Darrell N. Ueckert
Taxonomy and Biogeography
All junipers (cedars) are in the Gymnospermae class of seed-bearing or flowering plants (the Spermatophyta division). The gymnosperms are more or less resinous trees or shrubs whose ovules and seeds are not enclosed in an ovary (Correll and Johnston 1970). The gymnosperms include the pines, firs, bald cypress, cypress, junipers, and ephedra (Mormon tea). The junipers and Arizona cypress are in the Cupressaceae family.
Redberry juniper (Juniperus pinchotii Sudw.) is a basal-sprouting, evergreen conifer that usually has several stems arising from the base to form a dense clump. Its bark is thin, ashy-gray, and longitudinally fissured into persistent scales. The sapwood is nearly white, while the heartwood is light-brown-reddish in color. The branch tips on these shrubs or small trees are erect or ascending. Small flecks of a white, wax-like substance are almost always present on redberry junipers’ yellowish-green leaves, but they are not abundant. The reddish- or copper-brown fruits contain 1 or 2 lustrous chestnut-brown seeds. The plants are almost all dioecious (male and female flowers on separate plants). They inhabit gravelly, rocky, limestone or gypsum soils on open flats or dry hills, in arroyos and canyons, and on caprocks as well as deep, fertile soils on lowland sites. Redberry juniper occurs in southwestern Oklahoma, western Texas, southeastern New Mexico, southern Arizona, and northeastern Mexico (Correll and Johnston 1970). It is also commonly called redberry cedar or Pinchot juniper.
Redberry juniper is believed to be a stabilized hybrid of alligator juniper (J. deppeana) and one-seed juniper (J. monosperma) that developed during the Pleistocene (Hall and Carr 1968). Redberry juniper is a somewhat variable taxon (species), with some quite divergent populations in the Trans-Pecos region, where it is morphologically similar to one-seed juniper. Some taxonomists have recognized J. erythrocarpa, J. texensis, and J. monosperma var. pinchotii as separate taxon, but others consider these synonyms of J. pinchotii (Correll and Johnston 1970). There is some evidence of hybridization between redberry juniper and one-seed juniper in Palo Duro Canyon of the Texas Panhandle, but redberry juniper and ashe (blueberry) juniper (J. ashei) do not hybridize (Adams 1972).
Male redberry junipers produce anther cells during late summer through mid autumn as evidenced by their golden coloration and the increased incidence of allergies among nearby human populations. The fruits (conelets) on the female plants ripen a yr later. The shrubs are capable of reproducing when they are about 12 yr old, but reproductive maturity is believed to be delayed by competing vegetation until the plants are about 25 yr old (41 in. tall) on upland sites and 16 yr old (47 in. tall) on lowland sites (McPherson and Wright 1987).
Ripe berries of redberry juniper are eaten and the seeds are dispersed by birds, coyotes, foxes, raccoons, jackrabbits, cottontails, other small mammals, deer, and livestock. The seedcoat is apparently impermeable to water or it may contain germination inhibitors. Soaking redberry juniper seeds in concentrated sulfuric acid for 45 min. increased germination slightly (Forest Service 1974), but germination was not enhanced by passage through the digestive tracts of small mammals (Smith et al. 1975). Redberry juniper obviously has adaptations which insure that all its seeds will not germinate at the same time. If conditions resulted in the death of all the seedlings, an adaptation that prevents all the seed from germinating at the same time insures that some seed are still available for later germination. Redberry juniper seeds germinate and emerge best at a temperature of about 64F in moist soils (Smith et al. 1975). This suggests that under natural conditions, germination and emergence would be greatest in wet spring and autumn seasons. Emergence of seedlings is greatest when the seeds are planted on the surface or to a depth of 0.8 in.
Establishment of redberry juniper seedlings after germination and emergence depends upon adequate precipitation and favorable growing conditions. A study conducted near Snyder, Texas revealed that redberry juniper establishment was about twice as great during the second year of a 2-yr period of above-average cool-season precipitation as during other periods (McPherson and Wright 1990). Above-average precipitation in successive years may be the trigger factor for accelerated redberry juniper establishment in grasslands.
