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Woody vegetation of the dry, sandy uplands of eastern Texas.

ABSTRACT. -- The plot method was used to obtain information on the woody vegetation of dry, sandy upland forest communities in eastern Texas. Based on importance values, Quercus stellata, Carya texana, and Quercus marilandica were the overall dominant species, contributing 75 percent of the total basal area. In the Post Oak Savannah Vegetational Area, Q. stellata is the overwhelming dominant, whereas the numerical importance of dominant species is more equitable in the Pineywoods Vegetational Area, where species diversity and basal area were highest. Key words: dry uplands; woody vegetation; species diversity; eastern Texas.


The forested region of eastern Texas represents the southwestern terminus of the Deciduous Forest Formation (Braun, 1950) of eastern North America. Within this forested region, different combinations of environmental factors produce different combinations of plants. These varying assemblages were broadly classified by Nixon (1985). One such assemblage needing study involves the communities associated with dry, sandy upland sites. The purpose of our study, therefore, was to describe the composition and structure of dry, sandy upland communities in eastern Texas and to determine geographical floristic variation.


Sandy, dry upland sites in eastern Texas occur on rounded to flat-topped hills. Forests occupying these sites are relatively open. Shrub density is usually low and there is a sparse herbaceous layer. The forest floor is characterized by a thin to moderate litter layer to much exposed sand. This investigation includes study sites in both the Pineywoods and Post Oak Savannah Vegetational areas of Texas (Gould, 1975). The latter lies immediately west of the Pineywoods (Fig. 1).

The study area is gently rolling to hilly country, with elevations ranging from 60 to 250 meters. The area has a rather mild, humid climate with persistent southerly and southeastwardly winds (Carr, 1967). Annual rainfall averages from 88 to more than 125 centimeters in the Pineywoods and from 88 to 113 centimeters in the Post Oak Savannah. Rainfall is farily uniformly distributed throughout the year in the Pineywoods; either May or June is usually the high rainfall month in the Post Oak Savannah. Average annual mean free-air temperature ranges from 21[degrees] C in the south of the study area to 18[degrees] C in the northernmost part.


To adequately describe the woody plant communities, emphasis was placed on a wide geographical distribution of vegetation samples (Fig. 1). Communities selected were as mature and free from disturbance as possible. The woody vegetation of each site was analyzed by the plot method consisting of 50 contiguous plots that were five meters square. These plots were situated in two belt transects and represented a total sample area of 0.125 hectare. The names and diameters of all woody species with diameters at about 1.4 meters above ground of half a centimeter or greater were recorded for each plot. From these data, frequency, density, and basal area were computed. An importance value for each species was calculated as the sum of relative frequency, relative density, and relative basal area. The importance values in turn were used to calculate community similarity coefficients (Cox, 1980) using the equation C = 2w / a + b, where w = the sum of the lower of the two quantitative values (importance values) for species shared by two communities, a = the sum of all values for the first community, and b = the sum of all values for the second community. A polar community ordination was established following techniques set forth by Cox (1980). Species diversity also was computed employing the Shannon-Wiener Diversity Index (Shannon and Weaver, 1949). The general formula for this index is H' = - [SIGMA] [p.sub.i] [log.sub.2] [p.sub.i], where [p.sub.i] = the proportion of all individuals in the sample that belong to species i.


Surface soil samples (0-15 centimeters) were collected at each study site. Soil samples later were analyzed for pH, Ca, P, K, and Mg at the Stephen F. Austin State University soil testing laboratory. Particle size was determined using the hydrometer method (Bouyoucos, 1962). Scientific nomenclature followed that of Nixon (1985).

A total of 20 communities was analyzed in 15 counties (Fig. 1). Seven study sites were located in the Post Oak Savannah and 13 study sites were located in the Pineywoods.


Surface Soils

Percent sand ranged from 73 to 92 percent for 19 of the 20 communities, with the majority of sites containing 80 to 85 percent. Surface soils of the study sites were sands (two sites), sandy loams (six sites) or loamy sands (11 sites). The soil of community 15 in Morris County contained only 33 percent sand and was classified as clay loam. The soils of the 20 communities are acid, with a pH range of 4.6 to 6.2 (mean). Phosphorus ranged from a trace to seven parts per million with a mean of 2.0, potassium ranged from 20 to 80 parts per million with a mean of 41.0, calcium ranged from 100 to 1200 parts per million with a mean of 485.0, and magnesium ranged from 10 to 130 parts per million with a mean of 66.0.


