GABBRO SOILS AND PLANT DISTRIBUTIONS ON THEM

2011·7.4 MB·English

by  Earl B Alexander

Madrono, Yol. 58, No. 2, pp. 113-122, 2011
GABBRO SOILS AND PLANT DISTRIBUTIONS ON THEM
Earl B. Alexander
Soils and Geoecology, 1714 Kasba Street, Concord, CA 94518
[email protected]
Abstract
Gabbroisamaficrockwithmanyspeciesthatdonotoccuronsoilsofgraniticorultramaficrocks.
Althoughsomegabbro soilsharbormanyunique orendemic species, othersdo not, asbotanistshave
notedfromtheAppalachianPiedmont tothemountainsofCaliforniaandOregon. Gabbrosoilswere
sampledinthePeninsularRanges and in.thefoothills ofthe SierraNevadatoidentifyspecialfeatures
of the gabbro soils with unique plant species distributions. No soil morphological or chemical
differenceswerefoundbetweengabbro soilswithandwithoutuniqueplantspeciesthatmightexplain
the differences in plant distributions. Althoughmany unique speciesmay occur only on gabbro soils,
apparently their distributions cannot be explained primarily by soil differences among gabbro soils.
Key Words: Gabbro, rare plant species, rocks, soils.
Some soils with gabbro parent materials have differences in the Sierra Nevada and in the
unique plant associations (Hunter and Horen- Peninsular Ranges and to evaluate soil properties
stein 1992; Oberbauer 1993), or have plant that might explain why some gabbro soils have
communities that are intermediate between those unique vegetation and others do not (Fig. 1).
of soils on granitic and on ultramafic rocks
(Whittaker 1960). Gabbro is a mafic plutonic The Nature ofGabbro
rock with mineralogy and chemistry that spans
the range between granitic rocks, which are silicic Gabbro is a petrologic term for plutonic rocks
rather than mafic, and ultramafic rocks. There- that consist of olivine or hornblende, pyroxene,
fore, it is reasonable to expect plant communities and calcium-feldspar. Its composition spans the
ofgabbro soils to have some aspects ofthose on range between diorite and peridotite. Among the
granitic soils and some aspects of those on plutonic rocks, from granite to peridotite, diorite
ultramafic rocks, as described by Whitaker yields themostfertile soilsbecauseithas themost
(1960). However, no reasons have been found favorable balance ofmagnesium, potassium, and
for some endemic species occurring on some calcium. Soils with peridotite parent materials
gabbro soils and not on others. have very high magnesium and low calcium
Botanists in eastern North America have concentrations, making them much less favorable
recognized that some soils with mafic soil parent for most plants. Gabbro has higher concentra-
materials have unique plant associations, espe- tions of all ten of the first transition elements
cially on the Idedell soils (Dayton 1966). The than more silicic rocks (Table 1), and some of
Iredell series are common soils with mafic parent those elements can be toxic to plants. Peridotite,
material on the Piedmont of eastern North however, has much greater concentrations of
America. Several pedons in soils mapped as those elements that are most likely to be toxic
Iredell were sampled in NRCS (Natural Resourc- (Cr, Co, and Ni). Because the composition of
es Conservation Service) soil surveys and ana- gabbro spans the range between diorite and
lyzed in an NRCS laboratory. Exchangeable peridotite, gabbro soils might be expected to
Ca/Mg ratios <1 were found in most of those range from quite fertile to unfavorable for many
pedons, but the soil parent material was not plants.
