HISTORICAL
IN THE
DECLINE
OF
RIVERSIDE-PERRIS
COASTAL
PLAIN,
SAGE
SCRUB
CALIFORNIA
RICHARD A. MINNICH, Department of Earth Sciences, University of California,
Riverside, California 92521
RAYMOND J. DEZZANI, College of Liberal Arts, Boston University,Boston, Massachusetts
02215-1401
Californian coastal sage scrub (CSS), which consistsof dense stands of
soft-leaved drought-deciduoussubshrubs0.5-1.5 m tall, has been extensivelyclearedfor agricultureand urbanization(Westman1981). The state of
California has initiated a regionally focused conservation-planningprocess
for natural communities,includingCSS, in southern California (O'Leary et
al. 1992). To protect two endangered species of CSS, the Stephens'
Kangaroo Rat (Dipodornys $tephen$i) and the California Gnatcatcher
(Polioptila californica), Riverside County has developed a habitat-conservation plan for CSS in the Riverside-Perris
Plain, an area of rapid presentand
future urbanization.This has resultedin protectionof CSS through purchase
of private lands surroundingpreexistingpublic lands, largely through political and economic incentives(Feldman 1995).
Few studies have examined the landscape-scaledynamics of surviving
CSS, particularlyin relation to the invasionof exotic annualsintroduced
from the Mediterraneanbasin and Middle East since the late 18th century.
While it is widely reported that exotic annuals have displacedindigenous
herbaceous ecosystems(McNaughton 1968, Gulmon 1977, Heady 1988,
Drake and Mooney 1986, Huenneke et al. 1990, D'Antonio and Vitousek
1992), relationshipsbetween the spread of these annualsand the dynamics
of shrublandcommunitiesare not well understood.O'Leary and Westman
(1988) and O'Leary (1990) demonstratedthat CSS has been reduced by
frequent fire, grazing, and the invasion of exotic annuals, as well as air
pollution.These trendshave specialimportancefor the endangeredCalifornia Gnatcatcher, whose habitat requirements include dense stands of CSS
with high shrub-species
diversity(Atwood 1993). Planning efforts have not
taken into account whether CSS is a static system.
From 1929 to 1934 the vegetationof Californiawas inventoriedby the
VegetationType Map (VTM) Survey under the CaliforniaForest and Range
ExperimentStation (Minnich et al. 1995). During the survey, 78 plots of
CSS were surveyedin the Riverside-Perris
Plain. The objectiveof our study
is to replicatethe VTM for this area to quantifychangeof CSS over the past
60 years and to evaluate the role of exotics, specifically, whether the
invasionof exotics is dependenton frequent burning or whether exotics
promote frequent burning. The answer to this question affects whether
managementshouldfocuson disturbanceor the controlof exotics.Addressing the question requires the examination of such processesassociatedin
habitat change, including disturbance, grazing, competitive replacement,
and air pollution.
366
Western
Birds29:366-391,1998
HISTORICAL
STUDY
DECLINE
OF
COASTAL
SAGE
SCRUB
AREA
The Riverside-Perris Plain is a fault-bound alluvial basin with scattered small
hills (elevation600-1000 m) 80 km east of Los Angeles,Califomia (Figure1).
Although the floor of the plain has been largely cultivatedsince ca. 1900,
hillsidesare coveredwith CSS dominatedby Artemisia californica, Eriogohum fasciculatum, Salvia mellifera, S. apiana, and Encelia farinosa (plant
names after Hickman 1993). The climate is Mediterraneanwith winter rain
from frontalcyclonesand summerdrought.The rain shadowof the Santa Ana
Mountainsto the west leavesthe plain with mean annual precipitationof 25
to 30 cm, falling mostly between November and April. The Santa Ana
Mountains also isolate the basin from cooling sea breezes, resulting in
maximum temperatures•35øC from June to September.
METHODS
We mapped CSS in the Riverside-PerrisPlain from Kodak Type 2443
color positivetransparencies
taken from an aircraftin 1990 (scale1:20,000),
using a roll-film stereoscopewith 3 and 8)< magnification and a standard
pocket stereoscope.CSS is recognizedfrom the blue-gray color and low
stature of subshrubs.We identifiedthree types on the basis of cover: (1)
dense coastal sage scrub (subshrubsforming contiguousstands),(2) mixed
coastal sage scrub/exotic grassland(open stands of subshrubswith understow of exotic annuals),and (3) exotic grasslands(continuousexotic annuals
Figure1. Coastalsagescrubin the Riverside-Perris
Plain,withlocations
of California
VegetationType Maps (VTM) plots.
367
HISTORICAL
DECLINE
OF
COASTAL
SAGE
SCRUB
with subshrubcover <20%). Boundary data were transferredonto 1:24,000
topographicsheetsby means of a zoom transferscope. The vegetationmap
and VTM data were entered into the Arc-Info geographic-information
system installed on a Sun workstation. General methods are given in
Minnich (1987).
We compileddata from VTM plots from originalrecords.The plots consist
of 20 field quadratscovering0.005 acres(0.04 ha, on file at the Department
of EnvironmentalScience, Policy, and Management, University of California, Berkeley).AlthoughVTM workersdid not leave permanentmarkers,we
believethat plots could be relocatedwithin a radiusof 100 m from localities
givenon manuscriptmaps (Minnichet al. 1995). We sampledeach site three
times, subjectively
scatteringreplicateplots over an area of 1.0 ha, with each
matchingthe slope and aspect conditionsgiven on original field sheets.The
three replicateswere averagedto achieve one modern compositesample to
be comparedwith originaldata. In each plot the dominantshrub speciesis
identifiedfor each 0.001 acre (ca. 2 m2). No speciesare recordedfor
individual quadrats lacking shrub cover. Eighty-two plots were visited in
1930, but four had been cleared for urbanizationor fuelbreaks,leaving a
total sampleof 78 plots. Some changebetween1930 and 1992 may be due
to sampling error in relocation, but we believe that actual changes will
emerge as a result of the large sample size.
RESULTS
Published VTM vegetation maps (Riverside, San Jacinto quadrangles;
Weislander 1934, 1938) show that CSS covered ca. 74,950 ha, concentrated on hilly terrain of the Riverside-Perris Plain. Most stands were
apparentlydense, as only a few hills were shown as "semibarren."This is
confirmedby aerial photographs
taken in 1931, 1948, 1952, and 1962 (on
file, Riverside County Department of Public Works). VTM field sheets
disclosethat most semibarrensites were recenfiybumed. A few sites are too
rocky to support dense cover. VTM vegetation maps give a hierarchical
classificationby life-form and species dominance (species_+20%cover). In
the northern Riverside-Perris Plain, stands were dominated by Encelia
farinosa on southern exposures, Eriogonurn fas½iculatum and Artemisia
½alifornica on northern exposures. Farther south, most stands comprised
mixtures of Eriogonurn fas½iculatum, Artemisia ½alifornica, and Salvia
mellifera. Monotypic stands of Eriogonurn fas½iculaturncovered hillsides
in the far south. Salvia mellifera was dominant in the Santa Ana Mountains
and the badlandsof the northeast.Other standsof $. mellifera were found
in the central Riverside-PerrisPlain in associationwith underlyingMesozoic
basic intrusiverocks (gabbrobasalts,Calif. Div. Mines 1969).