A polar ordination for the 20 communities is shown in Figure 2. Although the ordination generally shows a continuum, the communities of the Pineywoods generally occur in the upper part of the figure and those of the Post Oak Savannah in the lower part. Communities 16 and 18 of the Pineywoods, where Quercus velutina attained its greatest importance, plotted out together in the uppermost portion of the ordination. Three communities (9, 11, 20) of the Pineywoods were somewhat separate due to the importance of Quercus incana.

Community Dominants

Composition tables developed from each of the 20 vegetation samples were examined and the top one, two, or three species, depending upon importance values, were used to distinguish types of communities present in the dry uplands. Using this arbitrary procedure, 11 types of communities based on 10 community dominants were recognized (Table 1).

Dominants consisted primarily of species of the overstory, exceptions being Ilex vomitoria, Cornus florida, Ulmus alata, and Sassafras albidum. The latter two species are potentially medium-sized trees but, with the exception of one or two trees of S. albidum, were representatives of the understory. The four understory species are among the community dominants primarily because of high frequency and density.

With the exception of the Quercus stellata, Quercus stellata-Carya texana-Sassafras albidum and Quercus stellata-Quercus marilandica communities, the types of communities present in the Pineywoods did not occur in the Post Oak Savannah. Study sites with Q. velutina, C. florida, and Pinus palustris as community codominants were confined to the Pineywoods, whereas sites with I. vomitoria and U. alata as community codominants occurred only in the Post Oak Savannah.



The dry upland forests in eastern Texas are basically two-layered. Q. stellata is the overwhelming overstory dominant in the Post Oak Savannah, whereas the numerical importance of overstory species within the Pineywoods is more equitable (Table 2). Further comparisons indicate that Q. marilandica was nearly proportionate for the two vegetational areas, whereas C. texana and Q. incana had greater values in the Pineywoods. Q. velutina, Pinus echinata, and P. palustris were not recorded from the Post Oak Savannah and Pinus taeda was insignificant. Comparisons within the Pineywoods reveal a significantly greater importance of Q. incana and P. taeda in the southern part and an absence of Q. velutina there. Conversely, a greater importance of Q. stellata, C. texana, and P. echinata occurs in the northern part of the area. P. palustris was absent from the northern part. Only rarely in the dry upland sites were Liquidambar styraciflua, Morus rubra, Bumelia lanuginosa, and S. albidum found in the overstory.

Importance values of the major understory species are given in Table 3. In the Post Oak Savannah, I. vomitoria and U. alata had the highest importance values. Other characteristic understory species in the Post Oak Savannah were Vaccinium arboreum, C. florida, and Vitis aestivalis.

Important understory species of the Pineywoods are generally the same as those of the Post Oak Savannah excluding I. vomitoria and U. alata (Table 3). S. albidum and C. florida have the highest importance values. V. arboreum, Callicarpa americana, and V. aestivalis were also important in the Pineywoods understory.

Community Composition--Eastern Texas Forest Region

For the Eastern Texas Forest Region as a whole, Q. stellata was the most important woody species of the dry uplands (Table 4). Q. stellata had the highest frequency (60 percent) and density (580 plants per hectare) values and accounted for nearly half (48.1 percent) of the total basal area of the 20 communities. The top three dominants, Q. stellata, C. texana, and Q. marilandica contributed 75 percent of the total basal area. Other major species of the overstory were Q. incana and Q. velutina. Characteristic shrub and small tree species were I. vomitoria, S. albidum, U. alata, C. florida, V. aboreum, C. americana, V. rufidulum and B. lanuginosa. I. vomitoria and C. florida were among the dominant species (Table 4). The most important vines were V. aestivalis, and V. rotundifolia. The top 18 species (Table 4) best characterize dry upland sites in eastern Texas. These species combined represent 90 percent of the relative frequency, 94 percent of the importance value, and 98 percent of the relative basal area.

Basal area for the Eastern Texas Forest Region averaged 31.7 square meters per hectare. The Post Oak Savannah had 32.5, the Pineywoods 31.3. Density averaged 3012 plants per hectare for the region as a whole, 3736 in the Post Oak Savannah, and 2632 in the Pineywoods.

