Rob Mitchell, Carlton Britton, David Wester, and Susan Mullins
Introduction
Redberry juniper (Juniperus pinchotii Sudw.) is a noxious brush species that occupies over 11 million acres of Texas rangeland (Soil Conservation Service 1985). Redberry juniper is an evergreen, multi-stemmed basal sprouter which historically occurred on northwest exposures of rocky, shallow slopes in limestone and gypsum soils (Correll and Johnston 1970). Due to the reduction of natural fires, redberry juniper has expanded onto adjacent slopes and lowland areas, and has become a major problem on many range sites in the Texas Rolling Plains (Steuter and Britton 1983).
Redberry juniper is common in southwestern Oklahoma, western Texas, southeastern New Mexico, southern Arizona, and northeastern Mexico (Ueckert et al. 1994). Redberry juniper is considered an invader on most Texas range sites and has little economic value. However, redberry juniper is desirable on some range sites because it stabilizes soil and provides food and cover for wildlife (Scifres 1980).
This paper is a synopsis of two research projects currently being conducted at Texas Tech University. The first project deals with the seasonal foliage moisture dynamics of redberry juniper. The second project deals with redberry juniper establishment on rangeland. Although the results presented are preliminary, they will likely be representative of what will be observed throughout the conclusion of these studies.
SEASONAL FOLIAGE MOISTURE DYNAMICS
Introduction
Fire was an important factor in the development of grassland ecosystems. Recurrent fires suppressed woody vegetation and maintained the character of grassland ecosystems (Sauer 1950). In its original habitat, redberry juniper was historically protected from these fires by the lack of fine fuel and the topography of the steep, rocky slopes. The suppression of fire in the Texas Rolling Plains that came about with settlement promoted the encroachment of redberry juniper from the steep, rocky slopes into adjacent rangeland.
Prescribed burning is an important tool for managing junipers in grassland ecosystems, and has been used to manage redberry juniper. However, due to the basal sprouting characteristics of redberry juniper, results have been variable. An important characteristic in the response of redberry juniper to fire is the position of the bud zone. Redberry junipers with bud zones elevated above the soil surface had 70% mortality following fire (Steuter and Britton 1983). Conversely, redberry junipers that had bud zones partially below the soil surface had only 3% mortality. To maximize redberry juniper mortality with fire, the foliage must be ignited and a crown fire generated.
An important variable in the successful ignition of junipers with fire is foliage moisture content. Junipers are readily ignited by fire when foliage moisture content falls below 70%. However, the seasonal changes in redberry juniper foliage moisture are not well understood. The objectives of this study were to determine the seasonal changes in redberry juniper foliage moisture content, and determine the relationship between foliage moisture content and soil moisture content on three range sites in the Texas Rolling Plains.
Materials and Methods
This study was conducted at the Texas Tech Experimental Ranch in Garza County near Justiceburg, Texas. Five mature redberry junipers on each of three range sites (sandy bottomland, clay flat, and shallow clay) were sampled at approximately 14-day intervals from September 1995 through September 1996. Redberry juniper foliage was hand-stripped from 1 to 4 feet above the soil surface from the terminal 4 inches of branches around the perimeter of each tree to determine foliage moisture content on a dry weight basis. Tree height ranged from 5 to 12 feet. Soil samples were taken beyond the drip-line around the perimeter of each tree to a depth of 12 inches.
Results and Discussion
Preliminary results indicated redberry juniper foliage moisture content on all range sites followed similar trends (Fig. 1). Foliage moisture content was generally highest on the sandy bottomland site and lowest on the clay flat site. Redberry juniper foliage moisture content was below 70% on all range sites and sample dates after 24 January 1996, except for the sandy bottomland site in mid-May.
Soil moisture content was always highest on the clay flat site, which was likely due to the higher water holding capacity of the heavy clay soil (Fig. 2). The coarse-textured nature of the soils on the sandy bottomland and shallow clay sites, coupled with the generally low herbaceous vegetation cover, likely resulted in increased evaporation from the soil surface on these sites. Precipitation events in August 1996 resulted in very high soil moisture contents on all sites (Fig.2).
