Ecological history

Abstract

This paper reviews the long­term effects of geology, climate, and human immigration on California's Mediterranean landscapes. The ecological history of the California landscape starts with the process of plate tectonics, which has created a complex geology of sea­floor sediments, metamorphic, and volcanic rocks. These processes, under the influence of the cold California ocean current, developed the characteristic Mediterranean climate over the past 3 million years. California's native flora and fauna represent groups selected from the larger array of species adapted to the summer rainfall climate present before the Pliocene throughout western North America. Climatic fluctuations over the past 150,000 years have been dramatic, with extended periods of glacial advance and cooler and wetter climates, although still with pronounced summer drought, and interspersed with periods warmer and drier than at present. These climatic shifts, which have extended into the Holocene, probably have had important recent impacts on the extent and structure of Quercus dominated vegetation types and coastal scrub, but not on chaparral or grasslands. Human influences began after 12,000 BP, the end of the last major glacial advance, and included possible extinctions of large herbivores and increased fire frequency. Human impacts in the last 150 years have included cultivation, urbanization, and introduction of new species. Of these factors, cultivation and plant species introductions appear most important in changing patterns of landscape dynamics. Interpretations of landscape change are strongly influenced by spatial and temporal scale, the differing causes of ecosystem degradation and improvement, and regional variability. The reliability of ecological interpretations and management recommendations can be improved using an approach which links holistic and reductionist explanations for landscape dynamics. http://rangelandarchive.ucdavis.edu/Annual_Rangeland_Handbook/

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UniversityofCalifornia
EcologicalHistory
byJamesBartolomeandSheriSpiegal
Summary
Thispaperreviewsthelongtermeffectsofgeology,climate,andhuman
immigrationonCalifornia'sMediterraneanlandscapes.
TheecologicalhistoryoftheCalifornialandscapestartswiththeprocessof
platetectonics,whichhascreatedacomplexgeologyofseafloorsediments,
metamorphic,andvolcanicrocks.Theseprocesses,undertheinfluenceofthe
coldCaliforniaoceancurrent,developedthecharacteristicMediterranean
climateoverthepast3millionyears.California'snativefloraandfauna
representgroupsselectedfromthelargerarrayofspeciesadaptedtothe
summerrainfallclimatepresentbeforethePliocenethroughoutwesternNorth
America.
Climaticfluctuationsoverthepast150,000yearshavebeendramatic,with
extendedperiodsofglacialadvanceandcoolerandwetterclimates,although
stillwithpronouncedsummerdrought,andinterspersedwithperiodswarmer
anddrierthanatpresent.Theseclimaticshifts,whichhaveextendedintothe
Holocene,probablyhavehadimportantrecentimpactsontheextentand
structureofQuercusdominatedvegetationtypesandcoastalscrub,butnoton
chaparralorgrasslands.
Humaninfluencesbeganafter12,000BP,theendofthelastmajorglacial
advance,andincludedpossibleextinctionsoflargeherbivoresandincreased
firefrequency.Humanimpactsinthelast150yearshaveincludedcultivation,
urbanization,andintroductionofnewspecies.Ofthesefactors,cultivationand
plantspeciesintroductionsappearmostimportantinchangingpatternsof
landscapedynamics.
Interpretationsoflandscapechangearestronglyinfluencedbyspatialand
temporalscale,thedifferingcausesofecosystemdegradationand
improvement,andregionalvariability.Thereliabilityofecological
interpretationsandmanagementrecommendationscanbeimprovedusingan
approachwhichlinksholisticandreductionistexplanationsforlandscape
dynamics.

