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UniversityofCalifornia
EcologicalHistory
byJamesBartolomeandSheriSpiegal
Summary
Thispaperreviewsthelongtermeffectsofgeology,climate,andhuman
immigrationonCalifornia'sMediterraneanlandscapes.
TheecologicalhistoryoftheCalifornialandscapestartswiththeprocessof
platetectonics,whichhascreatedacomplexgeologyofseafloorsediments,
metamorphic,andvolcanicrocks.Theseprocesses,undertheinfluenceofthe
coldCaliforniaoceancurrent,developedthecharacteristicMediterranean
climateoverthepast3millionyears.California'snativefloraandfauna
representgroupsselectedfromthelargerarrayofspeciesadaptedtothe
summerrainfallclimatepresentbeforethePliocenethroughoutwesternNorth
America.
Climaticfluctuationsoverthepast150,000yearshavebeendramatic,with
extendedperiodsofglacialadvanceandcoolerandwetterclimates,although
stillwithpronouncedsummerdrought,andinterspersedwithperiodswarmer
anddrierthanatpresent.Theseclimaticshifts,whichhaveextendedintothe
Holocene,probablyhavehadimportantrecentimpactsontheextentand
structureofQuercusdominatedvegetationtypesandcoastalscrub,butnoton
chaparralorgrasslands.
Humaninfluencesbeganafter12,000BP,theendofthelastmajorglacial
advance,andincludedpossibleextinctionsoflargeherbivoresandincreased
firefrequency.Humanimpactsinthelast150yearshaveincludedcultivation,
urbanization,andintroductionofnewspecies.Ofthesefactors,cultivationand
plantspeciesintroductionsappearmostimportantinchangingpatternsof
landscapedynamics.
Interpretationsoflandscapechangearestronglyinfluencedbyspatialand
temporalscale,thedifferingcausesofecosystemdegradationand
improvement,andregionalvariability.Thereliabilityofecological
interpretationsandmanagementrecommendationscanbeimprovedusingan
approachwhichlinksholisticandreductionistexplanationsforlandscape
dynamics.
Introduction
ThispaperoutlinestheecologicalhistoryoftheCalifornianlandscape,linking
naturalfactorstohumanimpacts.Thereviewdescribeslongtermchanges,
emphasizinggeology,vegetation,andclimate;andconcludesbyidentifying
someweaknessesininformationandtheory.
Ecologiststendtouseafavoritesetoftemporalandspatialscalestodetect
pattern.Thisstrategyisacceptableiftheeffectsonecologicalconclusionsare
recognized(Bartolome1989;Friedel1994).Normallytrendsandfluctuations
aredetectableonlyatspecificscales.Astemporalscalebecomeslongerthe
appropriatespatialscaletodetectpatternalsobecomeslargerthuslongterm
changesaremosteffectivelydescribedoverlargeareas.
Definingahierarchyofthescalesisusefultodetect,describe,andunderstand
thepatterns.Herewedefinealandscapeasadiversecollectionofmore
homogeneoussubunitscalledecosystems(FormanandGodron1986).
Ecosystemsareinturncomprisedofbioticcommunitiesandtheir
environment.Definingalandscapeinthiswayexplicitlyallowsforthe
detectionandanalysisofspatialandtemporalpatternbetweenandamong
ecosystemsandinferenceaboutcausalrelationships,acentralaimfor
ecologists.Althoughnestingecosystemswithinlandscapesmaynotalways
beappropriate(Allen1998;Wiensetal.2002),itisusefulforourdiscussion.
Changesinlandscapesimplylongtermandoftenimpreciselyunderstood
processes,suchasclimaticchangeandprehistorichumanimpacts.
Landscapesbydefinitionincludetherangeofnaturaldisturbancesinbiotic
communities(Urbanetal.1987).
FormostofhumanhistorytheCalifornianlandscaperemainedpristine,freeof
humaninfluence.OncepeoplediscoveredNorthAmericaabout12,000BP,
theybegantheextensiveuseofabundantnaturalresourcesthatcontinued
throughabout1800AD.Inthelate18thcenturytheSpanishpastoralsystem
wasimportedviaMexico,beginningaprogressiontowardsmoreintensive
resourceusethatcontinuestoday(Burcham1957).
