Abstract. -- The role of shade tolerance in the dynamics of a sandy
upland pine-oak forest in Big Thicket National Preserve, southeast Texas
was investigated. Using a forest dynamics modeling framework, radial
growth of saplings as a function of light availability and mortality as
a function of recent growth history for species with a range of shade
tolerance levels was investigated. In low light, shade-tolerant species
grew faster than shade-intolerant species. However, in high light,
shade-intolerant species did not grow faster than shade-tolerant species
possibly because some of them are adapted for drought resistance. They
did not survive better, either, perhaps because of recent increases in
canopy shading. Mesic, shade-tolerant species had better performance at
the dry site than at the mesic site, possibly because of a difference in
the competitive environment of the two sites. An implication of invasion
and higher growth and survival of the mesic species is that these
species may have been limited to a larger extent by fire than by site
conditions on this site in the past.
Broad patterns in species dominance across the landscape are well
known for the southeastern United States (Christensen 1988; Ware et al.
1993), and these are consistent with general understanding of
physiological tolerances of the major tree species. In southeast Texas,
interspecific differences in response to light are consistent with
trends in species dominance at a mesic site (Lin et al 2001; 2002), and
thereby help provide mechanistic underpinning for observed species
dominance on mesic sites. At a wet site, light was important in helping
to explain species dominance, but only if response to flooding was
considered, as well (Hall 1993; Hall & Harcombe 1998; 2001; Lin et
al. 2004). In the study reported here, analysis of the light response to
a dry site is extended, partly to further investigate the effects of
site differences on light responses, and partly also to determine
whether differences in light response among species help explain changes
in species dominance.
The approach is based on the general understanding that light, soil
moisture and nutrients are important factors that determine species
composition of many terrestrial plant communities (e.g. Huston &
Smith 1987; Smith & Huston 1989; Pacala et al. 1994; Knox et al.
1995; Sipe & Bazzaz 1995; Grubb et al. 1996; Catvosky & Bazzaz
2000). Mortality-growth-light relationships based on the forest dynamics
model, SORTIE (Pacala et al. 1993; 1994; 1996; Kobe et al. 1995) are
used. The model assumes resource competition among coexisting species,
as do most forest dynamics models (e.g. Botkin et al. 1972; Shugart
1984; Smith & Huston 1989; Pacala et al. 1996). Through repeated
iterations of the model, light competition results in shifting dominance
from shade-intolerant species to shade-tolerant species over the course
of stand development. Extending SORTIE by incorporating soil moisture
into the mortality-growth model, Caspersen & Kobe (2001) found that
species ranks in mortality-growth relationship shifted substantially
across soil moisture gradient, resulting in shifting dominance.
Although competition for soil moisture provides a possible
process-level explanation for the broad pattern of species segregation
across the landscape in southeast Texas (Marks & Harcombe 1981;
Harcombe et al. 1993) and across the southeastern United States
(Christensen 1988; Ware et al. 1993), fire also plays a role (Harcombe
et al. 1993; 1998). Under the fire scenario, sites with longleaf pine
(Pinus palustris), a species highly tolerant to fire, would not support
mature hardwood forests. One way to investigate the question of the
relative importance of soil moisture and fire is to compare
growth-mortality relationships of species under different moisture
regimes. In essence, this is asking whether consistency can be found
between process (growth/mortality) and pattern, and tie it to a
mechanism (competition for light and/or mois-ture). If growth and
mortality for species present at different sites are lower at the dry
site, the inference that soil controls vegetation pattern cannot be
ruled out. If, on the other hand, growth and mortality are higher at the
dry site under the current fire suppression scenario, then fire may have
been the major limiting factor at the dry site in the past.