Species Richness and Diversity

A total of 54 woody taxa was encountered in this investigation. Of this number, 26 were trees, 16 were shrubs or small trees, and 12 were woody vines.

Comparison of tree species in the Pineywoods with those in the Post Oak Savannah reveals little difference in species richness (Table 5). However, variation in species richness becomes apparent within the Pineywoods. The species richness of the southern part (communities 9, 10, 11, 12) is less than half that of the northern part. This apportionment of species applies to tree, shrub and small tree, and vine categories, as well as totals for the two forest areas.

Species diversity for all woody species was highest (H' = 2.88) in the Pineywoods (Table 5). The average Shannon-Wiener index for the Eastern Texas Forest Region was 2.69 (Table 5). Species diversity also was determined separately for tree species and for small tree and shrub species in each vegetation area (Table 5). Average values for both layers were highest in the Pineywoods and lowest in the Post Oak Savannah.


Community Comparisons

Dry upland communities in eastern Texas are similar to communities recognized on approximate sites throughout much of the Deciduous Forest Formation. As a whole, dry uplands of the region compare best to Braun's (1950) Oak-Hickory Forest Region. Dominants of this region are Carya texana, Quercus stellata, and Q. marilandica. These were dominant species in eastern Texas, comprising 75 percent of the relative basal area.

More specifically, Adams and Anderson (1980) found Q. marilandica, Q. velutina, and C. texana to be important species on xeric sites in central Illinois, which is in the Northern Division of the Oak-Hickory Forest Region (Braun, 1950). Quercus velutina was the leading dominant in two communities in eastern Texas.

The Southern Division of the Oak-Hickory Forest Region (Braun, 1950) shows even greater affinity to eastern Texas with regard to dry upland communities. There, Q. stellata, Q. marilandica, Q. velutina, and Carya texana are dominants. These species are also important in the Shawnee Hills region of southern Illinois (Fralish, 1976), the Missouri Ozarks (Erenreich and Crosby, 1960), northwestern Arkansas (Turner, 1935), and in the central and eastern parts of Oklahoma (Rice and Penfound, 1959; Penfound, 1963). In the Chautauqua Hills of southeastern Kansas, Q. stellata, Q. marilandica, and Q. velutina dominate rocky ridges and exposed hilltops, with Carya sp. reported as accessory species (Hale, 1955).

Elsewhere, Q. stellata and Q. marilandica are reported among the community dominants on dry sites from eastern Texas to New Jersey. Quercus velutina is a codominant with Q. stellata and Q. marilandica on the Coastal Plain of North Carolina (Sechrest and Cooper, 1970) and in the Monongahela National Forest in West Virginia (Clarkson, 1966). Pinus echinata, an associate species in the forests of eastern Texas, shares dominance with Q. stellata and Q. marilandica on the Alabama Piedmont (Golden, 1979) and in the New Jersey Pine Barrens (Buell and Cantlon, 1950). Quercus stellata-Q. marilandica communities also occur west of the Deciduous Forest Formation in the Wichita Mountains in southwestern Oklahoma (Buck, 1964) and as far southwest in Texas as Medina County (McBryde, 1933). In addition to the Pineywoods and Post Oak Savannah, the type occurs on the Edwards Plateau (Whisenant, 1982) and in the Cross Timbers (Dyksterhuis, 1948; Kroh and Nisbet, 1983) vegetational areas of Texas (Gould, 1975).

Dry upland vegetation of the southern portion of the Pineywoods of our study correlates closely with other southeastern Texas communities. The sandhill pine forests characterized by Marks and Harcombe (1981) along Village Creek in southeastern Texas were almost identical in description to the sandhill communities reported by us. Wilkinson (1982) analyzed comparable communities on deep, xeric sands in the upper Big Thicket.

