178 Northeastern Naturalist Vol. 16, Special Issue 5 Soil and Vegetation Differences from Peridotite to Serpentinite Earl B. Alexander* Abstract - Pedologists and ecologists generally consider peridotite and serpentinite together as common serpentine soils or substrates. A detailed survey in the Klamath Mountains, CA, with separate soil map units on peridotite and serpentinite revealed appreciable differences in geomorphic and pedologic features between these types of ultramafic rocks. Slopes tend to be steeper on peridotite and the soils redder. More Alfisols (Luvisols) were found on peridotite, and more Mollisols (Phaeozems) on serpentinite. Very shallow soils (7% of the area), which are mostly Mollisols and Entisols (Leptosols), are more common on serpentinite. Barrens are commonly fragmental colluvium (talus) on peridotite and erodible, slightly to moderately stony summits and slopes on serpentinite. The most obvious vegetation differences related to parent material are on very shallow and shallow soils, with more coniferous trees (Pinus jeffreyi [Jeffrey Pine] > Calocedrus decurrens [Incense Cedar] > Pseudotsuga menziesii [Douglas Fir]) and Quercus durata (Leather Oak) on peridotite, and more Ceanothus cuneatus (Buckbrush) and Vulpia microstachys (Annual Fescue) on serpentinite soils. Introduction Pedologists and ecologists generally consider peridotite and serpentinite together as common serpentine soils or substrates (Brooks 1987, Kruckeberg 1984). This approach is easy to understand from a botanical perspective, because there are few obvious vegetation differences from peridotite to serpentinite. Topographic and soil differences are more obvious, but not so obvious that they are perceived in casual observations. Peridotite is a precursor of serpentinite. Serpentinite is produced by the addition of water to peridotite in a metamorphic process called serpentinization. The complete alteration of peridotite to serpentinite requires the addition of about 13 or 14% water. Generally, calcium and silica are lost, but otherwise the chemical composition is seldom changed in the metamorphosis (Coleman 1971). The mineralogy, however, is almost completely altered from olivine and pyroxenes in peridotite to serpentine and accessory magnetite in serpentinite. Generally, only chromite remains unaltered from the complete serpentinization of peridotite (Coleman 1971). A 33% expansion during serpentinization, from heavy peridotite (3.2–3.3 Mg/m3) to light serpentinite (2.4–2.6 Mg/m3), is commonly accompanied by extensive fracturing and shearing. Consequently, most serpentinite breaks into smaller blocks than does peridotite. The shearing commonly smooths and polishes fracture surfaces in serpentinite, reducing its shear strength and *Soils and Geoecology, 1714 Kasba Street, Concord CA 94518, USA; alexandereb@ att.net. Soil and Biota of Serpentine: A World View 2009 Northeastern Naturalist 16(Special Issue 5):178–192 2009 E.B. Alexander 179 making it more susceptible than peridotite to mass failure. With these profound structural differences, we might expect to find differences in peridotite and serpentinite landscape topographies. Mineralogy is very important in weathering and soil formation. Olivine and the orthorhombic pyroxenes in harzburgite, the most common variety of peridotite in the Rattlesnake Creek terrane (RCT), are more readily weatherable than serpentine. Serpentinite formed from dunite (nearly all olivine), however, contains much brucite, which is even more readily weathered than olivine (Alexander 2004). Dunite is not extensive in the RCT. Even though the parent material mineralogy is quite different, soils on peridotite and serpentinite are commonly considered to be similar. The same soils are mapped on both peridotite and on serpentinite in soil surveys of Washington, Oregon, and California (e.g., Lanspa 1993). Perhaps because of their lack of economic importance, serpentine soils have not been investigated intensively. Soil differences from peridotite to serpentinite were recognized only in detailed soil survey. Detailed soil survey is generally for agriculture; nearly all serpentine soils in North America are in hilly or mountainous areas where agriculture is seldom practiced because the slopes are steep