Soil and Biota of Serpentine: A World View
2009 Northeastern Naturalist 16(Special Issue 5):111–120
Adiantum viridimontanum, Aspidotis densa, Minuartia
marcescens, and Symphyotrichum rhiannon: Additional
Serpentine Endemics from Eastern North America
Tanner Harris1 and Nishanta Rajakaruna2,3,*
Abstract - Serpentine outcrops around the world are known to harbor disproportionately
high rates of plant endemism. Remarkable cases of serpentine endemism occur in
New Caledonia and Cuba, with 3178 and 920 endemic taxa, respectively, found solely
on serpentine. Despite the patchy occurrence of serpentine in eastern North America
from Québec and Newfoundland south to Alabama, only one taxon, Cerastium velutinum
var. villosissimum, has been broadly recognized as a serpentine endemic for the
region. Based on reports in the literature, we suggest that Adiantum viridimontanum,
Minuartia marcescens, and Symphyotrichum rhiannon be considered endemic to
serpentine soils from the east coast of North America. Aspidotis densa, with several
disjunct populations on and off serpentine in western North America, is known solely
from serpentine soils where it occurs in eastern North America and should be considered
endemic to the substrate there. The geobotany of eastern North America in general
is poorly understood, and additional taxonomic studies on the region’s unique geologic
substrates will likely yield further edaphic endemics.
Introduction
Narrow endemism can result from any number of biological and environmental
interactions. However, within a regional climate, geological
discontinuities, both topographic and geochemical, are the most common
and striking influences of narrow endemism (Kruckeberg 1986, Kruckeberg
and Rabinowitz 1985). Among the endemic species resulting from
geological discontinuities are edaphic endemics, those species restricted
to chemically and/or physically unique soils (Rajakaruna and Boyd 2008).
Edaphic endemics are often either products of recent in situ evolution resulting
from divergence following colonization of a new substrate (i.e.,
neoendemics; Baldwin 2005, Ramsey et al. 2008) or are relicts that have experienced
a reduction in their geographic or ecological ranges as a result of
altered climatic or biotic conditions (i.e., paleoendemics; Kruckeberg 1986,
Raven and Axelrod 1978). Recent investigations of edaphically restricted
taxa provide compelling evidence in support of both modes of origin for
edaphically endemic taxa (Gottlieb 2004, Mayer and Soltis 1994, Mayer et
al. 1994, Pepper and Norwood 2001).
1Department of Plant, Soil, and Insect Sciences, University of Massachusetts, Fernald
Hall, 270 Stockbridge Road, Amherst, MA 01003. 2College of the Atlantic, 105
Eden Street, Bar Harbor, ME 04609. 3Current address: Department of Biological Sciences,
San José State University, One Washington Square, San José, CA 95192-0100.
*Corresponding author - Nishanta.Rajakaruna@sjsu.edu.
112 Northeastern Naturalist Vol. 16, Special Issue 5
Serpentine soils, with their low Ca:Mg ratios, high levels of heavy metals,
and generally low levels of essential nutrients, are known to harbor
disproportionately high numbers of edaphic endemics compared to regional
floras (Alexander et al. 2007). The tropical islands of New Caledonia and
Cuba provide remarkable cases of serpentine endemism with 3178 and 920
cases, respectively (Borhidi 1992, Jaffré 1992). California alone, in western
North America, is home to 176 species endemic to serpentine (Safford et al.
2005), 12.5% of the total taxa endemic to that state (Hickman 1993).
Although serpentine occurs in patches along the Appalachian orogen
from Québec and Newfoundland south to Georgia and Alabama, little is
known about plant life on serpentine in eastern North America compared
to serpentine floras from other regions (Rajakaruna et al. 2009). Only one
taxon, Cerastium velutinum Rafinesque var. villossissimum (Pennell) J.K.
Morton (Caryophyllaceae; syn. C. arvense var. villosissimum Pennell)
(Octoraro Creek Chickweed), has been broadly recognized as a serpentine
endemic in eastern North America (Gustafson et al. 2003, Morton 2004).
