Soil and Biota of Serpentine: A World View
2009 Northeastern Naturalist 16(Special Issue 5):121–130
Climate Gradients, Climate Change, and Special
Edaphic Floras
Susan Harrison1,*, Ellen Damschen2, and Barbara M. Going1
Abstract - Serpentine endemics and other soil-restricted taxa may be presumed to
face extraordinarily high risk from climate change because their narrow edaphic
niches limit their possibilities to adapt through migration. However, their distinctive
life-history traits and their competitive relationships with faster-growing soil
generalists may complicate this picture and produce unexpected outcomes. Here
we propose a conceptual framework for how serpentine endemics will fare under
climate change, together with three potential tests of its predictions. We believe
climate change should be embraced by serpentine plant ecologists as a critical area
for greater study.
Introduction
To date, surprisingly little attention has been given to how serpentine
floras, and other special edaphic floras, may be affected by global climate
change. This issue is important for two reasons. First, the world’s special
edaphic floras are major contributors to global biodiversity. Examples include
the rich floras of limestone grasslands in southern Europe, dolomite
glades in the Ozarks, shale barrens in Appalachia, or serpentine outcrops in
the Mediterranean, Cuba, New Caledonia, and California (Anderson et al.
1999, Kruckeberg 2005). In California, one of the world’s botanical diversity
hotspots (Myers et al. 2000, Stein et al. 2000), 612 of 1742 rare plants are
associated with serpentine, limestone, volcanic outcrops, vernal pools, or
other special substrates (Skinner and Pavlik 1994), and plants restricted to
serpentine comprise >10% of species unique to the state even though serpentine
is <2% of the state’s area (Kruckeberg 1984, Safford et al. 2005).
Second, their naturally fragmented distributions make special edaphic
floras potentially exceptionally vulnerable. Plants confined to outcrops of
special soils might be expected to have far lower chances of successful migration
to suitable new sites and thus far greater risks of extinction in the face of
climate change, than plants that are soil generalists. This problem is of general
importance. Forecasts of biotic change over the coming decades are typically
made by modeling current versus future climatic envelopes of species, and
these models assume that even if a species is incapable of migration, it will at
least survive in the geographic areas of overlap between its present and future
climatic envelopes (Loarie et al. 2008, Schwartz et al. 2006, Thomas et al.
1Department of Environmental Science and Policy, University of California – Davis,
Davis, CA 95616. 2Department of Biology, Washington University, 1 Brookings
Drive, Campus Box 1137, St. Louis, MO 63130. *Corresponding author - spharrison@
ucdavis.edu.
122 Northeastern Naturalist Vol. 16, Special Issue 5
2004). Edaphic endemics are perfect examples of why this assumption is too
optimistic; it underestimates the risks faced by species whose geographic distributions
are constrained by other niche requirements besides climate.
Nonetheless, in an informal survey at the Sixth International Serpentine
Conference, only about 15 of the 80 participants agreed with the statement
that they had given thought to the possible effects of climate change on
serpentine floras. Of these 15, only one agreed with the statement that serpentine
plants are likely to be at higher risk than other plants, while the others
agreed that the risks faced by serpentine plants ought to be lower. We believe
this view base on the perception that because serpentine plants are less sensitive
to temperature and water availability than other plants, as we describe below.
We first propose a conceptual framework for considering the possible effects
of climate change on special edaphic floras. We then discuss ongoing
work to test this model. As we will explain, we believe that the geographic
variation in serpentine versus nonserpentine (or other special edaphic versus
“normal”) plant communities is one important source of evidence that is readily
available. Other sources of evidence include field manipulations of water
availability, and comparisons of historical and modern vegetation data.
Conceptual Model
Assumption 1: Edaphic restriction is relative
We begin by assuming that few species are 100% specialized on any
particular substrate. For example, in the unusually well-studied Californian
serpentine flora, the 200+ taxa that are considered strong “endemics” are those
with roughly >85% of their known occurrences on serpentine; another several
hundred taxa show lesser degrees of affinity, and have been called “indicators”
(Kruckeberg 1984, Safford et al. 2005). Similarly, species that are “endemic”
to limestone glades may only be restricted to glades within a limited portion
of their range and species that are “characteristic” to limestone glades are not
necessarily confined to glade habitats (Nelson and Ladd 1983).