During establishment, redberry juniper seedlings are weak competitors and relatively susceptible to damage from a number of factors. Competition from associated grasses greatly reduces shoot and root growth of the seedlings, retards reproductive development, and possibly predisposes them to mortality during prolonged drought. Clipping the seedlings above the cotyledonary node during the first 2 months after emergence killed 18 to 66% of the seedlings (average = 58%) whereas clipping at ground level killed essentially all of the seedlings and saplings until they were about 8 yr old (Smith et al. 1975). Seedlings and saplings clipped above the cotyledonary node usually resprout profusely from the axil of the cotyledons. The cotyledonary nodes of many saplings become covered by soil after about 8 to 12 yr. This characteristic suggests that redberry juniper seedlings less than 8 to 12 yr old should be fairly susceptible to grassland fires or to cutting at ground level (Smith et al. 1975).
Redberry junipers may grow any time during the year if ambient temperatures and soil water contents are favorable, but maximum growth normally occurs during June through September (McPherson and Wright 1989). Growth during April and May may approach or exceed that of the June – August period in some years. Based on tree-ring analysis to estimate age of the plants, redberry junipers occupying upland sites grow about 2.3 in./yr in height during yr 1-10, 2.0 in./yr during yr 11-20, and 1.7 in./yr during yr 21-30 (McPherson and Wright 1987). Redberry junipers occupying deeper soils grow more rapidly, i.e. about 3.1, 2.7, and 2.2 in./yr in yr 1-10, 11-20, and 21-30, respectively. There is some concern that junipers in arid and semiarid environments cannot be accurately aged by tree-ring analysis because several growth rings are produced in certain years (R.Q. Landers, Jr., pers. comm.). Thus, there is a possibility that the growth rates given above may under estimate the real growth rates.
Redberry junipers resprout profusely if the aboveground portion of the plants above the bud zone is killed or damaged by fire, hand cutting, shredding, etc. This resprouting characteristic makes redberry juniper one of the most difficult shrubs to control and manage. Growth rates of resprouts are faster than those of undamaged plants because the large root systems and food reserves are in place to support the resprouting branches. Growth rates of redberry juniper resprouts are largely controlled by temperature and availability of soil water, whereas competition from adjacent shrubs and herbaceous plants have little effect (McPherson and Wright 1989).
Prior to development of the range livestock industry in western Texas, redberry juniper populations were primarily restricted to rocky outcrops and rocky, north-facing slopes where they were protected from intense grass fires (Ellis and Schuster 1968). Its encroachment into adjacent grasslands since the late 1800’s and early 1900’s is attributed largely to the reduced frequency and intensity of grass fires, along with overgrazing which increased the abundance of safe sites (bare ground) for juniper seedling establishment and diminished competition between deep-rooted perennial grasses and juniper seedlings. Researchers at Texas Tech University recently documented that a grazed High Plains site dominated by sod-forming and unpalatable grasses and several forbs supported 859 redberry junipers per acre compared to only 117/acre on an adjacent ungrazed site dominated by bunchgrasses and a few forbs (McPherson et al. 1988).
There is evidence that increasing carbon dioxide concentrations in the atmosphere during the last century may be benefitting junipers. Elevated carbon dioxide concentrations give the competitive advantage to plants that utilize the C3 photosynthetic pathway, such as junipers and mesquite, whereas plants that utilize the C4 photosynthetic pathway, such as our warm-season grasses, are at a competitive disadvantage. Conifers exhibit pronounced growth increases with increasing carbon dioxide concentrations in the atmosphere, suggesting that carbon dioxide enhancement may have played a role in the recent increases in the distribution and abundance of junipers throughout North America (Mayeux et al. 1991).
Redberry juniper has dramatically altered the structure and function of a substantial portion of our State’s rangeland ecosystems in a relatively brief time span. A 1982 survey by the Soil Conservation Service indicated that redberry juniper occurred on about 11.7 million acres of rangeland in Texas (Soil Conservation Service 1985). Of this acreage, 73% was characterized as light juniper canopy cover, 23% as moderate, and 4% as dense canopy cover. The acreage infested by redberry juniper in a 65-county area in northwest Texas increased by about 60%, or 3.8 million acres, during the period 1948-1982 (Ansley et al. 1995).
Redberry junipers have a dramatic, debilitating impact upon grassland plant communities they invade, and they seriously reduce the carrying capacity for livestock and wildlife. We recently found that annual herbage production (1,156 lb/acre) on a low stony hill range site near San Angelo supporting 117 mature redberry junipers/acre was about 40% lower than the potential production of the site in the absence of mature junipers (1,909 lb/acre) (Dye et al. 1995). Projected annual herbage yield for this site when the juniper plants currently present mature to create a closed-canopy woodland was only 283 lb/acre, an 85% decrease from the site’s potential. In the absence of mature junipers, this site had an estimated carrying capacity of 1 animal unit to about 20 acres, compared to 1 animal unit to 135 acres when the site becomes a closed-canopy woodland. The current total juniper density on this site, 2,639/acre, suggested that it will become a closed-canopy juniper woodland in the foreseeable future.
