366 Northeastern Naturalist Vol. 16, Special Issue 5
Geologic and Edaphic Controls on a Serpentine Forest
Community
Jerry L. Burgess1,*, Steven Lev2, Christopher M. Swan3,
and Katalin Szlavecz1
Abstract - This study examined woody vegetation, edaphic factors, bedrock
geochemistry, petrography, and outcrop structure to evaluate some of the community-
structuring factors in an ultramafic terrain of Maryland. Analyzing the dynamic
nature of combined geological and ecological processes can detect correlative relationships
between factors that are typically considered as independent such as
tectonically driven bedrock fracturing and ecological community interaction. This
study provides evidence for structural variation in fracture density of bedrock as
a partial control on tree species distribution in an ultramafic woodland/forest ecosystem.
Increases in the number of bedrock fractures correlates negatively with
plot-level volumetric soil moisture. Additionally, the degree of serpentinization of
the ultramafic parent material results in compositional variation in Ca, Mg, and Ni
of parent materials and soils. The combination of these factors provides a significant
level of control on the distribution of xeric tree species.
Introduction
Plant community structure and diversity is controlled by many factors.
Among the many factors that work in concert with biotic interactions
(competition, predation, etc.) are the abiotic mechanisms that shape species
diversity and distributions. Spatial and temporal heterogeneity in the
physical environment also plays a key role in structuring community composition.
One such mechanism that influences plant diversity is the fundamental
structure and geochemistry of the parent materials–the underlying geodiversity.
The overall patterns of vascular plant diversity on the landscape are a
function of the biodiversity and geodiversity (Mutke and Barthlott 2005). As
geodiversity increases, the number of niche dimensions afforded for organismal
distribution also increases (Silvertown 2004). Geodiversity has many
components, but with respect to plant community compositional control, one
geological factor that is often overlooked is the importance of the structure
of the parent rock to plant community composition. In this case, geological
structure refers to the macroscopic features shown by rocks. Such features
1Department of Earth and Planetary Sciences, Johns Hopkins University, 301 Olin
Hall, 3400 North Charles Street, Baltimore, MD 21218. 2Department of Physics,
Astronomy, and Geosciences, Towson University, 8000 York Road, Towson, MD
21252-0001. 3Department of Geography and Environmental Systems, University of
Maryland, 211 Sondheim Hall, 1000 Hilltop Circle, Baltimore, MD 21250. *Corresponding
author - jerry.burgess@jhu.edu.
Soil and Biota of Serpentine: A World View
2009 Northeastern Naturalist 16(Special Issue 5):366–384
2009 J.L. Burgess, S. Lev, C.M. Swan, and K. Szlavecz 367
are jointing, foliation, cleavage, shear fabrics, fractures, faults, and other
fissures. These features provide preferred pathways for agents of weathering
such as water or root structures. Other structural features such as shear zones
are generally accompanied by grain-size reduction via dynamic recrystallization
in the host rock, and accordingly, these differences in parent-material
grain size will also affect soil genesis, creating spatial heterogeneity that is
fundamental to the dynamics of ecosystems (Levin 1992).
The relationship between geodiversity and forest woody species distribution
remains obtuse in many ecosystems. Areas with serpentinized rocks
are model habitats for geoecological studies as they are considered extreme
environments with plants having special adaptations or where the rock may
serve as refugia for many species (Kruckberg 2002). The tight coupling
between rock, soil, and flora evident in these areas provides a unique opportunity
to explore relationships between geodiversity and biodiversity. In
addition, the temporal component of heterogeneity supplemented by anthropogenic
disturbances plays an equally important role in determining tree
species distribution creating patch mosaics in forest communities (Denslow
and Hartshorn 1994). Semi-natural grassland and savannah ecosystems,
such as serpentine barrens and associated woodlands, are of considerable
interest for their number of endemic species, rare and threatened species,
biodiversity, and landscape value. These habitats have become reduced and
fragmented throughout Eastern North America due to changes in management,
grazing, and the absence of frequent and widespread fires formerly
promoted by Amerindians (Marye 1955a, b, c, Tyndall and Hull 1999). The
subsequent extirpation of Amerindians, a lack of livestock grazing, and selective
clearing has allowed the once expansive xerophytic grasslands and
savannahs to become forested; however, remnants of these grasslands are
still present (Tyndall 2005, Tyndall and Hull 1999). It is these successional
forested regions that are of interest to this work.
Unusual species assemblages are typically equated with the eastern serpentine
barrens, but serpentine savannas, woodlands, and forests may also
contain regionally unique communities (Brush et al. 1980, Latham 1993).
Typically, these local assemblages are equated to edaphic physical and chemical
properties such as shallow soil depth and low water-holding capacity.
Chemical attributes which are characteristic of serpentine soils worldwide
include: elevated levels of heavy metals such as Cr, Ni, and Co that are toxic
to many plants; near-neutral pH values; and Ca:Mg ratios <1 (Alexander et
al. 2007; Rajakaruna et al. in press; Roberts and Proctor 1992).
The influence of parent rock material on soil formation and flora has
long been evident to ecologists. However, small-scale investigations of such
phenomena are rarely available. Local variation in geologic substrate and
derived soils within an ultramafic lithotype may act as abiotic templates that
correspond to associated tree-community compositions and successional
trajectories. In this paper, we investigate edaphic properties derived from
ultramafic parent materials in relation to xeric tree species abundance. The
368 Northeastern Naturalist Vol. 16, Special Issue 5
results are discussed in terms of the factors controlling xeric species abundance.
Specifically, this study investigates the following question: How is
woody plant community composition affected by the spatial distribution of
structural and chemical discontinuities across a single ultramafic lithotype?
Materials and Methods
Field area
The Pilot Serpentine Barrens (PSB) is located approximately 1 km east
of the town of Pilot, in the Piedmont of western Cecil County, MD. The barrens
lie within the Conowingo Dam Susquehanna River watershed as part
of the Upper Western Shore basin and extend to the Pennsylvania state line.
Mean annual temperature is 12.2 ºC with 122.1 cm precipitation (MSCO
2008). Bedrock consists of a northeast-trending group of undifferentiated
serpentinite and other mafic-ultramafic bodies that may be an extension
of the Baltimore Mafic Complex and that were tectonically emplaced into
the metasediments of the Wissahickon Formation (Crowley 1976, Hanan
1980).
