butincreased
survivalin milderwinters;
no change inamountof habitat
fornonforest
species, but
compositionalchanges in
communities
mayoccur.
Figure3. Potential effectson theGreaterYellowstoneEcosystemof a climate that is warmerand drierthan
thepresentclimate,but witha compensatingincrease in plant water use efficiency(scenario 2).
and 6). An upwardshiftin theforestedzone also would
have a disproportionateeffecton the extent of highelevationforesttypes.For example,whitebarkpine forests presentlyoccur in a zone from2600 to 2900 m,
which occupies an area ofabout 250,000 ha withinYNP
and on the adjacent highpeaks (Despain, in press; unpublished YNP data). The upper elevational limit of
whitebarkpine apparentlyis controlledby low temperatures;the lower limitmaybe set partlyby competition
with otherconifers,particularly
lodgepole pine. If vegetation zones shiftedby 460 m, then whitebarkpine
would be foundfromabout 3060 to 3360 m, with an
area ofonly27,000 ha (Figs. 5 and 6). This representsa
90% decrease in available habitatforwhitebarkpine.
Such a reductionwould have additionalramifications,
because thisspecies not onlyformsa distinctiveforest
type at the upper timberlinebut also is an important
food source forClark'snutcrackers(Nucifraga colum-
biana), red squirrels(Tamiasciurus hudsonicus), and
grizzlybears (Ursus arctos).
Quite a different
situationis projectedforthe lowerelevationtreespecies,Douglas-fir.
The lower timberline
presentlyis around 1900 m,and Douglas-fir
occursfrom
about 1900 to 2200 m (Patten 1963; Waddington&
Wright 1974). A 460-m upward shiftin elevational
zones would resultin a largerpotentialrange forthis
species withinthe borders of Yellowstone Park since
mostoftheparkarea lies above 2000 m (Figs. 5 and 6).
However, Douglas-firwould probably disappear from
lower-elevationareas elsewhere in the GYE, so its regional abundancewould remainthe same or decrease.
Furthermore,
even withinYNP this species mightnot
occupy all of the sites thatwere climaticallysuitable,
because itrequireshighcalciumlevels thatare available
mainly in soils derived from calcareous substrates
(Patten 1963; Despain, 1991). Most of the higher-
ConservationBiology
Volume 5, No. 3, September1991
378
of ClimateChange
Implications
Romme& Turner
WARM,WET SCENARIO(No. 3)
Increased
temperature
]
Increased
PET
Large increase
inWUE
Increasedprecipitation
Reducedwaterstressat
lowertimberline
Increasedgrowing
season at upper
timberline
Downwardshiftof lower
ttimberline
__________
[Upwardshiftofuppe]
timberline
_]
of
Contraction
alpinecommunities
Decreased area and
increasedhabitat
forobligate
fragmentation
alpinespecies (e. g., rosy
arctic
finch,
waterpipit,
gentian,alpinechaenactis);some localextinctions.
Largeincreaseintotal
forestedarea; decrease in
to prefirefrequency;
shift
older
~~~~~dominantly
age
classes.
Decrease inarea of
forestcommunities.
lweeainnn
__l
Increasedarea of habitatfor
forestspecies (e.g.,
old-growth
marten,
goshawk,orchids,twindecrease inwhitebark
flower);
pine;increasein Douglas-fir.
_l
Decrease inarea ofnonforested
winter
ungulaterangein
protectedParksand Forests,
butpossiblyincreasedforage
production
;decrease in habitat
fornonforest
species withsome
localextinctions;
semi-desert
elementsdisappearalmostentirely.
Figure 4. Potential effectson theGreaterYellowstoneEcosystemof a climate that is warmerand wetterthan
thepresentclimate (scenario 3).
elevation areas in YNP are underlainby rhyoliteand
other extrusiveigneous rocks containinglow calcium
levels (Despain, 1991).