identified specifically for any ofthe pedons. Ogg Gabbro has 10 to 90% feldspar, 5 to 90%
and Smith (1993) identified theparent material of pyroxene, and 5 to 90% olivine orhornblende(Le
a soil mapped as Iredell to be pyroxenite, and Maitre 2002). The feldspar is a calcic plagioclase
exchangeable Ca/Mg ratios <1 prevailed in it, and the pyroxene is mostly augite (monoclinic
but the ratios were not as low as in ultramafic pyroxene). If the pyroxene is mostly enstatite or
soils sampled for the same investigation. hypersthene (orthorhombic pyroxenes) the rock
Although botanists have recognized unique is called norite. Gabbro, with monoclinic pyrox-
vegetation on some gabbro soils and not on enes, is much more common than norite in
others, no soil differences have been discovered western North America. Rock compositions are
that might explain vegetation differences. A commonly shown on triangular diagrams (Le
gabbro soils investigation was conducted to Maitre 2002). Only three minerals can be shown
discovery soil differences related to vegetation on a triangular diagram; therefore separate
MADRONO
114 [Yol. 58
Table ElementalCompositionofGabbroand
its Nei1g.hbors—More Silicic Diorite and Less
Silicic Peridotite. Mean composition of major
elements from Le Maitre (1976) and first transition
elements from Vinogradov (1962), who included
andesite with diorite, basalt with gabbro, and dunite
with peridotite. The first transition elements have outer
electrons that are in d-orbitals. They are transitional
from group 1 and 2 elements with outer electrons in s-
orbitals (columns on left side of periodic table) and
group 3 to 8 elements with outer electrons in p-orbitals
(right side ofperiodic table ofelements).
Major Diorite Gabbro Peridotite
element g/kg (ppt)
Si 269 234 198
A1 88 82 22
Ti 6 7 5
Fe 55 80 76
Mn
Mg 221 461 1883
Ca 47 67 36
Na 21 14 3
K
12 7 2
P 1.2 1.0 0.4
First trans.
element mg/kg (ppm)
Sc 2 24 5
Ti 8000 9000 300
V 100 200 40
Cr 50 200 2000
Mn 1200 2000 1500
Fe 58,500 85,600 98,500
Co 10 45 200
Ni 55 160 2000
Cu 35 100 20
Zn 72 130 30
Fig. 1. LocationsofthePine HillPreserve(PH)inthe plagioclase is only 4.4 moles/kg while Mg in the
fGouotahtilalys oafretahe (SCieGr)ra iNnevtahdea PaenndinTshuelaCruyRaamnagceas- olivine is 27.7 mol/kg.
Province, CA.
Gabbro Compositions in California
Gabbro occurs as components in ophiolites of
triangles are displayed for olivine gabbro (Fig. 2) accreted terranes, in stocks and Alaska-type
and gabbro with hornblende (Fig. 3). Molar intrusions, and in laterally extensive layered
Ca/Mg ratios are plotted on Figs. 2 and 3. This bodies. Botanists have described plant communi-
requires assumptions about the compositions of ties on gabbro in the intrusive bodies of the
the minerals. The compositions were assumed to Peninsular Ranges (Beauchamp 1986; Oberbauer
b(ewhCiacoh.6Nisa0d.4eAslii.g6nSait2e.4d08Afnor60cablyciupmetprloalgoigoisctlsa)s,e t1h9e93S;ieCrhreanNgev20a0d4a) faonotdhitlhles P(iHnuentHeilrlacnodmpHloerxeno-f
CMFaego)i.A.4l8M2FgSeoi.06.402FS2e2i(0O.024HSi)f02or3ffooorlrihvcionlrein,nbolpeaynnrddoex.eCnTaeh2eM(agumg3oi(tlAeal)r,, bsLteaeerinsne1ns9t9u(2d1)i9.e4d8G)a,cbobmaprnrdoehoWefanltshlieavsweeelyinndtberyrusiM(vi1el9l7be6or)di(ei1ns93ht7a)hs,e
Ca/Mg ratios, based on these formulas are Peninsular Ranges and by Springer (1980) in the
infinite for plagioclase, 1.0 for clinopyroxene, 0 Pine Hill complex.
