Historic vegetation of Camp Bullis and Camp Stanley, southeastern Edwards Plateau, Texas.
Proper planning of ecological restoration requires that the nature of historic landscapes be known to the greatest degree possible. The Edwards Plateau of central Texas has been and continues to be a major focus of study for plant ecologists and range managers (Amos & Gehlbach 1988; Taylor 1997) interested in understanding and managing this region for biodiversity and economic benefits to society. However, little quantitative treatment of the subject of its original condition has been published (Weniger 1984; 1988; Wills 2005). Narrative accounts, though historically valuable as qualitative statements about a particular locality, or (in aggregate) a region, are too often substituted for quantitative analyses of the early composition and structure of regional plant communities.
In 1839, surveyors began delineating tracts of land along Salado Creek in northern Bexar County granted by the Republic of Texas. John Leonard Riddell (Breeden 1994:58-59) described the contemporaneous landscape seen by surveyors of this area in the following manner: "The musquit tree now disappears and is replaced by live oak, post oak etc.... Land sparsely timbered, but no uninterrupted large prairies. Real thickets occur only in the canadas or ravines of water courses." Spaight (1882:25) provides additional detail, stating that the "timber in the northern portion [of Bexar County] is live oak on the hills and high plateaus, post oak on the flats, and elm, walnut, pecan, and hackberry along the streams."
Based on study of historic land surveys, an assessment of the amount of woodland in the southeastern edge of the Edwards Plateau region (Comal County and Kendall County data) occurring during the middle of the nineteenth century found that 47.0% (range 46.1-47.9%) of the landscape was wooded; historic vegetation in the Balcones Canyonlands as a whole was also 47.0% wooded (Weniger 1988). The purpose of this paper is to use site-specific historic land survey data, comprising the tree component of the vegetation, to test the results of Weniger's analysis, and to describe the relationship of wooded versus grass-dominated vegetation to range site types.
MATERIALS AND METHODS
Study area. -- Camp Bullis (subpost of Fort Sam Houston) and Camp Stanley (part of the Red River Arsenal) are contiguous sites located in northern Bexar and western Comal counties approximately 29 km NNW of San Antonio, Texas (29[degrees]41'N, 98[degrees]34'W). These two sites, together formerly known as the Leon Springs Military Reservation, cover approximately 12,870.8 ha within the Balcones Canyonlands subregion of the Edwards Plateau. The topography is hilly, with numerous intermittent streams draining to the east and south. The most prominent of these are Cibolo Creek and Salado Creek. Mean annual precipitation is 74 cm; mean annual temperature is 19.8[degrees]C. Elevation ranges from 306-462 m. Many of the hills exhibit a terraced or "stair step" appearance due to alternating harder and softer strata. Most (74%) of the rock outcropping on Camp Bullis is Upper Glen Rose limestone. Lower Glen Rose limestone is exposed over 14% of the northern portion of Camp Bullis, while 12% (mainly in the southeastern corner of the site) is Kainer limestone of the Edwards group (Anonymous 1990). Soils belong primarily to the Tarrant-Brackett association; these are shallow to very shallow soils over limestone parent material. Specific soils within the study area include Bexar, Brackett, Crawford, Krum, Lewisville, Patrick, Tarrant, Frio, and Venus. These fall into nine range sites: Clay Loam (Lewisville, Venus), Loamy Bottomland (Frio), Low Stony Hill (Tarrant), Redland (Bexar, Crawford), Rocky Upland (Tarrant), Shallow (Patrick), Steep Adobe (Brackett), Steep Rocky (Tarrant), and Valley (Krum) (Anonymous 1990; Taylor et al. 1991).
Recently, the vegetation of Camp Bullis was estimated to be 18.3% grassland, 11.9% savanna, 62.7% woodland (including forest), and 7.1% other (sotol [Dasylirion texanum], yuccas [Yucca spp.], rock outcrop, other) (Hudler 2000). Grassland is more frequent at higher elevations, whereas oak savanna is more common at lower elevations (Hudler 2000). Riskind & Diamond (1986, 1988) described the region as comprising evergreen woodland, deciduous woodland, and floodplain forest (riparian woodland), and these types are all represented at Camp Bullis. American sycamore (Platanus occidentalis) is the principal riparian species, although pecan (Carya illinoensis) is also present.