A second taxon once considered endemic to serpentine, Symphyotrichum
depauperatum (Fernald) G.L. Nesom (Asteraceae) (Serpentine Aster) has
subsequently been discovered on mafic diabase glades in North Carolina
(Gustafson and Latham 2005, Hart 1990, Levy and Wilbur 1990). Thus, S.
depauperatum may be considered a serpentine indicator (sensu Kruckeberg
1984) in eastern North America, but is not endemic to this substrate.
In a comprehensive review of the serpentine literature for eastern
North America (Rajakaruna et al. 2009), we identified three additional
taxa largely, if not entirely, restricted to serpentine in the region: Adiantum
viridimontanum Paris (Pteridaceae) (Green Mountain Maidenhair Fern),
Aspidotis densa (Brackenridge in Wilkes) Lellinger (Pteridaceae) (Indian’s
Dream), and Minuartia marcescens (Fernald) House (Caryophyllaceae)
(Serpentine Stitchwort). Subsequent to our review, a fourth taxon, Symphyotrichum
rhiannon Weakley and Govus (Asteraceae) (Rhiannon’s Aster),
was brought to our attention. It is a recently described species known only
from the Buck Creek serpentine barrens of North Carolina (Kauffman et
al. 2004). Here we make suggestions on the endemism status of all four
species and discuss the status of our knowledge of serpentine endemics in
eastern North America.
Adiantum viridimontanum Paris (Pteridaceae)
Adiantum viridimontanum is an allotetraploid hybrid of A. aleuticum
(Ruprecht) Paris (Aleutian Maidenhair Fern) and A. pedatum Linnaeus
(Northern Maidenhair Fern) (Paris 1991). Of the proposed serpentine endemics
in eastern North America, this species has received the most attention (e.g.,
O’Connor 1995, Paris 1991, Paris and Windham 1988, Ruesink 2001).
The species has been verified only from serpentine soils in Vermont and
Québec (Paris 1991, Ruesink 2001, Tyndall and Hull 1999). In Vermont, it
is known from 7 locations in the northern part of the state, with population
2009 T. Harris and N. Rajakaruna 113
sizes ranging from 25–1000 individuals (Ruesink 2001). The species occurs
at 14 sites in Québec between the Québec/Vermont border and the Thetford
Mines area. We were unable to obtain specific information for the Québec
occurrences. In total, it is thought that there are approximately 2000 individuals
of this species in existence (www.centerforplantconservation.org).
A possible additional station for A. viridimontanum was confirmed on
Deer Isle, Hancock County, ME in June 2008. Based on observations of
morphological features made by Arthur Haines (New England Wild Flower
Society, Framingham, MA) and Geoffrey Hall (Appalachian Corridor Appalachien,
Lac-Brome, PQ, Canada) and its occurrence on serpentine, it was
determined that the specimens, discovered in 2004 by the senior author, were
likely A. viridimontanum. However, a final determination cannot be made
until spores have been examined.
Because A. viridimontanum has only recently been described (Paris
1991), it is likely that a reexamination of many of the documented serpentine
occurrences of A. pedatum and, in particular, A. aleuticum (e.g., those cited
by Reed 1986), will yield new localities for A. viridimontanum (G. Hall,
pers. comm.). The possibility for further unknown occurrences of A. viridimontanum
is also supported by the presence of primary diploid hybrids in
populations of A. pedatum and A. aleuticum, suggesting the possibility for
repeated origins of A. viridimontanum (Ruesink 2001). Repeated origins via
polyploidy and hybridization are not uncommon in ferns (Shinohara et al.
2006, Trewick et al. 2002, Vogel et al. 1998).
Aspidotis densa (Brackenridge in Wilkes) Lellinger (Pteridaceae) Syn.
Cheilanthes siliquosa Maxon, Cryptogamma densa (Brackenridge) Diels,
Onychium densum Brackenridge, Pellaea densa (Brackenridge) Hooker
In western North America, Aspidotis densa is known as a “faithful indicator”
of serpentine (Kruckeberg 1979) ranging from British Columbia south
to California and east to Montana, Wyoming, and Utah (www.natureserve.
org). In eastern North America, it is known exclusively from serpentine sites
in the Mt. Albert and Thetford Mines region of Québec, Canada (Bouchard
et al. 1983, Brooks 1987, Reed 1986, Tyndall and Hull 1999). In these two
areas, there are seven known occurrences, each with less than 100 individuals
(Thériault 1999). Three of these sites occur in protected areas, and two
sites are threatened by mining (www.mddep.gouv.qc.ca). Although some
work has been done on gender expression in gametophytes of this species
(Greer 1993), we were unable to locate any conservation-relevant studies on
its biology or ecology. Thériault (1999) produced an unpublished technical
report on the species for the government of Québec; however, it has never
been circulated.