and serpentine soil chemistry limits plant productivity. A detailed soil survey in which peridotite and serpentinite were separated in mapping revealed appreciable differences in geomorphic and pedologic features between these two types of ultramafic rocks. These differences may have been observed before, but they were not recognized in reports from previous soil surveys. Nor have botanists reported vegetation differences between serpentinite and peridotite, but some obvious vegetation differences and trends from peridotite to serpentinite were observed in data from sites described for soil and landscape characterization in the RCT (Alexander 2003). My objective was to indicate major differences between peridotite and serpentinite soils and vegetation recognized in a detailed soil survey area of the RCT in the Klamath Mountains (Fig. 1). Pedologists generally do not considered soil differences from periditite to serpentinite, and because most plant distributions are similar on peridotite and serpentinite, the possible differences are generally not considered by ecologists (Kruckeberg 1984). Although soil mapping in the RCT was not designed to obtain a definitive comparison of vegetation on peridotite and serpentinite soils, data from it serve as a basis for retrospective evaluation of the differences. Field-site Description The area of detailed serpentine soil survey is in the southern exposure of the RCT (Fig. 1). This is an allochthonous terrane annexed to the North American continent during the Mesozoic era. It contains serpentinized harzburgite, gabbro, basalt, graywacke (clastic marine sediments), chert, and minor limestone. Some small granitic plutons intruded the terrane after it was accreted to the continent (Wright and Wyld 1994). The RCT extends about 120 km, from the southeast corner to the western edge of the Klamath Mountains in a strip that is about 6 to 16 km wide. It is 180 Northeastern Naturalist Vol. 16, Special Issue 5 in mountainous terrain, with some small valleys. A peneplain that extended across the Klamath Mountains during the Tertiary (Diller 1902) is no longer evident in the RCT, unless it is represented by the small summit flats of Red Mountain (1718 m asl) on the southeast portion of RCT and Sugar Pine Mountain (1202 m asl) on the northwest. The current climate is temperate with warm, dry summers and cold, wet winters. Mean annual temperatures range from 8 to 16 °C. Mean annual precipitation is mostly in the range from 100 to 150 cm, with extremes of about 75 cm on the southeast portion of RCT and 200 cm on the northwest. Soil temperature regimes are mesic and thermic, mostly mesic, with minor frigid regimes; and soil moisture regimes are xeric (Soil Survey Staff 1999). The vegetation is mostly open conifer forest (Fig. 2) and chaparral. Tree, shrub, and herb species distributions differ substantially from hotter and drier low elevations to cooler and wetter high elevations. Tree and grass species distributions, however, are somewhat similar from southeast to northwest at the intermediate elevations in the RCT. Methods A detailed soil survey of the approximately 12,000 ha of serpentine in the southern exposure of the RCT was conducted with stereo pairs of 1:16,000 aerial photographs (Alexander 2003). The minimum delineation was about one hectare, and most of the map polygons were observed on the ground. Soils chosen to represent those of the map units were described to bedrock, Figure 1. Location of the southern exposure of the Rattlesnake Creek terrane in the Klamath Mountains. 2009 E.B. Alexander 181 or to about 150 cm depth, at 208 sites: 73 on peridotite, 108 on serpentinite, 14 on other rocks associated with serpentine, and 13 on large landslides. Soils were classified according to Soil Taxonomy (Soil Survey Staff 1999). Soil depth classes are very shallow (depth <25 cm, except where noted as <18 cm), shallow (25–50 cm), moderately deep (50–100 cm), deep (100–150 cm), and very deep (>150 cm). Peridotite is easy to identify where rock surfaces have weathered to reddish brown and have a few millimeters of knobby relief with pyroxene highs and olivine lows (Figs. 1–2 in Alexander et al. 2007). Serpentinite is easy to identify where it has been tectonically sheared to reveal smooth greenish