The age class distribution
throughoutthe forestzone
if the climaticchanges also
could change substantially
alterthe disturbanceregime,particularly
thefrequency
and severityoffires.The occurrenceofextensive,standreplacing fires in the GYE is controlled largely by
weatherconditions;large firesonly occur duringsummersofbelow-normalprecipitation(Swetnam& Betancourt 1990; Despain, 1991; Turner& Romme,in press;
Renkin& Despain,in prep.). The subalpineforestlandscape of the GYE presentlycontainsa largecomponent
of old-growthstands that exceed 200 years in age
(Romme & Despain 1989). However,ifa warmer,drier
climateleads to an increasedfrequencyofsevere standreplacingfires,the landscape could be convertedinto
one dominatedbyyoungerstands,as is thecurrentconin partsof the CanadianRockiesand the subfiguration
arctic(e.g.,Johnson& Fryer1987; Tande 1979;Johnson
1979; see summaryin Turnerand Romme,in press).
ConservationBiology
Volume 5, No. 3, September1991
The combinationof a slightlysmallerforestzone and
a shiftto predominantlyyounger forest age classes
meansthathabitatforold-growth
species in theGreater
Yellowstone Ecosystemcould become smallerin area
and more fragmented
thanat present.Old-growthhabitatalreadyis beingreduced in portionsofthe GYE that
are managedfortimberproduction.Ifchangingclimatic
conditions and disturbanceregimes also reduce oldgrowthforestsin protectedparksand wildernessareas,
then some species could be threatenedwith local extinction.Examples of old-growthforestspecies in the
GYE include the northerntwinflower
(Linnaea borealis), fairyslipper(Calypso bulbosa), pine marten(Martesamericana), and goshawk(Accipitergentilis).
Withan upwardshiftin thelower timberline,
thearea
of low-elevationnonforestvegetationwould increase.
Animalscharacteristicof treeless landscapes, such as
pronghorn (Antilocapra americana) and badger
(Taxidea taxus), mightbecome more numerous.Sagedominatedby big sagebrush(Artemibrush-grasslands,
sia tridentata),bluebunchwheatgrass(Agropyronspi-
Romme
& Turner
catum), and Idaho fescue (Festuca idahoensis),
probablywould move to higherelevationsthan they
presentlyoccupy. At the lowest elevations,sagebrushgrasslandscould be replaced by semidesertvegetation,
characterized by saltbush (Atriplex gardneri) and
greasewood (Sarcobatus vermiculatus). This type of
vegetationpresentlyis rarein YellowstonePark,where
it is restrictedto soils developingin marineshales and
other extremelydroughtysubstrates(Despain, 1991).
Witha warmer,drierclimate,thesexerophyticcommunitiescould expand to occupy a greatervarietyof sites
at lower elevations,thoughtheymayremainlimitedby
edaphic conditions.
Compositionalchangeswithinvegetationzones could
be just as importantas the elevationalshifts.Species
changesberespond individuallyto the environmental
cause of differing
physiologicaltolerances,and altered
competitiveinteractionsbetween species are likelyto
result(Strain& Bazzaz 1983; Davis 1984; Neilson et al.
1989). Thus, communitycomposition and dynamics
could shift.In addition,a relativelysmall shiftin the
seasonalityof precipitationcould have dramaticeffects
on overallvegetationphysiognomyin the GreaterYellowstoneEcosystemeven iftotalprecipitationdoes not
change. Winterprecipitationfavorswoody vegetation,
whereas summerprecipitationfavorsgrassesand forbs
(Neilson et al. 1989). In YNP todaythereis a gradientin
westernportionsof the
the seasonalityofprecipitation;
Park have more winterprecipitation,characteristicof
the Great Basin region,while the eastern parts have
similarto
more springand earlysummerprecipitation,
the GreatPlains(Despain 1987).
These kinds of changes could lead to the developmentof plantcommunitiesin the GreaterYellowstone
Ecosystemquite unlikeanythatwe know today.Paleoecological reconstructionsof easternNorthAmerican
vegetationduringthe last 20,000 years have demonstratedthe widespread occurrence of species assemblages that have no modern analogues (Solomon &
of
Webb 1985; Davis 1984, 1989a). Our understanding
the biotic and abiotic processes thatpresentlycontrol
communitystructurein the GYE, and of the specific
waysthoseprocesseswould be alteredbyglobalclimate
change, are inadequate to permitany specificprojectionsof how communitiesmaychange in the next century.At present,all we can say is thatchange in communitystructureand compositionis likely,and thatthe
changescould be striking(Davis 1989a).