By 1990, only 30,118 ha or 40.1% of the stands mapped as CSS by
VTM workers still existed as contiguous stands (Figure 1), with most
occurring on outcrops of Mesozoic gabbro basalts in the south. Another
31,408 ha (41.9% of stands)were open CSS mixed with a continuouslayer
of exotic annual grasses.The remaining13,424 ha (18.0%) were entirely
convertedto exotic grassland,with most conversionsconcentratedin valley
edges and north-facingslopes.Cursory field observationsindicatethat pure
368
HISTORICAL
DECLINE
OF
COASTAL
SAGE
SCRUB
exotic grasslandsand exotic grasslandsmixed with CSS are dominated by
Bromus madritensis ssp. rubens on southern exposuresand B. diandrus
on northern exposures.Other important species include Avena barbata,
Brassica geniculata, Schismus barbatus, and Erodium cicutarium. Brassica tournefortii, native to the Sahara Desert, was introduced into the
southern California deserts in 1927 and began invading hillsides near
Riversideabout 1987 (Minnich and Sanders in press). Time-seriesground
photographsin Riversideshow that CSS declinedlargely after the 1950s
(Figure 2).
Replicatesof VTM plots revealedsignificantchangesin shrub cover and
species composition (Figure 3). VTM workers recorded 60 to 90% shrub
cover at most sites. Field sheetsshow that three plots with <40% were each
burned during the late 1920s. Most plots had two or three specieswith at
least 20% cover, although some were dominated by one species, usually
Artemisia californica on north-facingslopes, Eriogonum fasciculatum on
fiats or steep bluffs,Encelia farinosa on south-facing
slopesin the north, or
Salvia mellifera on gabbro basalts. Other species frequentlyrecorded on
VTM plots include Lotus scoparius, Keckiella antirrhinoides, b/lalacothamnus fasciculata, Opuntia littoralis, and Bebbia juncea.
Our replicatesshowedthat shrubcoverhad declinedto an averageof 36%
(Figure 3) and that dominance was usually restrictedto a single species,
accordingto VTM criteria. Plots with <30% shrub cover frequenfiyhad no
dominantspecies.The cover of Salvia apiana decreasedfrom 7.4 to 1.7%,
that of S. mellifera from 13.9 to 6.1%, that of Artemisia californica from
17.7 to 6.1%, that of Eriogonum fasciculatum from 20.2 to 9.0%.
Average cover of Encelia farinosa increasedfrom 4.6 to 5.2%.
The change in total shrub cover varied greatlyfrom site to site, ranging
from stability(<5% losses)to total displacementof dense cover by annual
grassland (>90% losses, Figure 4). Modal shrub loss was 40%. Among
dominant species, Artemisia californica experienced the greatest loss,
althoughon many plots its decline was <20%. Modal loss was 10-30% for
all speciesexcept Encelia farinosa. The frequencyof plots with shrub losses
was far greaterthan of plots with gains.
Total shrub cover declinedon most substrates(Figure 5). Modal declines
were 50% on granitoid rocks and Pauba (Pliocene)and Pleistocenesandstones, 30-60% on Santiago Peak volcanics and the Jurassic Bedford
Formation, 20% on gabbro basalts.The greatestlosseswere of Artemisia
californica and Eriogonum fasciculatum on alluvium and Salvia mellifera,
Artemisia californica, and Eriogonum fasciculatum on Santiago Peak
volcanicsand the JurassicBedford Formation. All species except Encelia
farinosa experiencedat least a 10% declineon granitoidrocks. Shrub losses
have resultedin shiftsin speciescompositionon variousrock units (Table 1).
Eriogonum fasciculatum is no longer dominant on granitoid rocks, while
on Santiago Peak volcanicsand the JurassicBedford Formation Artemisia
californica/Eriogonum fasciculatum codominance has shifted to monotypic dominance of Eriogonum fasciculatum. Encelia farinosa experienced relative gains to other specieson all rock units.
The magnitudeof total shrub loss correlateswith the decline of individual
species(Table 2). Plots with >35% cover lossesare typically associatedwith
369
HISTORICAL
37O
DECLINE
OF COASTAL
SAGE SCRUB
HISTORICAL
DECLINE
OF COASTAL
SAGE SCRUB
11.
371
HISTORICAL
DECLINE
OF COASTAL
SAGE SCRUB
lOO
-
1930
8O
60
4O
2O
Ac
Ef
En
Sm
Total
Ef
En
Sm
Total
o lOO
-
1992
80-
60-
40-
20-
AC
Figure3. Cover of coastalsage scrubin the Riverside-Perris
Plain in 1930 and 1992.
large declinesin Artemisia californica and Eriogonum fasciculatum and
virtual disappearanceof Salvia apiana and S. mellifera. S. mellifera was
stable only in plots with <35% decline. All speciesexcept S. apiana were
stable in plots having declinesof <15%. Encelia farinosa was stable at all
rates of stand-thinning.
372
HISTORICAL
DECLINE
OF COASTAL
SAGE SCRUB
15
10
5
0
15
10
5
15
o
5
•- •5
•
•o
5
0
5
0
15
5
0
-80
-60
-40
-20
0
20
Change in Cover (%)
Figure 4. Change in cover by species.Ac, Artemisia californica; Err, Encelia
farinosa;Els,Eriogonumfasciculatum;
Sa, Salviaapiana;Sm, Salviamellifera.
373
HISTORICAL
DECLINE OF COASTAL SAGE SCRUB
Mixed Granitoid Rocks
ø1111=
Pauba Sandstone
Pleistocene Sandstone
F_o
0
•'
5
Alluv•[••••
o'
•
Santiago
PeakVolcanics
Jurassic
Bedford
Formation
0
5
Ultrabasic Gabbro Basalt
oL....
-80
-60
-40
-20
........
0
20
Change in Cover (%)
Figure5. Changein coverof coastalsagescrubby substrate
(Calif.Div. Mines 1969).
The majorityof VTM plots had burnedwithin the previous20 years.