Introduction
ThispaperoutlinestheecologicalhistoryoftheCalifornianlandscape,linking
naturalfactorstohumanimpacts.Thereviewdescribeslongtermchanges,
emphasizinggeology,vegetation,andclimate;andconcludesbyidentifying
someweaknessesininformationandtheory.
Ecologiststendtouseafavoritesetoftemporalandspatialscalestodetect
pattern.Thisstrategyisacceptableiftheeffectsonecologicalconclusionsare
recognized(Bartolome1989;Friedel1994).Normallytrendsandfluctuations
aredetectableonlyatspecificscales.Astemporalscalebecomeslongerthe
appropriatespatialscaletodetectpatternalsobecomeslargerthuslongterm
changesaremosteffectivelydescribedoverlargeareas.
Definingahierarchyofthescalesisusefultodetect,describe,andunderstand
thepatterns.Herewedefinealandscapeasadiversecollectionofmore
homogeneoussubunitscalledecosystems(FormanandGodron1986).
Ecosystemsareinturncomprisedofbioticcommunitiesandtheir
environment.Definingalandscapeinthiswayexplicitlyallowsforthe
detectionandanalysisofspatialandtemporalpatternbetweenandamong
ecosystemsandinferenceaboutcausalrelationships,acentralaimfor
ecologists.Althoughnestingecosystemswithinlandscapesmaynotalways
beappropriate(Allen1998;Wiensetal.2002),itisusefulforourdiscussion.
Changesinlandscapesimplylongtermandoftenimpreciselyunderstood
processes,suchasclimaticchangeandprehistorichumanimpacts.
Landscapesbydefinitionincludetherangeofnaturaldisturbancesinbiotic
communities(Urbanetal.1987).
FormostofhumanhistorytheCalifornianlandscaperemainedpristine,freeof
humaninfluence.OncepeoplediscoveredNorthAmericaabout12,000BP,
theybegantheextensiveuseofabundantnaturalresourcesthatcontinued
throughabout1800AD.Inthelate18thcenturytheSpanishpastoralsystem
wasimportedviaMexico,beginningaprogressiontowardsmoreintensive
resourceusethatcontinuestoday(Burcham1957).
Managementtechnologyandresourcemanagementobjectiveshavechanged
remarkablyduringthelast200years.TodayCaliforniahassomeofthemost
productivecroplandsintheworld.Recently,increasingconflictsamongurban
development,intensiveagriculture,extensivepastoralism,andprotectionism
haveledtoincreasinglyrigiddistinctionsamongthesevariousresource
managementobjectives.Thelackofalongtermperspectiveonthecauses
andinterpretationofchangesinthelandscapecontributestoconflictinggoals
andobjectivesforthepublicandforecologists.
GeologicHistory
MostofthefollowinginformationistakenfromanexcellentsummarybyAlt
andHyndman(1975),NorrisandWebb(1976),andHarden(2004).Geologic
historyformsthefoundationforunderstandinglandscapedevelopment.The

PacificOceanandplatetectonicsarethefundamentalfactorsshapingthe
Californialandscape.IntheTriassicperiod,about220millionyearsago,the
NorthAmericanandPacificPlatescollided,forcingoceanfloortosubduct
undertheoverridingwesternedgeofthecontinent.Thisactioneventually
wouldproducethemetamorphicandvolcanicrocksofthemajormountain
ranges,theSierraNevadaandtheKlamathMountains(Fig1).

Fig1.GeologicregionsofCalifornia.
AstheoldseafloorsurfaceofthePacificwasscrapedoff,itaccretedtoNorth
America’swesternmarginandformedthemostimportantbasicrocksofthe
CoastRanges,thesandstones,shales,andchertsoftheFranciscan
formation.Ultrabasicrocksfromdeeperoceanbottomwereforcedupwards
bythesubductionofthePacificPlatebeneaththeNorthAmericanPlate,
throughtheFranciscansedimentarylayerstoformserpentinesoilswiththeir
oftenuniquefloras.Continuedactionofthetwoplatesstilldistortsand
elevatesthecoastalmountainswhichformthewesternedgeoftheCentral
Valley.
TheCentralValleybeganfillingwithsediments,nowthousandsofmeters
deep,intheCretaceousperiodabout80millionyearsago.Originallyaninland
sea,itfilledwithenoughsedimenttobecomemostlydrylandby1.5million
yearsago.TheGreatValleySeapersistedlongerintheSanJoaquinValley
thantheSacramentoValley(Harden2004).
Alreadyalowmountainrangeformedasaresultofthesubductionofthe
PacificPlate,theSierraNevadatiltedandbecameseveralthousandmeters
higherinthePlioceneperiod.Thisuplift,beginningabout5millionyearsago,
hadmajoreffectsontheregionalenvironment.Alsoimportant,andrelatively
recent,wasthebeginningofmovementoftheSanAndreasFault,whichin15
millionyearsmovedgranitesfromthesouthernSierraNevadatotheirpresent
locations500kmfarthernorth,asfarasSanFrancisco.
Geologicprocessesatacontinentalscalecontinue,butwithinthepast
150,000years,globalclimaticeventshavemodifiedthebasicgeologic
structureandassociatedlandscapesinwayswhichmodifytheslower
processesofplatetectonics.Importanthavebeenthechangesinsealevel
associatedwithiceages,which,withupliftofcoastallands,hasproduced
distinctivecoastalterracelandscapes.Themostrecentmajorglacialretreat
beganabout14,000yearsago,endingtheTiogaglacialperiodintheSierra
(Phillipsetal.1996),whichgenerallycorrespondswiththelateWisconsinin
NorthAmericaandtheWurmintheAlps(Wood1975).Asglaciersmelted,the
risingsealeveldrownedthemouthoftheSacramentoRivertoformSan
FranciscoBay.
BecausesomuchofCalifornia'sMediterraneanlandscaperestsonthe
unstablesedimentaryrocksoftheFranciscanformation,massmovementof
soiliscommon.Thesemovementsareanintegralfeatureofthelandscape,
whichtypicallyexhibitsmultiplelandslidesofvariedagesbutassociatedwith
intenserainfallevents(SmithandHart1982).Removalofwoodyvegetation
oftenleadstodisastrousmassmovement(HeadyandPitt1979;Gabetand
Dunne2002).NorrisandWebb(1976)claimthatslidesonFranciscan
substratetransportmorematerialintheNorthCoastrangesthanstreams.
Pre‑HoloceneClimateandVegetation
AccordingtoAxelrod(1977),CaliforniasvegetationintheearlyMiocene