Anotheraspect of the warm,dryscenarioto be conunsidered is its implicationforthe large,free-ranging
gulatesthatconstituteone of the special featuresof the
GreaterYellowstone Ecosystem.The total numbersof
elk,bison,and othernativeungulatesare limitedprimarilyby the availabilityofwinterforage(Meagher 1973;
Houston 1982). Nonforestedareas at low elevations
provide the majorwinterhabitatforthese animals.
ofClimate
Change 379
Implications
Milderwintersand a largernonforestarea at low elevationscould mean higherpopulationsof ungulatesin
the GYE. Of particularsignificancewould be the increasedwinterhabitatwithinprotectedparks,whichlie
at relativelyhighelevations.Largenumbersofungulates
presentlytravelout ofYNP everywinterintosurrounding landswhere theyare subjected to hunting,artificial
feeding,and otherkindsofmanagementthatcomplicate
theoverallmanagementoftheseanimals(Berger 1991).
With milderwintersand more habitatwithinYNP, a
greaternumberofanimalsmightremainwithinthepark
boundaries.However, the associated drier conditions
also could depressplantproduction,and elevatedatmospheric CO2 could alter C/N ratios in plant foliage
(Strain & Bazzaz 1983). The resultcould be reduced
foragequantityand qualityand a lesseningof the improvement in winter ungulate habitat produced by
milderwinterweatherand a largerwinterrange.
utilizethevegetationin partsof
Ungulatesintensively
the GYE, influencingplant growthformand possibly
plant communitycomposition. For example, heavy
browsing appears to prevent regenerationof aspen
(Populus tremuloides)standsexcept followingextensive fires(Jones 1974; DeByle 1985; Mueggler& Bartos
1977). Changesin ungulatehabitatselectionor foraging
behaviorin response to vegetationchangeswould furtheralterthe compositionand structureof plant communitiesat low elevationsin the GYE.
Scenario
(2) The Intermediate
In the intermediatescenario,a large,compensatinginis hypothesizedto accomcrease in wateruse efficiency
pany the increased temperature,increased potential
and reduced or unchangedprecipievapotranspiration,
tation described above. This combinationof climatic
and physiologicalchangesprobablywould produce the
same effectsat highelevationsas were justpresentedfor
the warm,dry scenario: lengthof the growingseason
would increase,the upper timberlinewould move upward,the alpine zone would be reduced,and local extinctionof some obligate alpine species could occur
(Fig. 3). The rangeofwhitebarkpine also would shiftto
a higherelevationalzone with a substantiallysmaller
area (Fig. 6).
The positionof the lower timberlinemightnot shift,
however,because the effectsof higherPET would be
compensatedfor by increased WUE. Thus, the elevacould expand,since itsupper
tionalrangeofDouglas-fir
limitsapparentlyare determinedby low temperatures
and snow depth (Leverenz & Lev 1987), which would
be amelioratedin thisscenario,while itslowerlimitsare
set bydroughtstress,whichcould remainthesame (Fig.
mightnot move intoall
6). As noted earlier,Douglas-fir
ofthesitesthatwere climaticallysuitable,because ofits
edaphic requirements.
ConservationBiology
Volume 5, No. 3, September1991
380
Romme& Turner
Implications
of ClimateChange
4000
3000-
.0
>
2000
1000-
0
2000
4000
6000
8000 10000 12000
Area (square km)
Figure5. Cumulative area above each elevation
zone withinYellowstoneNational Park and theadjacent mountains along its easternborder.(Data
courtesyof theNational Park Service.)
With a higherupper timberlineand no change in
lower timberline,the total forestarea would increase
of the for(Fig. 6). However,the age class distribution
estsprobablywould shiftto a predominanceofyounger
age classes just as in the warm,dry scenario. This is
because the increase in WUE could compensate for
physiologicaldroughtstressbut would not affectthe
expected increasein firefrequencyand severityunder
warmerand drierclimaticconditions.The resultof increased forestarea and a shiftin age class distribution
could be eitherno change or a decrease in old-growth
habitat.