In this study, light competition in a mixed pine-oak stand in the
Turkey Creek Unit of the Big Thicket National Preserve, southeast Texas
was investigated. In addition, growth and mortality of species common to
both this dry site and a nearby mesic site were compared. Compared with
the mesic site, the dry site is characterized by coarser soils and lower
soil moisture availability (Caird 1996). Widespread presence of charcoal
on stumps and the prevalence of longleaf pine indicates that the dry
site probably burned relatively frequently (Harcombe et al. 1993). Under
the current fire suppression scenario, the site is being invaded by
mesic species (Harcombe et al. 1998). The invasion of mesic species
suggests that they may have been limited by fire in the past, and not by
low soil moisture. The following questions are addressed: Do differences
in mortality-growth-light relationship among species within and between
sites explain differences in dominance between the dry site and the
mesic site? Can species responses to site conditions explain differences
in species composition or must historical disturbances (e.g., fire) be
STUDY SITES AND SPECIES
The dry study site is located on a low, sandy ridge in the Turkey
Creek Unit of the Big Thicket National Preserve about 10 km southeast of
Warren, Tyler County, Texas (30[degrees]35'N,
94[degrees]24'W). The climate of the area is humid subtropical with
an annual rainfall around 1475 mm. The soil is a sandy loam of Landman
series, loamy, siliceous thermic Grossarenic Paleudalf (Caird 1996).
Light measurements obtained from hemispherical photos taken at plot
centers (100 plots in total) indicated a light range in the understory
from 1.7% full sun to 33.5% full sun with a mean of 12.8%.
The vegetation is dominated by oaks and pines. Ranked in decreasing
order of relative abundance, post oak (Quercus stellata Wang.), southern
red oak (Quercus falcata Michx.), black hickory (Carya texana Buckl.),
longleaf pine (Pinus palustris Mill.), loblolly pine (Pinus Taeda L.)
and shortleaf pine (Pinus echinata Mill.) form a relatively open canopy
15-20 m tall. Basal area increased from 21[m.sup.2]/ha in 1982 to 28
[m.sup.2]/ha by 1999. Red maple (Acer rubrum L.) and sweetgum
(Liquidambar styraciflua L.) are minor canopy components. The understory
is a moderately dense mixture of tree saplings and shrubs; flowering
dogwood (Cornus florida L.), yaupon (Ilex vomitoria Ait.) are abundant.
Saplings of mesic species, such as Southern magnolia (Magnolia
grandiflora L.) and American holly (Ilex opaca Ait.) have become more
abundant since 1980 (Harcombe et al. 1998). American holly and flowering
dogwood are very shade-tolerant; sweetgum and most dry-site species are
shade-intolerant. The above shade tolerance categories are based on
conventional wisdom regarding shade tolerance as summarized by Burns
& Honkala (1990). These shade tolerance classifications are based
largely on field observations regarding the relative abundance of
different species in the forest understory.
The dry site was logged in 1930 but the stand is not strongly
even-aged (Harcombe et al. 1993; Kaiser 1995); apparently many old
hardwoods and older pines were left in the site. Exactly how long ago
fire occurred on this site is unknown. The presence of charcoal on
stumps implies relatively frequent fire prior to 1930 and relatively
infrequently after that until 1974. Fire has been absent since 1974
(Kaiser 1995; P. Harcombe, personal communication).
A nearby mesic site was chosen for comparison. The mesic site is
located in Hardin County, Texas (30[degrees]16'N,
94[degrees]12'W) approximately 14 km away from the dry site.
Species composition of this site represents many typical mesic sites
throughout the Coastal Plain area of the southeastern U.S. (Marks &
Harcombe 1981). The site is dominated by loblolly pine (Pinus taeda L.),
water oak (Quercus nigra L.), white oak (Quercus alba L.), American
beech (Fagus grandifolia Ehrh.) and southern magnolia (Magnolia
grandiflora L.). Red maple (Acer rubrum L.), blackgum (Nyssa sylvatica
Marsh.) and sweetgum (Liquidambar styraciflua L.) are abundant as small
to medium stems but are infrequent as large trees. Important understory
trees include American holly (Ilex opaca Ait.) and flowering dogwood
(Cornus florida L.). Basal area has varied between 33.7 [m.sup.2]/ha
(after hurricane) and 35.1 [m.sup.2]/ha over the last 20 years. More
detailed description can be found in Glizenstein et al. (1986) and Lin
et al. (2001; 2002). See Table 1 for shade tolerances and affiliations
of species with sites.