It follows, then, that any given species may be more or less “endemic”
to a special soil depending on the ecological circumstances, including the
climate and the surrounding plant community. For example, species that specialize
on serpentine in some parts of their ranges, but are more generalized
elsewhere, have been termed “regional endemics” (Rune 1953) or “regional
indicators” (Kruckeberg 1984). The reasons for geographic variability in
substrate restriction have never been examined, to our knowledge. The review
of observational studies below suggests a pattern, such that species and
assemblages are more soil-restricted in more climatically favorable regions.
Assumption 2: Edaphic restriction is influenced by competition
We next assume that competition with other plant species is one of the
major reasons why species show restriction to special soils. When freed from
competitors that grow faster and taller on more fertile soils, but that are less
tolerant of serpentine, it has been found that serpentine specialists grow
equally well (in some cases, better) on the more fertile soils (Kruckeberg
2009 S. Harrison, E. Damschen, and B.M. Going 123
1954). Similar patterns have been observed in species endemic to other
soils (e.g., Anderson et al. 1999, Baskin and Baskin 1998, Kruckeberg 2005,
Sharitz and McCormick 1973, Tansley 1917, Ware 1991).
A widely held paradigm is that edaphic endemics are tolerant of either
resource-poor or excessively cation-rich (“toxic”) soils, but are incapable of
fast growth on more fertile soils, while many soil generalists (“bodenvag”
species) show the opposite set of traits . If this is true, we would expect that as
the climate becomes more favorable (i.e., when water or degree-day limitations
on plant growth are relaxed), edaphic endemics will receive relatively
modest direct benefits, because of their limited capacity for resource uptake
and growth. Their major competitors, soil generalists, will benefit much more
from climatic favorability, but only on fertile soils, whereas their growth will
remain strongly limited by scarce nutrients or toxic elements on special soils.
Assumption 3: The degree of edaphic restriction may vary with climate
The degree of restriction to special soils may vary along climatic
gradients. The most common pattern is for species to be more confined to
serpentine in more climatically favorable regions. The general explanation
that has been proposed for this pattern (Brooks 1987, Raven and Axelrod
1978, Rune 1953, Whittaker 1960) is that, as Brooks (1987) stated,
“competitive pressure restricts some plants either to the edaphically harsh
environment of serpentine, or to climatically harsh environments,” which
corresponds to our conceptual model below.
For example, a number of species are serpentine specialists in the mesic
coastal mountains of California, although they are soil generalists in colder
or more arid zones; examples include Pinus jeffreyi Balf. (Jeffrey Pine;
Pinaceae), Calocedrus decurrens Torr. (Incense Cedar; Cupressaceae),
Arctostaphylos nevadensis A. Gray (Pinemant Manzanita; Ericaceae), and
Quercus vaccinifolia Kell. (Huckleberry Oak; Fagaceae). Likewise, several
species from southern Californian deserts reach their northern range limits
on serpentine in the mesic coastal mountains, e.g., Juniperus californica
Carrière (California Juniper; Cupressaceae), Eriogonum wrightii Torr. ex
Benth. (Wright’s Buckwheat; Polygonaceae), Fremontodendron californicum
(Torr.) Coville (California Flannelbush; Malvaceae), and Arctostaphylos
glauca Lindl. (Bigberrry Manzanita; Ericaceae) (Kruckeberg 1984,
Whittaker 1960).
Similar examples have been noted in eastern North America, northern
and central Europe, Japan, and the neotropics (Borhidi 1991, Brooks 1987,
Rune 1953). Boreal and subarctic species may reach their southern limits on
serpentine (Brooks 1987, Rune 1953). Montane species often extend their
elevational distributions downward on serpentine, sometimes by 1000 m or
more (Borhidi 1991, Brooks 1987, Whittaker 1960).
An interesting historical example comes from Sweden, where Arenaria
norvegica Gunnerus (Arctic Sandwort; Caryophyllaceae) is restricted to
serpentine only south of 66°N latitude. Rune (1953) explained that this
small herb was outcompeted on more favorable substrates as forest invaded
124 Northeastern Naturalist Vol. 16, Special Issue 5
southern Sweden in late postglacial times. At higher latitudes, A. norvegica
remains widespread on other soils, presumably because competitive pressure
is lower where forest is sparse or absent.