The area of nonforestcommunitiesat low elevations
would not change in this scenario. However, there
could be dramatic changes in species composition,
since plant species would respond individuallyto the
These
effectsof CO2 on WUE and net photosynthesis.
physiologicalchanges would alter species tolerances
unguand competitiveabilities.The area ofnonforested
latewinterrangealso would notchangein thisscenario,
but more of the range could be accessible forlonger
effect
periodsifwintersbecame milder.The fertilization
of elevated CO2 could potentiallyincreasenet primary
and forageproduction,leadingto a further
productivity
increasein the numberof ungulatesthatcould be supported on the winterrange,althoughsoil nutrientlimitationsand alteredC/Nratiosmightreduce the importance of the lattereffect(Strain& Bazzaz 1983).
(3) The Warm,WetScenario
In this as in the previous scenarios,warmertemperatures probablywould lead to an upward shiftin the
a reductionin area ofthe alpinezone,
uppertimberline,
and local extinctionofsome alpinespecies and commu-
ConservationBiology
Volume 5, No. 3, September1991
nities(Fig. 4). The rangeofwhitebarkpine also would
shiftup the mountainsand occupy a smallerarea (Fig.
6). Withincreasedprecipitationaccompanyingthe elehowever,even the remainingsubvated temperatures,
alpine environmentin the GYE could become unsuitable forthisspecies because of increasedcompetition.
Whitebarkpine is near the southernlimitof its distributionin thisarea (Arno & Hoff1989). The mechanisticexplanationforits southernrange limithas not
been demonstrated,but examinationof its range map
showsthatitis absentin thecentraland southernRocky
Mountainswhere moisture-ladenair masses fromthe
precipitation(the
GulfofMexico augmentlate-summer
"Arizonamonsoon"). In fact,the northernlimitof the
Arizona monsoon marksthe biogeographicboundary
for several plant species (Mitchell 1976; Neilson &
Wullstein1983). Whitebarkpine also is absent in the
wet coastal mountainranges of the PacificNorthwest
(Arno & Hoff1989). Ifwhitebarkpine presentlyis reor via competitiveinterstricted,eitherphysiologically
actions, to the drier summerclimates that presently
characterizethe northernRockies,thena climaticshift
to wettersummersin the GYE could resultin further
reductionor even local extinctionofwhitebarkpine in
thisarea.
Withincreasedprecipitationand wateruse efficiency,
droughtstressat low elevationswould be alleviated,and
the lower timberlinecould shiftdown slope to a lower
elevation.The range of Douglas-fircould expand both
upwardand downwardin thisscenario,unless it is limited by edaphic factors,as discussedabove (Fig. 4). The
totalforestedarea would increaseas the upper timberline moved upward and the lower timberlinemoved
downward (Fig. 6). Pollen data indicate that such an
expansionofsubalpineforestbothup and down slope in
CentralColorado occurred duringa mid-Holocenperiod (ca. 7000 to 4000 yrs ago) that apparentlywas
warmerand wetterthantoday(Fall 1985). Wetterconditions,especiallyin summer,could lead to a decrease
in firefrequencyand severityand a shiftin forestageclass distribution
to older age classes. The successional
mosaic of subalpineforestsin the GYE could become
morelike thatseen todayin the subalpinelandscapesof
the Colorado Rockies,where stands>500 yearsold are
common (e.g., Peet 1981; Veblen 1986; Aplet et al.
1988; see summaryin Turnerand Romme,in press).
With increased forest area and a shiftto older age
classes, the habitat available for old-growthspecies
would increase(Fig. 4).
The nonforestarea at low elevationswould be reduced if the lower treelinemoved down slope. Semidesertspecies and communities,
presentlyrestrictedto
severe substratesat the lowest elevations,could disappear entirelyfromYNP. Nonforestedwinterrangefor
ungulateswould be reduced, which potentiallycould
lead to smallernumbersofanimalson thewinterranges.
