Comparison of cedar glades and associated woodlands of the southern Edwards Plateau.
Cedar glades have been recognized as separate communities for many years (Harper 1926; Freeman 1933; Steyermark 1940; Erickson et al. 1942; Quarterman 1950a; Baskin & Baskin 1975; Nelson & Ladd 1980). They are defined as open grassy or herbaceous areas with shallow soils, little woody vegetation and surrounded by a Juniperus woodland or forest (Harper 1926; Freeman 1933; Quarterman 1950b; Baskin & Baskin 1978). There may be a few scattered small trees or islands of woody vegetation present within the cedar glades (Kucera & Martin 1957).
Cedar glades and glade like communities have been studied extensively in the southeastern and southcentral United States. Several studies in Missouri (Erickson et al. 1942; Kucera & Martin 1957) and Tennessee (Quarterman 1950a) indicated that soil and bedrock formations affected the character of cedar glades. In addition, Freeman (1933) suggested that soil depth, fertility and water content were the main influences on the ecology and flora of Tennessee cedar glades. Baskin & Baskin (1985a) found that temperature was influential in the germination stage of annual plant life cycles in cedar glades of the southeastern United States. The most common species occurring in the cedar glades of Kentucky (Baskin & Baskin 1975; 1978; 1985a) and Tennessee (Harper 1926) were in the Compositae, Leguminosae and Gramineae. Baskin & Baskin (1978; 1986) suggested that cedar glades have been sources of speciation due to the high number of endemics found in these communities. Furthermore, many cedar glade endemics require high light levels and are shade intolerant (Baskin & Baskin 1988).
[FIGURE 1 OMITTED]
Although there are over ten million hectares of Juniperus woodlands in the Edwards Plateau of central Texas (Gould 1969), only the study by Redfern (1980) addressed Edwards Plateau cedar glade vegetation. He compared the floristic composition of the bryophyte communities in Tennessee, southwest Missouri and the Edwards Plateau of Texas and suggested that differences found in the Texas cedar glades maybe due to their lower soil moisture levels.
[FIGURE 2 OMITTED]
There appear to be two types of central Texas cedar glades. Hillside cedar glades are more or less concentric narrow bands of herbaceous vegetation that alternate with bands of Juniperus woodlands. When viewed from the air, these bands form a "bulls-eye" around the hill (Fig. 1) and seem to correspond to a "stair-step" topography (Riskind & Diamond 1988; Woodruff 1993), commonly seen in the Edwards Plateau (Fig. 2). Cedar glades generally occur on the outer edge of the steps while the woodlands occur on the inner edge. This topography is due to different weathering and erosion rates of alternating bands of soft marl and hard limestone and dolomite which compose the Glen Rose Formation, the bedrock material of the area (Woodruff 1993). Kucera & Martin (1957), Hendrickson & Davis (1980) and Gates et al. (1982) also reported the presence of a similar "stair-step" topography and vegetational banding associated with cedar glades in Missouri. The second type of glade found in the Edwards Plateau region maybe described as a microglade. They are usually smaller than the hillside glades and are generally found on more level topography. They are normally circular in appearance and have a solid rock center.
The presence of the cedar glades on the Edwards Plateau has been noted in the past (Steyermark 1934) and although the geology and soils have been described (Marsh & Marsh 1993a; 1993b; Wilding 1993; Woodruff 1993), comparative vegetational studies appear to be lacking. The objective of this study was to describe and compare the phytosociological, physical and chemical characteristics of narrow hillside cedar glades and the surrounding woodlands of the Edwards Plateau in southcentral Texas.
MATERIALS AND METHODS
The study area is located in Bexar County, northwest of San Antonio, Texas on the southern edge of the Edwards Plateau (98[degrees]37'W and 29[degrees]37'N). Area climate is classified as subtropical-subhumid with a mean annual temperature of 20[degrees]C (Arbingast et al. 1976). The highest mean monthly temperatures occur in July and August (28.9[degrees]C) and the lowest in January (10.9[degrees]C). Average annual precipitation is 71.1 cm with peaks in May and September (Gale Research 1992). Cedar glades and their associated woodlands selected for this study were considered representative of those communities in the area. None of the selected cedar glades or their associated woodlands had indications of fire, cutting, or other major disturbances. Site selection was based on cedar glade size (>40 m long and >5 m wide), slope (<15[degrees]) and the lack of open, glade-like areas in the Juniperus woodland uphill from the cedar glade. Sample adequacy was not addressed because the cedar glades and associated woodlands were small enough that the entire community was sampled in many cases.