Managementtechnologyandresourcemanagementobjectiveshavechanged
remarkablyduringthelast200years.TodayCaliforniahassomeofthemost
productivecroplandsintheworld.Recently,increasingconflictsamongurban
development,intensiveagriculture,extensivepastoralism,andprotectionism
haveledtoincreasinglyrigiddistinctionsamongthesevariousresource
managementobjectives.Thelackofalongtermperspectiveonthecauses
andinterpretationofchangesinthelandscapecontributestoconflictinggoals
andobjectivesforthepublicandforecologists.
GeologicHistory
MostofthefollowinginformationistakenfromanexcellentsummarybyAlt
andHyndman(1975),NorrisandWebb(1976),andHarden(2004).Geologic
historyformsthefoundationforunderstandinglandscapedevelopment.The
PacificOceanandplatetectonicsarethefundamentalfactorsshapingthe
Californialandscape.IntheTriassicperiod,about220millionyearsago,the
NorthAmericanandPacificPlatescollided,forcingoceanfloortosubduct
undertheoverridingwesternedgeofthecontinent.Thisactioneventually
wouldproducethemetamorphicandvolcanicrocksofthemajormountain
ranges,theSierraNevadaandtheKlamathMountains(Fig1).
Fig1.GeologicregionsofCalifornia.
AstheoldseafloorsurfaceofthePacificwasscrapedoff,itaccretedtoNorth
America’swesternmarginandformedthemostimportantbasicrocksofthe
CoastRanges,thesandstones,shales,andchertsoftheFranciscan
formation.Ultrabasicrocksfromdeeperoceanbottomwereforcedupwards
bythesubductionofthePacificPlatebeneaththeNorthAmericanPlate,
throughtheFranciscansedimentarylayerstoformserpentinesoilswiththeir
oftenuniquefloras.Continuedactionofthetwoplatesstilldistortsand
elevatesthecoastalmountainswhichformthewesternedgeoftheCentral
Valley.
TheCentralValleybeganfillingwithsediments,nowthousandsofmeters
deep,intheCretaceousperiodabout80millionyearsago.Originallyaninland
sea,itfilledwithenoughsedimenttobecomemostlydrylandby1.5million
yearsago.TheGreatValleySeapersistedlongerintheSanJoaquinValley
thantheSacramentoValley(Harden2004).
Alreadyalowmountainrangeformedasaresultofthesubductionofthe
PacificPlate,theSierraNevadatiltedandbecameseveralthousandmeters
higherinthePlioceneperiod.Thisuplift,beginningabout5millionyearsago,
hadmajoreffectsontheregionalenvironment.Alsoimportant,andrelatively
recent,wasthebeginningofmovementoftheSanAndreasFault,whichin15
millionyearsmovedgranitesfromthesouthernSierraNevadatotheirpresent
locations500kmfarthernorth,asfarasSanFrancisco.
Geologicprocessesatacontinentalscalecontinue,butwithinthepast
150,000years,globalclimaticeventshavemodifiedthebasicgeologic
structureandassociatedlandscapesinwayswhichmodifytheslower
processesofplatetectonics.Importanthavebeenthechangesinsealevel
associatedwithiceages,which,withupliftofcoastallands,hasproduced
distinctivecoastalterracelandscapes.Themostrecentmajorglacialretreat
beganabout14,000yearsago,endingtheTiogaglacialperiodintheSierra
(Phillipsetal.1996),whichgenerallycorrespondswiththelateWisconsinin
NorthAmericaandtheWurmintheAlps(Wood1975).Asglaciersmelted,the
risingsealeveldrownedthemouthoftheSacramentoRivertoformSan
FranciscoBay.
BecausesomuchofCalifornia'sMediterraneanlandscaperestsonthe
unstablesedimentaryrocksoftheFranciscanformation,massmovementof
soiliscommon.Thesemovementsareanintegralfeatureofthelandscape,
whichtypicallyexhibitsmultiplelandslidesofvariedagesbutassociatedwith
intenserainfallevents(SmithandHart1982).Removalofwoodyvegetation
oftenleadstodisastrousmassmovement(HeadyandPitt1979;Gabetand
Dunne2002).NorrisandWebb(1976)claimthatslidesonFranciscan
substratetransportmorematerialintheNorthCoastrangesthanstreams.