Implications
of ClimateChange
Romme& Turner
381
Alpine Zone
3500
(1,23
3000 -
Today
Whitebark Pine Zone
3500
(1,2,3)
3000 -
j oday
Forest Zone
3500
(2)
3000
2500
E
-
2000 -
Today
.0
(3)
C,
wL
Douglas-fir Zone
3000 2500 2000 -
Today
(2)
1500 -
(3)
1000 -
Low-elevation
2500 2000
Today, (2)
Nonforest Zone
(1)
(3)
2000
4000
8000
6000
Area (square kilometers)
10,000
12,000
Figure 6 Presentand projectedareas and elevational ranges of vegetationzones in YellowstoneNational Park
and theadjacent mountains along its easternborder.Scenario (1) is elevated temperature,
CO2, and drought
stress.Scenario (2) is elevated temperature,
but no change in droughtstress.Scenario (3) is elevated temperatureand CO2, but reduceddroughtstress.See textfor details.
However, ungulatesare quite adaptable and probably
would increasetheiruse offorestsin thissituation(Murie 1951; Meagher 1973; Geist 1982; Houston 1982).
Milder winter temperaturesand possible increases in
forageproductionalso would compensateforthereduction in nonforesthabitatforelk and bison. Populations
of obligate grasslandsspecies such as pronghornand
badger,however,probablywould be reduced in the
GYE and some could become locally extinct.
The combination of warmer temperatures,longer
growingseasons, increasedprecipitation,and elevated
proCO2 could produce substantialincreasesinprimary
ductivity throughout the vegetation zones of the
GreaterYellowstoneEcosystem.The potentialaugmen-
ConservationBiology
Volume 5, No 3, September1991
382
of ClimateChange
Implications
tationof productivitymightnot be realized,however,
because other limitingfactorssuch as nutrientsmight
become more importantunder these conditions.Patternsin herbivoryalso would undoubtedlybe altered,
with probable but presentlyunpredictableeffectson
primaryproductivity.
Again,because of individualplant
species responses to all of the changes in limitingfactors,some dramaticchangesin community
composition
could occur throughoutthe vegetationof the GYE.
Discussion
The threeclimatescenariossharesome similarities.
The
upper treeline in the GYE is likely to move toward
higher elevations in response to increased temperatures, regardless of the direction and magnitude of
changes in precipitationand water use efficiency,
and
the distribution
of Douglas-firis likelyto expand. Concomitantly,the alpine and whitebarkpine zones decrease in extentand become morefragmented
underall
scenariosconsideredhere (Fig. 6). Thus,it appearsthat,
under these climatescenarios,severalor manyspecies
and communitiesthatare restrictedto the alpine zone
are likelyto become locallyextinctwithinYellowstone
Park and possiblythe GreaterYellowstone Ecosystem
duringthe nextfewcenturies.However,the totalnumber of species withinYNP and the GYE actuallymay
change little.Semidesertvegetation,which is currently
rare and restrictedto specialized habitats,may expand
in lower-elevation
portionsoftheGYE,especiallyunder
the warm,dryscenario.
The simplisticprospect of a smooth northerlyand
upward migrationof plant species and communitiesis
complicatedby individualspecies responsesand by the
rate at which climatechange mayoccur (Davis 1989b;
Davis & Zabinski,in press). Neilson et al. (1989:76)
state thatthermalregimescould move 4 to 6 degrees
northwardwithin100 years;thisis a rate of 70 to 100
km per decade. Similarly,
elevationalzones could shift
65 to 90 m per decade (Neilson et al. 1989:56). By the
timea slow-growing
treereaches reproductiveage, the
environment
mayno longerbe suitableforseedlingsurvival (Davis 1989b; Davis & Zabinski,in press). Probablythe species thatwill trackthemovingthermalzones
are those with short,rapid lifehistories,for example,
introducedweeds, or species with a broad distribution
such as lodgepole pine (Strain & Bazzaz 1983). The
species thatwill respondleast effectively
are the longlived species that reproduce late or irregularlyand
thosewithalreadylimited,fragmented
for
distributions,
example,whitebarkpine and alpine species. Competitive interactionsbetween species also would be complicated as new species fromlower elevationalzones
would have to become establishedin the higherzones
where adults of the formerlydominantspecies would
ConservationBiology
Volume 5, No. 3, September1991
Ronume
& Turner
stillbe presenteven iftheywere no longerreproducing
locally.
Mature individualsof many long-livedspecies may
persistin theirpresentlocationsforas muchas decades,
even centuries,afterthe climatebecomes unsuitablefor
survivalof theiroffspring
(Davis 1984; Brubaker1986).