Chronosequences
of plot replicatesshowthat averageshrubcoverincreased
from 22% for 0-9 years after a fire to 48% for 30-39 years afterward
(Figure6). Standslast burned>40 years ago still had lower averagecover
(31%)than recordedin the originalVTM survey.Postfirespeciescomposition comparedagainstVTM data appearsto be uniformthroughtime, as all
speciesseem to reestablish
themselvesearly in succession
(Table3). Encelia
farinosa reachedmature cover in 10 years, Artemisia californica and
Salvia apiana approachedhalf of VTM values only after 20 years,
EriogonurnfasciculatumreachedVTM valuesin 20 to 40 years,and dense
Salvia rnelliferastandswerefoundprimarilyin areaslastburned>40 years
ago.
Althoughstand-thinning
may lead to a declinein shrubdiversity,the
numbersof speciesrecorded
in plot replicates
are similarto thoseduringthe
originalsurvey,with two to five dominantsoccurringin 71% of plotsin 1932,
in 68% of plotsin 1992. The coverof extirpatedspeciesreportedin 1932,
however,was consistently
greaterthan for immigrating
speciesseentoday.
The patterns of immigrationsand extinctionsvary by species(Table 4).
Artemisiacalifornicaimmigrated
into more sitesthan it was extirpatedfrom,
but sitesof extirpationhad 42% coverin 1932 while immigrants
produced
only 5% coverin 1992. Eriogonumfasciculatumand Salvia mellifera were
374
HISTORICAL
DECLINE
OF
COASTAL
SAGE
SCRUB
Table 1 Change in Average Shrub Cover in the Riverside-PerrisPlain by
Substrate
Coverby species(%)
Substrate
Artemisia
californica
Encelia
farinosa
Eriogonum
fasciculatum
Salvia
apiana
Salvia
mellifera
Quaternaryalluvium
1932
32.2
(15,5)ø
1992
10.5
0.1
22.8
5.6
(0.3)
(4.9)
(8.1)
2.5
(9.8)
(6.2)
UndividedPliocenenonmarinesedimentary
1932
1992
9.0
1.8
(10.1)
(3.4)
0
0
-
20.0
5.7
17.5
1.0
30.0
(28.1)
(8.8)
(21.2)
(2.2)
(23.1)
7.8
3.7
6.8
(13.6)
(6.9)
(12.8)
0
5.5
-
(8.0)
Jurassic marine and metavolcanic
1932
1992
21.7
5.9
21.7
11.2
4.9
(19.5)
(13.2)
(15.7)
(12.5)
(8.5)
5.1
7.0
9.4
4.9
2.2
(5.8)
(10.9)
(9.6)
(10.4)
(5.5)
21
Mesozoic basic intrusive
1932
11.9
(17.9)
1992
0
-
17.9
3.5
38.6
(13.0)
(8.4)
(25.8)
8.1
1.2
14.1
0.1
30.6
(7.8)
(3.6)
(12.8)
(0.3)
(21.6)
10
Granitoid
1932
1992
15.7
6.9
22.8
8.9
11.6
(18.4)
(11.8)
(16.9)
(12,1)
(20.7)
5.6
7.1
8.3
0.7
2.8
(6.5)
(10.3)
(8.7)
(1.5)
(6.4)
32
øStandarddeviations in parentheses.
extirpatedat twice as many sitesas they immigratedto, and the 1932 cover
of extirpatedpopulations
was twice that of 1992 immigrantpopulations.$.
apiana died out at 42% of the originalVTM plots, and new populations
developedat only three localities.For œnceliafarinosa, local immigrations
were greaterthan extirpations,with an averagecover of 5% for each.
Twenty-twoplots (28.2% of the sample)had at least 10% cover of dead
mature shrubs.High mortalitywas most commonin plots last burned<40
years ago but also occurredin plots in early postfiresuccession
(data not
shown).
DISCUSSION
Replication of 78 VTM quadrats in the Riverside-PerrisPlain reveals
significantstand-thinningover the past 60 years. At many sites, formerly
denseshrubcover becameopen with a continuous
layer of exoticannuals
dominatedby eitherBromusdianaltusor B. madritensis.Amongdominant
375
HISTORICAL
DECLINE
OF
COASTAL
SAGE
SCRUB
Table 2 Change in AverageCover by Speciesin the Riverside-PerrisPlain
Compared with Change in Total Shrub Cover
Cover by species(%)
Change in
total shrub
cover(%)
Year
<-55
1932
Arternisia Encelia
californica farinosa
26.3
(26.5)a
1992
-35 to -55
1932
1992
-15 to -35
1932
1992
>-15
1932
1992
Eriogonurn
fasciculaturn
Salvia
apiana
Salvia
rnellifera
3.9
23.3
10.9
12.8
(7.2)
(16.3)
(11.8)
(21.8)
3.3
4.4
2.8
0.6
0.5
(4.3)
(9.1)
(4.2)
(1.4)
(1.2)
23.1
6.0
17.7
6.0
13.7
(19.1)
(12.4)
(16.4)
(9.8)
(20.3)
6.5
7.5
7.3
1.0
3.2
(6.2)
(9.7)
(7.7)
(1.8)
(5.2)
11.9
7.0
27.7
8.2
7.1
(12.0)
(13.3)
(14.3)
(15.5)
(16.2)
7.0
6.3
13.7
1.3
6.4
(9.6)
(11.0)
(11.5)
(2.8)
(12.1)
9.8
1.1
16.3
10.1
20.9
(13.7)
(3.1)
(9.9)
(13.6)
(25.8)
10.3
2.2
23.2
6.2
18.3
(9.8)
(4.9)
(14.7)
(13.4)
(21.3)
18
27
21
12
aStandard deviations in parentheses.
shrubs, only Encelia farinosa has been stable, while Eriogonurn fascicufaturn, Salvia apiana, S. rnellifera, and Arternisia californica have decreased since the original survey. Arternisia californica experienced the
greatestlosses,perhapsbecauseit was abundanton north-facingslopesnow
denselycoveredwith exotic annual grassland.The decline of Eriogonurn
fasciculaturnappears to be moderatedby the high diversityof habitatsit
occupies, including flats and cliffs, where exotics are discouraged.Salvia
rnellifera losseswere large except on gabbro basalts, and $. apiana has
experienced both widespread declines and local extirpations. Dense CSS
now persistsmostly on steep, rocky slopestoo porous for the establishment
of annualsand on gabbro basalts(Figure 1).
Other broad-scalesurveysof vegetationchangein CSS give mixed results.