The quadrat technique (Clements 1905; Van Auken & Bush 1995) was used to collect data from October 1993 through January 1994. Quadrats (5 by 5 m) were placed contiguously along a single transect line in the center of each cedar glade and each associated woodland, perpendicular to the slope. All woody and succulent plants [greater than or equal to]50 cm in height and [greater than or equal to]3 cm basal circumference were identified to species (following Correll & Johnson 1970) and their basal circumference was measured and recorded. Opuntia phaeacantha, Echinocereus caespitosus, Yucca rupicola, Dasylirion texanum and Mammillaria vivipara were considered succulents.
Densities of O. phaeacantha, Y. rupicola and D. texanum were determined but basal areas were not measured because of the growth habit of these species. Basal areas of Acacia roemeriana and Berberis trifoliolata, multistemmed species, were determined by measuring the outer circumference of the stem clump rather than individual stems. The number of quadrats measured in each cedar glade and associated woodland ranged from 8 to 12, depending on the length of the community.
Five quadrats (1 [m.sup.2]) were used to count and identify seedlings <50 cm tall. Five quadrats (0.1 [m.sup.2]) were used to visually estimate percent grass cover and percent herbaceous plant cover. At the time of the study, grasses were past flowering and herbaceous plants were desiccated, thus preventing identification of individuals to species. Percent grass cover (grasses and sedges) included both live and dead plant material. The 1 [m.sup.2] and 0.1 [m.sup.2] quadrats were located in the corners and center of each 25 [m.sup.2] quadrat.
Slopes of the cedar glades and associated woodlands were measured using a Brunton[R] compass. Aspect was measured in each cedar glade and associated woodland with a Sylva[R] compass. Light levels were measured with a LI-COR[R] LI-188 integrating quantum sensor in one cedar glade and associated woodland between 1:00 p.m. and 2:00 p.m. on a cloudless day (19 May 1994). Eighteen measurements were recorded in the cedar glade and seventeen in the associated woodland. Soil depths were measured along each transect using a five mm diameter iron bar driven into the soil every 0.5 m. Soil samples were collected in each cedar glade and each associated woodland at the beginning, middle and end of each transect. The samples were air dried and sieved through a number 10 mesh sieve. A total of 42 samples, three from each cedar glade and three from each associated woodland were sent to Texas A & M University Soil Testing Laboratory for measurement of calcium, magnesium, potassium, sulfur, sodium, phosphorous, nitrogen, salinity and pH. Soil organic carbon was determined using the loss-on-ignition method (Broadbent 1965). Samples were oven dried at 90[degrees]C, cooled, weighted, heated to 600[degrees]C for three hours and reweighed. The loss of weight was considered the amount of organic carbon in the sample.
Phytosociological, physical and chemical differences between cedar glades and associated woodlands were tested using a paired Student's t test (Ott 1993). All analyses were conducted with the SAS computer application system (SAS Institute 1982).
Visually, cedar glades and the surrounding woodlands are physically and biologically different allowing for easy distinction between the two communities (Figs. 1 & 2). A total of 20 species were present in the two communities, 15 woody species and five succulents. Two species were confined to the cedar glades, eight species were confined to the woodlands and 10 were found in both communities (Table 1). Only Opuntia phaeacantha was found in all seven cedar glades, but Juniperus ashei, Diospyros texana, Acacia roemeriana, Quercus fusiformis, Sophora secundiflora and Berberis trifoliolata were found in all seven woodlands (Table 1). Mean total woody plant and succulent density of the cedar glades was 1,146 [+ or -] 60 plants/ha ([bar.x] [+ or -] SE) and was significantly different (t = 5.45, P = 0.002) from the mean total woody plant and succulent density of the woodlands with 10,716 [+ or -] 1,752 plants/ha (Table 2). High density species in the cedar glades were O. phaeacantha, J. ashei, Q. fusiformis, Echinocereus caespitosus and A. roemeriana. High density species in the woodlands were J. ashei, D. texana, S. secundiflora, Q. fusiformis and Rhus virens.
Mean total woody plant and succulent basal area of the cedar glades was 2.59 [+ or -] 0.91 [m.sup.2]/ha and was significantly different (t = 6.62, P = 0.001) from the woodlands at 57.58 [+ or -] 8.73 [m.sup.2]/ha. High basal area species in the cedar glades were J. ashei, R. virens, E. caespitosus and A. roemeriana. High basal area species in the woodlands were J. ashei, Q. fusiformis, B. trifoliolata, D. texana, R. virens and S. secundiflora (Table 2).