Given A. densa’s disjunct distribution between eastern and western North
America, the taxon provides a unique subject for biogeographic investigations.
It is possible that the taxon dispersed from serpentine outcrops in
western North America where it is more abundant and subsequently spread
114 Northeastern Naturalist Vol. 16, Special Issue 5
among outcrops in eastern North America. Alternatively, the taxon may have
had a more extensive range on and off of serpentine across the continent but
is now restricted to serpentine in eastern North America, populations off
serpentine having been eradicated due to unfavorable changes in climatic or
biotic factors. While the first scenario implies long-distance dispersal, the
latter scenario would suggest vicariance (Perrie and Brownsey 2007, Wolf
et al. 2001).
Minuartia marcescens (Fernald) House (Caryophyllaceae) Syn. Arenaria
laricifolia var. marcescens (Fernald) Boivin, A. marcescens Fernald
Minuartia marcescens is known from ultramafic ledges and barrens in
Newfoundland and Québec, Canada, and Vermont (Brooks 1987; Cook 1959;
Dearden 1979; Roberts 1980, 1992; Tyndall and Hull 1999; Zika and Dann
1985). There are 20–30 occurrences totaling 20,000–50,000 individuals in
Gros Morne National Park, Newfoundland. There are only two known occurrences
on Mt. Albert in Québec (Reed 1986), for which we were unable to find
any specific data on population size. In Vermont, there is only one known occurrence,
with approximately 100 individuals (www.natureserve.org).
Symphyotrichum rhiannon Weakley and Govus (Asteraceae)
Symphyotrichum rhiannon was described in 2004 (Kauffman et al. 2004),
ending over two decades of confusion about this species. It is thought to
have been first collected and studied by Mansberg (1981), who ultimately
labeled it as “unidentifiable aster” (Kauffman et al. 2004). Kauffman et al.
(2004) suggested the taxon is not a recent or stabilized F1 hybrid, noting that
it does not appear to be morphologically intermediate to any two species of
Symphyotrichum. Although common garden or hybridization studies have
not been performed, they note that no intergrades have been found between
S. rhiannon and what they believe is its closest relative, S. puniceum (Linnaeus)
Á. Löve & D. Löve (Purplestem Aster), despite the geographic range
of S. rhiannon being imbedded within that of S. puniceum. If S. rhiannon was
the product of a recent hybridization, one would expect to find morphological
intergrades within its geographic range.
Discussion
We suggest that A. viridimontanum, M. marcescens, and S. rhiannon
be considered serpentine endemics, sensu stricto, from eastern North
America. Aspidotis densa should be considered endemic to serpentine
only within eastern North America as it occurs both on and off serpentine
in western North America. Including C. velutinum var. villosissimum, the
total of number of known serpentine endemics in eastern North America
increases to five, in stark contrast to the 176 serpentine endemics that have
been identified in western North America (Safford et al. 2005) and even
higher numbers in other serpentine regions of the world.
2009 T. Harris and N. Rajakaruna 115
The limited number of serpentine endemics in eastern North America
may be due to unique ecological and evolutionary factors, including the
smaller area and disjunct distribution of serpentine there relative to western
North America. However, the limited number of identified endemics also
likely stems from a poor understanding of the flora (Rajakaruna et al. 2009).
The flora of northeastern North America was extensively studied during
the 1800s and early 1900s (Fernald 1924). However, our knowledge of the
southeastern flora is more limited (A. Weakley, University of North Carolina
Herbarium, Chapel Hill, NC, pers. comm.). In addition, comparatively little
taxonomic work has been done in eastern North America as a whole since
the development of modern phylogenetic techniques.