Natural resource and mining reports combined with historical aerial
photographs spanning 1937 to 2007, indicate some portions of the PSB and
surrounding serpentinite were mined for albite (Na feldspar) or building
stone in the mid–late 1800s, and there was selective cutting of oaks for charcoal
production. However, in the 20th century, there is only limited evidence
of farming on these typically nutrient-poor soils. Nearly all of the forested
area was extensively logged by clear-cutting during the 1920s and early
1930s. Since that time, the forested areas have remained virtually intact,
with most of the forested area today held by private landowners.
Ecology
Serpentine outcrops in the Mid-Atlantic region are characterized by
prairie or savanna grassland if maintained by periodic disturbance such
as fire. In the absence of fire, the prairies vary from of patch mosaics of
prairie-like openings and pine scrub or greenbrier thickets to mixed pinedeciduous
woodlands to oak-dominated forests in areas of deeper soils. The
deeper soils on serpentine can support vegetation similar to that supported
by soils derived from silica-rich parent materials (Brush et al. 1980, Dearden
1979, Shreve 1910). The PSB is a federally endangered barren community
protecting rare and endangered wildflower species such as Talinum teretifolium
Pursh (Fameflower), Aster depauperatus Fern (Serpentine Aster), and
Sporobolus heterolepis Gray (Prairie Dropseed), among other species.
Of the serpentinite-dominated rocks of the Mid-Atlantic States, afforestation
of nearly all of the serpentine barrens has occurred since the
mid-twentieth century (Anderson et al. 1999, Latham 1993). The major
threat to this ecosystem is the invasion of woody plants, primarily Juniperus
and Pinus species (Tyndall 2005). In Eastern North America, the broadly
termed Conowingo Barrens of Pennsylvania and Maryland have received
2009 J.L. Burgess, S. Lev, C.M. Swan, and K. Szlavecz 369
considerable attention. Of the original 21,000 ha of serpentinite lands, only
1100 ha have persisted (Kruckeberg 2004). On a few extant barrens, efforts
to restore the habitats have been undertaken. In a study from the Conowingo
Barrens of Pennsylvania, Barton and Wallenstein (1997) reported that
Pinus virginiana Mill. (Virginia Pine) does not, at the stand level, change
soil chemical imbalances characteristic of serpentine soils, but cautioned
that the pine’s positive influence on soil depth alone could promote further
succession towards forests typical of non-serpentine sites in the region.
Though these studies document the influence of pine on serpentine soils, the
PSB are also heavily encroached upon by Juniperus virginiana L. (Eastern
Red Cedar). However, mixed deciduous woodlands and forests cover much
of the PSB area. The forests of the PSB include xeric woodlands of open
stands of Virginia Pine intermingled with Quercus marilandica Meunch.
(Blackjack Oak) and Quercus stellata Wang. (Post Oak). Mesophytic forests
are composed of these oaks and others such as Quercus rubra L. (Red Oak),
Q. alba L. (White Oak), Q. montana Willd (Chestnut Oak), and Q. velutina
Lam (Black Oak) along with numerous other species, most notably Carya
spp. (hickory), Prunus spp. (cherry), Fagus grandifolia Ehrh (American
Beech), Acer rubrum L. (Red Maple), Sassafras albidum Nutt (Sassafras),
Betula lenta L. (Sweet Birch), and Robinia pseudoacacia L. (Black Locust).
Based on observational data, such as diameter at breast height, and historical
records (Shreve 1910), the forests are undergoing successional change
towards maple and beech domination. On exposed rock surfaces, common
serpentinophiles are saplings of Blackjack Oak, Cerastium arvense L. var.
villosum (Darl.) (Serpentine Chickweed), and Symphyotrichum depauperatum
Fern (Serpentine Aster).
The majority of PSB soils are Chrome Series soils that are typical of the
other Maryland serpentine areas (shallow with sandy textures). PSB soils are
mostly chrome silt loams and chrome clay loams, with lesser amounts from
the Neshaminy Series silt and clay loams (USDA 1973). The soils are mostly
alfisols, classified as fine-silty or fine-loamy, mostly serpentinic, mesic Typic
Hapludalfs or Lithic Hapludalfs, with loess added, which has given them
properties atypical of serpentine soils (Rabenhorst et al. 1982).
Geology
There are a number of fault-bounded, tectonically emplaced, and variably
metamorphosed ultramafic bodies in the Maryland Piedmont. The Conowingo
Barrens and their southernmost expression, the PSB, are the southern extension
of the State-Line mafic complex, which underlies part of the Maryland
and Pennsylvania Piedmont. In turn, the State-Line mafic complex is the
northern-most segment of the Baltimore Mafic Complex (BMC). The BMC
is a large exposure of mafic and ultramafic rocks that includes Soldier’s
Delight, but not all of the complex need have the same tectonostratigraphic
interpretation. The northern part of the BMC is bounded to the northwest by
the Peters Creek - Westminster terrain (Gates et al. 1999, Muller et al. 1989).
This boundary is interpreted to be a thrust fault that was later reactivated as a
370 Northeastern Naturalist Vol. 16, Special Issue 5
dextral strike-slip fault (Gates 1992, Higgins and Conant 1990). Ultramafic
components of the BMC contain a basal unit of serpentinized peridotite with
relict websterite and dunite kernels with an overlying massive and layered
gabbronorite that contains minor silica unsaturated peraluminous to aplitic
igneous bodies. The PSB portion also displays an upper mafic component that
varies from gabbro to quartz gabbro to diorite. The unit is approximately 3 km
thick and is tectonically placed against rocks of the Wissahickon formation
that is in the Glenarm series of early Paleozoic metasedimentary rocks (Hanan
and Sinha 1989, Higgins and Conant 1990). The Glenarm series rests unconformably
on Proterozoic basement of Baltimore gneiss (Hanan and Sinha
1989). Work by Shaw and Wasserburg (1984) and Hanan and Sinha (1989)
provide compelling evidence that the State-Line, Conowingo Barrens portion
of the BMC intruded continental crust at approximately 490 Ma. Other ultramafic bodies in the area (i.e., Soldier’s Delight) are petrologically distinct, and
these units likely represent an ophiolitic island arc complex that was obducted
onto Laurentia during the closure of the Iapetus at the time of the Taconic
orogeny, approximately 455 Ma (Muller et al. 1989). In a detailed study of
the State-Line Complex of Pennsylvania, Gates (1992) documented a series
of NE-trending dextral transcurrent shear zones of 0.2–1.4 km width creating
well-foliated L-S tectonites across the serpentinite. The shear zones contain
lenticular pods of weakly deformed serpentinite between the anastomosing
mylonitic bands. These pods range in size from several millimeters to several
hundred meters.