Basal Area

Total basal area values have been reported in the literature for various forest systems. Held and Winstead (1975) reported a range of 25.8 square meters per hectare to 32.2 square meters per hectare total basal area for trees over 10 centimeters at breast height in mature deciduous forests. Rice and Penfound (1959) indicated that upland forest stands rarely exceed a total basal area of 25 square meters per hectare. The average tree basal area for dry uplands in eastern Texas was 30.7 square meters per hectare. The highest basal area (26.4 square meters per hectare) in Oklahoma upland forests occurred in the relatively mesic northeastern section of the state (Rice and Penfound, 1959). Buck (1964), however, indicated that basal areas ranged from 9.1 to 28.0 square meters per hectare for Quercus stellata-Q. marilandica communities in the Wichita Mountains of south-central Oklahoma. In north-central Texas, mean basal area of all species combined in a Quercus stellata-Q. marilandica forest was 24.6 square meters per hectare (Kroh and Nisbet, 1983).

Species Diversity

Monk (1967) compared Shannon-Wiener diversity indices for trees in the eastern deciduous forest. He concluded that no other association of trees is as diverse as those of the Mixed Mesophytic Forest Region (Mean [H.sup.1] = 2.73). His results showed a decrease in community diversity with increased distance from this region. The Oak-Pine Forest Region to the west has a diversity index of 2.51, whereas the Oak-Hickory Region located even farther westward has one of 2.02. Our diversity indices for communities in these two regions were generally higher (2.48 and 2.96, respectively). They are also higher than comparable communities in Oklahoma (Risser and Rice, 1971), the northern Ozark Highlands of Missouri (Zimmerman and Wagner, 1979), and central Illinois (Adams and Anderson, 1980). A likely reason for our higher values is that we included trees with a diameter breast high equal to, or greater than, one-half centimeter. Methods used in other studies generally included trees starting at about 10 centimeters in diameter at breast height.

Species diversity values for woody species in the southern part of eastern Texas (H' = 2.59), correlate quite closely with similar sites in southeastern Texas investigated by Ward and Hupp (1981) (H' = 2.23) and Wilkinson (1982) (H' = 2.7).

Species Richness

Species richness of southern Coastal Plain forests was compared with that in other forests by Marks and Harcombe (1975). Trees were defined as those species that are normally capable of reaching the canopy. All other woody species, including subcanopy species were classified as shrubs. They found an average of seven tree species and 14 shrub species for southeastern Texas upland communities. In the present study, communities of the southern Pineywoods contained an average of 7.5 tree species and an average of only 2.0 shrub and small tree species. Golden (1979), using the same arbitrary procedure to define trees and shrubs as Marks and Harcombe (1975), provided mean numbers of species of trees and shrubs for Quercus marilandica-Pinus sp. communities in the lower Piedmont of Alabama. The dry upland forests of eastern Texas had a lower average number of tree (10.2) and shrub (4.3) species than the Alabama Piedmont (trees, 12.0; shrubs, 7.0).
TABLE 1. Types of communities present and their locations in the East
Texas dry uplands based on importance values from composition tables.

 Post Oak
Community Savannah Pineywoods Total

Quercus stellata 1 2 3
Quercus stellata-Carya texana 1 1
Quercus stellata-Carya texana-
 Quercus marilandica 2 2
Quercus stellata-Carya texana-
 Quercus incana 3 3
Quercus stellata-Carya texana-
 Sassafras albidum 1 1 2
Quercus stellata-Quercus marilandica 2 1 3
Quercus stellata-Ilex vomitoria 2 2
Quercus stellata-Ulmus alata 1 1
Carya texana-Quercus marilandica-
 Quercus velutina 1 1
Carya texana-Quercus velutina-
 Cornus florida 1 1
Quercus incana-Pinus palustris 1 1
Total 7 13 20

TABLE 2. Average importance values for major species of the overstory of
eastern Texas dry uplands. Species are listed in order of importance for
the Eastern Texas Forest Region.

 Eastern Texas
 Post Oak Forest
Species Savannah Pineywoods Region

Quercus stellata 114.1 69.3 85.3
Carya texana 14.9 50.5 36.7
Quercus marilandica 27.5 32.1 30.3
Quercus incana 4.2 30.5 20.5
Quercus velutina 17.0 10.7
Pinus echinata 10.9 6.8
Pinus taeda 0.3 5.3 3.4
Pinus palustris 3.4 2.2

TABLE 3. Average importance values for major species of the understory
of eastern Texas dry uplands. Species are listed in order of importance
for the Eastern Texas Forest Region.