From aerial photographsrepeated in 1947 and 1989 near Santa Barbara,
Callaway and Davis (1993) found that grasslandwas convertedto coastal
sage scrubbut that Quercusagrifolia woodlandwas convertedto grassland
in the absenceof fire becauseof suppression
of the shrublayer.O'Leary and
Westman(1988) found that CSS was more resilientunder periodicfire along
the coast in the Santa Monica Mountainsthan in the inlandvalleysof the
Riverside-Perris Plain. Near Ramona in San Diego County, Bradbury
(1974) found that CSS invadedchaparralin areas of chronicdisturbancebut
found little changein CSS cover and speciescomposition.In a vegetationmapping survey from aerial photographs in 1928-36
and 1980,
Freudenbergeret al. (1987) found that grasslandhad increasedin some
376
HISTORICAL
DECLINE
OF COASTAL
SAGE SCRUB
lOO
80
ß
= 40
20
0-9
10-19
Years
20-29
30-39
>40
Since Fire
193o
Shrub
Cover
Figure 6. Chronosequenceof postfire shrub cover.
placesof the Los AngelesBasin but that CSS replacedgrasslandin others.
Seedlingdensitieswere high in CSS standsbut low outsidetheir boundaries.
CSS appears to be susceptibleto profoundchange, even over short time
scales, because of its extensive coexistence with invasive exotic annual
grasses. Invasions of exotic annuals into CSS have been attributed to
anthropogenic disturbance (Gulmon 1977, Freudenberger et al. 1987,
Heady 1988, O'Leary and Westman 1988, O'Leary 1990). Since invasions
are assumedto be closelyassociatedwith disturbance,it is arguedthat the
degradationof CSS dependson excessiveburning and overgrazing,which
open up stands to penetrationby exotics. CSS is able to resist biological
invasions because the herbaceous vegetation (including exotics), which
proliferates after a fire, is assumed to decline with the maturation of the
shrub layer. Alternatively,the invasionby exoticsinto CSS may also be due
to competitive replacement through processes such as competition for
moistureand nutrients.Exoticsmay also shortenfire intervalsby increasing
the habitat'sflammability(Drake and Mooney 1986, Huennekeand Mooney
1989, D'Antonio and Vitousek 1992).
Whether coastal sage scrub is degraded by disturbance or by exotic
invasions requires the examination of three factors: (1) the successional
dynamics of exotic herbaceous cover, (2) whether the invasion of exotics
requiresfire disturbance,grazing,or air pollution,and (3) whetheranthropogenic ignition rates influence intervals of fire recurrence. Explanations of
habitat change must also considerthe history of invasion of exotics that
compete directly with CSS.
377
HISTORICAL
DECLINE
OF
COASTAL
SAGE
SCRUB
Table 3 Change in Average Shrub Cover in the Riverside-Perris Plain
Compared with Time Since Plot Last Bumed
Cover by species(%)
Change in
total shrub
cover (%)
Year
0-10
1932
Artemisia
californica
25.4
(23.7)a
1992
10-20
1932
1932
1992
>40
1932
Eriogonurn
fasciculatum
Salvia
apiana
Salvia
mellifera
5.8
22.8
7.8
11.7
(14.7)
(15.5)
(12.0)
(18.1)
4.6
5.4
4.4
0.6
3.6
(7.2)
(10.0)
(6.4)
(1.5)
(8.1)
15.4
9.7
22.0
6.5
14.6
(16.4)
(13.3)
(17.0)
(9.5)
(23.4)
1992
21-40
Encelia
farinosa
3.3
9.5
9.2
0.7
2.3
(3.7)
(11.3)
(11.3)
(1.7)
(6.6)
23.1
1.4
14.5
18.4
2.6
(26.2)
(4.2)
(10.5)
(13.2)
(7.8)
10.0
5.7
14.5
(8.8)
(11.3)
(11.3)
4.2
2.1
(8.5)
(4.9)
17.1
2.3
22.1
5.1
17.7
(16.4)
(4.9)
(14.5)
(9.4)
(23.8)
1992
8.8
2.1
10.5
3.7
10.4
(8.8)
(4.4)
(10.3)
(9.5)
(18.4)
16
23
10
29
øStandard deviations in parentheses.
Land Clearing
The
assertion
that
as much as 90%
of CSS
has been
removed
for
urbanizationand agriculturehas been based on the assumptionthat fertile
soils of the Los Angeles basin were largely coveredby woody vegetation
(Westman 1981, Freudenbergeret al. 1987). Grasslandswere presumedto
be focusednear Indian settlementsand that frequentburningwas requiredto
maintain them (Timbrook et al. 1982). The record of indigenousvegetation
in the coastaland interior valleys of California, however,is virtually nonexistent becauseexotic annuals introducedwith Spanish colonizationin 1769
were displacing native herbaceouscover before scientists first visited the
region (Heady 1988). Accountsfrom the Portolfi and Anza expeditionsof
1769-1772 suggestthat the Los Angeles Basin was coveredby extensive
herbaceousvegetation (Bolton 1927, 1930, Minnich 1988). In 1796 Juan
Crespi and JuniperoSerra describedlarge areas of "pasture"in the plains of
San Diego (Minnich and Franco-Vizcaino1998). Hence estimates of CSS
losses may be greatly exaggerated.
History of Invasionsof Exotic Plants in California
An important question necessaryto evaluating the role of disturbance
versuscompetitivedisplacementis why the decline of CSS has occurredonly
during recent decades. Although European exotics have been present in
California since Franciscan mission times in 1769, invasive species have
378
HISTORICAL
DECLINE
OF COASTAL
SAGE
SCRUB
Talkie 4, Frequency and Average Cover of Shrubs Extirpated from and
Immigrating to Vegetation Plots in the Riverside-PerrisPlain
Species
No. of plotswith
speciesin 1932
No. of plotswhere
extirpatedin 1992
Percentageof plots
whereextirpatedin 1992
Averagecover(%)in 1932
in plotswhereextirpated
Artemisia
Encelia
Eriogonum
Salvia
Salvia
californica farinosa fasciculatum apiana mellifera
66
26
67
38
32
5
2
15
16
8
7.6
7.7
22.3
42.1
18.8
42.6
(25.6)a
5.0
(5.7)
14.9
(10.8)
12.8
(12.3)
25.2
(23.2)
11
8
3
3
42.3
11.9
7.9
9.4
5.0
(5.5)
4.3
(4.1)
2.3
-
7.3
-
No. of plots
where immigrated
11
Percentageof plots where
immigratedin 1992
16.7
Averagecover(%)in 1992
in plotswhereimmigrated 4.9
(4.3)
aStandarddeviations in parentheses.
come in "waves" over the past two centuries (Heady 1988), and the
ecologicalrequirementsof early arrivals were differentfrom later ones, with
divergent impacts on CSS.
The first exotics to spread extensivelyin southernCalifornia,notably,
Avena fatua, Brassicanigra, and Erodium cicutarium,were introducedby
Franciscanmissionaries
in 1769 or perhapsearlier as ballastfrom ships.