Sampling and measurement
Edaphic factors contributing to forest woody species community structure
were investigated using 21 randomly stratified plots (25 x 50 m) with
similar aspect (when possible) and elevation (>90 m) oriented along the
perceived environmental gradient (Fig. 1). No plots were overlapping.
At all plots, associated rock, soil, and floristic woody species canopy and
understory were surveyed. Geologic data such as rock type, cleavage, foliation,
kinematic indicators, mylonitic fabrics, faults, shear zones, and late
fractures/joints were measured. A fracture/joint inventory was conducted
by defining a representative region and measuring all structures and their
spatial orientations in that region. Measurements were taken perpendicular
to each set over 100 cm. Fracture density is calculated as the total number of
fractures or joints per 1 m3.
Tree species occurrence, diameter at breast height, vegetation cover,
and substrate characteristics (percent soil, litter, rock) were assessed. Soil
samples were collected from each site using a push probe or soil auger
to establish soil depth. Soil-moisture measurements were made using a
ThetaProbe® soil-moisture sensor. Composite soils for analysis were collected
1 m in from each corner, at 25 m from the end, and at the center of
each plot. Litter was brushed aside and samples were taken at 0–5 cm and
5–15 cm depth, referred to as “surface” and “deep,” respectively Soil characteristics
measured included soil depth, pH, soil organic matter content, major
2009 J.L. Burgess, S. Lev, C.M. Swan, and K. Szlavecz 371
and trace element analysis, C:N ratio, soil moisture, and textural analysis.
Moist-soil color was determined in the field using Munsell color charts.
Samples were combined, mixed, and split to give two samples of 200–400 g
that were lightly crushed, sieved to 2 mm, and sampled for textural analysis
(sieving followed by measurment with a hydrometer). A moist-soil subset
was taken for pH analysis. Soil pH was measured with a 20-g to 20-ml soil/
water slurry using a glass electrode. The remainder was oven dried at 30 °C
and powdered using an agate ball mill. A powdered subset was subsequently
used for X-ray fluorescence (XRF) and C:N analyses. Total N and C were
measured with an automated Perkin-Elmer 2400 Series II CHN analyzer.
Soil organic matter content was estimated as loss on ignition (LOI) at 550
°C for 3 hours. Total P, Na, K, Ca, Mg, Mn, Fe, Cr, Ni, Si, and Al were
measured by X-ray fluorescence using a Bruker AXS S4 Explorer wavelength-
dispersive X-ray fluorescence spectrometer following the procedures
outlined in Potts (1987). All samples analyzed by XRF were quantified using
a matrix-matched set of USGS- and NIST-certified soil standards. At least
one duplicate, one replicate, and one certified reference sample (SRM 2709,
San Joachin Soil) was run with every 10 samples analyzed to monitor for
external and internal reproducibility.
Values reported are typically means with the associated standard error.
To assess the consistency of the data and to identify the most strongly
Figure 1. Map showing study location along the Susquehanna River of Maryland.
Note that plot sizes are not to scale.
372 Northeastern Naturalist Vol. 16, Special Issue 5
discriminating variables, a principal component analysis (PCA) of the rock
and soil variables from the plots was performed. There were six variables in
the rock PCA: fracture density, Ca:Mg ratio, and P, Cr, Ni, and Al concentrations.
For the soils PCA, the following variables were assessed, for each
of the two depths: soil depth, texture (gravel, silt, and clay), C:N, organic
matter, pH, Ca:Mg ratio, and P, Cr, Ni, and Al concentrations.
Results
Mapped area
The major geologic boundaries, features, and contacts were mapped during
this study (Fig. 1). Though mapped in greater detail, the boundaries of
the serpentinite are consistent with previous mapping work of Higgins and
Conant (1986). Areas where the serpentinite was highly altered to a limonitic
laterite were recognized as a yellow-brown “honeycomb” outcrop or float
(Fig. 1). Using an approach similar to that of Gates (1992) in the Pennsylvania
State-Line mafic complex, we characterize the PSB site structurally as a series
of shear zones that anastomose around lenticular pods of less-sheared, though
still faulted and fractured, serpentinized peridotites. Measured fracture densities
vary from four fractures per m3 in the less sheared pods to 80 factures per
m3 in the relict barrens areas (Table 1). Sheared, folded, and fractured talcose
serpentinite were apparent underlying areas predominantly composed of upland
oaks (Fig. 2). Structural differences in terms of fracture density that are
typically encountered in the more-sheared and-fractured bedrock (Fig. 3) and
the less-fractured lenticular pods with pyroxenite (Fig. 4) were also apparent.
Chemically, the bedrock is typical of other ultramafic rocks with SiO2 around
40 percent by weight and high MgO, Cr, and Ni concentrations (Table 1).
Corresponding P, Ca, and Ca:Mg ratio values are very low. The limonitic chalcedony
or honeycomb lithology has similar trace-metal concentrations (high
Cr and Ni), but is nearly 80% SiO2. Ni concentrations are higher for the more
sheared and (or) serpentinized rocks.
Petrography
In areas that are highly sheared, such as the vicinity of the northwestern
fault contact and the relict barrens areas, the outcrop is a steatite or talcose
Table 1. Major and trace elemental data for plot-level bedrock, separated by the level of shearing
and fractures and by mineralogy for the limonitic parent materials. FD = fractive density.