DATA COLLECTION AND ANALYSES
Sapling growth. -- The dry study site is 4 ha divided into 100
contiguous tree plots. Each plot is 20m by 20m. Tree surveys were
performed in 1980, 1982, 1985, 1989, 1994, 1997 and 2000. During tree
surveys, stems with a Diameter at Breath Height (DBH) [greater than or
equal to] 2 cm are measured with a diameter tape. A subset of 16 plots
was chosen randomly for annual measurement of saplings (height [greater
than or equal to] 140 cm and DBH [less than or equal to] 4.5 cm), in
which DBH of all saplings was measured to the nearest 0.1 cm from
1980-2000. All trees and saplings are tagged with an identification
number. For each sapling (height [greater than or equal to] 140 cm and
DBH [less than or equal to] 4.5 cm), annual radial growth rate over
three years was calculated as the difference in radius between year 1999
and year 1996 divided by 3. The average over 3 years was used to reduce
measurement variation. Calculations of growth were made for all species
with more than 15 individuals in the sample.
As approximations of high-light growth and low-light growth, top
quartile growth rate (TQGR) and bottom quartile growth rate (BQGR) were
calculated. Approximations were chosen because it was not possible to
model mortality-growth-light relationships owing to small sample sizes
and/or insufficient range of light conditions, TQGR is a reasonable
approximation of high-light growth because saplings that have high
growth rates are unlikely to be growing in low light. Comparison of TQGR
and the actual high-light growth in the mesic site where both measures
are available showed a good agreement between the two (data not shown).
It is important to note that bottom quartile growth rate is only a rough
approximation of low-light growth because low growth could result from
many reasons other than low light.
Top quartile growth rate was computed as follows: First, the radial
growth rate over the first 3 years after the sapling first entered the
survey was calculated. After calculating growth rates of all first-year
saplings, growth rates were sorted in descending order. Then saplings
with growth rates in the top 25% were chosed and their growth rates were
averaged. To see whether TQGR of first-year sapling obtained this way
might underestimate maximum growth, it was compared with TQGR for all
saplings present in one period (1996-1999); it did not (results not
shown). The bottom quartile growth rates were obtained by taking the
bottom 25% growth rates and computing the average.
Light measurement. -- A subset of live saplings was selected from
the database for light measurements. In keeping with the protocols of
previous studies, the goal was to find at least 50 saplings per species
for light measurement. The final sample size ranged from 45 to 59
saplings per species. The six species are: red maple, sweetgum, loblolly
pine, post oak, Southern magnolia and American holly. Saplings were
selected in a stratified random fashion by plot to obtain a broad range
of light conditions. Fish-eye photographs were taken at the top of each
sapling (following Rich 1989; Pacala et al. 1994) in mid summer (late
June to mid July), 1999. To increase contrast, all photos were taken
early in the morning before sunrise and late in the afternoon after
sunset when skylight is evenly distributed. Moreover, all photos were
taken on Kodak TMAX ASA 400 (black and white) film and the film was
under-exposed by 1 f-stop to further enhance contrast. The images were
scanned, digitized and analyzed using CANOPY (Rich 1989). Threshold
values were set individually to minimize the "halo effects"
(Anderson 1964). The global site factor (GSF) was estimated from each
photo. GSF is an estimation of the fraction of total radiation (both
diffuse and direct) a sapling experienced during the growing season. The
GSF value was converted to percent of full sun by multiplying GSF by
100. Since no major canopy disturbances occurred during the 1996-1999
period, the light level captured in 1999 was considered to be a
reasonable representation of average light environment over the
three-year period at a given location.