Plantcommunitiesin theGYE mightappearto be stable
for a long time,but aftera disturbance(such as fire,
insectoutbreak,or windstorm)the matureforestcommunitycould be replaced by a completelydifferent
suite of species. Indeed, many of the vegetational
changes caused by climatechange would be mediated
bydisturbance(Brubaker1986; Neilsonet al. 1989; Sandenberget al. 1987), and thedisturbanceregimesthemselves will be alteredby the climaticchanges,as noted
in the discussion of the warm, dry,and intermediate
scenarios.There is paleoecological evidence for rapid
changes in communitycompositionfollowingdisturbance of mature communitiesthat had persistedfor
some time despite climatic change (Brubaker 1986;
Dunwiddie 1986; Cwynar1987).
Researchand Monitoring
Needs
The rangeofscenarioswe presentedand theirpotential
implicationssuggestsome directionsforresearchand
monitoringin the GreaterYellowstoneEcosystem.Because of the complexityand spatiotemporalscales of
potentialecological responses,it is importantto design
long-term
measurementscreativelyso thattheyare particularlysensitive to early indications of ecological
change.Forexample,species or individualsthatare near
the limitsof theirrange of tolerance are likelyto respondmorerapidlythanthosethatare well withintheir
physiologicalrange.Our analysessuggestthatthe locationof the upper and lower timberlinein the GYE will
probablyrespondto anyof the likelyclimatescenarios.
Researchin subarcticNorthAmericaand Europe and in
mountainrangesofthe PacificNorthwestand GreatBasin in westernNorthAmericahas shown thattimberlinescan respondquicklyeven to climatechangesofthe
magnitudeobservedin the last 100 to 500 years(Davis
1984; Brubaker1986). Therefore,upperand lower timberlinesshould be high-priority
sites forresearchand
monitoring.
A firststep in the GYE timberlinestudieswould be to
relatethe presentlocation of the upper and lower timberlinesto underlyingvariablessuch as geologic substrate,slope, aspect, and disturbanceand land-usehisis needed beforewe can select
tory.This information
specificsitesformonitoringor distinguishbetween local and global causes oftimberlinechange.Yellowstone
NationalPark'sgeographicinformation
systemis an excellenttool forthisanalysiswithinparkboundaries,althoughadjacent public and privatelands mustalso be
Romme& Turner
evaluated.Researchis thenneeded to clarifythe mechanisms that explain the site-specificcompositionand
location of timberlines.Differencesin soils, local climate, and species availabilitycontributeto variability
the present
among sites (Krebs 1985). Afterclarifying
timberlinepatternsand mechanisms,we recommend
the establishmentof permanenttransectsthattraverse
upper and lower timberlineareas.The transectsshould
span a gradientfromforestinteriorto nonforestzones.
Tree mortalityand seedling establishmentwould be
measuredalongthesetransectsat regulartimeintervals.
These long-termmeasurementsmust be coupled with
researchdesignedto understandthe underlyingmechanismsofanychangesdetectedin timberline
locationor
composition.Forexample,yearlyor decadal variationin
tree establishmentand mortalitycould be correlated
in temperature
withcomparablyscaled fluctuations
and
precipitation to clarifyclimatic constraintson tree
growthand survival.
Anotherearlyindicatorof global climatechangemay
be alterationsin the frequencyand severityof disturbances,which maybe theproximalcause of substantial
changes in the structureand functionof naturallandscapes. Giventhe importanceoffirein the GreaterYellowstone Ecosystem,particularemphasis should continue to be placed on increasingour abilityto predict
the occurrence and effectsof fire.Postfiresuccession
should be monitoredfollowingthe 1988 firesand after
futurefires,especiallyin areas neartheupperand lower
timberlines.Monitoringof disturbancesalso must be
coupled with research directed at understandingthe
mechanismsofpostdisturbancechange and the individualisticresponsesof species to disturbancesofdiffering
magnitudesand severities.For example,thewidespread
and abundantestablishmentof aspen seedlingsfollowing the 1988 fireswas surprising(Despain & Renkin,
unpubl.ms.). It was previouslythoughtthataspen seedlings could not become establishedunder currentclimaticconditionsin the northernRockyMountains.We
do not yet knowwhetherthisaspen establishment
was
a normalresponseto large-scalefiresor whetherit heralds a fundamental
change in postfiresuccession in the
GYE since the last extensivefiresthatoccurredduring
theeighteenthand nineteenthcenturies.The survivalof
these aspen seedlingsundera varietyof site conditions
mustbe studied.Additionalresearchshouldbe initiated
to understandthe potentiallimitsof aspen distribution,
abundance, and reproductionin the GreaterYellowstoneEcosystemin responseto local climate,soils,herbivory,disturbance,and competitiveinteractionswith
otherplants.
