J. Range Manage. 50:507512 Browsing and tree size influences on Ashe juniper understory SAMUEL D. FUEILENDORF, FRED E. SMEINS, AND CHARLES A. TAYLOR Authors at the time of this research were respectively postdoctoral research associate. professor, and associate professor, Department Rangeland Ecology and Management, Texas A&M University, College Station, Tex. 77843-2126. Currently Fuhlendorfis assistant professor, Department of Agronomy, Oklahoma State University, Stillwater, Okla. 74074-6028 and Taylor is professor Texas A&M University, College Station, Tex. 77843-2126. Abstract Mexico (Johnsen and Alexander 1974). On the semi-arid western divide portion of the Edwards Plateau, this species occurs on Ashe juniper (Juniperus ushei, Buckholz) is increasing on most shallow, rocky limestone derived soils in a QuercuslJuniperus sites across the Edwards Plateau of Texas. It is the purpose of Savannah/parkland. The Edwards Plateau is an important sheep this investigation 1) to document the influence of Ashe juniper and goat production region, it provides habitat for several endan- tree size on understory vegetation and 2) to evaluate how the gered plant and animal species, as well as game species, it is a interaction between tree size and browsing by domestic goats and growing recreational and second home area, and its rangelands white-tailed deer modifies overstory/understory relationships. are sources of water for several urban areas (Armstrong 1980, Trees were randomly selected from 2 long-term treatments Kroll 1980, Huston et al. 1994, Taylor and Smeins 1994, Thurow (browsed and unbrowsed) and analyzed with unlvariate analysis and Carlson 1994). The relative proportion of the landscape occu- of covariance and multivariate repeated-measures analysis. pied by Ashe juniper is controlled by fire frequency, intensity, Without browsing, Ashe juniper is more abundant and its indi- and season of occurrence, degree of soil/geologic heterogeneity, vidual influence increases as the size of the tree increases; trees weather/climatic variability, and kind and intensity of herbivory with a canopy diameter < 6.0 m expressed minimal influence on understory vegetation compared to larger trees. When browsers (Smeins and Merrill 1988, Archer 1994, Fuhlendorf et al. 1996). are present at sticient stocking rates to create a browse line on Understory relationships of other juniper species indicate an large trees, encroachment of Ashe juniper is slowed, rate of inverse relationship between juniper cover and herbaceous increase of all woody species is reduced, and huge trees cause a species diversity and production (Jameson 1967, Engle et al. shift in species composition directly under the canopy, however 1987, Armentrout and Pieper 1988, McPherson and Wright 1990, cover of all herbaceous species is not reduced. Immediately Pieper 1990). However, the significance of these community under the canopy of small browsed trees, herbaceous cover is changes for juniper species in western North America have been lower than for unbrowsed trees. Environmental variables questioned (Belsky 1996), which suggests that more data is need- responsible for these patterns were litter depth and light pene- ed to understand the influences of juniper. For example, browsing trating the canopy when the sun is at an angle (during the wln- animals can alter stem density, canopy cover, foliage density, and ter). The increased cover of several herbaceous species under the overall growth habit of juniper plants (Fuhlendorf 1992), which canopy of large browsed trees and at the canopy edge of browsed may modify overstorylunderstory relationships. Age/size rela- and unbrowsed trees, indicates the importance of the interaction tionships have been quantified and are influenced by browsing between canopy cover and the presence of a browse line. Browse animals (Fuhlendorf 1992, Smeins et al. 1994) but there has been lines on Large trees enhance growth and production of cool sea- son species, such as Texas wintergrass (St& leucotricha Trin. & no attempt to determine the understory relationships of different Rupr.) and reduce negative influences (low light, thick leaf litter, size/age trees. It is the purpose of this investigation 1) to docu- etc) on other herbaceous species. At this level of browsing many ment the influence of Ashe juniper tree size on understory vege- other palatable species could be reduced or lost from the plant tation and 2) to evaluate how the interaction between tree size community. and domestic goat and white-tailed deer browsing modifies over- storylunderstory relationships. Key Words: Juniperus ashei, herbivory, goats, Edwards Plateau, Materials and Methods grazing Ashe juniper (Juniperus ashei, Buckholz) is a fire-sensitive, This study was conducted on the Texas A&M University non-sprouting, evergreen small tree or shrub found primarily on Agricultural Research Station which is located on the southwest- the Edwards Plateau of Texas, but with populations