Of the eighteen species of seedlings found in the study, one unidentified species was restricted to the cedar glades (Table 3). Quercus stellata, Celtis laevigata, Salvia ballotaeflora, Ulmus crassifolia, Yucca rupicola and Forestiera reticulata were restricted to the woodlands. Eleven species were common to both communities (Table 3) and four species (two unidentified) were detected as seedlings but not adults. Seedling density in the cedar glades was 33 plants/100 [m.sup.2] and in the woodlands it was 941 plants/100 [m.sup.2] (t = 2.21, P = 0.01).
Mean cedar glade grass cover (11 [+ or -] 5%) was over three times higher than the mean woodland grass cover (3 [+ or -] 0.4%), although the two communities were not significantly different (t = 1.5, P = 0.18). The mean herbaceous plant cover was significantly greater in the cedar glades (2 [+ or -] 0.4%) than the woodlands (0.4 [+ or -] 0.2%) (t = 2.65, P = 0.04). Mean light levels were ten times greater in the cedar glades (1990 [+ or -] 9 [micro]mol/[m.sup.2]/s) than in the woodlands (189 [+ or -] 46 [micro]mol/[m.sup.2]/s) (t = 38.57, P = 0.0001).
There was no significant difference (t = 2.20, P = 0.07) between the mean slope angle of the cedar glades (7.7 [+ or -] 1.7[degrees]) and the woodlands (13.0 [+ or -] 1.8[degrees]). Aspect ranged from 113[degrees]-298[degrees] with four locations southwest, two southeast and one northwest. Mean soil depth in the woodlands was 8.1 [+ or -] 1.2 cm and was 2.4 times greater than the mean soil depth in the cedar glades (t = 3.8, P = 0.01; Table 4). Mean percent organic carbon, salinity and magnesium concentrations were all significantly higher (P < 0.01) in the woodlands compared with the cedar glades. Mean pH, sulphur and sodium content were all significantly higher (P < 0.01) in the cedar glades (Table 4). Mean calcium, potassium, phosphorus and nitrogen content were not significantly different (all paired t-tests had P > 0.05) between the two communities (Table 4).
Although never vegetatively described, cedar glades are common biotic features in the Edwards Plateau. They have a low woody species density with individuals of small stature but a relatively high grass cover and other herbaceous plant cover. While herbaceous species were not identified in this study, preliminary results from a study of the microglades suggest many are annual species.
Southeastern and southcentral cedar glades have many characteristics similar to the Edwards Plateau cedar glades, including a limestone bedrock. The most distinctive characteristics of cedar glades in these three areas of the United States are the low density of woody plants, shallow soil and the surrounding Juniperus woodland. Juniperus ashei was one of the most common woody species encountered in this study, while in the woodlands of the southeastern and southcentral United States, Juniperus virginiana was the most common woody species encountered (Harper 1926; Quarterman 1950b; Kucera & Martin 1957). The percent grass cover we observed in the cedar glades was similar to that reported in the cedar glades of the southcentral United States. Quarterman (1950b) indicated high grass and herbaceous cover in the open glades of Tennessee, but minor herbaceous cover in the associated woodlands. In Missouri cedar glades, Kucera & Martin (1957) reported higher grass cover than herbaceous cover. This study revealed a higher grass cover in the cedar glades when compared to the associated woodlands, although the herbaceous cover in this study was lower than expected. The low herbaceous plant cover reported in this study may be due to sampling in the fall and early winter months, when many winter annuals would be rosettes and summer annuals would have completed their life cycles (Baskin & Baskin 1985a).
In addition to the presence of annuals in the Edwards Plateau cedar glades, Nostoc commune and various bryophytes were common but not quantified. Nostoc commune and the bryophytes were especially common in areas with exposed rock or soil less than one cm deep. Quarterman (1950b) found that N. commune formed the basis for vegetation mats in shallow depressions in Tennessee cedar glades. Opuntia spp. were common in many Tennessee and Kentucky cedar glades (Freeman 1933; Quarterman 1950b; Baskin & Baskin 1978); O. phaeacantha was the only Opuntia species found in all of the cedar glades examined in this study.