AccordingtoAxelrod(1977),CaliforniasvegetationintheearlyMiocene
period(25millionyearsBP)wasatemperaterainforestontheshoreofa
tropicalsea.Summerrainfallwasapartoftheclimate,andtheplants,
includingcogenericancestorsofmanymodernspeciesweredistributedover
alessruggedlandscapewithneitherCoastRangesnorthecurrent
manifestationoftheSierraNevada.
SummerrainfallbegantodecreaseinthemiddleMiocene.Thisprocess
culminatedwithfulldevelopmentofthemoderntypeMediterraneanclimatein
thelatePliocene(3millionyearsBP)followingupliftoftheSierraNevadaand
theRockyMountainswhichblockedoffsummermoisture.
MostoftheplanttaxawhichdominatevegetationinCalifornia'sMediterranean
landscapeswerewidespreadinNorthAmerica.Quercuswoodlandsand
savannas(Griffin1977)hadspreadovermuchofCaliforniainmodernformby
5millionyearsBP,withbothdeciduousandevergreenQuercusspp.
extendingupanddownthecoastalareas.Extensivewoodlandsofinland
southernCalifornia,whichinthePlioceneextendedoverthedesertregion,
laterretreatedintohigherdesertmountains(Axelrod1977).
RelativesofchaparralshrubspecieswerewidespreadintheMiddleMiocene
periodoverthesouthwesternUnitedStatesandmanyrelativesofpresent
CaliforniaspeciesarefoundinshrublandsofArizonaandNewMexico.
AccordingtoAxelrod(1977),althoughchaparral(Hanes1977)wascommon
bytheearlyPleistocene,itwaspresumablyduringthedrywarminterglacials
inthepast100,000yearsthatthetypespreadtoitspresentextent.The
coastalscrubtypepoorlyrepresentedinthefossilrecordmayonlydatefrom
thepostTiogaHoloceneperiod.
HoloceneClimateandVegetation
DuringthelastthousandyearsoftheWisconsin/Tiogaglacialmaximum,
about14,000BP,theclimateinCaliforniawasapparentlyconsiderablycooler
thanatpresent.McCartenandVanDevender(1988)estimatethatthe
temperatureduringtheTiogaglacialmaximumatRobber'sRoost(inthe
easternSierraat1200melevation)wassimilartothatnowfoundat1750m,
whichsuggestsadecreaseof3degreesCinaveragetemperature.Rainfallat
Robber'sRoostsubsequentlydecreasedbyabout35percentafterglacial
retreat.Thechangesinamountofrainfallvariedbyregionwheredocumented.
CoastalandSouthernCaliforniawereconsiderablywetterduringtheglacial
maximum(AdamandWest1983,DavisandMoratto1988).
California'sclimatethenbecamehotteranddrier.Theclassical"altithermal"
describedforNorthAmerica,hotanddrybetween7000and4500BP(Antevs
1948),doesnotfitsomeareasofCalifornia,especiallythewesternSierra,
whichappeartohavehadmaximumariditybetween10,000and7000BP.
Desertswerewetuntil8000BP,thecoastalregionsuntil7000BPaccording
toDavisetal.(1985).Theseregionalandtemporalvariationsshouldnot
obscurethefactthattheclimatedidbecomeconsiderablywarmeranddrier