Mean of Mean of
sheared and lenticular pods Mean of
Rock highly fractured of low fracture limonitic
properties Mean all plots plots (FD >20/m3) density (FD < 20/m3) laterites
Si (%) 39.04 ± 3.07 33.56 ± 0.93 37.01 ± 1.22 76.31 ± 2.09
Al (%) 0.55 ± 0.28 0.02 ± 0.02 1.53 ± 0.40 0.00 ± 0.00
P (%) 0.00 ± 0.01 0.00 ± 0.01 0.00 ± 0.00 0.02 ± 0.00
Cr (ppm) 2215.37 ± 111.95 2256.68 ± 122.40 2229.80 ± 109.25 1937.64 ± 89.20
Ni (ppm) 1650.81 ± 209.73 2343.86 ± 133.13 762.48 ± 105.88 948.20 ± 39.28
Ca: Mg 0.12 ± 0.05 0.02 ± 0.01 0.30 ± 0.07 0.00 ± 0.00
2009 J.L. Burgess, S. Lev, C.M. Swan, and K. Szlavecz 373
Figure 2. Large-scale fold with fracturing. Associated trees include the dominate
Blackjack Oak with lesser numbers of Eastern Red Cedar and minor regresentation
of Sassafras.
Figure 3. High fracture density serpentinite associated with barrens areas.
374 Northeastern Naturalist Vol. 16, Special Issue 5
serpentinite with little evidence of original mineralogy. In thin section, these
rocks are fine-grained with strong foliations defined by talc, lizardite, and
chrysotile in a felted-mesh pattern typically cut by late serpentine veins. Serpentinized
rocks at the southeastern contact with the gabbro are also talcose,
with minor chlorite. Other minerals include minor magnesite and opaque
minerals such as magnetite and/or chromite. Relict igneous minerals, though
lacking in the highly sheared rocks, are visible in the less-sheared lenticular
pods and kernels. These massive rocks come in two end-member varieties. The
first is a completely serpentinized rock with a felted-mesh pattern of interlocking
grains of interpenetrating serpentine, possibly antigorite, with magnetite
and minor chromite, talc, or brucite. The other massive rock is a dense, partially
serpentinized, often pseudomorphic rock, with abundant relict minerals of
olivine, orthopyroxene, clinopyroxene, and amphibole in various proportions
indicating igneous protoliths of websterite, wehrlite, and pyroxenite.
Forest characteristics
The forest canopy was mostly deciduous trees of irregular height. The
understory included very dense greenbrier to more open, sapling-dominated
areas. In all plots, dense Smilax spp. (greenbrier) was restricted to
the more heavily serpentinized areas, with more open understory found
on the limonitic lateritic soils or massive peridotites. A total of 33 tree
species was observed across the 21 plots, with species richness ranging
from 5 to 19. Table 2 details the dominant species in each plot and the
Figure 4. Low fracture density partially serpentinized peridotite from a lenticular
pod. Dominant canopy species are White and Red Oak along with American Beech.
2009 J.L. Burgess, S. Lev, C.M. Swan, and K. Szlavecz 375
associated geology. Diversity values using the Q-statistic of Kempton
and Taylor (1976), which is less sensitive to the commonest species in the
sample, range from 1 to 7.7 for the plots.
Soils data are presented in Table 3. Soil depths varied from 8 cm to
greater than 40, cm with variable texture and soil color across the sites.
Soil colors ranged from dark-brown (7.5YR 3/2) to grey (7.5YR 6/1) in
the upper 1–3 cm of the soil profile to yellow-browns (10YR 5/6) and redbrowns
(2.5YR 4/4) with depth. The redder (2.5YR Hues) soils are located
in conjunction with the peridotites, and the reddest (2.5YR 4/6) soils with
the limonitic chalcedony. The talcose serpentinites yield more yellow (10YR
Hues) varieties. All soils, with the exception of plots 14 and 20, are texturally
gravel-dominated. Upon laboratory analysis and thin-sectioning, it was
shown that plots 14 and 20 are hosted by a mafic-ultramafic parent that is
not part of the serpentinized sequence and hence is excluded from further
analysis. The ultramafic rocks contain around 30% gravel by mass, but the
limonitic soils contain a smaller percentage of gravel and a larger component
of sand. Mg concentrations are high and corresponding Ca:Mg ratios low
(<0.7) across all sites. The Ca:Mg ratio is a measure of serpentine cation
imbalance. Soil organic matter content is consistently below 4% across all
sites. C and N analysis of the soils yielded low N concentrations and C:N
ratios between 16 and 22, with the higher values on the more fertile limonitic
Table 2. Plot-level (25 m x 50 m) data detailing the dominant geologic rock type, the tree species
that comprise the majority of the forest canopy, and biodiversity measures.
Percent
abundance
Species Q- xeric
Plot # Geology Forest canopy richness statistic species
Plot 1 Massive serpentinite Hickory-oak 14 4.74 6.24
Plot 2 Limonitic laterite Cherry-maple-oak-Tulip Poplar 11 5.01 0.00
Plot 3 Talcose serpentinite Sassafras-oak 12 5.00 12.11
Plot 4 Serpentinite Oak 7 1.67 83.09
Plot 5 Serpentinite Eastern Red Cedar-maple 9 3.42 34.07
Plot 6 Peridotite Oak-gum-beech 13 3.02 5.40
Plot 7 Serpentinite Eastern Red Cedar-oak-pine 11 3.08 66.03
Plot 8 Limonitic laterite Beech-oak 5 0.99 0.00
Plot 9 Serpentinite Beech-Eastern Red Cedar 15 5.41 36.20
Plot 10 Pyroxenite Maple-oak-cherry 12 4.35 1.80
Plot 11 Pyroxenite Maple-hickory-oak 19 7.74 1.65
Plot 12 Pyroxenite Beech-oak-maple 19 5.65 1.30
Plot 13 Massive serpentinite Maple-oak-hickory-ailanthus 19 6.14 0.00
Plot 14 Leuco-ultramafic Maple-beech-gum-Tulip Poplar 12 3.42 0.00
Plot 15 Talcose serpentinite Sassafras-oak-maple 16 6.14 21.40
Plot 16 Serpentinite Oak-sassafras-maple 13 5.99 16.50
Plot 17 Serpentinite Sassafras-oak 12 3.37 38.47
Plot 18 Mass Maple-hickory-oak 16 7.21 25.00
Plot 19 Talc serpentine schist Beech-birch-oak-aspen 13 6.37 0.00
Plot 20 Gabbro Tulip Poplar-walnut-hickory 16 6.83 2.10
Plot 21 Serpentinite Oak-maple-Sassafras 18 5.14 41.10
376 Northeastern Naturalist Vol. 16, Special Issue 5
soils. Cr concentrations are enriched in the laterites compared with the other
host lithologies. This finding is likely due to resistance of Cr-oxides to
weathering and suggests that these phases are not likely sources for Cr in soil
solutions and plants of serpentine soils (Oze et al. 2004). The pH of the soils
is typically low to slightly acid (c. 4–6), leading to a low cation exchange
capacity (McFee et al 1977).