Sapling mortality. -- In addition to periodic measurement, each
sapling was checked annually to see whether it was dead or alive.
Survival time was calculated as the length of time a sapling was
followed during the course of the study. If a sapling died, then its
survival time would be the difference between the year of death and the
year it entered the study. If a sapling was alive at the end of the
study (Year 1999), its survival time was the difference between the
ending year and the year it entered the study. Saplings that were alive
at the end of the study were flagged as right censored (Cox & Oakes
1984; Lee 1992). All saplings (dead or alive) that had been recorded
since the beginning of the long-term study (Year 1980) were included. To
model mortality as a function of recent growth, pre-mortality growth
rate was calculated for dead saplings as the difference in radius over
the last 3 years prior to death divided by 3.
Growth-light analysis. -- The goal of this analysis is to model
growth response from light availability using a Michaelis-Menten
function, as in previous studies (cf. Pacala et al. 1994; Wright et al.
1998). However, because of sampling limitations, the asymptote parameter
was replaced by TQGR, which is treated as a constant instead of a
parameter, because of inadequate range of conditions and small sample
sizes for some species. The one-parameter model takes the following
[mu] = [aL/a / S + L] (1)
Where [mu] is the mean growth response given light availability; a
is the TQGR; S is the slope at low light; L is the light availability (%
of full sun).
The maximum likelihood methods to estimate parameter S was used.
The final likelihood function is:
[n.[product]i=1] [1/[square root of (2[pi]C[aL/(a / S +
L).sup.D])] - exp(- [[Gi - aL/(a / S + L).sup.2]]/2C[aL/(a / S +
where [G.sub.i] is the radial growth rate of sapling i (3-year
average); C, D are two parameters that account for heteroscedasticity.
Confidence intervals of S were obtained by bootstrapping. Both
model fitting and bootstrapping were done using Splus 6.0 on Unix
(Mathsoft, Inc. 2000). A more detailed description of the maximum
likelihood estimation method can be found in Lin et al. (2002).
Mortality risk (annual death rate) as a function of growth. --
Survival analysis was used to model mortality risk as a function of
growth. The likelihood function for censored and non-censored saplings
is (Lee 1992):
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)
where r is the number of saplings that died during the study and
n-r is the number of saplings that are right-censored. [T.sub.i] and
[t.sub.i] are lifetimes of a non-censored and right-censored sapling i,
respectively; [lambda] is the parameter of mortality risk (annual
A negative exponential function was used to estimate [lambda] from
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)
where [X.sub.1] is the radial growth rate (mm/yr); [X.sub.2] is the
initial size (radius in mm). The parameters to be estimated are the
[beta]s. [theta] is the error term. Estimates of parameters
[[beta].sub.0], [[beta].sub.1] and [[beta].sub.2] were found by
maximizing the likelihood function (3).
Maximum likelihood estimation of annual death rate. -- To further
explore how mortality might be different among species with different
shade tolerance, annual death rate was also compared.
The maximum likelihood estimator of annual death rate is (Lee
[^.[lambda]] = [D/[[SIGMA].sup.D.sub.i=1] [T.sub.i] +
[[SIGMA].sup.N-D.sub.i=1] [t.sub.i]] (5)
Where D is the number of deaths during the time interval.
The 95% confidence interval of [lambda] is:
[^.[lambda]][+ or -][[^.[lambda]] X 1.96]/[square root of(D - 1)]
Growth response to light and inter-specific tradeoff. -- Growth
increased with light for all species (Figure 1). Except for sweetgum,
which showed higher growth than red maple, the pattern of low-light
growth was consistent with the expectation that shade-tolerant species
grow faster in low light than shade-intolerant species (Figure 1). The
low-light growth index, slope at low light, was highest for American
holly, followed by southern magnolia (Table 2). Two shade-intolerant
species, loblolly pine and post oak, ranked low in slope (Table 2). The
correspondence between low-light growth and shade tolerance ranks was
further supported by the comparison of bottom quartile growth rates
among species (Figure 2a): Shade-tolerant species ranked higher than
most shade-intolerant species in bottom quartile growth rates, though
bottom quartile growth rate of sweetgum and loblolly pine were higher
than expected based on standard shade tolerance ranks.