Pre‑HoloceneClimateandVegetation
AccordingtoAxelrod(1977),CaliforniasvegetationintheearlyMiocene
AccordingtoAxelrod(1977),CaliforniasvegetationintheearlyMiocene
period(25millionyearsBP)wasatemperaterainforestontheshoreofa
tropicalsea.Summerrainfallwasapartoftheclimate,andtheplants,
includingcogenericancestorsofmanymodernspeciesweredistributedover
alessruggedlandscapewithneitherCoastRangesnorthecurrent
manifestationoftheSierraNevada.
SummerrainfallbegantodecreaseinthemiddleMiocene.Thisprocess
culminatedwithfulldevelopmentofthemoderntypeMediterraneanclimatein
thelatePliocene(3millionyearsBP)followingupliftoftheSierraNevadaand
theRockyMountainswhichblockedoffsummermoisture.
MostoftheplanttaxawhichdominatevegetationinCalifornia'sMediterranean
landscapeswerewidespreadinNorthAmerica.Quercuswoodlandsand
savannas(Griffin1977)hadspreadovermuchofCaliforniainmodernformby
5millionyearsBP,withbothdeciduousandevergreenQuercusspp.
extendingupanddownthecoastalareas.Extensivewoodlandsofinland
southernCalifornia,whichinthePlioceneextendedoverthedesertregion,
laterretreatedintohigherdesertmountains(Axelrod1977).
RelativesofchaparralshrubspecieswerewidespreadintheMiddleMiocene
periodoverthesouthwesternUnitedStatesandmanyrelativesofpresent
CaliforniaspeciesarefoundinshrublandsofArizonaandNewMexico.
AccordingtoAxelrod(1977),althoughchaparral(Hanes1977)wascommon
bytheearlyPleistocene,itwaspresumablyduringthedrywarminterglacials
inthepast100,000yearsthatthetypespreadtoitspresentextent.The
coastalscrubtypepoorlyrepresentedinthefossilrecordmayonlydatefrom
thepostTiogaHoloceneperiod.
HoloceneClimateandVegetation
DuringthelastthousandyearsoftheWisconsin/Tiogaglacialmaximum,
about14,000BP,theclimateinCaliforniawasapparentlyconsiderablycooler
thanatpresent.McCartenandVanDevender(1988)estimatethatthe
temperatureduringtheTiogaglacialmaximumatRobber'sRoost(inthe
easternSierraat1200melevation)wassimilartothatnowfoundat1750m,
whichsuggestsadecreaseof3degreesCinaveragetemperature.Rainfallat
Robber'sRoostsubsequentlydecreasedbyabout35percentafterglacial
retreat.Thechangesinamountofrainfallvariedbyregionwheredocumented.
CoastalandSouthernCaliforniawereconsiderablywetterduringtheglacial
maximum(AdamandWest1983,DavisandMoratto1988).
California'sclimatethenbecamehotteranddrier.Theclassical"altithermal"
describedforNorthAmerica,hotanddrybetween7000and4500BP(Antevs
1948),doesnotfitsomeareasofCalifornia,especiallythewesternSierra,
whichappeartohavehadmaximumariditybetween10,000and7000BP.
Desertswerewetuntil8000BP,thecoastalregionsuntil7000BPaccording
toDavisetal.(1985).Theseregionalandtemporalvariationsshouldnot
obscurethefactthattheclimatedidbecomeconsiderablywarmeranddrier
forsignificantperiodsafter10,000BP.Theregionalclimatehasbecome
wettersince3000BP,probablyaccompaniedbycooling(Davisetal.1985).
Theseclimaticinfluencesareprimarilybasedonpollenrecordsandinference
fromcompositionofthosesamples,althoughglacialrecordsroughlysupport
thesedescriptions(Wood1975).FewpollenrecordsexistforMediterranean
typevegetationincoastalCalifornia.
Axelrod(1977)claimsthattwomajortypes,theQuercusagrifoliaforestand
theQuercusdouglasiiPinussabinianawoodlandhaverecentlychanged,the
formerextendingovermoreofSouthernCalifornia,thelattereliminatedover
muchofsouthernCaliforniaduringtheXerithermic(Holoceneperiod,7000
4500BP).HolocenereductionintheextentofQ.douglasiiissupportedbythe
apparentlyrelictualpopulationsinCalifornia'sChannelIslands(Muller1967).