forsignificantperiodsafter10,000BP.Theregionalclimatehasbecome
wettersince3000BP,probablyaccompaniedbycooling(Davisetal.1985).
Theseclimaticinfluencesareprimarilybasedonpollenrecordsandinference
fromcompositionofthosesamples,althoughglacialrecordsroughlysupport
thesedescriptions(Wood1975).FewpollenrecordsexistforMediterranean
typevegetationincoastalCalifornia.
Axelrod(1977)claimsthattwomajortypes,theQuercusagrifoliaforestand
theQuercusdouglasiiPinussabinianawoodlandhaverecentlychanged,the
formerextendingovermoreofSouthernCalifornia,thelattereliminatedover
muchofsouthernCaliforniaduringtheXerithermic(Holoceneperiod,7000
4500BP).HolocenereductionintheextentofQ.douglasiiissupportedbythe
apparentlyrelictualpopulationsinCalifornia'sChannelIslands(Muller1967).
California'sMediterraneanclimateinthepastfewmillenniahasbeenrather
poorlydescribed.Fromtreeringrecordsweknowthattherehavebeen
extendedperiods(severaldecades)ofbelowaveragerainfallinthepast500
years(FrittsandGordon1980).However,theeffectsofthoseperiodsonthe
extentandstructureofvegetationapartfromafewthousandtreeringsis
unknown.RecordsaremainlyfromIntermountainRegionlocationsathigh
elevation.
Between1600ADandthepresent,treeringanalysesrevealamajordryspell
from17601820sufficienttohavesignificantimpactsonvegetationstructure
anddistribution(FrittsandGordon1980).Anotherbetterdocumenteddry
periodfrom1860to1885isoftenconsideredasacontributortothedemiseof
thenativeperennialgrasslandandreplacementbyMediterraneanannuals
(Heady1977).
RecentlyCalifornia'sweatherhasbeen“normal”inthecontextof100yearsof
record.Theextremeandlargelyundocumenteddroughtattheendofthe18th
centurywasworsethanthatexperiencedinthewellpublicizeddroughtsofthe
late1970sandlate1980s.Generally,the20thcenturyhasbeenoneof
relativelyhighrainfallcomparedtothepast500years.Thisrecordoftree
ringsisalsosupportedbytherecent(700BPtopresent)glacialadvance
(Wood1976).
FirstHumanImpacts
HumanimmigrationintoCaliforniaabout12,000BPhadanunknowneffecton
thelandscape.Aswithotherareasoftheworldtheeffectsofhuman
immigrationintoNorthAmericaonfloraandfaunaareconfoundedwith
dramaticchangesinclimate.Speculationthatmassextinctionsofthemega
faunaafterthistimeweretheresultofhumanhuntersandtheiractivitiesis
supportedinferentiallybutbylittledirectevidence(Martin1973).Alargeand
diversefaunaofgrazingandbrowsinganimalswerepresentinCaliforniauntil
aseriesofextinctionsbetween20,000and10,000BP(Wagner1989;