These annualsquicklyreplacedindigenousherbaceouscover throughout
California(Table 5; Heady 1988). For example, the U.S./Mexican boundary surveyreports and the PacificRailroad Surveysin the 1850s found that
Avena fatua, Brassica geniculata, Erodium cicutarium, and Trifolium
spp. were extensivelynaturalizedin coastalsouthernCalifornia, including
the San Bernardino Valley (Parry 1859, Minnich 1988). These surveys
indicate, however, that exotic annuals were concentrated in bottomland
habitatsand disturbed
places,as seentoday,far from the CSS coveringthe
foothills. Indigenousherbaceousvegetationwas assumedto be perennial
bunch grasslandof Nassella spp., but historicaldescriptions,as well as
phytolith and archaeologicalevidence,indicatethat some herbaceouscover
in southernCaliforniaand neighboringnorthernBaja Californiamay have
consistedpredominantly of forb fields (Bartolome et al. 1986, Bean and
Lawton 1973, Timbrook et al. 1982, Minnich and Franco-Vizcaino1998).
The secondwave of exotic annuals,includingmostlyBromus spp. and
Brassicaspp., did not reach the Riverside-Perris
Plain until the early 20th
century.In the San BemardinoValley,the firstrecordsof Bromusmadritensis,
B. dianaltus,and Brassicageniculatadate to ca. 1890, when they were
foundmostlyalong roadsidesand in wasteplaces.These annualsgradually
379
HISTORICAL
DECLINE
OF
COASTAL
SAGE
SCRUB
Table 5 ApproximateArrival Dates of Exotic Annuals in California
a
Species
œrodium cicuta rium
Arena fa•ua
Brassicanigra
Brornus mollis
Brornusmadri•ensisssp. rubens
Brornus dianaltus
Brornus •ec•orurn
Arena barbara
Brassicageniculata
Brassica •ournefor•ii
Schismus barba•us
Year
1600s?
1769
1769
1860
1880
1880
1880
1880
1880
1927
1950
Native range
Mediterranean Europe
Mediterranean Europe
MediterraneanEurope
Mediterranean Europe
MediterraneanEurope
Mediterranean Europe
Mediterranean Europe
Mediterranean Europe
MediterraneanEurope
Northern Sahara Desert
Middle East
aAfter Heady (1988), Parish (1913, 1920).
expandedacrossvalley lowlandsonly by 1920 (Parish 1913, 1920). In
contrastwith the first wave of exotics,they spreadonto CSS-covered
hillsides.
They were later joinedby still other taxa capableof invadingCSS, including
Schismusbarbatusin the 1950s and Brassicatournefortii in the 1980s. The
evaluation of dynamicsin relation to fire, grazing, and air pollution must
considerthe historyof proliferationof exotics.
Fire
Studiesin Califomiahave shown that CSS is resilientunder periodicfire,
thoughmost of its plantsresproutpoorlyafter being burned.Seedlingsmust
establishthemselvesfrom a prefire seed cache (Salvia apiana, S. rnellifera)
or germinatefrom seed dispersedwidely by wind (Arternisia californica,
Encelia farinosa, Eriogonurnfasciculaturn).Resproutingspeciesflower
vigorouslythe first few years after a fire, providingnonrefractory
seedsthat
germinate in subsequentyears, leading to mixed-agedstands (Westman
1981). Recruitmentand growthto maturityis extremelyrapid (ca. 10-20
years) for most taxa (Westman1982).
Disturbance-dependentmodels state that although early successionis
characterizedby abundant herb cover, the probability of recurring fire
capableof breachingsuccession
and inducingpermanentvegetationchange
is avertedby the limited persistenceof indigenousherbs. Many herbs are
specializedfire annuals that sprout from a dormant seed pool whose
germinationis stimulatedby the heat of fire, charred wood, and light
(Westman1979, Keeley and Keeley 1984, O'Leary and Westman 1988).
The declineof the herb layer improvesthe chancesfor the long intervals
between fires compatiblewith the life-historytraits of CSS species.However, there is uncertainty whether natives are fire annuals sensu stricto.
Accountsby C. R. Orcutt and othersof extensiveflower fieldsbeforethe
secondinvasionof exotics (Minnich and Franco-Vizcaino1998) suggestthat
wildflowersmay be more generalized,or germinateto cues of precipitation
and temperature,
like closelyrelateddesertwildflowers,as well as from fires.
380
HISTORICAL
DECLINE
OF
COASTAL
SAGE
SCRUB
Table 6 Total and Native Biomass and Shrub Cover by Time Since Fire in
the Box Springs Mountains, Riverside
a
Biomass (tons/ha)
Site and year
Two •ees
Site 1
1989
1990
1991
1992
1993
1994
1995
Precipitation
(cm)
Yearssincefire
Total
Native
Shrubcover(%)
Canyon
16.4
1
1.59
0.81
1.1
14.0
2
0.62
0.51
_ b
26.3
28.1
52.8
23.7
48.0
3
4
5
1
2
2.56
1.80
3.31
2.49
2.92
2.13
1.15
1.34
0.52
0.41
12.4
0
0
16.4
14.0
26.3
28.1
52.8
23.7
48.0
8
9
10
11
12
1
2
2.32
0.57
3.39
2.31
2.53
2.41
2.31
1.45
0.02
0.08
0.08
0.07
0.04
0.09
29.3
12.4
0
0
14.0
26.3
28.1
52.8
1
2
3
4
1.06
2.22
2.11
1.64
0.18
0.41
0.08
0.10
0
5.4
23.7
48.0
1
2
1.45
2.03
0
0.06
0
0
10
11
12
13
14
15
0.37
2.90
1.49
0.55
0.49
0.95
0
0.27
0.09
0.03
0.01
0.03
29.7
44.9
44.8
S•e 2
1989
1990
1991
1992
1993
1994
1995
Box Springs
S•
1
1990
1991
1992
1993
1994
1995
S•e 2
1990
1991
1992
1993
1994
1995
Mean
25.4
øData from transectstaken annuallyat the end of spring growth flush betweenMarch and early
May.
bNo data.
Many forbs may decomposeand their stems may fragment during summer
drought because they lack the silica content critical to the curing and
flammability of grasses(D'Antonio and Vitousek 1992).