for olivine, and 0.67 for hornblende. The molar Larsen(1948) gavethemineralcompositions of
Ca/Mg ratios of seven gabbro samples from the 20 rock samples that are representative ofgabbro
Pine Hill complex reported by Springer (1980) and norite in the Santa Ana Mountains. All
ranged from 0.58 to 0.76 and averaged 0.69 mol/ contained hornblende and most of them con-
mol. Olivine has more affect than plagioclase on tained at least 10% hornblende, but only 5
the Ca/Mg ratios in Fig. 2, because Ca in the samples contained olivine. A hornblende-free
2011] ALEXANDER: GABBRO SOILS AND PLANTS 115
Fig. 2. Olivine-bearing gabbro. Molar Ca/Mg ratios (0.05-3.0) ofgabbro composed ofplagioclase (PI), olivine
co(Ohflr)ay,smopalhniitdbeop(lyMerg)oix.ae8nnFdee0o(l.2PiSyv)ii.nOe4P)l(,a1ag/3niodocflpayasrmeopxhisiebnroeelpber)ye.saeuSngptieetcdeifb(iyCcaCol.aa4b/MraMgdgo0r.r4iaFtteeioo.(s2CaSfoir.0o63mN).acoAh.me4pmAhilic1ba6loSlia2en.s4a0lay8rs)ee,saodlaidrveeidntehtoobsypeyfrroeropsxoteernrtieetde(2bo/yr3
Springer(1980)forthePineHillcomplex,byMiller(1937)andLarsen(1948)fortheSantaAnaMountains, andby
Wallawender (1976) for the Los Pinos complex. Values for the Pine Hill complex are means for four groups of
rocks that Spinger (1980) designated A, B, C, and D. Rock samples from the three Pine Hill soils were plotted by
the percentages ofplagioclase, clinopyroxenes, and olivine (symbols PH4, PH5, and PH9).
gabbro with more than about 15 to 20% olivine major components of aeolian dust (Pye 1995).
will have a molar Ca/Mg < 0.7 (Fig. 3), but none The main sources of aeolian dust have been the
of 8 rocks with chemical analyses had Ca/Mg < Sacramento Valley, about 40 km from the Pine
1.0 (Larsen 1948). Hill soil sampling sites, and the coast of the
Wallawender (1976) gave mineralogical and Pacific Ocean, about 60 km from the Peninsular
chemical analyses for 18 rock samples from the Ranges soil sampling sites. No appreciable
Los Pinos pluton. Ten are hornblende bearing amounts of coarse sand are likely to have been
gabbro, onewith a high Ca/Mg ratio (2.98) and a carried these distances, but appreciable accumu-
mean ratio of 1.35 for the nine others, plotted in lations ofvery fine sand are possible in the soils.
Fig. 3. One of the 18 samples is an anorthosite.
Six ofthe others have olivine >10% and four of Soil Parent Material Influences on Plant
them have molar Ca/Mg ratios <0.7 (Fig. 2). Species Distributions
One sample with Ca/Mg = 1.14 seems out of
place plotted between 0.3 and 0.4 (Fig. 2), and The chemical composition of gabbro is inter-
maybe it is, but it has nearly as much hornblende mediate between ultramafic and granitic rocks.
as olivine. Ultramafic rocks are represented by the base of
Springer (1980) showed analyses of29 samples the triangle in Fig. 2 and granitic rocks (quartz
ofgabbro. Onlytwo ofthemhad molarCa/Mg < diorite to granite) are more silicic than rocks
1.0 and one had Ca/Mg < 0.7. He arranged the represented by the left edge of the triangle.
gabbro into four groups, based on the calcium Anothosite, represented by the apex of the
cBo,nAcenn7t0r_a85t;ioCn,iAnnt5h5e_6p9l;aaginodclDa,seA:ng<ro5u5.pTAh,eAmne>an85s; tornilaynglien, tihseanSaanncGiaebnrtierlocakndfouOnrdocionpiCaaliMfoournni-a
ofthe four groups are shown in Fig. 2. tains; it is generally much less calcic than
There might have been appreciable additions anorthite, which is the plagioclase feldspar with
ofaeolian dust, orloess, to the soils developed on more than 90% Ca and less than 10% Na.