The issue of edaphic endemism is further complicated by the possibility
of edaphic races or ecotypes adapted to unique geologic substrates that
may not have recognized taxonomic standing. Despite the presence of a few
genera in eastern North America with known serpentine ecotypes in other regions
(Brooks 1987, O’Dell and Claassen 2006), few studies have examined
ecotypic differentiation in eastern North America (Rajakaruna et al. 2009).
Although the taxonomic standing of such ecotypes can be a matter of contention,
they clearly set the stage for speciation (Abbott and Comes 2007, Levin
1993, Rajakaruna 2004), and as such are worthy of closer study.
Despite the possibility for unrecognized edaphic races or ecotypes, the
number of serpentine endemic species in eastern North America is negligible
compared to serpentine regions in other parts of the world. Harrison et
al. (2004) have attributed patterns of serpentine endemism in western North
America to the age of exposure and outcrop size in addition to factors of
climate and topography. These factors all likely play a role in the low rates
of endemism for eastern North America; the serpentine areas of eastern
Canada and New England were glaciated as recently as 10,000–13,000
years ago and are small in size compared to serpentine areas in western
North America (Rajakaruna et al. 2009). Further, the serpentine soils in recently
glaciated regions of northeastern North America are often diluted by
glacial till which may ameliorate some of those aspects of serpentine soils
detrimental to plant growth.
However, it is interesting to note that, with the exception of C.
velutinum var. villosissimum, which occurs on the serpentine barrens
of Pennsylvania (Gustafson et al. 2003, Morton 2004), and S. rhiannon
from North Carolina, three of the five serpentine endemics in eastern
North America occur in the more recently glaciated, northern latitudes of
the serpentine belt. This distribution is in contrast to patterns of general
endemism in eastern North America, with a greater number of endemic species
concentrated in the south (Loehle 2006). The prevalence of serpentine
endemics in the northern, more recently glaciated latitudes may be attributable
to the greater area of serpentine in Québec and Newfoundland than in
the southern regions of eastern North America (Rajakaruna et al. 2009), as
would be predicted by the theory of island biogeography (MacArthur and
Wilson 2001) applied to the widely accepted view of serpentine outcrops as
ecological islands (Harrison et al. 2006).
116 Northeastern Naturalist Vol. 16, Special Issue 5
A number of authors have put forth possible explanations for the evolution
and maintenance of narrow endemism (e.g., Kolb et al. 2006, Lesica
et al. 2006, Poot and Lambers 2008); however, there has been little agreement.
As Drury (1980) and Stebbins (1980) have both suggested, there is
likely no single factor responsible for such forms of rarity. Thus, studies
to elucidate the influences of narrow endemism, including serpentine endemism,
must be done on a species-by-species basis and must incorporate
aspects of the local environment, genetic structure, and the history of both
the population and the particular site. Such studies are critical to the conservation
of these rare species.
In support of species-specific ecological research, García (2008) found
no significant population size-extinction risk relationship for narrow endemics,
suggesting that habitat preservation, as it relates to population size, may
not be sufficient for the long-term persistence of such species. Thus speciesspecific ecological research may be necessary to determine appropriate
management strategies beyond habitat preservation.
That is not to say land preservation is not a key component of any conservation
plan. The serpentine barrens of Pennsylvania and Maryland, once
covering nearly 20,234 ha (50,000 acres) but now reduced to a mere 1012
ha (2500 acres) (Kruckeberg 2004), serve as a reminder of the importance
of land conservation. Without such basic land preservation efforts, rare species
found in these unique habitats have little chance of survival. With their
extirpation, we stand to lose not only potentially critical biodiversity but also
the opportunity to understand important aspects of ecology and evolution
(Kruckeberg 2004, Kruckeberg and Rabinowitz 1985, Stebbins 1980, Whiting
et al. 2004).
Acknowledgments
The authors thank two anonymous reviewers for useful comments, Alan Weakley
for comments on S. rhiannon, Joe Pollard for bringing the species to our attention,
Arthur Haines and Geoffrey Hall for their observations and comments on A.
viridimontanum, and of course the attendees of the SICSE for their enthusiasm and
support—one could not hope to be part of a finer scientific community.
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