The Edwards Plateau Juniperus woodlands have higher woody species densities, higher basal areas, lower grass and herbaceous cover and deeper soils than the associated cedar glades. Species of Juniperus, Quercus, Rhus, Bumelia, Cercis and Ulmus were found in the woodlands in this study as well as those in Tennessee (Quarterman 1950b) and Missouri (Kucera & Martin 1957). In measuring successional stages of cedar glades in Tennessee, Quarterman (1950b) found Rhus aromatica and Foresteria ligustrina in the ecotone between the cedar glade and woodland. Field observations in the present study indicated that most of the Rhus virens and Foresteria reticulata plants were located at the woodland edge.
Soil characteristics were also similar between the cedar glades of the southcentral United States and the Edwards Plateau. The higher pH values in the cedar glades compared to the woodlands in this study were similar to the trend found by Kucera & Martin (1957) in Missouri, although their pH values were lower in both communities. The shallowness of the soils in the cedar glades was visually evident in this study and has been reported in previous studies of cedar glades of the southcentral United States (Harper 1926; Freeman 1933, Kucera & Martin 1957; Quarterman 1950b). Quarterman (1950b) and Kucera & Martin (1957) demonstrated that soil moisture in southeastern United States cedar glades was lower than in the soils of the surrounding woodlands. While not measured in this study, Wilding (1993) reported lower soil moisture, lower fertility and higher runoff in the cedar glades of the Edwards Plateau compared to the associated woodlands. Quarterman (1950b) found 28.3% soil organic content in the woodlands surrounding the cedar glades of Tennessee, which was similar to the 26 [+ or -] 5% reported here (Table 3).
The higher organic content and deeper soils of the woodlands are probably due to capture of run-off from the hillside cedar glade above (Wilding 1993). See Fig. 2. Initially, this would deepen the soils of the woodland and allow for establishment and growth of woody vegetation and secondarily, litter deposition from the woodland itself would continue the process.
The low soil moisture and high runoff rates reported by Wilding (1993) in combination with the high light levels at the soil surface recorded in this study, would cause high temperatures and increase evaporation from the shallow cedar glade soils (Freeman 1933). These factors probably promote higher grass and annual plant cover as well as low density of woody seedlings in the cedar glades compared to the associated woodlands.
The cedar glades of the southeastern and southcentral United States appear to be larger and wider than the Edwards Plateau cedar glades and often contain soil filled cracks that support adult trees (Quarterman 1950a; Baskin & Baskin 1985b). The Edwards Plateau cedar glades contained a few small cracks that supported only shrub sized individuals but no adult trees. These cracks maybe due to differences in limestone formations (Quarterman 1950b). Nelson & Ladd (1980) suggested that differences in the size of cedar glades maybe the result of different rates of erosion and runoff of bedrock materials, where dolomite based cedar glades could be as large as 2,000 ha but limestone based cedar glades were narrower and smaller.
The density of woody plants found in the woodlands was much higher than other reported values for the Edwards Plateau (Van Auken et al. 1981; Van Auken 1988). This discrepancy is probably due to differences in methods of data collection (quadrat versus the point-centered-quarter method) and because this study measured only the woodland and not interspersed openings. Basal area measurements were similar to previously reported values (Van Auken 1988).
The shallow soil depth, high light levels and seasonally low soil moisture are probably the most influential factors in determining woody plant density, basal area and community species composition in the Edwards Plateau cedar glades. High light levels and low soil moisture probably prevented woody seedlings from becoming established as well as preventing growth to any appreciable size. Conversely, in the woodlands, the deeper soils, higher soil moisture content, higher organic content and lower light levels due to shading, probably provide a better habitat for establishment and growth of many woody species, but to the detriment of the grasses and other herbaceous plants.