gabbro in the Pine Hill area and the Peninsular Ultramafic rocks and soils are well known for
Ranges. Very fine sand and coarse silt are the having unique plant communities and many
MADRONO
116 [Vol. 58
Fig. 3. Hornblende-bearing gabbro. Molar Ca/Mg ratios (0.8-3.0) of gabbro composed of plagioclase (PI),
bhoyrCnabl2eMngd3eA(lHFbe)A,la2nSdi60p2y2r(oOxHe)n2e,(Payn)d. Pplyargoixoecnlaesbeyisaruegpirtees(eCnateod^Mbgyo^laFberoa^dSoriiCtbe).(CaSoy.m6bNoaol.s4aAsl!in6SFii2g.4.028:),o,hoLronsblPeinndoes
complex; # Santa Ana Mountains. Rock samples from the three Guatay-Cuyamaca soils were plotted by the
percentages ofplagioclase, clinopyroxenes, and hornblende (symbols CGI, CG2, and CG3).
endemic species. The absence of many species tions on all ofthe soils and to P on most ofthem,
that arecommon on soils ofmore silicicrocks has except on an infertile anorthosite soil where there
been attributed to low exchangeable Ca/Mg was no response to a complete N + P + K
ratios (Alexander et al. 2007). Some gabbro has fertilizer either. Only on an anorthosite soil did
low Ca/Mg ratios, but the ratios were not the lettucerespondto Kaddition. Althoughplant
particularly low in the gabbro soils sampled in cover appeared to be related to soil productivity,
the Pine Hill area and Peninsular Ranges. there were no distinct differences in plant species
Vegetation differences from gabbro to quartz distributions among the soils. Other than sparse
diorite soils are evident, but not as dramatic as trees (Pinus coulteri D. Don and Quercus
the differences from gabbro to ultramafic soils chrysolepis Leibm.), only shrubs and Yucca sp.
(Whittaker 1960). More perfuse vegetation on were reported from the soil sampling sites; the
quartz diorite than on gabbro soils (Whittaker shrubs were Adenostoma fasciculatum Hook. &
1960) might be explained by greater fertility of Arn., Arctostaphylos spp., Ceanothus spp., Cer-
quartz diorite soils related to greater amounts of cocarpus sp., Eriodictyon sp., Lotus scoparius
K and P in diorite than in gabbro (Table 1). On a (Nutt.) Ottley, Quercus spp., Malosma laurina
mafic to silicic scale, fertility may be greatest on (Nutt.) Abrams, and Salvia spp.
diorite soils, which generally have a more
favorable balance of plant nutrients than either
mafic or granitic soils.
An anorthosite soil in the San Gabriel Moun- Gabbro soils and their parent rocks were
tains was sampled and characterized by Graham sampled at two sites with unique vegetation and
et al. (1988). Although the parent rock had more one without unique vegetation in both Research
than 90% plagioclase, the feldspar was andesine Natural Areas (RNAs) in the Peninsular Ranges
that has slightly more Na than Ca. Nevertheless and the Pine Hill Preserve in the foothills of the
the soil had much more exchangeable Ca than Sierra Nevada. Altitudes are 460 to 590 m at the
Na. It had low exhangeable K and presumably Pine Hill sites and 1270 to 1385 m at the Guatay
RNA
low P, although the analyses did not include P. and King Creek sites in the Peninsular
Vlamis et al. (1954) grew lettuce in 13 soils Ranges. The mean annual precipitation at these
mm
sampled on gabbro, anorthosite, diorite, and sites is in the 750 to 800 range. At a site on
N
granodiorite. The lettuce responded to addi- the upperone-halfofamoderately steep (21-36%
2011] ALEXANDER: GABBRO SOILS AND PLANTS 11
f gradient) convex slightly slope in each plant
community, three sets of soil and rock samples go GPN
coaw3poen0amre-rpe4tto5.ostcTiatthmkheeernedoseefopfimttlrehshostrwm.eeereErssseauuabcsbpshaaisrtmastepmu.splbslePtiedlhtsareanetttesasotikhtlweeoens0ranf-emir1pnio5eldmeecnmpmwteoiatifaesninrtedssad 00CPpPa3J sMjsPooU OPOoJ 7oyX3 'Xz2PpCsJ _'UcaCa0gp/J30 xOPp'i'iU^*PpP -x1O8PG1 >H£g0.3* TW+XpoX_3> §„™pkJ fufPctjt
by reference to The Jepson Manual (Hickman p
>>
1993) and cover areas were estimated visually. UP
x
Plant species in a list for the Pine Hill sites were X
vlMieasrtniffaiogeredmtbheeyntPG,ernaEiclnidseuollraaardHoiRnaHinslghlesa,swCs(AiBt)eusraewnaedurtehoovfesreLiafiinnedad 00f03t WSX3 oo W£CXJ tgO/5
by Todd Keeler-Wolf(California Department of
Fish and Game, Sacramento).