 Eastern Texas
 Post Oak Forest
Species Savannah Pineywoods Region

Ilex vomitoria 51.1 1.2 21.3
Sassafras albidum 7.3 20.5 15.3
Ulmus alata 24.4 0.5 10.1
Cornus florida 7.1 10.9 9.3
Vitis aestivalis 7.7 9.5 8.8
Vaccinium arboreum 6.2 5.9 6.0
Callicarpa americana 3.8 7.1 5.8
Viburnum rufidulum 1.6 4.9 3.6
Bumelia lanuginosa 3.8 2.1 2.7
Rhus copallina 0.4 2.6 1.8
Juniperus virginiana 2.3 1.1 1.5
Crataegus crus-galli 3.7 1.4

TABLE 4. Combined frequency, density, basal area, and importance value
data for woody species in the Eastern Texas Forest Region.

 Relative Relative
 Frequency frequency Density density
Species (%) (%) plants/hectare (%)

Quercus stellata 60.2 18.08 580 19.13
Carya texana 45.5 13.67 404 13.39
Quercus marilandica 25.4 7.63 148 4.90
Ilex vomitoria 16.2 4.87 444 14.73
Quercus incana 21.5 6.46 184 6.15
Sassafras albidum 23.8 7.15 204 6.76
Quercus velutina 10.7 3.21 68 2.25
Ulmus alata 8.9 2.67 208 6.85
Cornus florida 15.1 4.54 124 4.07
Vitis aestivalis 17.8 5.35 96 3.23
Others (2) 26.34 552 18.55
Total 99.97 3012 100.01

 Basal Relative
 area basal area Importance (1)
Species (m2) (%) value

Quercus stellata 38.14 48.12 85.33
Carya texana 7.66 9.66 36.72
Quercus marilandica 14.12 17.81 30.34
Ilex vomitoria 1.34 1.69 21.29
Quercus incana 6.23 7.86 20.47
Sassafras albidum 1.09 1.37 15.28
Quercus velutina 4.14 5.22 10.68
Ulmus alata .43 .54 10.06
Cornus florida .52 .66 9.27
Vitis aestivalis .14 .18 8.76
Others (2) 5.45 6.87 51.76
Total 79.26 99.98 299.96

(1) Sum of relative frequency, relative density and relative basal area.
(2) Other species present listed in order of decreasing importance value
were: Pinus echinata, Vaccinium arboreum, Callicarpa americana, Viburnum
rufidulum, Pinus taeda, Vitis rotundifolia, Bumelia lanuginosa, Pinus
palustris, Liguidambar styraciflua, Rhus copallina, Juniperus
virginiana, Crataegus crus-galli, Crataegus marshallii, Rhus
toxicodendron, Quercus falcata, Pinus elliottii, Carya tomentosa, Morus
rubra, Smilax rotundifolia, Fraxinus americana, Zanthoxylum
clava-herculis, Berchemia scandens. Castanea alnifolia, Diospyros
virginiana, Prunus serotina, Crataequs spathulata, Chionanthus
virginica, Ilex decidua, Parthenocissus quinquefolia, Acer saccharum,
Ilex opaca, Cercis canadensis, Forestiera ligustrina, Quercus nigra,
Ampelopsis arborea, Bignonia capreolata, Smilax smallii, Celtis
laevigata, Prunus umbellata, Acer rubrum, Smilax bonanox, Vitis
mustangesis, Smilax glauca, Asimina parviflora.

TABLE 5. A comparison of species richness and species diversity for all
woody species of each vegetation area. Species richness refers to number
of species.

 Eastern Texas
 Post Oak Forest
 Savannah Pineywoods Region

Species richness
Trees 19 23 26
Shrubs and small trees 12 13 16
Vines 9 7 12
Totals 40 43 54

Species diversity (H')
Trees 2.48 2.96 3.10
Shrubs and small trees 1.44 2.51 2.32
Combined--trees and shrubs 2.32 2.88 2.69


We wish to thank Jenny K. Sullivan for preparation of the map of eastern Texas.


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Department of Biology, Stephen F. Austin State University, Nacogdoches, Texas 75962-3003
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Author:Ward, John R.; Nixon, Elray S.
Publication:The Texas Journal of Science
Geographic Code:1U7TX
Date:Aug 1, 1992
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