California'sMediterraneanclimateinthepastfewmillenniahasbeenrather
poorlydescribed.Fromtreeringrecordsweknowthattherehavebeen
extendedperiods(severaldecades)ofbelowaveragerainfallinthepast500
years(FrittsandGordon1980).However,theeffectsofthoseperiodsonthe
extentandstructureofvegetationapartfromafewthousandtreeringsis
unknown.RecordsaremainlyfromIntermountainRegionlocationsathigh
elevation.
Between1600ADandthepresent,treeringanalysesrevealamajordryspell
from17601820sufficienttohavesignificantimpactsonvegetationstructure
anddistribution(FrittsandGordon1980).Anotherbetterdocumenteddry
periodfrom1860to1885isoftenconsideredasacontributortothedemiseof
thenativeperennialgrasslandandreplacementbyMediterraneanannuals
(Heady1977).
RecentlyCalifornia'sweatherhasbeen“normal”inthecontextof100yearsof
record.Theextremeandlargelyundocumenteddroughtattheendofthe18th
centurywasworsethanthatexperiencedinthewellpublicizeddroughtsofthe
late1970sandlate1980s.Generally,the20thcenturyhasbeenoneof
relativelyhighrainfallcomparedtothepast500years.Thisrecordoftree
ringsisalsosupportedbytherecent(700BPtopresent)glacialadvance
(Wood1976).
FirstHumanImpacts
HumanimmigrationintoCaliforniaabout12,000BPhadanunknowneffecton
thelandscape.Aswithotherareasoftheworldtheeffectsofhuman
immigrationintoNorthAmericaonfloraandfaunaareconfoundedwith
dramaticchangesinclimate.Speculationthatmassextinctionsofthemega
faunaafterthistimeweretheresultofhumanhuntersandtheiractivitiesis
supportedinferentiallybutbylittledirectevidence(Martin1973).Alargeand
diversefaunaofgrazingandbrowsinganimalswerepresentinCaliforniauntil
aseriesofextinctionsbetween20,000and10,000BP(Wagner1989;
Edwards2007).Theseextinctionswereconcentratedinmegavertebrate
genera,anddistinctlynonrandom(Marshall1988).Numbersofafewspecies,
antelopeandtuleelkremainedhighuntiltheearly1800s(McCullough1971).
AlthoughfirewasundoubtedlyatoolusedbyprehistoricCalifornians
(Sampson1944;Anderson2007),theimpactsofdeliberateburningon
vegetationarenoteasilydetermined(Headyetal.1991;Biswell1999;
Bartolomeetal.2007).FirefrequencyinMediterraneantypeshaschanged
significantlyinthepast300years,withfiresmuchmorefrequentinthe
Quercuswoodlandbetween1848(beforesettlement)and1948thaneither
beforeorafter(McClaranandBartolome1989a).
Duringthefirstperiodofhumanoccupation,severalwelldocumented
changesoccurredinmajorecosystemtypesadjacenttotheMediterranean
zone.IntheIntermountainregion,shrubandwoodlandtypesshowed
remarkablyrapidchangesindominantvegetationlinkedtoclimateandfire
(Mehringer1986).Artemesiatridentatadominanceshiftedoverashortperiod,
andJuniperusspp.movedhundredsofmetersinelevationoverafew
hundredyears.SimilardocumentationisnotavailableforMediterraneantype
vegetation,butsuchrapidchangeswerelikely(Axelrod1977).
Europeans:TheirLivestockandAnnualPlants
TheintroductionoflivestockandcropagricultureintotheCaliforniagrassland
hasbeenexhaustivelychronicled(Burcham1957,Heady1977,Headyetal.
1991,Bartolomeetal.2007).Theeffectsonwoodytypesismuchless
documented,althoughextensiveareasofchaparralandcoastalshrubhave
beenconvertedtocroporurban(FRAP2010).Areasofwoodlandhavebeen
thinnedforrangeimprovementorclearedforagriculture(Bartolomeetal.
1986).Inwoodlandareasneighboringtheurbanandagriculturalareas,the
frequencyoflightninginducedfirehasbeensuppressedinrecentdecades,
resultingindensegrowthburninginextremewildfireeventsthatsignificantly
affectwoodlandstructureandfunction(FRAP2010).