The model and its predictions
We predict that more favorable (e.g., wetter) conditions will favor the net
success of edaphic endemics on special soils, where the growth of their generalist
(bodenvag) competitors is limited by other factors, but these conditions
will reduce the net success of edaphic endemics on normal soils, because of
increased competition from generalists. Less favorable (e.g., drier) conditions
will have the opposite effects. These interactive effects will lead both individual
species and entire assemblages to show greater edaphic specialization in
more favorable climates (Fig. 1), which is the general pattern that is observed.
We think this three-way interaction between climate, competition, and
soils provides a testable ecological explanation for biogeographic patterns in
the floras of special soils. We also propose that this model provides a basis
for predictions about human-caused climate change.
Our research aims to test the following hypotheses:
1. Special edaphic floras will show higher distinctiveness from the floras of
nearby normal soils in more favorable climates (those with higher mean
rainfall, higher minimum or lower maximum temperatures, etc.).
Figure 1. Conceptual Model. Bold arrows and multiple ++ or -- signs indicate very
strong effects; dashed arrows and signs in parentheses (+ or -) indicate weak or absent
effects. Our hypothesized effect—that more favorable climates lead to more edaphically
specialized floras—depends on the differences in the strengths of the four arrows
on the left side of the figure.
2009 S. Harrison, E. Damschen, and B.M. Going 125
2. Field water manipulations will have lesser effects (additions beneficial,
reductions harmful) on serpentine endemics than related non-endemics.
Water manipulation effects will also depend on whether competitors are
present or absent, and whether the background community is a serpentine
or nonserpentine grassland.
3. The serpentine flora of the Siskiyou Mountains will show lesser effects
of climatic warming than the diorite flora, when we compare vegetation
data collected in 2007 with data collected by ecologist Robert Whittaker
in 1949–1951.
Testing the Conceptual Model
1. Observational studies of Californian serpentine and Ozark glade floras
In California, serpentine is found almost entirely within the boundaries of
the California Floristic Province, i.e., the zone of Mediterranean climate. Its
vegetation ranges from conifer woodland and forest through chaparral (shrub)
to grassland, along latitudinal and elevational moisture gradients (Alexander et
al. 2006, Grace et al. 2007, Harrison et al. 2006, Kruckeberg 1984).
The species richness of serpentine endemics declines sharply along California’s
north-to-south gradient of decreasing precipitation, both in absolute
terms and as a percentage of total species richness (Harrison et al. 2000).
Interestingly, serpentine endemics also diminish to the north and south of the
California Floristic Province; there are almost none in the remainder of western
North America, despite an abundance of serpentine (Alexander et al. 2006).
In previous work, we sampled species and >70 environmental variables at
109 serpentine sites across California (Harrison et al. 2006). The sites spanned
1200 km in latitude, 0–2750 m in elevation, 21–257 cm in average annual precipitation,
and 28.9–38.6 °C in maximum July temperature.
For this study, we will sample a paired non-serpentine site for every serpentine
site in the previous study. Sites will be chosen by examining geologic, vegetation,
and road maps and identifying an accessible non-serpentine area that is
within a relatively constant distance (e.g., 0.5–5 km) from each serpentine site,
and represents the typical vegetation for the area.
To test Hypothesis 1 at the assemblage level, we will calculate assemblage-
level dissimilarity for each pair of sites (one minus the Jaccard
similarity coefficient) and regress it on mean annual rainfall and other climatic
variables. We will use partial Mantel tests to ask if such effects are significant
after spatial and environmental distances (e.g., based on soil variables) are
taken into account.
To test Hypothesis 1 at the individual species level, we will use our previous
data and Safford et al. (2005) to divide the species we find into avoiders,
tolerators, and weak and strong endemics. For each species, we will use
combined data from the present and previous study to calculate the proportion
of its total abundance that is on serpentine. We will regress this measure
on mean annual rainfall and other climatic variables, across all sites where
the species was found.
We want to generalize beyond the flora of serpentine, and the glades of the
Ozark Plateaus Province provide an ideal opportunity. Ozark glades are open
126 Northeastern Naturalist Vol. 16, Special Issue 5
herbaceous communties found on several different substrates (limestone,
dolomite, sandstone, rhyolite) within the generally forested Ozark Plateaus
Province, in the southern half of Missouri and portions of Arkansas, Illinois,
Kansas, and Oklahoma.