Field sampling for herb cover in the Box Springs Mountains, near
Riverside, from 1989 to 1995 (Table 6) shows that annuals form high
biomasswithin the first growing season after fire, reducingbare zones for
shrub establishment.Native annuals(mostly Phacelia distans, Cryptantha
intermedia, and Emmenanthe penduliflora) proliferated after a spring
381
HISTORICAL
DECLINE
OF
COASTAL
SAGE
SCRUB
burn at one site but were replacedwithin 5 years by a densecoverof exotic
annuals (Bromus madritensis, Arena barbata, œrodium cicutarium,
Schismus barbatus, Brassica geniculata, and B. tournefortii), further
preventingthe developmentof the bare zones necessaryfor shrub establishment. Successionafter a summer bum in Box Springs Canyon was dominated by exotic annuals from the start. œrodium cicutarium and Schismus
barbatus were dominant the first two years, Bromus madritensis and
Arena barbata thereafter. The persistence of herbaceous biomass was
associatedwith shrub cover of 5 to 12% after 4 or 5 years. Shrub cover
decreasedfrom 29 to 12% after 10 years at one site from the combined
effects of drought and freeze mortality. At yet another site, exotic annuals
produced 1.5-3.0 tons/ha for 12 years after fire until the shrub layer
reached canopy closure (shrub cover increasedfrom 27 to 44% between
1990 and 1995).
Some studiesindicatethat fire at intervals<5-10 years will likely eliminate
sage scrub, leading to domination by nonnative grasses (Wells 1962,
Kirkpatrick and Hutchinson 1980, Keeley 1981, Zedler et al. 1983,
Malanson 1984, Freudenbergeret al. 1987, O'Leary and Westman 1988),
although others have come to the opposite conclusion(Griffin 1978, Davis
et al. 1988, Callawayand Davis 1993). It has been arguedthat the invasion
of exotics has been prompted by anthropogenicburning becausethe rarity
of lightningat low elevationsmakes natural fire too rare (Keeley 1981).
Prehistoricfire intervals in CSS are unknown. However, electromagnetically
recordedlightning-detection
rates in the region are 0.5 to 1.0 strikes per
kilohectareper year, and a 2-4% successrate in the initiating fires (suppressed lightning fires/lightning detections),typical of the nearby mountains, yields a fire-establishment rate requiring suppression of 1 per
kilohectareevery 30 years (Minnichet al. 1993). Fires in grasslandcan be
much largerthan in sagescrubbecausegrasslands
constitutea fire hazardat
the landscapescaleregardlessof their history.Hence relativelyfew ignitions
can lead to extensiveburning.In the Box SpringsMountains,exotic annuals
supportedfires in 1989, 1993, and 1995, with the biomasslevelsof +1.5
tons/ha. Since this level of biomasswas reachedwith annual precipitationas
low as 16.4 cm, 65% percentof normal (Table 6), herbaceousfuel build-up
may be sufficientto carry fire most years. Three samplingsites had burned
two to four years within the seven-yearsamplingperiod. Similarly,frequent
fires in San Diego County were encouragedby deliberate seeding of the
inflammableexotic grass Lolium multiflorum (Zedler et al. 1983).
In the River$ide-Perri$Plain, California Division of Forestry records (on
file at Perris)and Landsatimagery (Minnich 1983) show that fires carried
primarily by exotic annual grasslandhave been frequentand extensiveover
the past 40 years,with the majorityof VTM plots havingbumed duringthe
past 20 years. VTM replicatechronosequences
show that all CSS dominants
are capable of reestablishingthemselvessoon after a fire. Only œncelia
farinosa, however, has achieved the cover values reported in the original
survey,and often within 10 years after fire. Other shrubshad not reached
VTM levels by 40 years.
The divergencein successionpatterns by slope is seen on Box Springs
Mountain (Table 7). South-facingslopes experiencerapid establishmentof
382
HISTORICAL
DECLINE
OF
COASTAL
SAGE
SCRUB
virtually monotypic stands of Encelia farinosa, with shrub cover increasing
to 60% and shrub densityto 9687 per hectareby 14 years after a fire.
Althoughwe observedlow rates of resproutingin Encelia farinosa (_+10%),
as many as 11,000 seedlings/haestablishedthemselveswithin a few years,
apparentlythrough long-rangewind dispersalof seeds. Seedling establishment diminishedwith increasingshrub cover, especiallyafter 5 years. The
resilienceof œ. farinosa may be relatedboth to its resproutingabilityand to
its preference for xeric southern exposures where herbaceous cover is
limited. On north-faring slopes, CSS cover was _+2.2%and densitieswere
+500 stems/ha, even in standsas old as 12 years. Eriogonum fasciculaturn,
ß
Arternisia californica, and Salvia apiana, which were mapped as dominants on these sites by the VTM survey, apparently were burned almost
totally, and the establishmentof seedlingsof these speciesthat must regrow
from seeds is apparentlylimited by the dense cover of Brornus clianclrus.
Moreover, there is little evidence that canopy closure reduces herb cover.
Hence another grassfire is a virtual certaintybefore the developmentof a
shrubcanopy.Postfiresuccession
may also be further postponedby increasing lag times due to the need for long-rangeseed dispersalfrom increasingly
scarce surviving populations.
These trends are similar to postfiresuccessions
in inland Riversidiansage
scrub recorded in other studies(Kirkpatrickand Hutchinson 1980, O'Leary
'l¾,l•le ? Sequence of Postfire Successionof Coastal Sage Scrub in the Box Springs
Mountains
Cover(%)b
Years
Site
since fire
En
South-facingslopes
•
Sug94S
1
24.2
Ef
Total
Stemdensity
c
En
Ef
Ac
Seedlingdensity
d
Total
En
Ef
0
24.2
2760
0
0
1760
2
4.8
0
4.8
900
0
0
900
3000
Sug88S
Sug83S
7
12
35.6
42.4
0
0
35.6
42.4
2297
4386
100
0
0
0
2397
4386
1000
0
Belv81S
14
35.4
0
35.4
2300
0
0
2300
1100
1.6
67.8
9203
242
242
9687
0
20
20
200
240
0
0
0
0
0
500
0
0
0
0
20
20
TT93S
Sug81S 14
66.2
North-facingslopes
TT93N
2
0
0
0
Sug88N
Sug83N
Sug81N
7
12
14
0
0
0
0
2.2
0
0
2.2
0
Belv81N
14
0
0
0
160
11,000
0
500
0
0
2220
0
200
0
Ac
Total
0
0
11,000
0
0
0
0
3000
1100
0
0
0
0
0
0
0
1100
0
0
0
100
100
0
0
100
0
100
0
0
0
0
0
0
aEn, Encelia farinosa; El, Eriogonum fasciculatum; Ac, Artemisia californica.
bBasedon 50-m line-intercept
transects.
CExpressedas stems per hectare and based on 50-m point-center quarter transects.
alExpressed
and seedlings
per hectareand basedon 1.8-m circleplots on 10 pointsof point-center
quarter
transects.Seedlingswere <10 cm tall with no charred stems at the root axis that would indicateresprouting.
eSlope aspect divided hemisphericallywith northern exposures (clockwise) from 270 ø to 90 ø and southern
exposuresfrom 90 ø to 270 ø.