Data collection and analysis. -- Witness tree data were obtained from field notes of 64 original land surveys (Whitney & DeCant 2001) of the study area conducted during the years 1839-1862. These survey reports are held in the Archives and Records Division, General Land Office of Texas, Austin; a list is available from the author. Data collected included survey number, year, tree species, tree diameter, bearing of tree from survey corner, and distance of tree from survey corner. All survey corners were inside, or within 750 m of, the Camp Bullis/Camp Stanley boundary. Abbreviations for tree species, as used by surveyors, were interpreted as follows: L.O. = Plateau (live) oak (Quercus fusiformis), P.O. = post oak (Quercus stellata), and Sp.O. = Texas (Spanish) oak (Quercus buckleyi). Other species included blackjack [oak] (Quercus marilandica) and [cedar] elm (Ulmus crassifolia). Diameters, recorded in whole inches, were converted to meters. Distances, recorded by surveyors in fractional varas, were converted to meters by multiplying by 0.84667 (Reasonover 1946), and the tree radius (in meters) was added to the converted distance. The Texas General Land Office apparently did not specify how far surveyors should go from a survey corner to record witness (bearing) trees. Maximum tree distance reported for the study area was 85.6 m. Most survey corners with reported trees had two witness trees, but a few indicated one or three. Only those with two or three witness trees could be used in computing mean tree distance at a corner. Corners with a single tree were assigned to a coarser vegetation type (grass-dominated). Corners with no reported trees were scored as open grassland. One hundred and seventy-two corners were included in the analyses. Most surveys having shared corners agreed with respect to presence/absence of trees, tree species, diameters, bearings, and distances at a given corner. In the few cases where any of these data differed, information from the oldest survey was given priority to eliminate changes due to settlement (e.g., tree cutting) and possible field note transcription errors.
Trees per hectare were calculated according to the following formula: 10,000/[d.sup.2] where d is the mean tree distance in meters (Smeins & Slack 1982). Weniger's (1988) distance criteria were adopted to determine if a given survey corner (point) represented savanna (>21 m), woodland (7-21 m), or forest (<7 m). For some analyses, corners were grouped into wooded (woodland and forest) and grass-dominated (savanna, grassland, and single trees) categories. The only differences between the methods used in this paper and those of Weniger (1988) are that data on plant communities are reported by five categories herein instead of two, and that one of these five categories (single trees) was eliminated from consideration by Weniger.
Using ArcView 9.02 GIS, a digital Texas General Land Office original land survey map (RRC 2000, slightly modified sensu Boggs & Giles 1932) was superimposed on a digital NRCS range site map to provide a basis for locating survey corners and their witness trees within range site types on the study area. Some range sites were combined into larger groups if they had similar potential vegetation and/or similar topographic positions (Taylor et al. 1991). The following range sites were combined into range site groups (percent of study area in parentheses): Steep Adobe and Steep Rocky (35.9%), Clay Loam and Shallow (1.6%), and Rocky Upland and Low Stony Hill (27.3%). This process reduced the number of range sites or range site groups from nine to six, including Loamy Bottomland (4.7%), Redland (15.7%) and Valley (14.9%). Chi-square analysis was used to compare the results of this study with those of Weniger (1988). A similar analysis comparing range site groups with respect to wooded and grass-dominated vegetation was performed (Bruning & Kintz 1968). The sample sizes for Clay Loam/Shallow and Loamy Bottomland were too limited; these sites were excluded from the analysis.
Surveyors found five species of trees on the study area, including four oak species and cedar elm (Ulmus crassifolia). Almost 99% of the 294 trees reported were oaks. Riparian trees were absent from the survey notes, as was Ashe juniper (Juniperus ashei), the most abundant species in the contemporary woodland (Van Auken 1988). Post oak (Quercus stellata) was the largest species (mean diameter 38 cm), Plateau oak (Quercus fusiformis) was intermediate in size (mean diameter 27 cm), and blackjack oak (Quercus marilandica), Texas oak (Quercus buckleyi), and cedar elm were the smallest species (mean diameters 20, 18, and 22 cm, respectively). Plateau oak was the dominant tree (ca. 85% of all trees), post oak and blackjack oak (between 5-8% each) were uncommon, and Texas oak and cedar elm (<2% each) were rare (Table 1).