Since many of the data are related covariates, a PCA was used to reveal
the internal structure of the data. For the rock-related PCA (Fig. 5), both axes
account for 68% of the variation. Fracture density and Ni correlate positively
Table 3. Soil data partitioned by rock parent material. Soils are separated into shallow (0–5 cm
below ground surface) and deep (5–15 cm below ground surface) soils.
Serpentinite Partially serpentinized Limonitic
and steatite websterite, wehrlite, chalcedony
Soil parent materials and pyroxenite parent parent material
properties (n = 12) materials (n = 5) (n = 2)
SOM (LOI)
Shallow 2.56 ± 0.42 1.55 ± 0.21 2.64 ± 0.10
Deep 1.23 ± 0.35 1.11 ± .025 1.67 ± 0.26
C:N
Shallow 19.73 ± 3.49 19.12 ± 2.54 21.91 ± 3.74
Deep 16.41 ± 2.93 17.18 ± 0.42 18.54 (1 Plot)
pH
Shallow 4.99 ± 0.38 5.48 ± 0.29 4.61 ± 0.28
Deep 5.27 ± 0.31 5.11 ± 0.15 4.70 ± 0.20
Gravel %
Shallow 30.18 ± 7.55 31.10 ± 1.12 7.85 ± 6.35
Deep 37.47 ± 9.72 28.50 ± 1.44 0.80 (1 Plot)
Cr (ppm)
Shallow 2526.00 ± 883.37 1891.74 ± 748.92 4904.60 ± 1855.19
Deep 2092.87 ± 648.13 1912.82 ± 745.95 4841.84 ± 2212.33
Ni (ppm)
Shallow 1055.18 ± 346.47 453.46 ± 187.42 999.68 ± 276.53
Deep 1182.28 ± 371.19 434.03 ± 196.87 1107.76 ± 402.59
P (%)
Shallow 0.05 ± 0.01 0.03 ± 0.01 0.07 ± 0.00
Deep 0.04 ± 0.01 0.02 ± 0.01 0.06 ± 0.01
K (%)
Shallow 0.32 ± 0.19 0.51 ± 0.19 0.53 ± 0.22
Deep 0.33 ± 0.21 0.70 ± 0.15 0.54 ± 0.26
Ca (%)
Shallow 0.69 ± 0.65 1.57 ± 0.28 0.08 ± 0.04
Deep 0.69 ± 0.74 1.83 ± 0.60 0.04 ± 0.02
Mg (%)
Shallow 4.95 ± 1.64 3.94 ± 1.00 0.54 ± 0.03
Deep 5.92 ± 2.07 4.17 ± 0.99 0.56 ± 0.04
Ca:Mg (total)
Shallow 0.25 ± 0.30 0.46 ± 0.06 0.14 ± 0.06
Deep 0.20 ± 0.24 0.49 ± 0.13 0.07 ± 0.04
Al (%)
Shallow 1.97 ± 0.69 1.93 ± 0.39 2.81 ± 0.22
Deep 2.29 ± 0.92 1.92 ± 0.48 2.98 ± 0.26
2009 J.L. Burgess, S. Lev, C.M. Swan, and K. Szlavecz 377
with component one, and the Ca:Mg and Al content correlate negatively.
Component two has most of the variation accounted for by fracture density and
Ca:Mg. For the soil-related PCA, the first three components explained only
67% of the variance (Fig. 6). Ca:Mg, soil moisture, and Al accounted for the
bulk of the eigenvector totals for principal component one, while SOM, C, and
Figure 5. Principal component analysis of initial rock variables.
Figure 6. Principal component analysis of initial soil variables.
378 Northeastern Naturalist Vol. 16, Special Issue 5
Figure 7. Fracture density verses the xeric tolerant woody species. Graph details the
percent abundance of typical barrens/savanna and woodland xeric species (Blackjack
Oak, Post Oak, Virginia Pine, Eastern Red Cedar) at each ultramafic plot and the corresponding
fracture density of the bedrock.
C:N comprise the majority of the discriminatory power of principal component
two. Plots of fracture density versus percent xeric tree species (Fig. 7) and
Figure 8. Plot of fracture density versus synoptic volumetric moisture content of all
ultramafic plots. Data points are the mean and standard error of a minimum of twelve
plot-level data points per quadrat.
2009 J.L. Burgess, S. Lev, C.M. Swan, and K. Szlavecz 379
versus synoptic volumetric moisture content (Fig. 8) display predominately
linear trends, with xeric tree species having a positive correlation and moisture
content a negative correlation with respect to fracture density.
Discussion
The dominance of certain tree species such as the oaks, hickories, Eastern
Red Cedar, Red Maple, and American Beech with the associated rock formations
and their derived soils is strikingly shown by Table 2. Oaks such as
Blackjack, Post, and Chestnut Oak are commonly found in serpentine areas
of the Mid-Atlantic. Prior to the cessation of fire by the Amerindians, many
of these woodlands and forests were likely savanna or barrens areas. Further,
between 70–100 years ago, the serpentinized areas of the PSB were extensively
logged, though not farmed. This anthropogenic disturbance provides
the canvas whereon the flora competes for scant resources such as moisture
and nutrients. Edaphic factors and disturbance play an important role in
the unusual vegetation properties of these xeric oak forests and woodlands.
Geochemical data are consistent with other serpentinites containing elevated
Cr, Ni, V, and Mg with low Ca, Na, Al, and K.