mottled surfaces, or where rocks have weathered to smooth gray surfaces (Alexander 2004). Where the ultramafic rocks are massive and lack distinctive weathering surfaces, partially serpentinized peridotite can be recognized by the cleavage faces of pyroxenite in freshly broken rocks. All of the ultramafic rocks in the RCT are at least partially serpentinized (except pyroxenite, which is not extensive)—the presence or absence of pyroxene cleavage faces seemed to be a good criterion for distinguishing between peridotite that has been only partially serpentinized and serpentinite lacking pyroxene (Alexander 2003). Surface boulder (nominal diameter >60 cm), “stone” (25–60 cm), and cobble (7.5–25 cm) cover were estimated by 100 step-point counts at each soil-description site. Soils were given color designations from a Munsell chart (Munsell 1988). These color designations were converted to redness ratings (Rr) by subtracting the prefix to YR from 15 (or 15 minus 12.5 for 2.5Y hues), multiplying by the chroma, and dividing by the value (Alexander 2004). The redness ratings were assigned to classes, allowing a comparison of differences between soils on peridotite and serpentinite by chi-square (χ2) statistics. Plant species cover (percentage) was estimated visually at each of the 208 soil-description sites. Because the sites were chosen to represent soiltopography map units, vegetation bias in choosing the sites was minimal. The soil site data are suitable only for indicating the distribution of the more common plant species. Serpentine endemic species (6 species) distributions in the RCT were investigated by Williams et al. (2009). Soil horizon samples from two pedons were analyzed in a laboratory. These pedons represent the most extensive shallow soils on peridotite (Wildmad series) and serpentinite (Bramlet series). Cations were extracted with 0.5 M KCl, and Ca and Mg were measured by EDTA titration (Heald 1965). Acidity was extracted with KCl-nitrophenol buffer, pH 7.0, and with BaCl2- TEA buffer, pH 8.2, and the buffer solutions were titrated to a bromocresol green endpoint with standard hydrochloric acid, following procedure modifications from Schofield (1933) and Peech (1965). Soil pH was ascertained with a glass electrode after 30 minutes in 1:1 water to soil suspensions. Iron and Mn were reduced and extracted with Na-dithionite in 0.3 M Na-citrate (Holmgren 1967) and analyzed by atomic absorption spectrometry. Soil organic matter (SOM) is approximately equal to the loss on ignition (LOI) 182 Northeastern Naturalist Vol. 16, Special Issue 5 at 360 °C. It was estimated from the equation SOM = 0.88 LOI - 0.14 Fed, where Fed is the citrate-dithionite extractable Fe. This equation is based on data from Alexander et al. (1989) for LOI and for SOM by weighing CO2 produced by dry combustion. Soil organic matter was assumed to be twice the weight of organic carbon. Mean differences for tree, shrub, and grass cover on soils in two parent material classes and four soil habitat classes were compared by two-way analysis of variance (ANOVA). Because the number of samples in each soil parent-material class and in each soil habitat class were different, comparisons among individual classes were not appropriate (Havicek and Crain 1988, Kachigan 1986). Results Topography and soils The frequencies of very steep slopes (gradient >60%) on soils mapped in peridotite landscapes are thought to be much greater than those in serpentinite landscapes (Figs. 2 and 3). Soil and rock detritus that has accumulated downslope in landslides is more common in serpentinite landscapes than in peridotite landscapes. Peridotite colluvium that has accumulated on steep or very steep slopes, or near the base of those slopes, is generally expected to be coarser than serpentinite colluvium. Although some of the soil description data support this conjecture, too few sites were described in these colluvial positions to test this idea objectively. Obvious extremes are blocky talus of dunite lacking particles <7.5 cm in High Camp basin at the head of the Trinity River and fine serpentinite slopes of the Lassics area, about 18 km southwest of the RCT, lacking particles >7.5 cm. In the RCT, the mean stone cover was 1% boulders, 4% “stones,” and 13% cobbles