In contrast, for high-light growth, the order of TQGR did not
correspond to shade tolerance expectation: First, shade-intolerant post
oak and loblolly pine showed low TQGR; second, shade-tolerant southern
magnolia and American holly grew more rapidly than expected (Figure 1,
Table 2). Top quartile growth rates of xeric dominants (e.g., post oak,
black hickory, southern red oak) were significantly lower (P < 0.05;
ANOVA followed by Tukey's multiple comparison adjustment) than
mesic invaders (e.g., American holly, Southern magnolia, sweetgum). Even
within the six mesic species, top quartile growth rates did not conform
to expectation: shade-tolerant southern magnolia grew significantly
faster than shade-intolerant sweetgum and loblolly pine (Figure 2b).
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
Mortality risk as a function of growth. -- Mortality risk as a
function of growth was used to characterize shade tolerance in previous
studies (e.g., Kobe et al. 1995; Lin et al. 2001). In this study, the
low number of dead saplings of American holly, southern magnolia and red
maple made survival analysis on these species unreliable (e.g., there
was only one dead American holly sapling and two dead southern magnolia
saplings found in the long-term study data base). Thus, at this site,
the only shade-tolerant species included in survival analysis was
flowering dogwood. In contrast to results of a previous study performed
at the mesic site (Lin et al. 2001), both growth and size were
significant predictors of mortality risk in the dry site. Overall,
mortality risk decreased as growth increased and decreased with
increasing size (Table 3). The mortality-growth relationship was not
consistent with the expectation that shade-intolerant species have
higher mortality risk at zero growth and steeper slope than
shade-tolerant species (Table 3).
Annual death rate. -- Interpretation of the above mortality-growth
responses in terms of shade tolerance expectation was limited by the
fact that only one shade-tolerant species (dogwood) was involved in the
analysis. Therefore, annual death rates among species were also compared
(Figure 3). Mesic species such as American holly, southern magnolia, red
maple exhibited extremely low annual death rate (Figure 3), which is
consistent with the previous finding that they have become more abundant
and species typical of dry sites have experienced dramatic decline
(Harcombe et al. 1998). Death rates of dry site dominants (longleaf
pine, post oak, southern red oak) were consistently higher than mesic
Cross-site comparisons. -- Growth-light curves of southern magnolia
and American holly were significantly higher at the dry site than at the
mesic site over the light range (Figure 4a and b): For red maple, growth
rates were significantly higher only above 60% full sun (Figure 4c). For
sweetgum, there was no significant difference between sites (confidence
interval overlapped, not shown) (Figure 4d). Annual death rates were
significantly higher at the mesic site than at the dry site for all
species common to the two sites except flowering dogwood (Figure 5).
Growth, mortality and tolerance. -- Results show that growth
responses to low light are roughly consistent with one of the
expectations regarding shade tolerance: in low light, shade-tolerant
species grow faster than shade-intolerant species, even on dry sites.
However, growth responses to high light do not correspond to the
expected pattern. Instead, two shade-intolerant species, post oak and
loblolly pine, have lower highlight growth than expected. Why loblolly
pine showed lower high-light growth than expected remains an interesting
question for further investigation. The low growth of post oak can
possibly be explained by drought tolerance. The inherent conflict
between carbon uptake and water loss of plant has been widely documented
and intensively studied (e.g., Field & Mooney 1986; Huston &
Smith 1987). Adapted to soil water deficiency, drought-tolerant species
are reported to develop traits that minimize water loss but limit growth
rates (Delucia et al. 1988; Kozlowski et al. 1991; Barton & Teeri
1993). Indeed, the three xeric dominants (post oak, black hickory and
southern red oak) in this study ranked the lowest in both top quartile
growth rates and bottom quartile growth rates (Figure 2) indicating slow
growth of drought-tolerant species (Chapin 1991).