Table 1. Percent frequency of species in seven cedar glades and seven woodlands in northern Bexar County, Texas. A bar (-) indicates species was not found in that community type. Taxon Cedar Glades Woodlands Opuntia phaeacantha 100% 57% Juniperus ashei 86% 100% Diospryros texana 71% 100% Acacia roemeriana 57% 100% Rhus virens 57% 71% Echinocerous caespitosus 43% - Quercus fusiformis 43% 100% Sophora secundiflora 43% 100% Yucca ripicola 43% 43% Dasylirion texanum 29% 14% Eysenhardtia texana 29% 14% Mammillaria vivipara 14% - Berberis trifoliolata - 100% Bumelia lanuginosa - 57% Foresteria reticulata - 29% Celtis laevigata - 29% Quercus stellata - 29% Ulmus crassifolia - 14% Cercis canadensis - 14% Ungnadia speciosa - 14% Table 2. Mean density (plants/ha) and mean basal area ([m.sup.2]/ha) for woody and succulent species, >50 cm tall, found in cedar glades (GL) and woodlands (WD) in northern Bexar County, Texas. A bar (-) indicates species was not present in that community. A (**) indicates species presence in those communities but basal area was not included in the table because of the species growth habit. Taxon Mean Density Mean Basal Area GL WD GL WD Opuntia phaeacantha 300 54 ** ** Juniperus ashei 157 2,363 0.76 22.56 Quercus fusiformis 137 1,167 0.03 11.18 Echinocerous caespitosus 102 - 0.46 - Acacia roemeriana 95 381 0.41 0.10 Sophora secundiflora 81 1,904 0.12 3.70 Rhus virens 73 1,007 0.54 4.25 Yucca ripucola 71 57 ** ** Dasylirion texanum 70 46 ** ** Diospryros texana 50 1,971 0.12 4.36 Berberis trifoliolata - 930 - 11.01 Quercus stellata - 694 - 0.34 Bumelia lanuginosa - 63 - 0.06 Other* 10 79 0.15 0.02 Total 1,146 10,716 2.59 57.58 * Species with densities less than 50 plants/ha and basal areas less than 0.10 [m.sup.2]/ha, including Eysenhardtia texana and Mammillaria vivipara found in the cedar glades and Cercis canadensis, Ungnadia speciosa, Celtis laevigata, Forestiera reticulata, Eysenhardtia texana and Ulmus crassifolia found in the woodlands. Table 3. Mean density (plants/100 [m.sup.2]) of woody and succulent seedlings (<50 cm tall), for cedar glade and woodland communities in Northern Bexar County, Texas. A (+) indicates a density less than one plant/100 [m.sup.2] and a bar (-) indicates that species was not present in that community. Taxon Cedar Glades Woodlands Juniperus ashei 15 311 Quercus fusiformis 7 369 Opuntia phaeacantha 3 3 Acacia roemeriana 2 17 Berberis trifoliolata 1 70 Bumelia lanuginosa 1 3 Diospryros texana 1 3 Rhus virens 1 15 Sophora secundiflora 1 107 Eysenhardtia texana + 3 Quercus stellata - 15 Celtis laevigata - 12 Salvia ballotaeflora - 1 Ulmus crassifolia - 1 Yucca ripucola - 1 Other* 1 10 Total 31 941 * Foresteria reticulata, isolated in the woodlands and two unidentified species, one isolated in the cedar glades and one found in both communities. Table 4. Comparison of cedar glade and woodland soils in the southern part of the Edwards Plateau. Values are [bar.x] [+ or -] 1 SE. An * indicates a significant difference (Student's paired t-test P [less than or equal to] 0.01). Parameter Cedar Glades Woodlands Salinity (mmol/ 0.52 [+ or -] 0.03 0.75 [+ or -] 0.06* [cm.sup.3]) Organic Content (%) 14 [+ or -] 1 26 [+ or -] 2* pH 8.0 [+ or -] 0.0 7.6 [+ or -] 0.1* Soil Depth (cm) 3.4 [+ or -] 0.7 8.1 [+ or -] 1.2* Calcium (mg/kg) 26,561 [+ or -] 60 24,105 [+ or -] 1,677 Sulphur (mg/kg) 617 [+ or -] 107 326 [+ or -] 58* Magnesium (mg/kg) 279 [+ or -] 37 379 [+ or -] 37* Potassium (mg/kg) 230 [+ or -] 16 349 [+ or -] 65 Sodium (mg/kg) 30 [+ or -] 3 22 [+ or -] 2* Phosphorus (mg/kg) 13 [+ or -] 2 17 [+ or -] 2 Nitrogen (mg/kg) 8 [+ or -] 2 15 [+ or -] 2
We thank Dr. Loyce Collenback and Mr. William G. Collenback for financial support during the project. We also thank E. M. Gese for helpful suggestions and manuscript review and J. K. Bush and J. M. Baskin for helpful suggestions.
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P. A. Terletzky and O. W. Van Auken
Division of Life Sciences, The University of Texas at San Antonio
San Antonio, Texas 78249
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|Author:||Terletzky, P.A.; Van Auken, O.W.|
|Publication:||The Texas Journal of Science|
|Date:||Feb 1, 1996|
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