at 73 peridotite sites, and much less than 1% boulders, 1% “stones,” and 5% cobbles at 108 serpentinite sites. The ground cover at one peridotite colluvium site with a well-vegetated Palexeralf was about 10% “stones” and 75% cobbles. Peridotite sites with greater stone cover were observed, but not sampled, because they generally lack fine earth (particles <2 mm) and vegetation. A typical horizon sequence in both peridotite and serpentinite soils of the RCT is Oi-A-Bt-C-R, with relatively few soils having Bw rather than Bt horizons. The most obvious difference between the peridotite and serpentinite soils is that the peridotite soils are redder. Table 1 contains a summary of soil redness for all shallow and moderately deep to deep serpentine Argixerolls and Haploxeralfs sampled in the RCT. Very deep soils and soils lacking argillic horizons were not included in this summary. The χ2 statistic for a comparison of redness class differences for peridotite and serpentinite soils is 70.6 with 3 degrees of freedom, which is highly significant (α < 0.01). Table 2 contains data from representative shallow soils with mesic soil temperature regimes on peridotite (Wildmad series) and serpentinite (Bramlet series). There are data from too few pedons to make many comparisons of chemical differences. It is obvious from the data (Table 2), however, 2009 E.B. Alexander 183 that dark colors (low values and chroma) are not entirely related to soil organic matter content. Greater Fe oxide concentrations in soils on peridotite increases chroma and tends to diminish the effects of organic matter on soil colors. Free Fe and Mn were extracted from three soil samples: Wildmad A Figure 2. Typical serpentine soils and vegetation on a steep east-facing slope in the southern exposure of RCT. Although logs were taken from some of the sites many years ago, the forests on serpentinite were never dense. On the left is a shallow Bramlet soil with a Jeffrey Pine–Incense Cedar/Buckbrush plant community, and on the right is a moderately deep Hyampom soil with a Jeffrey Pine–Incense Cedar/ fescue plant community. Table 1. Soil redness based on Munsell colors (moist) of subsoils in shallow (18–50 cm) and moderately deep to deep (50–150 cm) serpentine soils with argillic horizons. The expected value is the product of the number of samples in a row and the number of samples in the column divided by the total number of samples (126). The χ2 value is 70.6, with 3 degrees of freedom (α < 0.01). Number of samples (pedons) Peridotite Serpentinite Redness class Observed Expected Observed Expected Low, Rr <5 2 18.2 44 27.7 Moderate, Rr 5.1–10 11 15.1 27 22.9 Mod. High, Rr 10.1–15 20 9.5 4 14.5 High, Rr >15 17 7.1 1 10.9 Total (126 pedons) 50 50.0 76 76.0 184 Northeastern Naturalist Vol. 16, Special Issue 5 (4.0% Fe, 0.13% Mn) and Bt (4.5% Fe, 0.12% Mn) horizons and Bramlet Bt (1.3% Fe, 0.05% Mn) horizon. X-ray diffractograms of clay from Wildmad (National Soil Survey Laboratory, NSSL pedon 04N0625) and Bramlet (NSSL pedon 03N0075) soils indicate that the main clay mineral is serpentine, apparently with a bit of chlorite/vermiculite mineral as found by Lee et al. (2003) elsewhere in the Klamath Mountains and in the Bramlet pedon minor talc. Table 2. A comparison of shallow peridotite and serpentinite soils with mesic soil temperature regimes in the Rattlesnake Creek terrane. Exchangeable cations Soil horizon (mmol+/kg) Depth Color Field Soil OMB Bases Acidity Ca:Mg Sym. (cm) moist gradeA (g/kg) Ca Mg pH 7 pH8 (mol/mol) pH Wildmad series, a Lithic Haploxeralf, pedon TS42 Oi 3–0 Slightly matted pine needles. A 0–5 7.5YR 2/2 GrSL 95 61 166 44 105 0.37 6.3 AB 5–12 7.5YR 3/3 GrSL 42 35 175 18 77 0.20 6.4 Bt 12–33 5YR 3/4 vGrL 29 22 161 15 66 0.14 6.6 R 33+ Moderately fractured partially serpentinized peridotite Bramlet series, a Lithic Ultic Argixeroll, pedon TS26 Oi 3–0 Slightly matted pine needles. A1 0–2 10YR 2/1 vGrSL 32 16 83 8 38 0.19 6.2 A2 2–12 10YR 2/1 vGrSL 23 11 82 2 29 0.13 6.4 Bt 12–26 10YR 2/2 vGrSL 18 11 96 3 29 0.11 6.5 R 26+ Highly fractured serpentinite AField grade symbols: GrSL, gravelly sandy loam; vGL, very gravelly loam; vGrSL, very gravelly sandy loam. BOrganic matter estimated from loss-on-ignition at 360 °C, with an adjustment for citratedithionite extractable iron. Figure 3. Slope d i s t r i b u t i o n s of serpentine soils as mapped in the southern exposure of the R a t t l e s n a k e Creek terrane. The slope classes are landslide, m o d e r a t e l y steep (15–30%), steep (30–60%), and very steep (>60%). 