[FIGURE 3 OMITTED]
With respect to mortality, the positive association of initial size
and survivorship has also been reported in other studies (e.g., Clark
& Clark 1992; Condit et al. 1995; Sheil & May 1996; Kobe 1999).
Compared with the mesic site (Lin et al. 2001), where a significant
effect of size was not detected, saplings at the dry site span a wider
size range, so the significant effect of size on mortality in this study
may be attributable to relatively large size variation (cf. Kobe 1999).
In addition, the decline of mortality with size may be an indication
that larger saplings with more extensive root systems suffer less
drought-induced mortality on dry sites, as suggested by Caspersen &
[FIGURE 4 OMITTED]
The higher death rate for xeric species than most mesic species
(Figure 3) can possibly be explained in terms of stand dynamics and
change in light environment over the last 20 years. Stem density
increased about 15% from the early 1980s to the 1990s, and most of the
increase in total stem density was caused by increased density of
understory dominants, such as yaupon (Ilex vomitoria), southern magnolia
and American holly (Kaiser 1995). A direct consequence of an increase in
density of understory species is reduced light penetration to the
understory, which would cause the high death rates of shade-intolerant
As an exception to the pattern of low death rate of shade-tolerant
species, flowering dogwood had a higher death rate than even
shade-intolerant species. This high mortality is consistent with a
declining trend of this species over its range, which is associated with
the exotic fungus, anthracnose (Discula destructiva) in the Great Smoky
Mountains, but not elsewhere (Schrope 2001). It was noted that fire
suppression, which results in thicker canopy and increased moisture,
help the fungus to thrive (Schrope 2001).
[FIGURE 5 OMITTED]
Cross-site comparison and implications for stand dynamics. --
Previous studies have shown that the combined effect of soil moisture
and light on plant performance (growth and survivorship) may largely
depend on the balance between the improvement allowed by one
environmental factor (e.g., light) and the reduction imposed by
deterioration in another factor (e.g., soil moisture) (Berkowitz et al.
1995; Holmgren et al. 1997). At drier sites, if the negative effects of
soil moisture deficiency on plant performance do not outweigh the
positive effects of more light penetration resulting from the more open
canopy, then better performance at drier sites would be expected. In
fact, many studies have reported such "facilitative" effects
at drier sites (Parker & Muller 1982; Barton 1993; Belsky et al.
1993; Berkowitz et al. 1995; Kobe & Coates 1997). In an experiment
testing the effects of community composition on growth and survival of
tree seedlings, Berkowitz et al. (1995) noted that in sites that were
physically unfavorable, surrounding vegetation had few negative effects
(competition) on seedling growth. In the case of sugar maple in their
study, surrounding vegetation actually facilitated growth of sugar maple
seedlings. So growth performance was not only influenced by site
suitability, but depended on surrounding vegetation, as well. This
conclusion may provide an explanation for what was observed. For mesic
species (magnolia, American holly and red maple) in this study, saplings
at the dry site may benefit from less competition for soil resources
from slow-growing neighboring vegetation, and thereby maintain a
favorable growth and survival status, even though there is more total
available water at the mesic site than at the dry site (Caird 1996). The
exception, sweetgum, failed to exhibit higher growth at the dry site
possibly because it is less drought-tolerant than others (Marks &
Harcombe 1981) and therefore suffered more drought-induced growth
The better performance of shade-tolerant mesic species at the dry
site is not consistent with the idea that there is trade-off between
shade tolerance and drought tolerance (e.g., Smith & Huston 1989).