Experimental evidence suggests that glade endemics fit our conceptual
model. They may grow well off of glade soils in the absence of competition
(Baskin and Baskin 1988, Sharitz and McCormick 1973, Ware 1991). The
limestone glade endemic Talinum calcaricum Ware (Limestone Fameflower;
Portulacaceae)was much less affected by drought stress than its generalist
competitors when growing together on limestone, supporting our model (Ware
1991). Increasing water availability shifted competitive dominance from the
shallow-soil granite endemic Minuartia uniflora (Walt.) Mattf. (One-flower
Stichwort; Caryophyllaceae) to the deeper soil endemic Sedum smallii (Britt.
ex Small) Ahles (Elf Orpine; Crassulaceae) to the larger species that form the
dominant community of normal soils (Sharitz and McCormick 1973).
We will employ the same sampling protocols used in California to sample
species and environmental variables at 75 paired glade and non-glade sites
across the Ozark Plateaus Province. We will test the same hypotheses using
the same techniques. We will also determine whether the relationship between
climate and edaphic endemism is the same or different for dolomite, limestone,
and sandstone. We expect our model to fit best for dolomite, the harshest
substrate, and worst for sandstone, the most benign of the three soils.
2. Experimental study of 3 serpentine endemics and 3 non-endemics
Serpentine grasslands are ideal for experimental tests of the climatecompetition-
soil interaction, because they are dominated by annual species
that are amenable to manipulation. Dominant functional groups in both
serpentine and other grasslands in California are annual grasses (nearly all
exotics) and forbs (native and exotic) (Harrison 1999).
At our study site in northern California (the University of California’s
McLaughlin Reserve), the vernal annual flora of serpentine grasslands include
a small number serpentine endemics, all of which are annual forbs;
they include Clarkia gracilis ssp. tracyi (Jepson) Abdel-Hamee &R. Snow
(Tracy’s Clarkia; Onagraceae), Navarretia jepsonii V. Bailey ex Jepson
(Jepson’s Pincushionplant; Polemoniaceae), and Calycadenia pauciflora A.
Gray (Smallflower Western Rosinweed; Asteraceae). Each has a close relative
in the nearby nonserpentine flora (Clarkia purpurea (Curtis) Nelson &
J.F. Macbr. [Winecup Fairyfan; Onagraceae], Navarretia pubescens (Benth.)
Hook & Arn [Downy Pincushionplant; Polemoniaceae], and Hemizonia congesta
DC. [Hayfield Tarweed; Asteraceae]). In this part of the study, we will
focus on precipitation, because it is well known that the amount and timing
of rainfall plays a central role in determining the composition and dynamics
of Mediterranean-climate annual systems such as Californian grasslands.
To test Hypothesis 2, we will use rainout shelters, supplemental watering,
and competition manipulations in the field. We will set up 60 plots in
serpentine and non-serpentine grasslands. Each of the three endemic species
2009 S. Harrison, E. Damschen, and B.M. Going 127
and three non-endemic species will be planted into one 30- x 30-cm subplot
that is cleared of competing species by clipping all aboveground material,
and one 30- x 30-cm subplot that is uncleared.
We will impose 2 levels of total water availability: 50% of normal and
200% of normal. These will be compared to a control with normal rainfall.
We will impose each of these treatments in combination with two levels of
soil (serpentine, non-serpentine) and competition (with, without) in our field
experiment. Competitor removal will be repeated monthly by carefully clipping
all aboveground material except the focal plants. Plant performance
will be assessed by measuring plant height, seed number, seed weight, and
root and shoot dry biomass.
The experiment will be repeated for two years to ensure that the results
are robust against variability in annual climate. We anticipate setting up
similar experiments in the Ozark Glades.
3. Resampling Robert Whittaker’s historic Siskiyou sites
Our most direct test of our conceptual model’s applicability to modern
climate change is our resampling of sites studied by ecologist Robert Whittaker
in the Siskiyou Mts of Oregon in 1949–1951 (Whittaker 1954, 1960).
The Klamath-Siskiyou region includes North America’s largest exposure
of ultramafic rock, including serpentine and peridotite. The size if the area
and its great age (>50 ma), high rainfall, and topographic complexity of
ultramafic rocks combine to make it the continent’s leading hotspot of serpentine
endemism; of 246 plant taxa confined to serpentine and peridotite in
California, 97 are found here (Alexander et al. 2006, Coleman and Kruckeberg
1999, Safford et al. 2005).