Edwards2007).Theseextinctionswereconcentratedinmegavertebrate
genera,anddistinctlynonrandom(Marshall1988).Numbersofafewspecies,
antelopeandtuleelkremainedhighuntiltheearly1800s(McCullough1971).
AlthoughfirewasundoubtedlyatoolusedbyprehistoricCalifornians
(Sampson1944;Anderson2007),theimpactsofdeliberateburningon
vegetationarenoteasilydetermined(Headyetal.1991;Biswell1999;
Bartolomeetal.2007).FirefrequencyinMediterraneantypeshaschanged
significantlyinthepast300years,withfiresmuchmorefrequentinthe
Quercuswoodlandbetween1848(beforesettlement)and1948thaneither
beforeorafter(McClaranandBartolome1989a).
Duringthefirstperiodofhumanoccupation,severalwelldocumented
changesoccurredinmajorecosystemtypesadjacenttotheMediterranean
zone.IntheIntermountainregion,shrubandwoodlandtypesshowed
remarkablyrapidchangesindominantvegetationlinkedtoclimateandfire
(Mehringer1986).Artemesiatridentatadominanceshiftedoverashortperiod,
andJuniperusspp.movedhundredsofmetersinelevationoverafew
hundredyears.SimilardocumentationisnotavailableforMediterraneantype
vegetation,butsuchrapidchangeswerelikely(Axelrod1977).
Europeans:TheirLivestockandAnnualPlants
TheintroductionoflivestockandcropagricultureintotheCaliforniagrassland
hasbeenexhaustivelychronicled(Burcham1957,Heady1977,Headyetal.
1991,Bartolomeetal.2007).Theeffectsonwoodytypesismuchless
documented,althoughextensiveareasofchaparralandcoastalshrubhave
beenconvertedtocroporurban(FRAP2010).Areasofwoodlandhavebeen
thinnedforrangeimprovementorclearedforagriculture(Bartolomeetal.
1986).Inwoodlandareasneighboringtheurbanandagriculturalareas,the
frequencyoflightninginducedfirehasbeensuppressedinrecentdecades,
resultingindensegrowthburninginextremewildfireeventsthatsignificantly
affectwoodlandstructureandfunction(FRAP2010).
Thegrasslandwasaffectedbythecombinationofseveralfactorswhich
cannotbeproperlyevaluatedindependently:grazing,introductionofalien
plants,18601865drought,andcultivation(Headyetal.1991;Bartolomeetal.
2007).Grazing,beginninginthelate1700salongthecoastandspreading
inland,achievedheavystockinglevelsbythemid19thcentury.Exotics,
largelyannual,andfromtheeasternMediterranean,replacedthenatives
duringthesameperiodoflivestockincrease(Baker1989).
Cultivation,whichhadbeenlocalaroundthecoastalmissionsettlements,
spreadrapidlybeginninginabout1860,furtherrestrictingrangeareasand
eliminatingnativeplantsfromseveralmillionhectaresofgrasslandand
woodland(Burcham1957;Minnich2008).Unknownareasofshrubsandtrees
werecleared.
PrehistoricVegetationComparedtoPresent

ThefourmajorMediterraneanvegetationtypesinCaliforniahaveundergone
considerablechangesinceprehistorictimes.Humans,changesinfireregime
andcontinuinglongtermenvironmentaltrendshavehadsignificantbut
differingeffects.
Grasslands
GrasslandsinCaliforniaoccupyaclimatethatisnovelwithrespecttogeologic
time,withpronouncedsummerdrought,buttheyarefavoredbygeologic
history.Becausesomuchparentmaterialisderivedfromoldseafloor,
weatheringproducesfinetexturedsoilswhichoftensupportgrassland.The
oftenunstablesubstratesfavorherbaceousoverwoodyvegetation.Areaswith
rollingtopographyandanunderlyingclaypansupportuniquevernalpool
grasses(Heady1977).Theserpentinegrasslandsestablishedonsoils
derivedfromultrabasicrockformedbythesubductionofthePacificPlate
(MurphyandEhrlich1989).
Now,thegrasslandisalmostentirelydominatedbyannualsfromtheOld
World,whichformsanewstablevegetationtype.Appearanceandproductivity
ofthelandscapehasnotchangedmuch,butspecieswerereplaced.
Thegrassland'soriginalHoloceneextentandcomposition,beforeEuropean
settlement,isunknown.Mostgrasslandecologistssuggestanoverallextent
similartothepresentgrassland,exclusiveofareasundercultivation(Headyet
al.1991;Bartolomeetal.2007).Recentevidencefromphytoliths,silicaceous
bodiesshedbygrassesthatpersistinthesoilformillennia,revealsthat
grasseswereinfactpresentacrossthisextent,butthattheywerenotthe
dominantspeciesexceptincoastalareasandalongripariancorridors(Evett
andBartolome2013).Manycontendthatthepristinegrasslandwasprobably
dominatedbynativeannualforbs(Schiffman2007;Minnich2008).This
contentionrepresentsashiftfrompreviousthinking,inwhichnativeperennial
bunchgrasses,particularlypurpleneedlegrass(Stipapulchra),were
hypothesizedtobethedominants(Bartolomeetal.2007).Nativeannuals
werethoughttofillthespacesbetweenthebunchgrasses,withmoreannuals
indrierareasandfewerinwetterareas(Heady1977).Stillothersespouse
shrubsastheformerdominants,especiallyinthedriestinlandlocations
(Keeley1990,2002;HopkinsonandHuntsinger2005).
Reasonsfortheapparentdeclineofnativeperennialbunchgrasseshavebeen
sought.Majorlinesofconjectureincludelivestockgrazing,drought,and
cultivation(Bartolomeetal.2007).Thenativeperennialgrassesapparently
didnotevolveunderheavyyearlonggrazingpressure(Heady1977),butthis
hasnotbeenwellstudied(D’Antonioetal.2007).Adroughtthatoccurred
concurrentlywithintensivelivestockgrazingin18601865isthoughttobea
majorcontributortotheirdecline.Tillingsoilforcultivation,whichdirectlykills
thebunchgrasses,hasalsobeenimplicated(Strombergetal.2007).
Overthepast150years,thegrassland’sboundarieswithQuercussavanna