2009 E.B. Alexander 185 Conceptual transects from south to north across a very steep peridotite mountain (1334 m asl) and nearby steep peridotite and serpentinite mountains indicate the soils encountered (Fig. 4). On very steep slopes (gradient >60%), which are common only on peridotite, rock outcrops and very shallow Entisols (Lithic Xerorthents) are characteristic of upper slopes. Further down very steep slopes, there are some very deep north-facing Inceptisols (Haploxerepts) and very deep south-facing Alfisols (Haplic Palexeralfs). Soils on serpentinite can be readily compared to those on peridotite on steep slopes (30–60% gradient), because steep slopes are common on both kinds of rocks. Alfisols predominate on peridotite: mostly Lithic Haploxeralfs on south-facing slopes and moderately deep to deep Mollic Haploxeralfs on north-facing slopes. On serpentinite, moderately deep to deep Alfisols (Mollic Haploxeralfs) predominate on north-facing slopes, and there are some shallow Mollisols (Argixerolls). Shallow Mollisols (Argixerolls) predominate on south-facing serpentinite slopes, with some moderately deep Mollisols (Ultic Argixerolls) and Alfisols (Mollic Haploxeralfs). Moderately deep soils with mesic soil temperature regimes are predominantly in clayeyskeletal families on peridotite and in loamy-skeletal families on serpentinite. Otherwise, the soils with mesic soil temperature regimes are mostly in Figure 4. Profile sketches of very steep and steep slopes on peridotite (A) and steep slopes on serpentinite (B) in the Rattlesnake Creek terrain. 186 Northeastern Naturalist Vol. 16, Special Issue 5 loamy-skeletal families, and those with thermic soil temperature regimes are mostly in clayey-skeletal families. Soils grouped by four habitat classes that were defined to show vegetation differences related to soil depth and slope aspect are displayed in Table 3. The very shallow soils are mostly Entisols (Lithic Xerorthents) on peridotite and Mollisols (Lithic Haploxerolls) on serpentinite. Vegetation Differences related to longitude and elevation in the RCT are much greater for vegetation than for soils. These vegetation differences were minimized by restricting the comparison of vegetation to mesic soil temperature regimes in the southeastern half of the terrane where the vegetation was described at 84 sites on moderately steep to steep slopes: 44 sites on peridotite and 40 on serpentinite. Two-way ANOVA for tree, shrub, and grass cover (Table 4) indicates a significant difference (α < 0.05) for tree cover among the soil habitat classes, with very low tree cover on very shallow soils. Although there are large differences in grass cover on peridotite compared to serpentinite soils and in tree cover (Pinus jeffreyi (Grev. & Balf.) [Jeffrey Pine], Calocedrus decurrens (Torrey) Florin [Incense Cedar], and Pseudotsuga menziesii (Mirabel) Franco [Douglas Fir]) on very shallow soils, because of large variance these differences are not statistically significant. Figure 5 shows some large differences in species cover between peridotite and serpentinite soils. Conifer trees (Jeffrey Pine > Incense Cedar > Douglas Fir) are the dominant cover on north-facing serpentinite slopes and all peridotite soils, except very shallow ones. Quercus durata Jepson (Leather Oak) is the predominant shrub on very shallow peridotite soils and moderately deep to deep serpentinite soils that are not on north-facing slopes, but it is absent from very shallow serpentinite soils. Ceanothus cuneatus (Hook.) Nutt. (Buckbrush) is the predominant shrub cover on shallow serpentinite soils. Other than Buckbrush, Leather Oak, and Arctostaphylos viscida C. Parry (Whiteleaf Manzanita), other shrub species of Arctostaphylos spp. (manzanita), Garrya spp. (Silktassel), and Rhamnus spp. (Coffeeberry) are common in small amounts in the mesic soil habitats. Grasses, mainly perennial fescues (much Festuca idahoensis Elmer [Idaho Fescue], and less F. californica Vasey [California Fescue]), are more