Concentrations of potentially toxic metals such as Ni, Cr, and Co are
usually considered as one of the main causes of the vegetation dynamics of
ultramafic soils. However, in this study, the highest Cr values reside in the
limonitic laterites. Limonitic laterites often support typical Maryland Piedmont
tree species such as Liriodendron tulipifera L. (Tulip Poplar). These
soils (low in gravel content compared to the ultramafics) are also suitable to
be farmed in the PSB area. Proctor and Nagy (1992) and Roberts and Proctor
(1992) note that the role of metals in causing the infertility of ultramafic soils
are pH dependent and should be reconsidered with respect to hydrological
and nutritional stresses. Plant elemental stresses are broadly similar across
the landscape. However, outcrops with sheared and mylonitic fabrics tend to
have greater fracture densities and slightly higher Ca:Mg ratios (Tables 1, 3).
Upland oaks are known for their strategies and adaptations that promote
tolerance of drought and nutrient-poor soils. There are 60 species of oak in
North America, with Blackjack and Post Oak being the two most common
associates in dry, fire-prevalent, and nutrient-poor sites (Abrams 1992). On
xeric, serpentine soils of Maryland, Virginia Pine and Eastern Red Cedar
are also common. The upland xeric oaks seen at the PSB possess a suite of
ecophysiological adaptations for these conditions, but not for competing
in a closed-forest understory dominated by shade-tolerant species such as
American Beech. A number of the plots contain shade-tolerant species such
as the American Beech and Red Maple (Table 2). DBH measurements (J.L.
Burgess, unpubl. data) as a proxy for tentative age suggest that these forests
are undergoing continual transformation and in the absence of fire may be
on a trajectory towards typical Maryland piedmont flora in the future, when
only the most xeric and extreme Ca:Mg microsites will retain the serpentine
380 Northeastern Naturalist Vol. 16, Special Issue 5
character. The positive correlation of xeric, edaphic specialists as a function
of fracture density suggests higher dominance of xeric species with more
fracturing (Fig. 7). The presumed effect of the fracturing (a composite of
tectonic shearing and late jointing) is to create preferred pathways for water
drainage. The availability of soil water was once thought to be the main factor
limiting plant diversity and growth on the serpentine soils (Hughes et al.
2001, Rajakaruna et al. 2003). However, Hull and Wood (1984) measured
summer soil water and oak tree xylem potentials and concluded that the
availability of water does not appear to be the factor allowing Blackjack and
Post Oaks to replace White and Black Oaks in serpentine soils of Soldier’s
Delight, MD. Accordingly, the consensus has shifted toward the Ca:Mg ratio
as a major factor controlling which plants will grow on serpentine soils (E.B.
Alexander, unpubl. data; Brady et al. 2005). To test if there is any control of
moisture content with increasing fractures, a synoptic record of soil moisture
was measured at all plots and indicated a significant negative relationship
between soil moisture and bedrock fracturing (Spearman Rank Correlation
Rs = -0.76; Fig. 8).
These results suggest that, at least in part, the distribution and abundance
of woody species and the persistence of remnant “barrens” areas may be controlled
by the fracture density, associated soil texture, and soil moisture. Lutz
and Chandler (1946) were two of the first forest ecologists to state that the
differentiation effects of parent material are enhanced on immature soils that
are likely to be chemically unbalanced or deficient, especially when soils are
derived from a single geological formation. In a more modern treatment,
Roberts (1980) explores these same physio-chemical properties of mature
and immature soils as a function of parent material in Western Newfoundland.
Since most of the forest soils in this region are shallow, poorly sorted,
immature, and derived from the same ultramafic parent material, the geologic
controls on tree distribution may be more important than in many other habitats.
For example, the Ca:Mg ratio of all the ultramafics is low (mean of 0.12 ±
0.05), but the less sheared, less serpentinized peridotites have a significantly
higher Ca:Mg ratio of 0.30 ± 0.07 (t-test: P = 0.005). This same composition
is displayed in the soils derived from these parent materials (Table 3). Thus,
the increase in shearing or mylonitization of the parent rock correlates with
a lack of relict minerals such as olivine and clinopyroxene. The absence
of clinopyroxene (a Ca-bearing phase) and the subsequent replacement by
serpentine minerals in the more-sheared rocks is associated with the lowest
Ca:Mg ratios and the lowest moisture contents. Consequently, the more-xeric
forest tree species are more abundant in these areas.
Additionally, of the small, relict grasslands that occur at the PSB, none
of these barrens occur on the lenticular pods of peridotite. The decrease
in Ca due to serpentinization of ultramafics has been observed in many
serpentinized peridotites (Coleman 1963; Puga et al. 1999; Roberts and
Proctor 1992; Roberts and Rodenkirchen 1997; Rodenkirchen and Roberts
1993a, b; Shervais et al. 2005) and is attributed to the breakdown of
2009 J.L. Burgess, S. Lev, C.M. Swan, and K. Szlavecz 381
clinopyroxene during serpentinization by fluid infiltration. Such mobilization
may be accompanied by fracturing, faulting, and shearing, and hence
in the Conowingo Barrens, there is a direct relationship between: structural
features, Ca:Mg ratios, soil moisture, and ultimately floristic community
structure. Bulk rock and soil geochemistry are also important in those areas
where limonitic chalcedony occur. These “honey-comb” lithologies, though
Cr rich, do not contain abundant serpentinites or pyroxenites and do not host
remnant grasslands areas, or the xeric oaks, and historically are among the
first soils to be farmed.
Conclusion
Petrologic and structural diversity supports different plant communities
based on soils evolved from ultramafic materials. There are many confounding
factors and feedbacks in operation at the Pilot Serpentine Barrens
and the wider Conowingo Barrens. The integration of geologic, edaphic,
and biologic factors suggest that fracture density, the result of shearing
during orogenesis, and subsequent terrain uplift of the Baltimore Mafic
Complex, are exerting some level of control on forest tree community composition.
This tectonic influence has resulted in ultramafic lithologies and
derived soils with varying Ca:Mg ratios as well as different outcrop fracture
densities that, in some part, influence soil moisture content. Ultimately
the combination of these factors results in landscapes that host a higher
proportion of xeric woody species in areas of more-sheared and fully serpentinized
ultramafics. Plots with communities that more closely resemble
those on adjacent non-ultramafic sites have lower fracture density, higher
Ca, higher moisture, and lower Ni content.