383
HISTORICAL
DECLINE
OF
COASTAL
SAGE
SCRUB
and Westman 1988) and contrastwith the resilienceof sage scrub near the
coast (Malanson and O'Leary 1982, 1985, Malanson 1984, O'Leary and
Westman 1988). Along the coast,sage scrub shrubs,even some of the same
or closely related species (Artemisia ½alifornica, Salvia leucophylla, Eriogonum fas½iculatum, and Encelia californica) resprout better. Seedling
establishment
began in the secondgrowingseasonafter fire from the fruiting
of shrubs resproutingthe first year.
Grazing
Thinning of CSS has been attributedto livestockgrazing,which opens up
the shrub canopy to invasion by exotic annuals, disseminatesseeds, and
reducesthe ability of native forbs and perennial bunch grassesto compete
with exotics (McBride 1974, Freudenberger et al. 1987, O'Leary and
Westman 1988). Historically, grazing was heaviest during the late 19th
century. Livestock grazing in the Riverside-PerrisPlain consistedof winter
sheep drives beginningin the 1860s, reaching a peak near 1900 (Minnich
1988). Evidence is seen in criss-crossingtrails ("sheeptours")on hillsides.
However, grazing declined by the early 20th century--before the widespread naturalizationof "secondwave" exotics, includingBromus spp., and
before the decline in CSS-because the annual transhumancegrazing cycle
was interrupted by the prohibition of summer browsing in the mountains
with the establishmentof national forests of southern California (Lockmann
1981). During our resurvey only two plots were subject to intense sheep
grazing (Arlington Mountain, Massacre Canyon). Exotic grasses at these
sites were browsed to the ground, and shrub seedlings appear to be
eliminatedby domestic stock, recallingof the impacts of grazing by feral
goats and sheep on the Channel Islands(Brumbaugh1980, Minnich 1980,
Van Vuren and Coblentz 1987). Alternatively,the proliferationof exoticsin
southern California'sinland valleys suggeststhat these plants were able to
occupy suitable undisturbedhabitat. For example, there is evidence that
"first wave" invasions preceded settlement in the Central Valley (Wester
1975, 1981). Livestock disturbanceand agriculturalclearing have only a
short-termrole in the spread of these weeds (Biswell1956, Naveh 1967,
Heady 1988, Sauer 1988, Huenneke and Mooney 1989, Huenneke et al.
1990, D'Antonio and Vitousek 1992).
Competitive Exclusion
Aerial photographsand VTM-quadrat replicationsreveal shrub losses in
stands last burned •40 years ago. Hence, while fire and grazing may
facilitateconversionof CSS to grass,the deteriorationof CSS may be more
fundamentallyrelated to competitiveexclusion,in which exotics themselves
induce habitat changes. Means for this include resource competition, light
absorption, water uptake, altered water-holding capacity, and nutrient
uptake (D'Antonio and Vitousek 1992). Exotic grasslandsmay be inducing
nutrient and moisture regimes that inhibit the establishment,growth, and
persistenceof native shrubs(Westman 1981, O'Leary and Westman 1988).
In the Riverside-PerrisPlain, three lines of evidencesupport the view that
exotics outcompete CSS.
384
HISTORICAL
DECLINE
OF
COASTAL
SAGE
SCRUB
(1) High shrubmortalityduringthe 1992 replicatesurvey.In 22 plots (28%
of VTM plots)the cover of dead shrubswas greaterthan 10%. Mortality
followed drought between 1984 and 1990, during which time the water
demand of dense, shallow-rooted
exoticsmay have limitedthe water reaching
the shrubs'roots. Shrub mortality may be relatedto the ability of herbs to
producehigh biomass(high water demand)in winterswith subnormalprecipitation. Althoughherb biomasswas <0.6 tons/ha in the droughtyear of 1990,
annualproductivity
was 2.5 tons/hain the dry yearsof 1989 and 1994 (Table
6) owing to well-distributed
light rains. Mortality in old-growthstandsmay be
relatedto the largecanopyand leaf area (transpiration
load)of the shrublayer.
We observedlitfie shrub mortalityin plots with low annual cover,mosfiyon
rocky outcropsand on gabbro basalts.Soils overlyinggabbro basaltsmay
contain toxic chemicalsor be deficientin nutrients,adverselyaffectingboth
exotic and native annuals,like soils lackingvital nutrientsat Jasper Ridge in
central California (Huenneke et al. 1990).
(2) CSS/grasslandecotonesfollow slope lines independentof fire history.
These trends indicate a pattern of variable recruitmentrates with habitat
change unrelatedto disturbance(unpublisheddata).
(3) Dense, relatively diverse stands of CSS (Arternisia californica,
Eriogonurn fasciculaturn, Encelia farinosa) often grow on recent road cuts
and are often immediatelysurroundedby dense grasslandson undisturbed
soils. Apparently,the scrapingof topsoilsand exposureof coarserdecomposed regolith with nutrient characteristicsof the basementencouragesthe
rapid invasionof coastalsagespecies.Colonizationof road cutsby CSS, far
removed from neighboring stands, also suggests that long-range seed
dispersal is not limiting even with low resprouting rates (O'Leary and
Westman 1988) because these shrubs have tiny seeds capable of wind
dispersal.Perhapsthe absenceof Salvia rnellifera and $. apiana in road
cuts reflects their seeds' larger size (Hickman 1993) and limited dispersal
abilities. Except at sites occupied by Encelia farinosa, patterns of CSS
reestablishment
after fire on the Riverside-Perris
Plain are inconsistent with
the view that weedy invadersare eliminatedrapidly by shadingfrom larger,
longer-livednative species (Huston 1995).
Air
Pollution
Two major air pollutants,ozone and nitrogen oxides, may affect CSS
(Westman1979, 1981, Allen et al. 1996, Padgett et al. in press).Figure 7
shows a north-to-southgradient in the CSS lossesin the Riverside-Perris
Plain. For example,most VTM plots near Riversidehave <20% of former
cover, whereas cover was nearly stable at several plots to the south. The
concentrationof air pollutionin the region varies widely (Allen et al. 1996,
Padgett et al. in press).During summer,thermal gradientsproducedby the
combinationof daytime heating of the land and cool air overlyingthe ocean
draw the marine layer into the inland valleys,with vertical mixing limited by
strongtemperatureinversions.Blockageof onshoremarine air flow by the
Santa Ana Mountainsdividesthe air massinto more and lesspollutedhalves,
with polluted marine air moving into the north from the urbanized Los
AngelesBasin and less pollutedair enteringthe south from less populated
385
HISTORICAL
DECLINE OF COASTAL
SAGE SCRUB
Figure7. Changein the coverof coastalsagescrubon VTM plotsfrom 1932 to 1992.
coastalregions.The two air streamsfrequenfiyconvergenear Perris, 15 km
southeastof Riverside,yielding a discontinuityin air-pollution concentrations and visibility.