abundant on serpentinite than on Table 3. Dominant kinds of soils in four different soil habitat classes, based on 159 pedon descriptions in upland soils (66 on peridotite and 93 on serpentinite). Habitat class definitions: very shallow soils = depth <18 cm for soils with argillic horizons, or depth <25 cm for other soils; shallow soils = those not very shallow and with depth <50 cm on S-facing slopes or depth <30 cm on N-facing slopes; deep soils = depth 50–150 cm on S-facing slopes, and depth 30 to 150 cm on N-facing–slopes with aspect 300o to 120o azimuth. Habitat class Peridotite Serpentinite Very shallow soils Entisols Haploxerolls Shallow soils Alfisols Argixerolls Deep soils Alfisols Argixerolls and Alfisols North-facing slopes Alfisols Alfisols and Argixerolls 2009 E.B. Alexander 187 Table 4. Tree, shrub, and grass cover differences between peridotite and serpentinite and among soil habitat classes. A. Means and standard deviation by soil parent material and soil habitat class (mean ± standard deviation) Number of samples Tree cover Shrub cover Grass cover Soil habitat class Peridotite Serpentinite Peridotite Serpentinite Peridotite Serpentinite Peridotite Serpentinite Very shallow 5 5 14.4 ± 10.7 2.0 ± 1.9 14.2 ± 14.6 22.8 ± 28.2 2.0 ± 4.5 8.2 ± 8.9 Shallow 12 17 29.5 ± 12.9 18.3 ± 16.9 14.8 ± 15.9 18.2 ± 19.1 5.3 ± 9.7 5.7 ± 8.1 Deep, south 11 6 30.6 ± 18.2 22.5 ± 13.9 25.0 ± 11.6 39.2 ± 22.4 1.3 ± 1.9 10.2 ± 10.5 North-facing 16 12 35.9 ± 12.2 39.2 ± 14.1 11.2 ± 10.6 8.8 ± 10.8 2.4 ± 5.1 7.6 ± 8.3 B. Significance of differences rated by two-way analysis of variance (ANOVA) Degrees Trees Shrubs Grass Source of difference of freedom SS MS F SS MS F SS MS F Parent material 1 1104.1 1104.1 5.39 201.4 201.4 0.30 403.1 403.1 7.15 Soil habitat 3 7001.1 2333.7 11.38* 4167.0 1389.0 2.05 18.9 6.3 0.11 Interaction 3 492.3 164.1 0.80 3221.6 1073.9 1.28 187.1 62.7 1.11 Within subclasses 76 15,580.3 205.0 51,580.3 678.7 4284.7 56.4 *Significant (α < 0.05). 188 Northeastern Naturalist Vol. 16, Special Issue 5 peridotite soils. Vulpia microstachys Nutt. (Annual Fescue) is more abundant than perennial fescues on very shallow serpentinite soils and is common on shallow serpentinite soils, but it is practically absent from all other Figure 5. Major plant species distributions in soil habitat classes with mesic soil temperature regimes. A. Peridotite sites, n = 45. B. Serpentinite sites, n = 40. Plant species symbols for trees: DF = Douglas Fir (Pseudotsuga menziesii), IC = Incense Cedar (Calocedrus decurrens), JP = Jeffrey Pine (Pinus jeffreyi); for shrubs: Avi = Whiteleaf Manzanita (Arctostaphylos viscida), Ccu = Buckbrush (Ceanothus cuneatus), Qdu = Leather Oak (Quercus durata); and grass: Fescue (Festuca idahoensis and F. californica). 2009 E.B. Alexander 189 serpentinite and all peridotite soils. Two of the more ubiquitous but sparse forbs are Phacelia corymbosa Jepson (Serpentine Phacelia) and Streptanthus spp. (jewelflower). The Phacelia shows no habitat preference. Jewelflower is present on only 9% of the peridotite sites, but occurs on 36% of the serpentinite sites. These species of jewelflower are not Ni-hyperaccumulators. Angiosperm trees, with the rare exception of a Quercus chrysolepis Liebm (Live Oak) or an Arbutus menziesii Pursh (Madrone), both evergreen species, are absent from peridotite and serpentinite in the southeastern part of the RCT. Deciduous oak trees are absent from the peridotite and serpentinite, but a shrubby variety of Quercus var. garryana breweri (Engelm.) Jepson (White Oak) is common on pyroxenite soils. All of the more common shrub species on serpentinite and peridotite soils are evergreen. There are only a few hectares of pyroxenite soils in the southern exposure of the RCT. Discussion The greater frequencies of very steep slopes on peridotite and of landslides in serpentinite terrain reflect the greater rock strength of peridotite and the more extensive fracturing in serpentinite, respectively. Also, the lesser frequencies of large stones on serpentinite soils has been attributed to the more extensive fracturing in serpentinite than in peridotite. The main serpentine soil-forming processes are chemical weathering, oxidation of Fe, accumulation and migration of clay, and accumulation of