Thegrasslandwasaffectedbythecombinationofseveralfactorswhich
cannotbeproperlyevaluatedindependently:grazing,introductionofalien
plants,18601865drought,andcultivation(Headyetal.1991;Bartolomeetal.
2007).Grazing,beginninginthelate1700salongthecoastandspreading
inland,achievedheavystockinglevelsbythemid19thcentury.Exotics,
largelyannual,andfromtheeasternMediterranean,replacedthenatives
duringthesameperiodoflivestockincrease(Baker1989).
Cultivation,whichhadbeenlocalaroundthecoastalmissionsettlements,
spreadrapidlybeginninginabout1860,furtherrestrictingrangeareasand
eliminatingnativeplantsfromseveralmillionhectaresofgrasslandand
woodland(Burcham1957;Minnich2008).Unknownareasofshrubsandtrees
werecleared.
PrehistoricVegetationComparedtoPresent
ThefourmajorMediterraneanvegetationtypesinCaliforniahaveundergone
considerablechangesinceprehistorictimes.Humans,changesinfireregime
andcontinuinglongtermenvironmentaltrendshavehadsignificantbut
differingeffects.
Grasslands
GrasslandsinCaliforniaoccupyaclimatethatisnovelwithrespecttogeologic
time,withpronouncedsummerdrought,buttheyarefavoredbygeologic
history.Becausesomuchparentmaterialisderivedfromoldseafloor,
weatheringproducesfinetexturedsoilswhichoftensupportgrassland.The
oftenunstablesubstratesfavorherbaceousoverwoodyvegetation.Areaswith
rollingtopographyandanunderlyingclaypansupportuniquevernalpool
grasses(Heady1977).Theserpentinegrasslandsestablishedonsoils
derivedfromultrabasicrockformedbythesubductionofthePacificPlate
(MurphyandEhrlich1989).
Now,thegrasslandisalmostentirelydominatedbyannualsfromtheOld
World,whichformsanewstablevegetationtype.Appearanceandproductivity
ofthelandscapehasnotchangedmuch,butspecieswerereplaced.
Thegrassland'soriginalHoloceneextentandcomposition,beforeEuropean
settlement,isunknown.Mostgrasslandecologistssuggestanoverallextent
similartothepresentgrassland,exclusiveofareasundercultivation(Headyet
al.1991;Bartolomeetal.2007).Recentevidencefromphytoliths,silicaceous
bodiesshedbygrassesthatpersistinthesoilformillennia,revealsthat
grasseswereinfactpresentacrossthisextent,butthattheywerenotthe
dominantspeciesexceptincoastalareasandalongripariancorridors(Evett
andBartolome2013).Manycontendthatthepristinegrasslandwasprobably
dominatedbynativeannualforbs(Schiffman2007;Minnich2008).This
contentionrepresentsashiftfrompreviousthinking,inwhichnativeperennial
bunchgrasses,particularlypurpleneedlegrass(Stipapulchra),were
hypothesizedtobethedominants(Bartolomeetal.2007).Nativeannuals
werethoughttofillthespacesbetweenthebunchgrasses,withmoreannuals
indrierareasandfewerinwetterareas(Heady1977).Stillothersespouse
shrubsastheformerdominants,especiallyinthedriestinlandlocations
(Keeley1990,2002;HopkinsonandHuntsinger2005).
Reasonsfortheapparentdeclineofnativeperennialbunchgrasseshavebeen
sought.Majorlinesofconjectureincludelivestockgrazing,drought,and
cultivation(Bartolomeetal.2007).Thenativeperennialgrassesapparently
didnotevolveunderheavyyearlonggrazingpressure(Heady1977),butthis
hasnotbeenwellstudied(D’Antonioetal.2007).Adroughtthatoccurred
concurrentlywithintensivelivestockgrazingin18601865isthoughttobea
majorcontributortotheirdecline.Tillingsoilforcultivation,whichdirectlykills
thebunchgrasses,hasalsobeenimplicated(Strombergetal.2007).