Instead, these species appeared to be both shade-tolerant (i.e., grow
faster and/or survive better in shade than shade-intolerant species) and
drought-tolerant (i.e., better performance at dry site than at mesic
site). It may be, however, that differences in drought tolerance only
appear in years of more extreme drought or after saplings get large
enough to be exposed to the drying effect of full sun. Alternatively,
Caspersen et al. (1999) argued that whether species conform to a
trade-off between shade tolerance and drought tolerance may depend on
the relative importance of growth and survival in determining the
species ability to tolerate limiting resources. If the ability to
survive in the shade is achieved by allocation to defense and storage
(Kitajima 1994; Kobe 1997), then tolerance to shade may also confer
tolerance to other limiting resources, including soil moisture.
Pacala et al. (1996) argued that light competition can produce
successional patterns in forest communities because of different light
requirements of competing species. In a dry forest, light competition
has its apparent signature in growth and mortality of saplings, although
the correspondence between shade tolerance expectation and sapling
performance is weaker than it is at moister sites. The better growth
performance of shade-tolerant invaders in low light than
shade-intolerant dominants, and the correspondence between the decline
of shade-intolerant dominants and canopy closure clearly suggest that
this forest is undergoing successional changes driven by light
competition as suggested by Harcombe et al. (1998); i.e., mesic species
do not seem to be limited by low soil moisture in this forest. Instead,
they grow faster and survive better than at the moister site. While
light competition may be a major driving force of dynamics in this
forest, the fact that the light responses of some species (such as
flowering dogwood and sweetgum) do not conform to the expected pattern
of light competition points to the inadequacies of the SORTIE model. In
fact, aside from shade tolerance, tradeoffs involved in drought
tolerance, herbivore tolerance and fire tolerance may be of importance
to explain the observed deviations.
Returning to the question regarding the extent to which the effects
of site conditions and/or fire contribute to stand composition and
dynamics, the data showed that saplings of mesic species have better
performance at the dry site than at the mesic site in terms of both
growth and survivorship. Thus, mesic species do not seem to be limited
by site conditions under the current fire exclusion scenario. An
important implication is that mesic species may have been limited to a
larger extent by fire than by site conditions in the past (Harcombe et
al. 1998), and that the effect of site conditionson vegetation pattern
may be as much indirect via its effect on fire as it is direct via its
effect on differential growth and mortality among species.
We thank National Park Service for permission to carry out this
study in Turkey Creek unit. We thank all people participated in
collecting the long-term data set of this forest, especial thanks go to
Sandi Elsik who also manages the data set. Lisa Sweeney helped taking
hemispherical photos in the fields. Cherri Higgins scanned the photos.
Scott Baggett and Evan Siemann provided helpful suggestions on
statistical analysis. Funding for this study was provided by NSF grants
to Paul Harcombe (DEB-9726467) and Mark Fulton (DEB-9816493) and a
Wray-Todd Fellowship to Jie Lin. We thank Kyle Harms and an anonymous
reviewer for their comments that improve the manuscript.
JL at: firstname.lastname@example.org
(1) Department of Biology, Bemidji State University 1500 Birchmont
Dr. NE, Bemidji, Minnesota 56601
(2) Department of Biology, Auburn University at Montgomery 7300
University Drive, Montgomery Alabama 36117
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Table 1. Latin names, common names, name codes and shade tolerance of
major species. Species are arranged in ascending order of shade
tolerance according to Burns & Honkala (1990).
Latin Name Common Species Shade Site
Name Code Tolerance Affiliation
Quercus Post oak QUST Intolerant Dry
Carya texana Black hickory CATE Intolerant Dry
Pinus Longleaf pine PIPA Intolerant Dry
Pinus Shortleaf pine PIEC Intolerant Dry
Pinus Taeda Loblolly pine PITA Intolerant Mesic, dry
Liquidambar Sweetgum LIST Intolerant Mesic, dry
Quercus Southern red oak QUFA Intermediate Dry
Acer rubrum Red maple ACRU Tolerant Mesic, dry
Magnolia Southern magnolia MAGR Tolerant Mesic, dry
Ilex opaca American holly ILOP Very tolerant Mesic dry
Cornus Flowering dogwood COFL Very tolerant Mesic, dry
Table 2. Top quartile growth rates (TQGR, a in equation 2) and estimated
slope at low light (S in equation 2) with 95% confidence intervals (CI).