Soil samples were dried and sieved to separate
the fine-earth (particles <2 mm) for sand and
chemical analyses. Texture and consistence of X
wetted samples were estimated by feel and o <ou -<Ou ’oP
iSdteanftfif1i9e9d3).bySoUilSfDamAilnyocmleansscilfaitcautrieon(fSooillloSwsurSvoeiyl ^gCSJXPapS ^£M o<X5 G CXOJ •Pa g
TMianxeornalosmyweorfetheestUiSmaDteAd(iSnoil36SufriveeldysSotfaffvi1e9w99)i.n M1P3 xo 2(3D HH o %X X«CJ;Xr^X o aX 1xfC3tJ XCu>J =o3
CJ OJ
thin-sections ofrocks-one rock from each sample <*> o S-2 CO cj £ -2 aT 2p8
fsiotle.loSwainndg wtarseastempeanrtastedovferronmigfhitne-ienarthhoussaemhpolleds o0C3J CGSJ 003 ^ c2/3rXfPt ;L’ft CaO XPft PO -
bleach, decantation of clay, and overnight with
Na dithionite in Na citrate solution. Coarse silt
(30-62 pm) was obtained from the Pine Hill
samples by decantation. Sand separates were dry
sieved to obtain five size fractions, and the fine
sand (0.125-0.25 mm)fractionwas separated into
light and heavy grains with bromoform (SG =
2.85). Magnetic grains were separated from the
fine sand, very fine sand, and coarse silt fractions
with a hand magnet. Percentages of very fine
sand and coarse silt with low refractive indices 0 wo
(RI < 1.56) were ascertained by observing 300 CJ
grains from each Pine Hill soil.
Soil pH was ascertained with a glass electrode
in 1:1 water:soil suspensions. Calcium and
magnesium were extracted with molar KC1 and
measured by atomic absorption (AA) spectrom-
8*
etry. Exchangeable acidity was extracted with 0.5
molar KCl-triethanolamine pH 8.2 buffer and
back-titratedwith0.08 molarHC1to amethylred
endpoint (Peech 1965). Soil organic matter was
approximated byloss-on-ignition(LOI) at 360°C.
Results of chemical analyses are displayed as 00 in
means of three samples from each site with it>n Titn m00
standard deviations in parenthesis. One-way
ANOVA
was run for the three Pine Hill sites
and the three Peninsular Ranges sites; that is
three sites with three replications at each site in ffOtt ^abO
each run ofANOVA. Differences between means
in each set of three sites were evaluated by the
least significant differences (Snedecor and Co-
bfl
chrane 1967). Where the differences are signifi- Uo T(3U
cantatthe95% level ofconfidence (a < 0.05), the o
values are designated a and b ifthe means are on b<ful X£ 2XJ oX
two levels, or a, b, and c if they were on three s ft ft u
MADRONO
118 [Vol. 58
Table 3. Minerals (Percentby Volume) inthe Rocks. Sites PH are from Pine Hill, El Dorado Co., sites
CG are from Cuyamaca-Guataty, San Diego Co. aBlack, opaque minerals, mostly magnetite. bSome alteration of
feldspars, apparently to clinozoisite.