Long-termmeasurementsof net primaryproduction
and communitycompositionwould be most appropriate in the lower-elevationsagebrush-grasslands.
The
grasses and shrubs are likely to show more rapid
changesin productivity
and compositionin responseto
Implications
of ClimateChange
383
climatethan the subalpineforests.The grasslandsalso
are influencedbynativeungulates,so researchintovegetation-climate-herbivore
interactionsshould continue.
The use ofremotelysensed data and simulationmodels to complementlong-term
fieldmeasurementsis desirable.The detectionofbroad-scaleecological changes
is complicatedby practicallimitationssuch as the difficultyof replicatingextensiveexperimentsor sampling
regimesand extrapolating
local data to an entireregion
(Turneret al. 1989). Remotesensingholds greatpromise forgatheringsynopticdata on vegetationcomposition,structure,
and processes.Landsatimageryhas been
used to estimateproductivity
in thelow-elevationgrasslands (Merrillet al. 1988), and repeated observations
would permitthe detectionof directionalchangesover
largeareas.Models are ofparticularimportancebecause
of the timelags in forestresponses.In addition,simulation models can be used to explore hypothesesabout
sensitiveparameters,and
ecological responses,identify
extrapolatefromfinescales to the region.For example,
Pastorand Post (1988) explored the impactof climate
change on matureforestsin easternNorthAmericaby
linkingan individual-based
standsimulationmodel with
an ecosystemmodel. The nitrogencycle and waterbalance were modeled explicitly,and a varietyof soils
were simulated.Resultsillustratedhow broad-scalesoil
heterogeneity
influencedforestecosystemresponsesto
climateover long timeperiods.Similarly,
modelswould
be needed to predictlandscape changesin responseto
alteredfirefrequencyand severityover long timescales
in the GYE undervaryingclimateregimes.Fireregimes
are sensitiveto the average climate over decades to
ofwater
centuries,as well as the interannualvariability
balance (Clark 1990). Thus, a model linkingmeteorological conditions,fireinitiationand spread,and plant
reestablishment
could be extremelyhelpfulin projecting long-termlandscape dynamicsin the GYE.
Althoughthe inevitability
of global climatechange is
not assured,the potentialimplicationsare of sufficient
magnitudethatit would be foolishto ignorethem.The
conservationof biologicaldiversityin extensivenatural
areas such as the GreaterYellowstone Ecosystemwill
become increasinglydifficultas the broad-scale constraintson the biota undergo changes that are more
rapid thanthose experienced in the past. Explorations
of scenariossuch as those we have presentedcan provide usefulheuristictools to increaseour understanding
of ecological dynamicsin response to climate change,
and can stimulatediscussionregardingthestrategiesappropriateformaintaining
biologicaldiversityin theface
of environmental
change.
Acknowledgments
We thankJayE. Anderson,Joel Berger,David Challinor,
VirginiaH. Dale, RichardMarsten,Ronald P. Neilson,
ConservationBiology
Volume 5, No. 3, September1991
384
Implications
of ClimateChange
RobertV. O'Neill, James Schmidt,and an anonymous
reviewerforprovidingcommentson this manuscript.
This researchwas fundedby the Ecological Research
Division,Officeof Health and Environmental
Research,
U.S. Department of Energy,under contract no. DEAC05-840R2 1400 withMartinMariettaEnergySystems,
Inc.; and by the Ecology Program,National Science
Foundation,throughgrantno. BSR-8408181. Publication No. 3616 of the Environmental
Sciences Division,
ORNL.
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