In decreasing order of abundance, woodland, savanna, grassland, and forest plant communities were found to occur in the historic landscape (Table 2). Woodland and savanna were common and together accounted for 77.3% of the total landscape. Grassland was uncommon and appeared to be concentrated in the southern portion of the study area. Forest was rare and located mostly in the western part. Points having a single tree (ca. 5% of the total points) were not assigned to any plant community as no average tree distance could be calculated. Grass-dominated communities (grassland, savanna, and unclassified [single trees]) occurred at 91 points (52.9%), while wooded communities (woodland and forest) occurred at 81 points (47.1%). Comparable values reported by Weniger (1988) were 563 grass-dominated points (53.0%) and 500 wooded points (47.0%). There was no difference between the Weniger model and the data in this study ([X.sub.2] < 0.01, df = 1, n.s.). Grass-dominated and wooded communities were apparently equally abundant in the landscape of the study area during the historic period (Z = 0.76, n.s.).
Range sites and site groups appeared to vary in the extent to which they were wooded. Clay Loam/Shallow (n = 4, 3 wooded), Loamy Bottomland (n = 6, 4 wooded), and Redland (n = 32, 19 wooded) sites had more wooded points than grass-dominated points. Low Stony Hill/Rocky Upland (n = 38, 22 grass-dominated), Steep Adobe/Steep Rocky (n = 63, 36 grass-dominated), and Valley (n = 29, 17 grass-dominated) sites had more grass-dominated points than wooded points. However, differences in these two general categories of vegetative cover among range sites/site groups were insignificant ([X.sup.2] = 3.05, df = 3, n.s.). Much of the grassland (56.5% of all grassland points) was within the Steep Adobe/Steep Rocky site group. Woodland was the predominant community type in Redland (53.1% of its points) and Steep Adobe/Steep Rocky (41.3% of its points). The most common community type in the Valley site was savanna (55.2% of Valley points). Formation of mottes (small stands of trees in a grassy matrix) appears to have been uncommon in savanna. Only four of 59 savanna points (6.8%) had two Plateau oaks with similar distances and bearings from the survey corner stake, suggesting they were part of a tree cluster. These four points were all in the general vicinity of the confluence of Salado and Lewis Valley creeks.
Direct evidence of landscape disturbance is rare in the survey field notes, but there is some pertinent data. By 1860, three Plateau oaks in the vicinity of Salado Creek and the Pinta Trail were reported to have been killed. One 46-cm diameter tree had been reduced to a stump. Two others (diameters 13 and 25 cm) had been destroyed, one of them (13 cm) by fire.
The species composition of historic plant communities on the study area varied from that reported by Weniger (1984) for the Edwards Plateau as a whole. He found that 54% of the trees were oaks, about 7% were elms, 5% were baldcypress (Taxodium distichum), <1% were pecan, and about 33% other (the percentage of Ashe juniper was not given). Of the oak species, he reported that 40% were Plateau (live) oak, 33% were post oak, 9% were blackjack oak, 6% were Texas (Spanish) oak, and 12% other. While the rank order of these oaks was found to be the same in this paper, their relative abundances are quite different. These differences can be attributed partly to dissimilarities in methodology. Many of the surveys used by Weniger had two of their corners on a permanent stream, thus accounting for the presence of riparian species such as baldcypress and pecan. The high relative abundance of Plateau oak and low numbers of post oak, blackjack oak, and Texas oak on the study area is striking. It seems clear that Plateau oak dominated many Edwards Plateau uplands, and the high proportions of other oaks reported by Weniger are likely a reflection both of the larger scope of his study (13 counties) and differences in methods. Most of the surveys used in his study were concentrated along valleys where post oak and blackjack oak were more common (Weniger 1984). Points used in the present study are fairly well distributed between valley and upland areas, and apparently none fall in riparian corridors. On the other hand, the plant community results of this study are in close agreement with Weniger (1988), who found that there were historically about equal proportions of grass-dominated landscapes and wooded landscapes in the Edwards Plateau as a whole and in the Balcones Canyonlands subregion that includes Camp Bullis/Camp Stanley.