N is the sample size. NA stands for not available.
Species Shade N TQGR CI of S CI of S
Post oak intolerant 53 0.905 0.736-1.074 0.026 0.014-0.046
Black intolerant 78 0.718 0.641-0.795 NA NA
Loblolly Intolerant 59 1.720 1.643-1.798 0.058 0.033-0.099
Sweetgum Intolerant 58 2.006 1.912-2.099 0.654 0.357-1.100
Southern Intolerant 16 1.263 1.155-1.370 NA NA
Red maple Tolerant 45 1.728 1.599-1.857 0.347 0.232-0.530
Southern Tolerant 52 2.363 2.205-2.516 0.755 0.545-1.123
American Very tolerant 47 1.847 0.901-1.282 2.911 1.650-5.144
Flowering Very tolerant 33 1.944 1.831-2.057 NA NA
Fig. 2 Bottom quartile growth rates for different species (a) and top
quartile growth rates for different species (b). Values not sharing the
same letter are significantly different (ANOVA followed by Tukey's
multiple comparison adjustment, P < 0.05). N is the number of saplings.
Species are arranged in descending order of shade tolerance from left to
right. See Table 1 for key to species codes.
Bottom quartile growth rates of first year saplings
Radial growth rate (mm/yr)
Note: Table made from bar graph.
Top Quartile growth rate of first year saplings
Radial growth rate (mm/yr)
Note: Table made from bar graph.
Table 3. Parameter estimates of the mortality-growth model (equation 4)
with 95% confidence intervals (CI) for different species. N is the total
number of saplings (both dead and live); [beta]s are parameters in
equation 4. [lambda] is the mortality risk at zero growth at size class
Species Shade Sample [[beta].sub.0] [[beta].sub.1]
Post oak Intolerant 1563 2.15 1.46
Black Intolerant 441 3.50 0.28
Longleaf Intolerant 58 2.39 1.08
Shortleaf Intolerant 133 2.21 1.71
Loblolly Intolerant 1573 1.70 0.83
Sweetgum Intolerant 264 2.85 3.44
Southern Intermediate 178 3.17 1.18
Flowering Very 291 2.29 0.46
Species [[beta].sub.2] CI of [[beta].sub.0] CI of [[beta].sub.1]
Post oak 0.03 1.93-2.37 0.87-2.05
Black 0.06 2.89-4.10 -1.27-1.83
Longleaf 0.02 1.34-3.44 -0.32-2.48
Shortleaf 0.04 1.62-2.80 0.40-3.02
Loblolly 0.05 1.52-1.88 0.59-1.07
Sweetgum 0.03 2.17-3.54 1.73-5.15
Southern -0.01 2.59-3.75 0.25-2.10
Flowering 0.02 1.99-2.58 0.14-0.78
Species CI of [[beta].sub.2] [lambda] CI of [lambda]
Post oak 0.02-0.04 0.11 0.09-0.14
Black 0.03-0.09 0.03 0.02-0.05
Longleaf -0.02-0.05 0.09 0.03-0.27
Shortleaf 0.02-0.07 0.11 0.06-0.20
Loblolly 0.05-0.06 0.18 0.14-0.21
Sweetgum 0.00-0.06 0.06 0.03-0.11
Southern -0.03-0.00 0.04 0.02-0.08
Flowering 0.00-0.03 0.10 0.07-0.14
Fig. 5. Cross-Site comparison of annual death rates. Values not sharing
the same letter are significantly different between the two sites.
Annual death rate comparison
Annual death rate
Species Mesic Site Dry site
MAGR a b
COFL a b
ACRU a b
LIST a b
Note: Table made from bar graph.