Overthepast150years,thegrassland’sboundarieswithQuercussavanna
mayhavemovedduetotreeremoval,buttheareainvolvedisundetermined
(Bartolome1987).Suggestionsofshrubinvasionintoformergrasslands
likewiseaccountforonlyasmallpercentageofthetotalarea(Wells1962)and
arelikelytobecompensatedbygrasslandexpansion.
Knownclimaticchangesareinsufficienttoaccountforalterationofthe
grasslanddominants,asthepresentandpastdominantspecieshave
sufficientlybroadtolerance,asexhibitedbypresentgeographicdistribution
(Headyetal.1991;JacksonandBartolome2007)andphysiological
characteristics(JacksonandRoy1989)tosurviveinsituchangesinclimate
characteristicofthepast100,000years.
Changesingrazingpressureandfireareinsufficientlyknowntodeterminethe
impactsonplantcommunities,althoughasageneralization,firewouldfavor
nativeperennials,andseasonlongusebylivestockwouldfavortheannuals
(Heady1977).Thebiggestimpactonvegetationwastheplantintroduction
whichalteredtheflora.Thenextbiggestimpactwascultivation.
ChangesingrazingregimesresultingfromactivitiesofEuropeansarenot
necessarytoexplainthereplacementofthenativegrasslandbyexotic
annuals.Thebiggestchangesingrazingfaunaandgrazingimpactsoccurred
between20,000and10,000BP,withlossofmanywildlifeandplanttaxa,long
beforelivestockintroduction.California'sextantnativegrassesorclose
relativeshavebeenpresentsincethePliocene,thustheHolocenerelease
fromgrazingpressurebylargeungulateswasarelativelyrecentevent,which
wasonlyshorttermbecausegrazingwasshortlysubsumedbydomestic
livestock.Changesingrazingdonot.Webelievethatthenativegrasslands
wouldhavedisappearedwithintroductionsofexoticplantsevenwithout
livestockgrazing.Thereplacementofnativeperennialsbyannualswithout
grazinghasbeenoccasionallyobserved(BartolomeandGemmill1981).
QuercusSavannas
TheprehistoricextentofQuercussavannasisunknown,althoughthetype
wasprobablywidespreadbeforetheHolocene.Savannaswereanespecially
widespreadtypethroughsouthernCaliforniainthePliocene,retreatingsince,
althoughwithlocalextensionsofQuercusagrifolia(Axelrod1977).Quercus
speciesarewelladaptedtoavarietyofsubstrates,andCalifornia’supland
Quercusspecieswereselectedforsummerdroughttolerance,resistanceto
browsing,andabilitytoresproutfollowingfire.Pollenstudiesshowthatthe
glacialperiods’generalexpansionofconiferscontrastedwithinterglacial
dominancebyoaks(Millar1999,Byrneetal.1991).
Savannaswereclearedforcultivationinmanyspots,particularlytheQuercus
lobatadominatedtype(Burcham1957).MuchQ.douglasiisavannamayhave
beenclearedsincesettlement,raisingtheboundaryofspeciesdistributioninto
foothillsfromthecultivatedandsettledvalleys.Thelandscapehasbeen
altereddramaticallyinmanyareas:tocropland,tograssland,andtomore