Site Pyroxene Hornblende Olivine Feldspar Opaquea Other
PH4 64 2 10 11 13 Biotite, clinozoisiteb
PH5 58 trace 3 24 15 Green spinel
PH9 56 1 15 18 10 Biotite, green spinel
CGI 29 17 1 52 1
CG2 3 34 4 55 4
CG3 30 12 3 54 1
levels of magnitude. Where the differences are separates from sites PH4 and PH5 and about 1/5
significant at the 99% level of confidence (a < to 1/4 from site PH9 were black opaque grains
0.01), A, B, and C are the level designations. attracted to a hand magnet, suggesting that the
black opaque minerals were mostly magnetite.
The means of fine sand, very fine sand, and
Results
coarse silt from the 0-15 cm depth in soils at the
Pine Hill sites were 93, 71, and 33 g/kg and the
Pine Hill Preserve Sites
masses ofmagneticgrainswere 34, 12, and 2g/kg.
Soils at the three sites in the Pine Hill Preserve, Grains with low refractive indices (RI < 1.56)
two in the Pine Hill unit and one in the Penny averaged 5, 8, and 11% in these size fractions and
Lane unit, are all Alfisols. One site in the Pine about 1% ofthe grains in the coarse silt fraction
Hill unit has a very stony variant of Rescue soil wereisotropic(presumablyvolcanicglass)andthe
(site PH5) with a mixed chamise chaparral plant isotropic grains were subrounded to rounded, in
community and the other has a clayey variant of contrast to other grains which were mostly
Boomer soil (site PH4) with a black oak/toyon- angular to subangular. Most, or practically all,
Lemon ceanothus plant community (Table 2). ofthe grains with low refractive indices could be
The site in the Penny Lane unit has a clayey feldspars and vein quartz from the gabbro parent
variant ofRescue soil (site PH9) with a chamise- rock. Although the glass is presumed to be from
manzanita chaparral plant community. Rock an aeolian source, its mass is <0.1% ofthe mass
samples representing the soil parent materials of ofthe surface (0-15 cm) soil.
these sites are composed of predominantly The clayey Rescue variant at site PH9 had
clinopyroxene, with lesser amounts ofplagioclase vegetation that is common on Rescue soils and
feldspar and olivine (Table 3). The compositions the soils at sites PH4 and PH5 had unique
ofthese rocks plotted on Fig. 2 indicate that they vegetation (Table 4). MatureJeffreypine trees on
m
are expected to have Ca/Mg ratios about 0.5 to the north side of Pine Hill are about 38 tall,
1.0 mol/mol. Sand separates from the soils indicating moderate site productivity even with
showed medium (0.25-0.5 mm) and coarse (0.5- the relatively low precipitation on Pine Hill.
1.0 mm) sand modes. Light separates were about There are no Jeffrey pine trees on the Rescue
0.2 (20%) of fine sand fractions, comparable to soils.
the feldspar estimates in thin-sections (Table 3). Theplants that are localendemics are Pine Hill
About 1/3, or more, of the heavy fine sand ceanothus (Ceanothus roderickiiW. Knight), Pine
Table 4. Vascular Plants onthe Gabbro Soils. Botanical authorities are those given in Hickman (1993).
PH9 site: Pinus sabiniana and Quercus xvizlizenii occur on adjacent slopes. Abundance symbols: ++++, 30-100%;
+++, 10-30%, ++, 3-10%, +, 1-3%; -, trace; 0, none.