organic matter and its incorporation into soils (Alexander et al. 2007). A typical horizon sequence is Oi-A-Bt-C-R. Most of the organic matter is in the soils rather than in the Oi horizons of plant detritus. Peridotite soils are generally redder than serpentinite soils because peridotite has more Fe in readily weathered minerals and less in resistant spinel group minerals such as chromite and magnetite (Alexander 2004). Intense reddish colors, or high chroma, resulting from relatively high Fe-oxide contents may be the reason that more peridotite soils are Alfisols and more serpentinite soils are Mollisols, rather than because of differences in organic matter content. Munsell (1988) soil colors of high chroma (chroma >3 moist) are not mollic epipedon colors, even with dark values of 3 (Soil Survey Staff 1999), preventing many dark-colored peridotite soils from being Mollisols. The presence of clayey, moderately deep peridotite soils with mesic soil temperature regimes, where the majority of serpentinite soils are loamy, is not readily explicable. Perhaps the more weatherable silicate minerals in peridotite keep the free silicon concentrations higher in peridotite soil and promote the formation of silicate clay minerals. In wetter climates, much excess silicon is leached away as H4SiO4, leaving peridotite soils with very high concentrations of Fe oxides; for example, an Ultisol of the Littlered series in the California Coast Ranges, adjacent to the Klamath Mountains, has 33.2% citrate-dithionite extractable Fe (pedon S80CA-045-002, 25–66 cm depth), which would be 53% goethite (FeOOH). 190 Northeastern Naturalist Vol. 16, Special Issue 5 Because Ca is generally lost in the serpentinization process, serpentinite parent materials have less calcium than peridotite parent materials. The data in Table 2 indicate that the soil on serpentinite (Bramlet series) has lower Ca/Mg ratios than the soil on peridotite (Wildmad series), but there are data from too few pedons to assume that this is a general rule. It is not obvious why conifer tree cover might be greater on shallow peridotite soils than on shallow serpentinite soils (Fig. 5). Perhaps more water is lost by overland flow from serpentinite, rather than infiltrating into the soils. Peridotite soil depths appear to be less uniform than serpentinite soil depths; consequently, conifer trees and Leather Oak on very shallow peridotite soils may be exploiting inclusions of deeper pockets of soil. Other than rock outcrop and blocky talus in very steep peridotite terrain, most of the barrens with sparse vegetative cover are on very shallow serpentinite soils. Limited soil-water supplies, lack of shade, and high albedo of serpentinite, makes these soils poor environments for plants, especially if overland flow of water carries any of the scanty plant detritus away before it can be incorporated into the soils. Obviously, significant differences in tree cover related to soil habitat (Table 4) are mainly the result of less cover on very shallow soils, although a statistical comparison among soil habitat classes was not possible. Apparent greater tree cover on very shallow peridotite than on very shallow serpentinite soils and greater grass cover on serpentinite soils than on peridotite soils were not verified by statistical analyses, but some species differences are so definite that statistical analysis is not necessary to verify their significance. This clear significance is true for the dominance among shrubs of Leather Oak on peridotite and Buckbrush on serpentinite soils. It is also true for the occurrence of Annual Fescue almost exclusively on very shallow and shallow serpentinite soils; it is more abundant than perennial fescues on very shallow serpentinite soils. Summary Differences in soils from peridotite to serpentinite are more obvious than differences in vegetation, except for vegetation on very shallow soils. A large set of data from the RCT indicates that the slopes are generally steeper on peridotite and the soils are redder than on serpentinite. Although there are many other differences between soils on peridotite and serpentinite, the data for them are less definitive. Alfisols are the predominant peridotite soils, and both shallow Mollisols and moderately deep Alfisols are the predominant serpentinite soils with mesic soil temperature regimes. Apparent physiognomic differences of greater tree cover on very shallow peridotite than on very shallow serpentinite soils and greater grass cover on serpentinite soils than on peridotite soils were not verified by statistical analyses. The occurrence of Annual Fescue only on very shallow and shallow serpentinite soils, however, is an obvious distinction from peridotite soils. Also, on very shallow soils, the complete dominance of Leather Oak among shrubs on peridotite and Buckbrush on serpentinite is unequivocal. 