Areas of highest bedrock fracturing at the PSB survive as relict barrens,
while other bedrock variations may result in soil trajectories towards
savanna or wooded forest communities. Such heterogeneous geology may
explain why some barrens appear resistant to woody plant invasion.
Acknowledgments
The authors would kindly like to thank Dr. Richard Back for his aid in obtaining
the C:N data and The Nature Conservancy and the Girl Scouts of America for permission
to use their sites. Further, we would like to acknowledge thoughtful reviews by
B. Roberts, E. Hellquist, and E.F. Stoddard.
Literature Cited
Abrams, M.D. 1992. Fire and the development of oak forests. Bioscience
42(5):346–53.
Alexander, E.B., R.G. Coleman, T. Keeler-Wolfe, and S.P. Harrison. 2007. Serpentine
Geoecology of Western North America. Oxford University Press, New York,
NY, USA.
382 Northeastern Naturalist Vol. 16, Special Issue 5
Anderson, R.C., J.S. Fralish, and J.M. Baskin. 1999. Savannas, Barrens, and Rock
Outcrop Plant Communities of North America. Cambridge University Press, New
York, NY, USA.
Barton, A.M., and M.D. Wallenstein. 1997. Effects of invasion of Pinus virginiana
on soil properties in serpentine barrens in southeastern Pennsylvania. Journal of
the Torrey Botanical Society 124(4):297–305.
Brady, K.U., A.R. Kruckeberg, and H.D. Bradshaw, Jr. 2005. Evolutionary ecology
of plant adaptation to serpentine soils. Annual Review of Ecology, Evolution, and
Systematics 36:243–266.
Brush, G.S., C. Lenk, and J. Smith. 1980. The natural forests of Maryland: An explanation
of the vegetation map of Maryland. Ecological Monographs 50(1):77–92.
Coleman, R.G. 1963. Serpentinites, rodingites, and tectonic inclusions in alpinetype
mountain chains. Geological Society of America, Special Paper 73, Boulder,
CO, USA.
Crowley, W.P. 1976. The geology of the crystalline rocks near Baltimore and its
bearing on the evolution of the eastern Maryland piedmont. Maryland Geological
Survey Report of Investigations 27, Baltimore, MD, USA.
Dearden, P. 1979. Some factors influencing the composition and location of plant
communities on a serpentine bedrock in western Newfoundland. Journal of Biogeography
6(1):93–104.
Denslow, J.S., and G.S. Hartshorn. 1994. Tree-fall gap environments and forest dynamic
processes. Pp. 120–127, In L.A. McDade, K.S. Bawa, H.A. Hespenheide,
and G.S. Hartshorn (Eds.). La Selva: Ecology and Natural History of a Neotropical
Rain Forest. University of Chicago Press, Chicago, IL, USA.
Gates, A.E. 1992. Domainal failure of serpentinite in shear zones, State-Line mafic
complex, Pennsylvania, USA. Journal of Structural Geology 14(1):19–28.
Gates, A.E., P.D. Muller, and M.A. Kroll. 1999. Alleghanian transpressional orogenic
float in the Baltimore terrane, central Appalachian Piedmont. Pp. 125–139,
In D.W. Valentino and A.E. Gates (Eds.). The Mid-Atlantic Piedmont: Tectonic
Missing Link of the Appalachians. Geological Society of America Special Paper,
No. 330.
Hanan, B.B. 1980. The petrology and geochemistry of the Baltimore mafic complex,
Maryland. Ph.D. Dissertation. Virginia Polytechnic Institute and State University,
Blacksburg, VA, USA .
Hanan, B.B., and A.K. Sinha. 1989. Petrology and tectonic affinity of the Baltimore
mafic complex, Maryland. Geological Society of America, Special Paper
231:1–18.
Higgins, M.W., and L.B. Conant. 1986. Geologic map of Cecil County: Baltimore,
Maryland. Scale 1:62,500. Maryland Geological Survey. Baltimore, MD, USA.
Higgins, M.W., and L.B. Conant. 1990. The Geology of Cecil County, Maryland.
Volume 37. Maryland Geological Survey. Baltimore, MD, USA.
Hughes, R., K. Bachmann, N. Smirnoff, and M.R. Macnair. 2001. The role of drought
tolerance in serpentine tolerance in the Mimulus guttatus Fisher ex DC. complex.
South African Journal of Science 97:581–586.
Hull, J.C., and S.G. Wood. 1984. Water relations of oak species on and adjacent to a
Maryland serpentine soil. American Midland Naturalist 112(2):224–34.
Kempton, R.A., and L.R. Taylor. 1976. The Q-statistic and the diversity of floras.
Nature 262:818–820.
Kruckeberg, A. 2002. Geology and Plant Life: The Effects of Landforms and Rock
Types on Plants. University of Washington Press. Seattle, WA, USA.
2009 J.L. Burgess, S. Lev, C.M. Swan, and K. Szlavecz 383
Kruckeberg, A. 2004. The status of conservation of serpentinite sites in North
America. International Geology Review 46(9):857–60.
Latham, R.E. 1993. The serpentine barrens of temperate eastern North America:
Critical issues in the management of rare species and communities. Bartonia
57(supplement):61–74.
Levin, S.A. 1992. The problem of pattern and scale in ecology: The Robert H. MacArthur
award lecture. Ecology 73(6):1943–67.
Lutz, H.J., and R.F. Chandler. 1946. Forest Soils. John Wiley and Sons, New
York, NY, USA.
Marye, W.B. 1955a. The great Maryland barrens. Maryland Historical Magazine
50:11–23.
Marye, W.B. 1955b. The great Maryland barrens: II. Maryland Historical Magazine
50:120–42.
Marye, W.B. 1955c. The great Maryland barrens: III. Maryland Historical Magazine
50:234–53.
McFee, W.W., J.M. Kelly, and R.H. Beck. 1977. Acid precipitation effects on soil pH
and base saturation of exchange sites. Water, Air, and Soil Pollution 7(3):401–8.