The apparent absence of Ashe juniper in the historic landscape has four possible explanations: (1) Surveyors considered Ashe juniper to be a poor witness tree and ignored it, (2) Ashe juniper was completely missing from the region, (3) Ashe juniper was too small to make a good witness tree, or (4) Ashe juniper was fairly rare. The first two reasons can probably be rejected; this species has been recorded in survey notes in other parts of the Edwards Plateau and it was the second most common species on a site in Kerr County (Wills 2005). Whitney & DeCant (2001:155) believe "it is unlikely that [surveyor bias] obscured real differences in the relative abundances of the species." The third alternative appears somewhat more likely (Inglis 1964). However, small junipers are very sensitive to hot fires and would have been consumed quickly unless they occupied refuges, including rocky outcrops and. The fourth alternative, which appears to agree with the narrative description of Riddell (Breeden 1994), might be the best one. At the time of the surveys, this fire-sensitive species apparently had not expanded from those refugia due to recurrent fire at intervals of 13-25 yr (Frost 1998). Thus it covered a relatively small proportion of the landscape. Juniper fence post harvest and charcoal burning became major industries only after 1878 (Toepperwein 1950; Cartwright 1966), and their effect on the abundance of Ashe juniper was probably minimal prior to that time. The geographic proximity of disturbance to a travel corridor and places of early settlement suggests causality. However, considering the brief period of settlement ([less than or equal to]15 yr) and what appear to be fairly low numbers of livestock, it seems improbable that major changes in tree cover had occurred before 1862. Such changes likely began around the early 1880s with the advent of barbed wire fencing (Wills 2005).
Comparison of contemporary vegetative cover with the historic prevalence of plant communities suggests that juniper woodland (including forest) has increased (Van Auken 1993) on the study area over time at the expense of savanna (woodland/forest: 47.1% increasing to 62.7%; savanna: 34.3% decreasing to 11.9%). The true extent of this increase is somewhat speculative due to the different methodologies employed (land survey versus remote sensing). Land use change, primarily decreased fire frequency after 1947 and possibly lower levels of juniper harvest, is the likely cause of juniper population expansion. Climatic change is a possible mechanism for juniper increase (Smeins & Fuhlendorf 1997), but this has not been conclusively demonstrated. The 1950s drought of record caused 90% mortality in some Edwards Plateau populations of mature Ashe juniper and a 56% reduction in total woody canopy cover, including a loss of 30% of scalybark oak (Quercus sinuata) and 54% of Plateau oak on some sites. Junipers <2 m tall survived this bottleneck and might have been able to occupy the space vacated by the dead oaks. However, there was little change in community composition evident at the end of the drought (Young 1956; Merrill & Young 1959). Some features of historic vegetation patterns on the study area have apparently persisted over time, including savanna in the valleys and grassland on steep hills. The amount of grassland (including single tree points) does not appear to have decreased from 1862-2000. Indeed, a modest increase in the grassland percentage during that period is suggested when rosette plant patches are considered part of the grassland category. Expansion of grassland is likely due to clearing for agriculture, for flood control structures, and for military use.
Weniger (1988:22) argued that the "historic [Edwards] Plateau was thus a blend zone [(ecotone)]...." As such, it constituted an inviting region for nineteenth century settlers. There were woods for construction, fuel, posts, and shade, and abundant pasturage for grazing animals. Historical ecology and restoration ecology in the southeastern Edwards Plateau can benefit by taking into consideration that the region was not mainly grassland or savanna during the mid-nineteenth century settlement period, though these communities collectively accounted for [greater than or equal to]50% of the landscape.
I thank David Diamond, Lee Elliott, Robert Lonard, Peter Pagoulatos, and an anonymous reviewer for their timely comments on versions of the manuscript. Steven Gilbert and Justin Spelbrink performed the GIS analysis. The Nature Conservancy of Texas made possible access to digital soils data and staff support. Personnel of the Texas General Land Office Archives & Records Division provided copies of land surveys and Alfred Rodriguez assisted in finding legible duplicates of some of them in the Bexar County Archives.
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Table 1. Witness trees (1839-1862) and their diameters (nearest cm) on the study area. Range Mean Species n (Percent) (cm) (cm) Quercus marilandica 16 (5.4%) 13-30 20 Quercus fusiformis 249*(84.7%) 8-61 27 Quercus stellata 23 (7.8%) 23-71 38 Quercus buckleyi 2 (0.7%) 10-25 18 Ulmus crassifolia 4 (1.4%) 13-30 22 *no diameter available on one tree Table 2. Plant communities at survey corners on the study area, 1839-1862. Distance units are meters. Mean Mean Mean Distance Distance Density Community n Percent (range) (grand mean) (trees/ha) Grassland 23 13.4 -- -- Savanna 59 34.3 21.7-85.6 38.3 6.8 Woodland 74 43.0 7.4-20.5 13.6 54.1 Forest 7 4.1 2.6- 6.9 5.7 307.8 Single Tree 9 5.2 -- -- --
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|Author:||Wills, Frederick H.|
|Publication:||The Texas Journal of Science|
|Date:||Aug 1, 2006|
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