2009 E.B. Alexander 191 Acknowledgments A detailed soil survey of the RCT was possible only because Julie Nelson and Scott Miles of the Shasta-Trinity National Forest recognized the importance of soils information for managing the area for the endangered and sensitive serpentine plants and were able to obtain funds to support the soil survey. Susan Erwin, Trinity National Forest botanist, was very helpful in facilitating the soil survey. Literature Cited Alexander, E.B. 2003. Trinity Serpentine Soil Survey. Available online at www. fs.fed.us/r5/shastatrinity/. Shasta-Trinity National Forest, Redding CA, USA. Accessed 2003. Alexander, E.B. 2004. Serpentine soil redness: Differences among peridotite and serpentinite materials, Klamath Mountains, California. International Geology Review 46:754–764. Alexander, E.B., C. Adamson, P.J. Zinke, and R.C. Graham. 1989. Soils and conifer productivity on serpentinized peridotite of the Trinity ophiolite, California. Soil Science 148:412–423. Alexander, E.B., R.G. Coleman, T. Keeler-Wolf, and S.P. Harrison. 2007. Serpentine Geoecology of Western North America. Oxford University Press, New York, NY, USA. Brooks, R.R. 1987. Serpentine and Its Vegetation. Dioscorides Press, Portland, OR, USA. 454 pp. Coleman, R.G. 1971. Petrologic and geophysical nature of serpentinites. Geological Society America, Bulletin 82:897–918. Diller, J.S. 1902. Topographic development of the Klamath Mountains. US Geological Survey, Washington, DC. Bulletin 196. 69 pp. Havlicek, L.L., and R D Crain. 1988. Practical Statistics for the Physical Sciences. American Chemical Society, Washington, DC, USA. 489 pp. Heald, W.R. 1965. Calcium and magnesium. Pp. 999–1010, In C.A. Black (Ed.). Methods of Soil Analysis, Part 2, Chemical and Microbiological Properties. American Society of Agronomy, Madison, WI, USA. 1572 pp. Holmgren, G.G.S. 1967. A rapid citrate-dithionite extractable-iron procedure. Soil Science Society of America Proceedings 31:210–211. Kachigan, S.K. 1986. Statistical Analysis. Radius Press, New York, NY, USA. 589 pp. Kruckeberg, A.R. 1984. California Serpentines: Flora, Vegetation, Soils, and Management Problems. University of California Press, Berkeley, CA, USA. 180 pp. Lanspa, K.E. 1993. Soil Survey of Shasta-Trinity Forest Area, California. USDA, Forest Service, Shasta-Trinity National Forest, Redding, CA, USA. 521 pp. Lee, B.D., S.K. Sears, R.C. Graham, C. Amrhein, and H. Vali. 2003. Secondary mineral genesis from chlorite and serpentine in an ultramafic soil toposequence. Soil Science Society of America Journal 67:1309–1317. Munsell, A.H. 1988. Munsell Soil Color Chart. Kollmorgan Instruments, Baltimore, MD, USA. Peech, M. 1965. Exchange acidity. Pp. 905–913, In C.A. Black (Ed.). Methods of Soil Analysis, Part 2, Chemical and Microbiological Properties. American Society of Agronomy, Madison, WI, USA. 1572 pp. Schofield, R.K. 1933. The use of p-nitrophenol for assessing lime status. Journal of Agricultural Science 23:252–254. 192 Northeastern Naturalist Vol. 16, Special Issue 5 Soil Survey Staff. 1999. Soil Taxonomy: A Basic System for Making and Interpreting Soil Surveys. Agriculture Handbook No. 436. US Government Printing Office, Washington, DC, USA. 869 pp. Williams, J.N., C. Seo, J. Thorne, J.K. Nelson, S.Erwin, J.M. O’Brien, and M.W. Schwartz. 2009. Using species distribution models to predict new occurrences for rare plants. Diversity and Distributions 15:565–576. Wright, J.E., and S.J. Wyld. 1994. The Rattlesnake Creek terrane, Klamath Mountains,California: An early Mesozoic volcanic arc and its basement of tectonically disrupted ocean crust. Geological Society of America Bulletin 106:1033–1056.