Redberry juniper foliage moisture content appears to be more closely related to available soil moisture than to total soil moisture content. The lack of response in foliage moisture to precipitation events in August 1996 were likely due to inadequate subsoil moisture (Figs. 1 and 2).
The drought conditions that have persisted since 1992 in this area have likely caused soil moisture depletion below the sampling depth. Depletion of the subsoil moisture likely explains the minimal response of foliage moisture to precipitation in mid April and mid August. These precipitation events likely provided only adequate water for the soil surface and water did not sufficiently percolate to the lower portion of the redberry juniper root zone. Subsoil moisture recharge may not occur for one or two years of average precipitation. Consequently, redberry juniper foliage moisture may remain dangerously low during 1997 and 1998 in the Texas Rolling Plains.
REDBERRY JUNIPER ESTABLISHMENT ON RANGELAND
Introduction
Grazing pressure has been implicated as a factor which led to the increase in brush density in Texas (Scifres 1980). The apparent increase in juniper seedlings and young plants on areas excluded from grazing has given rise to serious questions concerning these theories. The commonly held idea that grazing increases brush density appears to have some flaws. The objective of this study was to determine the influence of livestock grazing on the establishment and density of redberry juniper.
Materials and Methods
This study was conducted at the Texas Tech Experimental Ranch in Garza County near Justiceburg, Texas atop the caprock escarpment. In 1982, about 50 acres of heavily grazed shortgrass prairie were fenced and livestock grazing excluded. Several years after livestock grazing was excluded, we began to notice redberry juniper seedlings throughout the area. By 1990 the young junipers were very evident and in great numbers despite a thick and vigorous stand of competing grass. In 1996 four large plots were located in the ungrazed exclosure and four plots in the adjacent grazed pasture. Redberry juniper density and canopy cover were determined by measuring five belt transects in each plot.
Results and Discussion
The ungrazed area averaged 1,411 redberry juniper per acre with 62% of these trees less than 30 inches tall (Fig. 3). The grazed area averaged 101 redberry juniper per acre with only 13% of these trees less than 30 inches tall. Redberry juniper canopy cover on the ungrazed area averaged 26.8%, whereas canopy cover on the grazed area averaged only 7.2% (Fig. 3). These numbers clearly indicate that removal of livestock and abundant grass competition did not inhibit juniper establishment. Further research is being initiated to assess the factors involved in redberry juniper establishment. In the coming year we will attempt to locate other areas that have been removed from grazing and sample the juniper populations. Additionally, we will be sampling the young redberry juniper to determine the age of the plants to verify the establishment year. These data appear to have profound implications concerning grazing management and brush establishment.
Summary
Redberry juniper is a severe problem on Texas rangeland. Understanding the seasonal dynamics of redberry juniper foliage moisture content will indicate when prescribed fire may be most effective for managing redberry juniper. Additionally, understanding the volatile nature of junipers, especially at very low foliage moisture contents, provides information for safety considerations during juniper burning. The exclusion of livestock grazing on Texas rangeland does not appear to deter redberry juniper establishment. On the contrary, redberry juniper density and canopy cover have increased following 14 years of exclusion of livestock grazing. Understanding the ecology of redberry juniper establishment will help us take a proactive management approach rather than a reactive management approach to the redberry juniper problem on Texas rangeland.
Literature Cited
Correll, D.S., and M.C. Johnston. 1970. Manual of the vascular plants of Texas. University of Texas Printing Div., Austin.
Sauer, C.O. 1950. Grassland climax, fire, and man. J. Range Manage. 3:16-21.
Scifres, C.J. 1980. Brush management: principles and practices for Texas and the Southwest. Texas A&M University Press, College Station.
Soil Conservation Service. 1985. Texas brush survey. USDA, Temple Tex.
Steuter, A.A., and C.M. Britton. 1983. Fire-induced mortality of redberry juniper (Juniperus pinchotii Sudw.). J. Range Manage. 36:343-345.
Ueckert, D.N., S.G. Whisenant, and R.J. Ansley. 1994. Biology and ecology of redberry juniper. Proc. Texas A&M Juniper Sym., Technical Rep. 94-2.
Paper number T-9-771, College of Agricultural Sciences and Natural Resources, Texas Tech University.
Comments: Allan McGinty, Professor and Extension Wildlife Specialist