openstandswithfewsmalltrees.Thesechanges,unlikethoseinthe
grassland,affectedthesustainabilityofresourceproductivity.Forexample,
soilstabilityandthesoilnutrientcycleshavebeenconsiderablydisrupted.
However,toputthesechangesinperspective,theHoloceneperiodhasseen
widespreadnaturalthinningofQuercusstandsfromwoodlandsintosavannas.
TheQuercussavannaunderstoryhasundergonethesametypeof
replacementbyexoticannualsasgrasslands,butthischangeis
undocumented,lackingabasisforgoodspeculationabouttheoriginal
savannaunderstory.Changesinfireregimeandgrazingwouldhaveaffected
treeregeneration(McClaranandBartolome1989a),butarenotwell
documented.Theregionalimpactsofclearingontheunderstoryareunknown.
Shorttermchangesinclimateareunlikelytohaveaneffectonmaturetrees,
assavannatypesarewideranging.Forexample,Quercusdouglasii
dominatedecosystemsarefoundfrom1000to300mmmeanannual
precipitation(McClaranandBartolome1989b),arangewhichspanschanges
inaveragerainfalloverthepastfewthousandyears.Seedlingsurvivaland
regenerationaremorelikelytiedtoclimaticshifts,assuggestedbythe
regionaldifferencesinregenerationstatusofQ.douglasii,whichis
regeneratingbetterinareaswithhigherrainfall(MuickandBartolome1986).
Q.douglasiisavannastandsatlessthan750mmannualrainfallcouldhave
establishedduringthewetterperiodsofthepastfewthousandyears,oreven
duringtheTiogaglacialperiod,withinfrequentnewestablishmentsince.The
treesthatarenowpresentrangeuptoover400yearsinnominalage
(Bartolomeetal.1986),butmayallbefromsproutsfollowingfire,notnew
plantsestablishedfromacorns.
Chaparral
Thechaparral,accordingtoAxelrod(1977),hasextendeditsrangeinthe
Holocenefromanunknownpriorextent.Themajorimpactonchaparralhas
beenthedeclineinsummerrainfalloverseveralmillionyears,possiblywith
accompanyingchangesinfireregime.Chaparralplantshavefoundaniche
wheresandstoneparentmaterialandtheresultantcoarsertexturedsoils
permitaccesstosummermoisture.Theshallowersoilsinageologicallyactive
landscapealsofavorchaparral.
Perhapsaboriginalburningplayedaroleinalteringchaparralstructureand
extentinsomeareas,althoughnonanthropogenicfireswerealsocommon.
Certainlyrecent,shorttermeffectsoffireregimeareimportant(Wells1962,
OdionandDavis2000).Compositionmayhavebeenalteredbychangesin
thefireregimeandmaybecontinuallyalteredbycurrentprescribedburning
systems.Obligatefirefollowingplants,narrowlyadaptedtospecificfire
regimes,maybeintrouble.Thelongtermsustainabilityoftheecosystemis
alsoundetermined.Theeffectsofmanagementonsoilstabilityandwater

yieldremaintobeproperlyevaluatedinthecontextofnaturaldisturbancesof
thechaparrallandscape.Faunahasapparentlynotbeenaffectedasmuchas
ingrasslandandsavannatypes.
Livestockgrazingimpactsareundetermined,asareshorttermclimatic
effects,butsomeecologistssuspectthatgrazingcouldaffectstructurein
wayssimilartofire(Axelrod1977).
CoastalScrub
ThistypeextendedintheHolocene,andiscommononcoastalterraces
exposedbysealevelchanges.Thetypehasnowretreatedbecauseof
widespreadcultivationandsubsequenturbanizationofcoastalterracesinthe
past15Oyears.Thetypeisnowheavilyaffectedbydevelopmentand
changesinthefireregime(WestmanandO'Leary1986).Grazingandrecent
climaticchangesseemtobelessimportantthaninothertypes.
ThechangesinlandscapestructureandextentsinceEuropeansarrivedwere
exceptionalinthegrasslands,rapidinsomewoodytypessuchasthe
Quercuslobatasavanna,butlessapparentinQ.douglasiisavannas.The
amountofchangeinshrubtypesisnotaccuratelydeterminable.
Conclusions
ChangepermeatesCalifornia'slandscape,frequentlyproducingshortterm
catastrophiceffects.Fire,drought,humanimmigration,floralandfaunal
immigration,climaticchangeandextinction,haveallbeenimportantsincethe
MediterraneantypeclimatefirstappearedfollowingthePlioceneupliftofthe
SierraNevada.Somechangesinthelandscapehavenotaccelerated
appreciablyduringhistorictimes.Forexample,theHolocenemegafaunal
extinctionsinvolvedreplacementofmorespeciesatnearlythesamerateas
thesubsequentchangesresultingfromintroductionoflivestockgrazing
followingsettlementbyEuropeans.
Threeattributesoflandscapes,ecosystems,andcommunitiesinfluence
interpretationofthefactorscausingchange.First,theinvestigator'schoiceof
spatialandtemporalscalewillaffectinterpretationofchange.Landscapesare
mosttypicallystudiedwithrespecttolongtermregionalorglobalfactors,
althoughtheecosystemscalealsomatcheswithclimaticchangeor
immigrationsofnewspecies(Heady1975).Evenwhenlandscapechangeis
abrupt,forexamplethelandslidescommonincoastalCalifornia,thebasic
causalfactorslieinthegradualdevelopmentofunstablesubstrateswhich
ultimatelycausetheevent.
Second,aprimarymethodusedtointerpretthecausesoflandscapechange
dependsonsimilaritiesbetweenrecoveryofanecosystemfollowing
disturbance,andtheprocessofdegradation.Ifdisturbanceisadiscreteevent
withanidentifiedcause,forexamplecultivation,thenthedifferencesbetween
degradationandrecoveryareobvious.Longtermchangesinecosystems,

