AN ECOLOGICAL COMPARISON OF UPLAND DECIDUOUS AND EVERGREEN FORESTS OF CENTRAL TEXAS

Scrub evergreen forests cover most of the cedar breaks region of the Edwards Plateau of central Texas; however, limited strips of deciduous forests are found on north-facing slopes just below the limestone caprock. Dominant species occurring in the deciduous forests in descending order of importance include Quercus texana, Q. glaucoides, Juniperus ashei, Diospyros texana, Prunus serotina, Aesculus pavia, Juglans nigra, and Fraxinus texensis. Nineteen woody species were found in the deciduous forests. Forty-eight percent of the species were exclusive to the deciduous forest community. The deciduous forests had a statistically greater number of species/stand and a higher density. The deciduous species found in the strip forests had combined relative density, dominance and importance values of 59, 93 and 72%, respectively. Dominants in the evergreen forest were J. ashei and D. texana, which had relative density, dominance, and importance values totaling 90, 97 and 93%, respectively. Total dominance was the same for both communities; however, J. ashei dominance was 20 times higher in the evergreen forests. Soils of the deciduous forests were significantly deeper and rockier. Also, these soils had higher organic contents and water retention capacities. The potential field capacity (PFC) was 3.47 times higher for the deciduous forest soils resulting in a 2.36-fold increase in the water availability index (WAI). Factors regulating distribution and composition of the deciduous forest communities appear to be insolation and PFC, resulting in part from differences in topographic position. THE EDWARDS PLATEAU of central Texas (Fig. 1) occupies an area of approximately 93,000 kM2, yet the biogeography and community ecology of that area are relatively unexamined. A number of general floristic studies have been completed in the Edwards Plateau, including early habitat descriptions of Hill and Vaughan (1898), Bray (1904a, b; 1906) and Tharp (1939), as well as the more recent species lists, descriptions and keys of Gould (1969) and Correll and Johnston (1970). Palmer (1920) noted the distinctive species composition of the plant. communities that occur in the upper canyons of this region and speculated on their cause, development, and maintenance. From this region, however, only three quantitative phytological studies have been identified. Buechner (1944) investigated the range management of the central Edwards Plateau. Van Auken, Ford and Stein (1979b) compared the plant composition of some upland forests and strip creek-bottom riparian communities of the southern Edwards Plateau; and Van Auken et 1 Received for publication 18 August 1980; revision accepted 6 December 1980. The authors wish to thank S. Daugherty, J. T. Rutledge, P. A. Schwarz, J. C. Smajdek and M. W. Stone for help with some of the fieldwork. Special thanks are due to R. Gazaway, H. B. Lively and T. Schneider for permission to study plant communities occurring on their ranches in central Texas. al. (1980) reported on similarities between upland forest communities occurring on the Edwards and Glen Rose Limestones. The Edwards Plateau has been treated as a separate physiographic unit, since it possesses a distinct flora including a number of endemics (Blair, 1950; Correll and Johnston, 1970). The western limits of many eastern species lie along the floodplains of the rivers of the Edwards Plateau including bald cypress (Taxodium distichum), sycamore (Platanus occidentalis), pecan (Carya illinoinensis), and others (Palmer, 1920; Blair, 1950). Southwestern species such as Juniperus ashei, Diospyros texana and Quercusfusiformis may be found along the dry ridges in the Edwards Plateau, but usually not much farther north or east. The upland Plateau flora shows some relationship to the flora of northern Mexico (Correll and Johnston, 1970). The cedar brakes region of the Edwards Plateau consists of deeply eroded limestones of the Glen Rose and Edwards Formations (Sellards, Adkins and Plummer, 1932). This region occurs along the southern and eastern third of the Plateau. Climatographically, the area is dry, subhumid to semiarid, and mesothermal (Thornthwaite, 1948). The approximate mean annual temperature for the area is 20 C with about 72 cm/yr rainfall (Arbingast et al., 1976). Area soils are usually shallow and underlain by limestone or caliche (Gould, 1969). Whit-