Overthepast150years,thegrassland’sboundarieswithQuercussavanna
Overthepast150years,thegrassland’sboundarieswithQuercussavanna
mayhavemovedduetotreeremoval,buttheareainvolvedisundetermined
(Bartolome1987).Suggestionsofshrubinvasionintoformergrasslands
likewiseaccountforonlyasmallpercentageofthetotalarea(Wells1962)and
arelikelytobecompensatedbygrasslandexpansion.
Knownclimaticchangesareinsufficienttoaccountforalterationofthe
grasslanddominants,asthepresentandpastdominantspecieshave
sufficientlybroadtolerance,asexhibitedbypresentgeographicdistribution
(Headyetal.1991;JacksonandBartolome2007)andphysiological
characteristics(JacksonandRoy1989)tosurviveinsituchangesinclimate
characteristicofthepast100,000years.
Changesingrazingpressureandfireareinsufficientlyknowntodeterminethe
impactsonplantcommunities,althoughasageneralization,firewouldfavor
nativeperennials,andseasonlongusebylivestockwouldfavortheannuals
(Heady1977).Thebiggestimpactonvegetationwastheplantintroduction
whichalteredtheflora.Thenextbiggestimpactwascultivation.
ChangesingrazingregimesresultingfromactivitiesofEuropeansarenot
necessarytoexplainthereplacementofthenativegrasslandbyexotic
annuals.Thebiggestchangesingrazingfaunaandgrazingimpactsoccurred
between20,000and10,000BP,withlossofmanywildlifeandplanttaxa,long
beforelivestockintroduction.California'sextantnativegrassesorclose
relativeshavebeenpresentsincethePliocene,thustheHolocenerelease
fromgrazingpressurebylargeungulateswasarelativelyrecentevent,which
wasonlyshorttermbecausegrazingwasshortlysubsumedbydomestic
livestock.Changesingrazingdonot.Webelievethatthenativegrasslands
wouldhavedisappearedwithintroductionsofexoticplantsevenwithout
livestockgrazing.Thereplacementofnativeperennialsbyannualswithout
grazinghasbeenoccasionallyobserved(BartolomeandGemmill1981).
QuercusSavannas
TheprehistoricextentofQuercussavannasisunknown,althoughthetype
wasprobablywidespreadbeforetheHolocene.Savannaswereanespecially
widespreadtypethroughsouthernCaliforniainthePliocene,retreatingsince,
althoughwithlocalextensionsofQuercusagrifolia(Axelrod1977).Quercus
speciesarewelladaptedtoavarietyofsubstrates,andCalifornia’supland
Quercusspecieswereselectedforsummerdroughttolerance,resistanceto
browsing,andabilitytoresproutfollowingfire.Pollenstudiesshowthatthe
glacialperiods’generalexpansionofconiferscontrastedwithinterglacial
dominancebyoaks(Millar1999,Byrneetal.1991).
Savannaswereclearedforcultivationinmanyspots,particularlytheQuercus
lobatadominatedtype(Burcham1957).MuchQ.douglasiisavannamayhave
beenclearedsincesettlement,raisingtheboundaryofspeciesdistributioninto
foothillsfromthecultivatedandsettledvalleys.Thelandscapehasbeen
altereddramaticallyinmanyareas:tocropland,tograssland,andtomore
openstandswithfewsmalltrees.Thesechanges,unlikethoseinthe
grassland,affectedthesustainabilityofresourceproductivity.Forexample,
soilstabilityandthesoilnutrientcycleshavebeenconsiderablydisrupted.
However,toputthesechangesinperspective,theHoloceneperiodhasseen
widespreadnaturalthinningofQuercusstandsfromwoodlandsintosavannas.
TheQuercussavannaunderstoryhasundergonethesametypeof
replacementbyexoticannualsasgrasslands,butthischangeis
undocumented,lackingabasisforgoodspeculationabouttheoriginal
savannaunderstory.Changesinfireregimeandgrazingwouldhaveaffected
treeregeneration(McClaranandBartolome1989a),butarenotwell
documented.Theregionalimpactsofclearingontheunderstoryareunknown.
Shorttermchangesinclimateareunlikelytohaveaneffectonmaturetrees,
assavannatypesarewideranging.Forexample,Quercusdouglasii
dominatedecosystemsarefoundfrom1000to300mmmeanannual
precipitation(McClaranandBartolome1989b),arangewhichspanschanges
inaveragerainfalloverthepastfewthousandyears.Seedlingsurvivaland
regenerationaremorelikelytiedtoclimaticshifts,assuggestedbythe
regionaldifferencesinregenerationstatusofQ.douglasii,whichis
regeneratingbetterinareaswithhigherrainfall(MuickandBartolome1986).
