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Comparative analysis of growth and mortality among saplings in a dry oak-pine forest in southeast Texas.
Subject:
Pine (Environmental aspects)
Pine (Comparative analysis)
Pine (Research)
Oak (Environmental aspects)
Oak (Comparative analysis)
Oak (Research)
Authors:
Lin, Jie
Harcombe, Paul A.
Fulton, Mark R.
Hall, Rosine W.
Pub Date:
11/01/2004
Publication:
Name: The Texas Journal of Science Publisher: Texas Academy of Science Audience: Academic; General Format: Magazine/Journal Subject: Science and technology Copyright: COPYRIGHT 2004 Texas Academy of Science ISSN: 0040-4403
Issue:
Date: Nov, 2004 Source Volume: 56 Source Issue: 4
Topic:
Event Code: 310 Science & research
Geographic:
Geographic Scope: Texas Geographic Code: 1U7TX Texas

Accession Number:
133485176
Full Text:
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.

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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 invoked?

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 form:

[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 + L)[].sup.D])] (2)

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 mortality risk).

A negative exponential function was used to estimate [lambda] from predictor variables

[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 1992):

[^.[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)] (6)

RESULTS

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 site species.

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).

DISCUSSION

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 & Kobe (2001).

[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 xeric dominants.

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 reduction.

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.

ACKNOWLEDGMENTS

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: jlin@mdanderson.org

Present address:

(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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Jie Lin, Paul A. Harcombe, Mark R. Fulton (1) and Rosine W. Hall (2)

Department of Ecology and Evolutionary Biology Rice University, Houston, Texas 77251-1892
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
  stellata

Carya texana   Black hickory      CATE     Intolerant        Dry

   Pinus       Longleaf pine      PIPA     Intolerant        Dry
 palustris

   Pinus      Shortleaf pine      PIEC     Intolerant        Dry
  echinata

 Pinus Taeda  Loblolly pine       PITA     Intolerant     Mesic, dry

Liquidambar      Sweetgum         LIST     Intolerant     Mesic, dry
styraciflua

  Quercus     Southern red oak    QUFA    Intermediate       Dry
  falcata

Acer rubrum      Red maple        ACRU      Tolerant      Mesic, dry

Magnolia      Southern magnolia   MAGR      Tolerant      Mesic, dry
grandiflora

 Ilex opaca    American holly     ILOP    Very tolerant   Mesic dry

  Cornus      Flowering dogwood   COFL    Very tolerant   Mesic, dry
  florida

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
             tolerance                  TQGR

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
hickory

Loblolly    Intolerant    59  1.720  1.643-1.798  0.058  0.033-0.099
pine

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 oak

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
magnolia

American   Very tolerant  47  1.847  0.901-1.282  2.911  1.650-5.144
holly

Flowering  Very tolerant  33  1.944  1.831-2.057   NA        NA
dogwood

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)

      ILOP     b
     (N=25)

      COFL     bc
     (N=33)

      MAGR     a
     (N=25)

      ACRU     bc
     (N=23)

      QUFA     c
     (N=16)

      LIST     bc
     (N=63)

      CATE     d
     (N=78)

      QUST     d
     (N=31)

      PITA     c
    (N=121)

Note: Table made from bar graph.

Top Quartile growth rate of first year saplings

Radial growth rate (mm/yr)

      ILOP     bc
     (N=25)

      COFL     bc
     (N=33)

      MAGR     a
     (N=25)

      ACRU     bc
     (N=23)

      QUFA     d
     (N=16)

      LIST     b
     (N=63)

      CATE     e
     (N=78)

      QUST     de
     (N=31)

      PITA     c
    (N=121)

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
0.5 mm.

Species       Shade      Sample  [[beta].sub.0]  [[beta].sub.1]
            tolerance     size
                          (n)

Post oak    Intolerant    1563        2.15            1.46

Black       Intolerant     441        3.50            0.28
hickory

Longleaf    Intolerant      58        2.39            1.08
pine

Shortleaf   Intolerant     133        2.21            1.71
pine

Loblolly    Intolerant    1573        1.70            0.83
pine

Sweetgum    Intolerant     264        2.85            3.44

Southern   Intermediate    178        3.17            1.18
red oak

Flowering      Very        291        2.29            0.46

Dogwood      tolerant

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
hickory

Longleaf        0.02            1.34-3.44            -0.32-2.48
pine

Shortleaf       0.04            1.62-2.80             0.40-3.02
pine

Loblolly        0.05            1.52-1.88             0.59-1.07
pine

Sweetgum        0.03            2.17-3.54             1.73-5.15

Southern       -0.01            2.59-3.75             0.25-2.10
red oak

Flowering       0.02            1.99-2.58             0.14-0.78

Dogwood

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
hickory

Longleaf       -0.02-0.05          0.09      0.03-0.27
pine

Shortleaf       0.02-0.07          0.11      0.06-0.20
pine

Loblolly        0.05-0.06          0.18      0.14-0.21
pine

Sweetgum        0.00-0.06          0.06      0.03-0.11

Southern       -0.03-0.00          0.04      0.02-0.08
red oak

Flowering       0.00-0.03          0.10      0.07-0.14

Dogwood

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

ILOP
MAGR           a             b
COFL           a             b
ACRU           a             b
LIST           a             b

Note: Table made from bar graph.
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