bothdegradationandrecovery,areoftenassumedtorepresentmirrorimages
ofthesamebasicprocess,withsimilarcausalfactors.Thisisrarelythecase.
Wearemostfamiliarwithexamplesofchangeinstructureandproductivityof
rangelandecosystemsgrazedbylivestock.Heretheprocessesofrange
deterioration(rarelymeasured)andrangeimprovement(sometimes
measured)areassumedtohavethesamebasiccausalmechanismsanda
pattern.Thisassumption,whichunderliesthebasictheoryofAmericanrange
science,isfalseformanyrangelandecosystems(Bartolome1985).The
ecologicalconceptofthresholdsprovidesatheoreticalfoundationfor
hypothesesabouttheecologicalprocessesthatcontributetolandscape
degradationandrecovery.Togaugewhereandwhenthethresholdconcept
canbeappliedtorangemanagement,rangelandecologistsaretesting
thresholdhypothesesinrangesettings(Bestelmeyeretal.2013).
InCalifornianQuercusspp.,thecausesforlackofregenerationandfor
successfulregenerationarelargelyindependent.Thecomplexof
environmentalinfluencesnecessaryforsuccessfulregenerationmaybe
unrelatedtothefactororfactorswhichcanpreventregeneration.
Regenerationinvolvesatleasttwodifferentsetsoffactorsandinfluences,
which,toaddtoconfusion,operateatdifferentspatialandtemporalscales
(Bartolomeetal.1987).Thepatternsandprocessesfordirectionalchange
towardmorenewtreesandfewernewtreesaredifferent.
Third,theuniquepropertiesofindividualecosystemslimitgeneral
interpretationsoflandscapechange.Evenwhencontrollingenvironmental
factorscanbeidentifiedatthepropertemporalandspatialscale,predictability
ofresponsecanbepoor.
AlthoughCaliforniandatasourcesarestillveryincomplete,theHolocene
recordofclimaticchangeandvegetationchangesuggestsconsiderable
regionalvariability.Oneinterpretationissimplelackofdata,butanotheristhat
similarecosystemsresponddifferentlytochangesinclimate.Theroleof
uniquehistoricalaccidentsinecosystemandcommunitychangeisalso
underestimated.InCaliforniangrasslandecosystems,theflorahascompletely
changedin15Oyears.Fewofitspresentpropertiescouldhavebeen
predictedbasedonpreviousstructureandfunction(HuennekeandMooney
1989).
Thethreeattributesoflandscapechangewhichcausedifficultiesforstudy
justifythevalueoflandscapeecologyforunderstandingchange(e.g.Liuand
Taylor.2002).However,thisapproachneedscomplementarysupportfrom
reductionists(e.g.Harper1967)todevelopanunderstandingofcauseand
effectwithinthecontextofwholesystems.Thepotentialbigloserswillbe
thosewhoattempttomanagelandscapes,ecosystems,communities,and
populationsbasedonpartialunderstandingofthepast.Themagnitudeof
futurelandscapechangesarelikelytosoonequalthoseofthedistantpast.
Thesechangescannotbeproperlymanagedbyrelyingonanalysisofpast

changestopredictthefutureundertheinfluenceofspatialandtemporal
scale,assumedsimilaritiesbetweenlandscapedeteriorationand
improvement,andchance.
Acknowledgements
WethankB.Allen.B.Holzman.L.Huntsinger.andC.Gonzalezfortheir
valuablecommentsonearlierdrafts.
Thispaperisanupdatedversionofareportoriginallypublishedin
ProceedingsoftheManandtheBiosphereSymposium,LandscapeEcology:
StudyofMediterraneanGrazedEcosystems,Nice,France.Oct.78,1989.
DepartmentofAgronomyandRangeScience,UniversityofCalifornia,Davis.
Pages215.
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