Q.douglasiisavannastandsatlessthan750mmannualrainfallcouldhave
establishedduringthewetterperiodsofthepastfewthousandyears,oreven
duringtheTiogaglacialperiod,withinfrequentnewestablishmentsince.The
treesthatarenowpresentrangeuptoover400yearsinnominalage
(Bartolomeetal.1986),butmayallbefromsproutsfollowingfire,notnew
plantsestablishedfromacorns.
Chaparral
Thechaparral,accordingtoAxelrod(1977),hasextendeditsrangeinthe
Holocenefromanunknownpriorextent.Themajorimpactonchaparralhas
beenthedeclineinsummerrainfalloverseveralmillionyears,possiblywith
accompanyingchangesinfireregime.Chaparralplantshavefoundaniche
wheresandstoneparentmaterialandtheresultantcoarsertexturedsoils
permitaccesstosummermoisture.Theshallowersoilsinageologicallyactive
landscapealsofavorchaparral.
Perhapsaboriginalburningplayedaroleinalteringchaparralstructureand
extentinsomeareas,althoughnonanthropogenicfireswerealsocommon.
Certainlyrecent,shorttermeffectsoffireregimeareimportant(Wells1962,
OdionandDavis2000).Compositionmayhavebeenalteredbychangesin
thefireregimeandmaybecontinuallyalteredbycurrentprescribedburning
systems.Obligatefirefollowingplants,narrowlyadaptedtospecificfire
regimes,maybeintrouble.Thelongtermsustainabilityoftheecosystemis
alsoundetermined.Theeffectsofmanagementonsoilstabilityandwater
yieldremaintobeproperlyevaluatedinthecontextofnaturaldisturbancesof
thechaparrallandscape.Faunahasapparentlynotbeenaffectedasmuchas
ingrasslandandsavannatypes.
Livestockgrazingimpactsareundetermined,asareshorttermclimatic
effects,butsomeecologistssuspectthatgrazingcouldaffectstructurein
wayssimilartofire(Axelrod1977).
CoastalScrub
ThistypeextendedintheHolocene,andiscommononcoastalterraces
exposedbysealevelchanges.Thetypehasnowretreatedbecauseof
widespreadcultivationandsubsequenturbanizationofcoastalterracesinthe
past15Oyears.Thetypeisnowheavilyaffectedbydevelopmentand
changesinthefireregime(WestmanandO'Leary1986).Grazingandrecent
climaticchangesseemtobelessimportantthaninothertypes.
ThechangesinlandscapestructureandextentsinceEuropeansarrivedwere
exceptionalinthegrasslands,rapidinsomewoodytypessuchasthe
Quercuslobatasavanna,butlessapparentinQ.douglasiisavannas.The
amountofchangeinshrubtypesisnotaccuratelydeterminable.
Conclusions
ChangepermeatesCalifornia'slandscape,frequentlyproducingshortterm
catastrophiceffects.Fire,drought,humanimmigration,floralandfaunal
immigration,climaticchangeandextinction,haveallbeenimportantsincethe
MediterraneantypeclimatefirstappearedfollowingthePlioceneupliftofthe
SierraNevada.Somechangesinthelandscapehavenotaccelerated
appreciablyduringhistorictimes.Forexample,theHolocenemegafaunal
extinctionsinvolvedreplacementofmorespeciesatnearlythesamerateas
thesubsequentchangesresultingfromintroductionoflivestockgrazing
followingsettlementbyEuropeans.
Threeattributesoflandscapes,ecosystems,andcommunitiesinfluence
interpretationofthefactorscausingchange.First,theinvestigator'schoiceof
spatialandtemporalscalewillaffectinterpretationofchange.Landscapesare
mosttypicallystudiedwithrespecttolongtermregionalorglobalfactors,
althoughtheecosystemscalealsomatcheswithclimaticchangeor
immigrationsofnewspecies(Heady1975).Evenwhenlandscapechangeis
abrupt,forexamplethelandslidescommonincoastalCalifornia,thebasic
causalfactorslieinthegradualdevelopmentofunstablesubstrateswhich
ultimatelycausetheevent.