PH4 PH5 PH9
A. Pine hill
Trees
Pinus sabiniana 0 0
Quercus kelloggii +++ 0
Quercus wislizenii 0 0
Cercis occidentalis ++
Shrubs
Adenostomafasciculatum 0 +++ ++H-T
Arctostaphylos viscida ++ +++
Ceanothus lemmonii + ++
2011] ALEXANDER: GABBRO SOILS AND PLANTS 119
Table 4. Continued
PH4 PH5 PH9
Ceanothus roderickii 0 + 0
Heteromeles arbutifolia ++ ++ 0
-
Quercus durata 0 0
—
Rhamnus ilicifolia + +
Rhamnus tomentella 0 ++ +
Salvia sonomensis 0 ++ ++++
Toxicodendron diversilobum 0 0
- -
Eriodictyon californicum 0
Forbs
Calochortus albus 0 0 +
- -
Chlorogalum grandiflorum 0
- -
Chlorogalumpomeridianum 0
Dichelostemma multiflorum 0 0 +
—
Dichelostemma volubile 0 0
-
Fritilaria micrantha 0 0
-
Galium californicum ssp. sierrae 0 0
Galium spp. + + +
-
Geranium molle 0 0
- -
Sanicula bipinnatifida 0
-
Senecio (Packera) layneae 0 0
-
Triteleia bridgesii 0 0
-
Wyethia angustifolia 0 0
— —
Wyethia reticulata 0
Grasses
-
Avenafatua 0 0
— —
Bromus diandrus 0
Bromus laevipes + 0 0
—
Bromus madritensis ssp. rubens 0 0
-
Cynosurus echinatus 0 0
Elymusglaucus + 0 0
—
Elymus multisetus 0 0
-
Fritilaria micrantha 0 0
-
Gastridium ventricosum 0 0
-
Melica sp. + 0
-
Nassella lepida 0 ++
B. Cuyamaca-Guatay CGI CG2 CG3
Trees
Callitropis stephensonii +++ 0 0
Callitropisforbesii 0 0 +++
Shrubs
Adenostomafasciculatum +++ ++ ++
-
Arctostaphylosglandulosa + +++
Ceanothusfoliosus 0 + 0
-
Ceanothusgreggiivar.perplexens 0 ++++
-
Cercocarpus betuloides 0 0
Ericameriaparishii +++ ++ 0
Heteromeles arbutifolia + 0 0
Rhamnus crocea — + 0
Quercus berberidifolia ++ 0 ++++
Salvia sonomensis + 0 0
Subshrubs and succulents
Eriophyllum confertifolium ++ 0 0
-
Trichostemaparishii + 0
-
Hesperoyucca whipplei 0 0
Forbs
-
Calystegia collina + 0
Dichelostemma capitatum + 0 0
Grasses
Calamagrostis koeleroides 0 + 0
MADRONO
120 [Vol. 58
Hill flannelbush (Fremontodendron californicum Exchangeable Ca and Mg in the surface and K
(Torr.) Coville ssp. decumbens (R. M. Lloyd) in the subsoil are relatively high in the Mollisol
HMuonozk).,&ElArDno.rassdpo. bsieedrsrtareaDwe(mGpasltieurm&calSitfeobrbniincs)u,m (tshieteMoClGl3i,solTaabtlesit5e).CAGs3ahtassitae PreHla5tiovenlyPitnheickHilOl-,
and El Dorado mule-ears (Wyethia reticulata horizon, but the surface soil organic matter
Greene). All four of these species are present in content (LOI, Table 5) is higher in the extremely
the vicinity of sites PH4 or PH5 in the Pine Hill stonyLasPosasvariantat siteCGI thathad been
unit, but only El Dorado bedstraw and El burned a year or two prior to sampling.
Dorado mule-ears are present in the Penny Lane Differences in element recovery by aqua regia
unit (G. Hinshaw, U.S. Bureau of Land Man- digestion, such as relatively low P content in the
agement personal communication). yellowish Las Posas variant, may not be related
Surface soil Ca concentrations are relatively to vegetation distributions.
high on the very stony Rescue variant (PH5) and Potentially toxic elements (Cr, Co, and Ni) are
the Mg relatively high on the clayey Rescue as high in the soil at site CG2 lacking cypress as
variant (PH9). The Ca/Mg ratios are significantly they are in the soil at site CG3 with Tecate
higher in the very stony Rescue variant than in cypress (Table 6). Although P is lower in the soil
the other soils, and the organic matter (LOI, at site CG2, that might be a result ofthe denser
Table 5) and exchangeable acidity are higher, vegetation and cypress trees at sites CGI and
also. The relatively high soil organic matter and CG3 recycling more P rather than an indication
high CEC (mostly exchangeable Ca and acidity) oflower inherent soil fertility at site CG2.