in the em edge of the Edwards Plateau about 56 km south of Sonora, Arbuckle Mountains of Oklahoma, the Ozark Mountains of Texas. Elevation of the station is approximately 732 m. Average Missouri and Arkansas. and the Sierra de1 Carmen Mountains of annual precipitation from 1918 to 1994 was approximately 600 mm. The range varied from 156 mm in 195 1, to 1,054 mm in 1937 with an annual median of 438 mm. Rainfall is bimodal, with Manuscript accepted3 Nov. 1996. peaks occurring in the spring and fall. Droughts are common. The JOURNAL OF RANGE MANAGEMENT 50(5), September 1997 507 growing season is about 240 days, with temperatures averaging beyond the canopy to twice the distance of the radius which was 30” C in July and 9” C in January (Station records). also divided by 5 to create a total of 10 sampling locations along Soils are primarily the Tarrant series which are in the thermic each line. For trees with a canopy radius of greater than 2.5 m, family of the Lithic Haplustolls (Thurow et al. 1988). Dominant the distance between locations was limited to 0.5 m to avoid are Tarrant stony clay soils, which are generally 15 to 30 cm missing transitional zones, and hence the total number of sample deep. These soils contain 5 to 70% limestone fragments or lime- locations for these trees exceeded 10 in order to reach twice the stone outcrops. The topography has gentle slopes of 3 to 4%. canopy radius. Vegetation is a savannah/parkland with individuals or clusters At each sample location, a line was established perpendicular to of woody species interspersed within a mid- and shortgrass the radius, and point quadrat measurements were taken at 3 points matrix (Kuchler 1964, Smeins et al. 1976, Smeins and Merrill 5 cm apart on that line, with the first point located directly next to 1988). Dominant woody plants are live oak (Quercus virginiana the line. Variables recorded under each point quadrat were litter var. virginiana Mill.), Vasey shin oak (Quercus pungens var. presence and depth, rock presence, foliar presence of understory vuseyunu Buckholz) and Ashe juniper. Dominant herbaceous species, and height from the ground to the nearest live juniper species include common curly mesquite (Hiluriu belungeri branch recorded in the following classes: 1 = 0 - 50 cm, 2 = 51 - Nash), three-awn (Aristidu purpureu Nutt.), sideoats grama 100 cm, 3 = 101 - 150 cm, 4 = 151 - 200, 5 > 200 cm. Percent (Boutelouu curtipendulu var. curtipendulu), Texas wintergrass light was measured with a quantum sensor (LI-COR , Inc., Model (Stipu leucotricha Trin. & Rupr.) and Texas cupgrass (Eriochloa LI-185B) in the winter (12.00 - l/15) and summer (607 - 7/10) sericeu (Scheele) Munro ex Vasey). Taxonomic nomenclature within an hour of midday. Light measurements were only taken follows Hatch et al. 1990. for open grown trees and at each sampling location. The station was established in 19 16 to study animal diseases. In For analysis, data were pooled from the 10 or more locations 1948 the primary emphasis was shifted to range management on each cardinal direction line to form 3 zones, the inner zone when multiple study units were established to study the influence which contained points nearest the stem and completely under the of stocking rates, kinds and mixtures of animals, and grazing sys- canopy, the mid zone contained locations at the canopy edge with tems on the vegetation. Also in 1948, an attempt was made to about 50% under the canopy of the target juniper and 50% out- remove all junipers on the station by hand cutting. Other woody side the canopy, and the outer zone which started approximately species were not removed, so many small junipers may have 1.5 times the canopy radius away from the target juniper. These escaped detection. Since 1948, no attempt has been made to man- zones varied for each tree, but averages were calculated to estab- age encroachment of woody species in any of the areas used in lish standardized zone limits for each size class (Table 1). this study, except as influenced by herbivores. Two long-term treatments were selected for this study. The Table 1. Ashe juniper canopy diameter size classes and standardized browsed treatment (32 ha) was heavily and continuously browsed zones based upon meters from the stem for each size class. by goats only at 5.4 ha/auy from 1948 to 1969, and then moder- ately grazed by cattle, sheep, and goats (60:20:20) at a stocking Size class Inner zone Mid zone Outer zone rate that has been variable with weather patterns (ca. 9 ha/auy). In (dia.) 1983, the animal ratio was changed to a 50:25:25 mixture of cat- >6m 0 - 2.75 2.16 - 4.15 4.76 - 6.00 tle, sheep, and goats. Cattle and sheep may utilize a significant 3.01 - 6.00 m 0- 1.80 1.81 - 3.00 3.01 - 4.80 portion of browse, but overall stocking rate decreases and the 1.51-3.OOm 0 - 0.70 0.71 - 1.50 1.51-2.00 reduction of the proportion of goats has reduced the browsing 0.76 - 1.50 m 0 - 0.35 0.35 - 0.75 0.75 - I .oo intensity since 1969.