Second,aprimarymethodusedtointerpretthecausesoflandscapechange
dependsonsimilaritiesbetweenrecoveryofanecosystemfollowing
disturbance,andtheprocessofdegradation.Ifdisturbanceisadiscreteevent
withanidentifiedcause,forexamplecultivation,thenthedifferencesbetween
degradationandrecoveryareobvious.Longtermchangesinecosystems,
bothdegradationandrecovery,areoftenassumedtorepresentmirrorimages
ofthesamebasicprocess,withsimilarcausalfactors.Thisisrarelythecase.
Wearemostfamiliarwithexamplesofchangeinstructureandproductivityof
rangelandecosystemsgrazedbylivestock.Heretheprocessesofrange
deterioration(rarelymeasured)andrangeimprovement(sometimes
measured)areassumedtohavethesamebasiccausalmechanismsanda
pattern.Thisassumption,whichunderliesthebasictheoryofAmericanrange
science,isfalseformanyrangelandecosystems(Bartolome1985).The
ecologicalconceptofthresholdsprovidesatheoreticalfoundationfor
hypothesesabouttheecologicalprocessesthatcontributetolandscape
degradationandrecovery.Togaugewhereandwhenthethresholdconcept
canbeappliedtorangemanagement,rangelandecologistsaretesting
thresholdhypothesesinrangesettings(Bestelmeyeretal.2013).
InCalifornianQuercusspp.,thecausesforlackofregenerationandfor
successfulregenerationarelargelyindependent.Thecomplexof
environmentalinfluencesnecessaryforsuccessfulregenerationmaybe
unrelatedtothefactororfactorswhichcanpreventregeneration.
Regenerationinvolvesatleasttwodifferentsetsoffactorsandinfluences,
which,toaddtoconfusion,operateatdifferentspatialandtemporalscales
(Bartolomeetal.1987).Thepatternsandprocessesfordirectionalchange
towardmorenewtreesandfewernewtreesaredifferent.
Third,theuniquepropertiesofindividualecosystemslimitgeneral
interpretationsoflandscapechange.Evenwhencontrollingenvironmental
factorscanbeidentifiedatthepropertemporalandspatialscale,predictability
ofresponsecanbepoor.
AlthoughCaliforniandatasourcesarestillveryincomplete,theHolocene
recordofclimaticchangeandvegetationchangesuggestsconsiderable
regionalvariability.Oneinterpretationissimplelackofdata,butanotheristhat
similarecosystemsresponddifferentlytochangesinclimate.Theroleof
uniquehistoricalaccidentsinecosystemandcommunitychangeisalso
underestimated.InCaliforniangrasslandecosystems,theflorahascompletely
changedin15Oyears.Fewofitspresentpropertiescouldhavebeen
predictedbasedonpreviousstructureandfunction(HuennekeandMooney
1989).
Thethreeattributesoflandscapechangewhichcausedifficultiesforstudy
justifythevalueoflandscapeecologyforunderstandingchange(e.g.Liuand
Taylor.2002).However,thisapproachneedscomplementarysupportfrom
reductionists(e.g.Harper1967)todevelopanunderstandingofcauseand
effectwithinthecontextofwholesystems.Thepotentialbigloserswillbe
thosewhoattempttomanagelandscapes,ecosystems,communities,and
populationsbasedonpartialunderstandingofthepast.Themagnitudeof
futurelandscapechangesarelikelytosoonequalthoseofthedistantpast.
Thesechangescannotbeproperlymanagedbyrelyingonanalysisofpast
changestopredictthefutureundertheinfluenceofspatialandtemporal
scale,assumedsimilaritiesbetweenlandscapedeteriorationand
improvement,andchance.
Acknowledgements
WethankB.Allen.B.Holzman.L.Huntsinger.andC.Gonzalezfortheir
valuablecommentsonearlierdrafts.
Thispaperisanupdatedversionofareportoriginallypublishedin
ProceedingsoftheManandtheBiosphereSymposium,LandscapeEcology:
StudyofMediterraneanGrazedEcosystems,Nice,France.Oct.78,1989.
DepartmentofAgronomyandRangeScience,UniversityofCalifornia,Davis.
Pages215.
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