Climatic warming and reduced snowpack, leading to drier conditions in
the spring and summer growing season, have been documented throughout
the Pacific Northwest (Mote et al. 2003). Flowering phenology has become
significantly earlier since the 1950s at the Oregon Caves National Monument
at the center of the study region (J. Roth, Oregon Caves National Monument,
National Park Service, Cave Junction, OR, unpubl. data).
Robert Whittaker’s goal was to quantify plant community variation along
three environmental gradients: elevation, soil type, and the local variation
from xeric to mesic microsites that he called the “topographic moisture gradient.”
A very mesic slope is a streamside or a shallow north slope, while
a very xeric site is a steep south-facing slope. We used the topographic
moisture gradient as a way to interpret climate-related change. Specifi-
cally, we predicted that the plant species composition of any site sampled
in 2007 would resemble that of a warmer (more xeric, south-facing) site in
1949–1951.
Whittaker collected vegetation data from 290 plots on diorite soils in 1949
and from 55 plots on serpentine and 50 plots on gabbro soils in 1950–1951. At
each 50- x 20-m plot, he obtained an estimate of percent areal cover for each
herb species, a total herb species list, counts of tree individuals by diameter at
breast height (dbh) and species, and counts of shrubs by species.
128 Northeastern Naturalist Vol. 16, Special Issue 5
We entered Whittaker’s data into a database with herb count, herb cover,
shrub count, and small and large tree counts, by species and plot number.
To locate sites as similar as possible to his (e.g., his plot number 268), we
followed the same road or trail on the same substrate (e.g. Wimer Road, serpentine),
stopped at the same elevation (e.g., 702 m [2400 ft]), and sought
the nearest place with the same slope and aspect (e.g., 25°E).
In 2007, we sampled all of Whittaker’s serpentine plots, and 53 plots representing
a subset of Whittaker’s diorite that match the serpentine plots in
elevation. We followed Whittaker’s sampling methods exactly.
We are just beginning to analyze the data. Our preliminary results show
that floras have shifted, such that the flora of any given site in 2007 resembles
the flora of a warmer slope and aspect in 1949–51. But our preliminary
results do not support Hypothesis 3, because this shift to a more “xeric”
species composition (i.e., toward identities and abundances of species characteristic
of a warmer slope/aspect) appears to have occurred equally in
serpentine and diorite floras.
Conclusions
Special edaphic floras are important contributors to both regional and
global biodiversity, but their potentially extreme vulnerability to climate
change has been little studied. If one ignores any life-history differences that
might distinguish edaphic endemics from other species, as well any role ecological
interactions may play in producing edaphic endemism, then the fate of
these floras looks very bad. Restricted to tiny edaphic islands, they are much
likelier to go extinct under climate change, and much less likely to successfully
migrate to suitable new habitats, than plant species that are soil generalists.
Our conceptual model, in contrast, considers two additional aspects of
edaphic endemism that may be very important in predicting climate-change
responses. The first is that endemics may be intrinsically slow-growing, even
when adequate resources are available, and thus they may be less sensitive to
temperature and water availability than other species regardless of what soils
they are growing on. The second is that the restriction of edaphic endemics
to special soils may be the outcome of soil-dependent competition with
faster-growing generalist species, whose growth is more strongly influenced
by temperature and water availability than that of edaphic endemics, but
which can only grow well on fertile soils.
Putting these two pieces together, our model predicts that in regions
where climate change leads to a warmer and wetter environment, serpentine
or other edaphic endemics should be outcompeted by soil generalists on all
but the harshest “special” soils, and thus endemics may become less common.
However, in regions where climate change leads to a warmer and drier
environment, edaphic endemics will be less adversely affected than other
species, and may even expand their ranges into marginal (less harsh) soil
habitats at the expense of generalist species. In the latter case, shifts in the
relative competitive abilities of edaphic endemics and generalists could, to
some extent, offset the extra risks faced by endemics under climate change.
2009 S. Harrison, E. Damschen, and B.M. Going 129
We are still far from knowing whether our conceptual model provides a
good framework for understanding climate-driven changes in either space or
time, in serpentine and other edaphic floras. However, we hope it will at least
encourage other scientists studying these floras to begin addressing the greatest
environmental challenge of our time: human-induced climate change.
Acknowledgments
The authors thank Nishanta Rajakaruna and Bob Boyd for organizing the Sixth
International Serpentine Conference; David Ackerly, Bruce Baldwin, and Rod Myatt
for reviewing this paper; and anonymous reviewers for helpful and consistently favorable
comments on the proposed work.
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