at site PH5 may be related to a thick O-horizon
ofpredominantly live oak, toyon, and coffeeber- Discussion and Conclusions
ry leaves. The O-horizon was not analyzed
chemically. Aqua regia digestion recovered rela- The chemistry of gabbro soils is sufficiently
tively low amounts ofCa for the soil at site PH5, different from others that some plants that grow
raising doubts about the usefulness of results on them do not grow on soils ofeither granitic or
from the digestion. Aqua regia is an aggressive ultramafic rocks. In both the Pine Hill area and
solvent, but it does not recovery the total the Peninsular Ranges, chemical differences
amounts ofall elements in soils. betweenthegabbro soilswithcommonvegetation
and those with unique vegetation do not appear
Peninsular Ranges, Cuyamaca-Guatay Sites to explain the differences in vegetation. The soil
differences, other than stoniness, appear to be
Soils attwo sites sampledin the Cuyamaca area related to differences in theplantcommunities on
(CGI and CG2) are Alfisols and the one on them, ratherthanthereverse. Thehypothesisthat
Guatay Mountain (CG3) is a Mollisol. Site CGI the exchangeable CaMg ratios ofgabbro soils are
site is on an extremely stony variant ofLas Posas important in the distributions ofunique species is
soil with a Cuyamaca cypress/chamise-golden- not verified from the chosen sites.
yarrow plant community; site CG2 is on a Perhaps theuniqueplantswouldgrow on all of
yellowish red variant of the dark reddish brown the gabbro soils sampled, but they have not all
Las Posas soil with an Eastwood manzanita plant been colonized by those plants. The sites lacking
community; and site CG3 on a soft, or friable, unique species are the hottest and driest sites,
black soil (a Mollisol) with a Tecate cypress/scrub which may have limited colonization of them.
oak-cupleaf lilac plant community (Table 2). Site PH9 is lower in elevation than sites PH4 and
Rock samples representing the soil parent materi- PH5 that have more unique plants, and site GC2
als of these sites are predominantly plagioclase lackingcypress is on a steep south-facing slope at
feldspar, with substantial amounts ofclinopyrox- a lowerelevation than siteGC1 whichis also ona
ene and hornblende, and only minor olivine south-facing slope.
(Table 3). Thecompositions ofthese rocksplotted Some aeolian dust, or loess, may have been
on Fig. 3 indicate that they are expected to have added to the coarse silt fractions of the gabbro
Ca/Mg ratios about 1.5 mol/mol. Sand separates soils, but these fractions are small. Only 3.3% of
from the soils showed fine sand (0.125-0.25 mm) the Pine Hill surface soils were coarse silt, not
modes. Light separates are about 0.6 (60%) offine much more than the 1.0% coarse silt at the 15-
sand fractions, comparable to the feldspar esti- 30 cm depth. The small amounts ofalkali (K and
mates in thin-sections (Table 3). About 0.3 (30%) Na > Ca) feldspars and sparse quartz in the
oftheheavyfinesandseparateswereblackopaque coarse silt and other particle-size fractions
grains attracted to a hand magnet, suggesting that suggests that the contributions ofloess have been
the black opaque minerals are mostly magnetite. minor. Any possible aeolian inputs do not alter
The yellowish red Las Posas variant at site the fact that four plants are endemic on the Pine
CG2 hasvegetation thatiscommon on LasPosas Hill gabbro (Wilson et al. 2010) and that in San
soils and the soils at sites CGI and CG3 have Diego Co. Cupressus arizonica Greene ssp.
unique vegetation (Table 4). stephensonii (C.B. Wolf) R. M. Beauch (Cuya-
—
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MADRONO
122 [Vol. 58
maca cypress) and Cupressus forbesii Jeps. Le Maitre, R. W. 1976. The chemical variability of
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