Muller, P.D., Candela, P.A. and A.G. Wylie. 1989. Liberty Complex: Polygenetic Melange
in the Central Maryland Piedmont. Pp. 113–135, In J.W. Horton and N. Rast
(Eds.). Melanges and Olistostrome of the US Appalachian. Geological Society of
America Special Paper 228. Boulder, CO, USA.
Maryland State Climatologist Office (MSCO). 2008. Department of Atmospheric and
Oceanic Science [database online]. University of Maryland, College Park, MD,
2008. Available online at http://www.atmos.umd.edu/~climate/conowingodam.
html. Accessed July 12, 2008.
Mutke, J., and W. Barthlott. 2005. Patterns of vascular plant diversity at continental to
global scales. Biologiske Skrifter 55:521–37.
Oze, C., S. Fendorf, D.K. Bird, and R.G. Coleman. 2004. Chromium geochemistry in
serpentinized ultramafic rocks and serpentine soils from the franciscan complex of
California. American Journal of Science 304(1):67–101.
Potts, P.J. 1987. A Handbook of Silicate Rock Analysis. Chapman and Hall, New York,
NY, USA.
Proctor, J., and L. Nagy. 1992. Ultramafic rocks and their vegetation: an overview. Pp.
469–494, In A.J.M. Baker, J. Proctor, and R.D. Reeves (Eds.). The Vegetation of
Ultramafic (Serpentine) Soils. Intercept Ltd., Andover, Hampshire, UK.
Puga, E., J.M. Nieto, A. Díaz de Federico, J.L. Bodinier, and L. Morten. 1999. Petrology
and metamorphic evolution of ultramafic rocks and dolerite dykes of the betic
ophiolitic association (mulhacén complex, SE Spain): Evidence of alpine subduction
following an ocean-floor metasomatic process. Lithos 49(1–4) (10):23–56.
Rabenhorst, M.C., J.E. Foss, and D.S. Fanning. 1982. Genesis of Maryland soils
formed from serpentinite. Journal of the Soil Science Society of America
46:607–16.
Rajakaruna, N., G.E. Bradfield, B.A. Bohm, and J. Whitton. 2003. Adaptive differentiation
in response to water stress by edaphic races of Lasthenia californica (Asteraceae).
International Journal of Plant Science 164(3):371–376.
Rajakaruna, N., T.B. Harris, and E.B. Alexander. In press. Serpentine geoecology of
eastern North America: A review. Rhodora.
Roberts, B.A. 1980. Some chemical and physical properties of serpentine soils from
western Newfoundland. Canadian Journal of Soil Science 60:231–240.
384 Northeastern Naturalist Vol. 16, Special Issue 5
Roberts, B.A. 1992. The serpentinized areas of Newfoundland, Canada: A brief review
of their soils and vegetation. Pp. 53–66, In A.J.M. Baker, R.D. Reeves, and
J. Proctor (Eds.).The Vegetation of Ultramafic (Serpentine ) Soils. Proceedings of
the First International Conference on Serpentine Ecology. Intercept Ltd., Andover,
Hampshire, UK. 509 pp.
Roberts, B.A. and J. Proctor (Eds). 1992. The Ecology of Areas with Serpentinized
Rocks. A World View. Kluwer Academic Publishers, Dordrecht, The Netherlands.
427 pp.
Roberts, B.A., and K.W. Deering 1995. Chemical properties of soil leachate measured
with porous cup lysimeters from two sites with serpentinized rocks, Newfoundland,
Canada. Paper presented at the Second International Conference on
Serpentine Ecology, Noumea, New Caledonia, July 31–August 5, 1995. Abstract
published. P. 42, In T. Jaffré, R. Reeves, and T. Becquer (Eds.). Proceedings of
The Second International Conference on Serpentine Ecology. Centre Orstom de
Noumea, BP A5, 98848 Noumea Cedax. Nouvelle-Caledonie.
Roberts, B.A., and H. Rodenkirchen. 1997. Soil and plant nutrition on a serpentinized
ridge in South Germany. Pp. 211–212, In T. Jaffré, R. Reeves, and T. Becquer
(Eds.). Proceedings of The Second International Conference on Serpentine
Ecology. Centre Orstom de Noumea, BP A5, 98848 Noumea Cedax. Nouvelle-
Caledonie.
Rodenkirchen, H., and B.A. Roberts 1993a. Soils and plant nutrition on a serpentinized
ridge in south Germany. I. Soils. Journal of Plant Nutrition and Soil Science
156:407–410.
Rodenkirchen, H., and B.A. Roberts 1993b. Soils and plant nutrition on a serpentinized
ridge in south Germany. II. Foliage macro-nutrient and heavy metal concentrations.
Journal of Plant Nutrition and Soil Science 156:411–413.
Shaw, H.F., and G.J. Wasserburg. 1984. Isotopic constraints on the origin of Appalachian
mafic complexes. American Journal of Science 284(4–5):319–49.
Shervais, J., Kolesar, P., and K. Andreasen. 2005. A field and chemical study of
serpentinization, Stonyford, California: Chemical flux and mass balance. International
Geology Review 47(1):1–23.
Shreve, F. 1910. The ecological plant geography of Maryland, midland zone, lower
midland district. Pp. 199–219, In F. Shreve, M.A. Chrysler, F.H. Blodgett, and
F.W. Besley (Eds.). The Plant Life of Maryland. Johns Hopkins University Press.
Baltimore, Maryland.
Silvertown, J. 2004. Plant coexistence and the niche. Trends in Ecology and Evolution
19(11):605–11.
Tyndall, R.W. 2005. Twelve years of herbaceous vegetation change in oak savanna
habitat on a Maryland serpentine barren after Virginia Pine removal. Castanea
70(4):287–97.
Tyndall, R.W., and J.C. Hull. 1999. Vegetation, flora, and plant physiological ecology
of serpentine barrens of eastern North America. Pp. 67–82, In R.C. Anderson,
J.S. Fralish, and J.M. Baskin (Eds.). Savannas, Barrens, and Rock Outcrop Plant
Communities of North America. Cambridge University Press. Cambridge, UK.
United States Department of Agriculture (USDA). 1973. Soil survey - Cecil County,
Maryland. US Department of Agriculture, Soil Conservation Service, Washington,
DC.