How specificair pollutantsmay influenceCSS is unclear.In the greenhouse, Preston (1986) and Westman et al. (1986) found simulatedambient
summer ozone concentrationsto reduce the growth of well-watered CSS
seedlings.They concludedthat air pollution weakens the shrubs,lowering
their resproutingability. However, there is no baselinedata on the sprouting
abilityof CSS in the Riverside-Perris
Plain beforeair pollution.Altematively,
differencesbetweencoastaland inland sites in resproutingrates may reflect
local genetic variability, as well as climatic differences, including higher
humidity,lower rates of leaf fall, and more greenfoliageduringthe summer
drought along the coast. Postfiresuccessional
responsemay also be related
to climatic gradients.CSS resiliencealong coast may be related to higher
shrub productivity sustained by warmer temperatures through the winter
rainy season.Growth persistsinto late spring becauseof the cooler summer.
In the Riverside-Perris Plain, cold winters limit productivity until March/
April when warmingtemperaturescombinedwith moist soilsresultin a brief
flush of growth. Growth is terminated by high temperatures beginning
usually in May. Air-pollution effects may also be limited because summer
ozone concentrations do not reach high levels until after shrubs have
experienceddesiccation,stomatal closure, and leaf senescence(Allen et al.
1996, Padgett et al. in press).
386
HISTORICAL
DECLINE
OF
COASTAL
SAGE
SCRUB
Anthropogenic nitrogen deposition may alter the composition of soil
microorganismsand encouragedisplacementof native forbs by nitrogendemandingexotics.In the Central Valley of Califomia,the invasionof exotics
was encouragedby high nutrient availability,independentof physicaldisturbance (Huennekeet al. 1990). Hence, increasedsoil nitrogenmay make CSS
more susceptibleto invasionby exotic plants. CSS may decline becauseof
elevatednitrate plus ammoniumlevelsin the soil and the air (Allen et al. 1996,
Padgett et al. in press). Studies have yet to control for air pollution by
examiningthe effectsof exoticsgrowingin areasof low pollution,suchas the
Califomia deserts and coastalnorthern Baja California.
Alternatively,differencesin CSS losses in the Riverside-PerrisPlain may
be related to substrate.Gabbro basalt, the substrateon which CSS is most
stable,occursover large areas in the southernbasin but is virtuallyabsentin
the north. Granitic substrateis widespreadin pollutedareas north of Perris.
Exotic annualshave also spread extensivelyinto the CSS of northern Baja
California, where air pollution is virtually absent (Minnich and FrancoVizca•no1998). Future researchon the effectsof air pollution shouldcontrol
for the role of substrateand changesin soil chemistry.
MANAGEMENT
IMPLICATIONS
A fundamental question in the management of CSS is whether the
present decline is due to anthropogenic disturbance or to competitive
displacementby exotic annuals. Limited chronosequencesamplingof shrub
and herb cover in the Box Springs Mountains supports both models,
depending on the site. Encelia farinosa recoversrapidly, with corresponding decreasesin herbaceouscover, even with short intervalsbetween fires,
but this shrub is only locally abundantin the northern Riverside-PerrisPlain.
In contrast, stands dominated by Arternisia californica, Eriogonurn fasciculaturn, Salvia rnellifera, and S. apiana recover poorly even with little
disturbance.The widespread decline of CSS in the Riverside-PerrisPlain
independentof disturbancesupports the competitive-displacement
hypothesis. The future of the Riverside-PerrisPlain may see the displacementof
CSS by exotic annuals.
To protect species diversity, the Riverside County Habitat-Conservation
Plan for the Riverside-Perris
Plain has purchasedprimarilylands contiguous
with preexistingpublic lands, includinglakes Matthews, Perris, and Skinner
(Feldman 1995). These measures may not serve to protect the California
Gnatcatcher. The assumption in preserve design and land management of
static CSS distributions,stand structure, and species diversity is refuted by
our replication of the VTM. Therefore, critical examination this dynamic
one-way processof CSS conversionto exotic annual grasslandis needed in
preserveselection.Otherwise, land-protectionmeasureswill result in grassland preserveswith little CSS flora or fauna. Most CSS on publicland is now
experiencing stand-thinning and potential displacement by exotic annual
grassland.The plain's largest remainingdense CSS, on gabbro basalts,are
less susceptibleto grassinvasionand have stableshrub communitiesbut are
in private ownership and endangeredby urban development.
387
HISTORICAL
DECLINE
OF
COASTAL
SAGE
SCRUB
SUMMARY
Californian coastal sage scrub (CSS) has been extensively cleared for
agriculture and urbanization. Few studies have examined the landscapescale dynamicsof surviving CSS, particularly in relation to invasive exotic
annuals.From 1929 to 1934 the vegetationof Californiawas inventoriedby
the VegetationType Map (VTM) Survey. We replicatedthe VTM to quantify
the changesin the CSS in the Riverside-Perris
Plain over the past 60 years.
The cover of Encelia farinosa cover was stable, but that of Salvia apiana,
$. mellifera, and Artemisia californicawas reducedgreafiy.At many sites,
former dense shrub cover had become open standswith a continuouslayer
of exotic annualsdominatedby either Bromus ClianClrus
or B. maclritensis
ssp. rubens. Encelia farinosa reestablisheditself rapidly on south-facing
slopes, but on north-facing slopes the dense cover of the exotic grass
Bromus ClianClrus
limited reestablishmentof Eriogonum fasciculatum and
Artemisia californica. CSS has declined under limited or no grazing
pressurefrom livestock.Hence, while fire and grazing may facilitateCSS-tograss conversion, in some cases the deterioration of CSS may be more
fundamentallyrelated to competitive exclusion. Exotics may alter nutrient
and moistureregimes, inhibitingthe establishment,growth, and survivalof
native shrubs.Our data refutethe assumptionthat CSS is still a stablehabitat
in this area. Attemptsto conservethe CaliforniaGnatcatcherby extending
public landsin regionsexperiencingthis type of habitatdegradationmay be
insufficient and need to be reexamined.
LITERATURE
CITED
Allen, E. B., Padgett, P. E., Bytnerowicz,A., and Minnich, R. A. 1996. Nitrogendepositioneffectson coastalsage vegetationof southernCalifornia,in Proceedings of the InternationalSymposiumon Air Pollutionand Climate Change Effects
on Forest Ecosystems,5-9 February, Riverside (A. Bytnerowicz, M. Arbaugh,
and S. Schilling,tech. coords.).U.S. Forest Service Rep. GTR-164.
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Accepted 18 June 1995
391