Graves (1973) found that herbage production between redberry junipers that had been killed with picloram sprays was 88% greater than between live trees in a study in Lynn and Garza Counties on the Texas High Plains. Herbage production beneath dead junipers was 108% greater than that beneath live junipers.
Gerbolini (1996) quantified the relationship between redberry juniper canopy cover and forage production on a very shallow range site in Nolan County, Texas. He found a curvilinear relationship between redberry juniper canopy cover and forage production (Fig. 1). Adapted from Gerbolini, 1996). Forage production decreased at an increasing rate until the juniper canopy cover reached about 34%, then decreased at a decreasing rate as juniper canopy cover continued to increase.
McPherson and Wright (1990) found that grass production decreased by only 4.3% (33 lb/acre) as redberry juniper canopy cover increased from 0 to 5% on an ungrazed site in the Texas High Plains, compared to a 26% (124 lb/acre) decrease on a grazed site. They attributed this difference to the fact that grazing shifted the competitive advantage to the redberry juniper.
The aboveground biomass of juniper leaves, twigs, and wood on a north-Texas site supporting 300 mature redberry junipers per acre was 40,000 lb/acre (R.J. Ansley, unpublished data) or almost 1 lb/ft2. The canopies of redberry juniper intercept precipitation and block sunlight from the desirable grasses and forbs.
The interference of mature redberry junipers with the herbaceous understory intensifies with increasing proximity to the juniper trunks (Fig. 2) (Dye et al. 1995). Beneath mature juniper canopies 55 to 97% of the soil surface is covered by a dense mat of dead juniper leaves, densities of herbaceous plants are 65 to 90% lower than in the area beyond the canopies, and total numbers of herbaceous species are only 60 to 72% as great as in the area beyond the canopies. The yield of grasses and forbs decreases dramatically from 20 ft beyond the edge of mature redberry juniper canopies to the juniper trunks. The sphere of influence of mature juniper plants was more extensive on shallow, rocky soils than on deep soils. Significant increases in yields of grasses and forbs occurred out to about 20 ft beyond mature juniper canopy edges after the junipers were killed on a shallow, rocky Kimbrough soil (Fig. 2 Different lower case letters within a sampling location indicate significant differences [P0.05]). Yields of grasses and forbs increased only to the canopy edges or to 3 ft beyond the canopy edges of junipers killed on deeper Angelo clay loams and Tulia loams (Fig. 2).
Interception of precipitation, competition for soil water and nutrients, shading, and allelopathy (a chemical given off by one plant that adversely affects another plant species) are all potential explanations for the observed interference of mature junipers with the herbaceous understory. The greatest increases in herbage production following killing of mature redberry junipers occurred beneath juniper canopies where the juniper litter cover was thickest and where shading had been most intense (Fig. 2). Herbage yields beneath live juniper canopies was only about 50 to 300 lb/acre, compared to about 1,500 to 2,300 lb/acre beneath the canopies of junipers that were killed 2 yr earlier (Dye et al. 1995). Results from our study suggested that competition for soil water and/or nutrients and for sunlight (or interception of rainfall) were the primary mechanisms of interference between mature junipers and the understory grasses and forbs. We found no evidence that redberry juniper litter was allelopathic to herbaceous plants.
Mature redberry junipers appear to serve as “nurse plants” or to facilitate establishment of certain shrubs and other plants. Algerita, littleleaf sumac, lime pricklyash, Mormon tea, pricklypear, and juniper seedlings were more abundant beneath mature juniper canopies than in interspaces between junipers in the Edwards Plateau (Dye et al. 1995). In the Rolling Plains and High Plains, algerita, littleleaf sumac and catclaw mimosa were more abundant in areas with large redberry junipers than in areas without large junipers (McPherson et al. 1988). McPherson et al. (1988) suggested that large mesquite trees facilitated the establishment of the first redberry junipers on some High Plains sites. It is well known that the soil beneath large mesquite trees is considerably more favorable for plant growth than that in the interspaces because of mineral redistribution by mesquite’s extensive root system, leaf fall, nitrogen fixation, and partial shading.
Two studies have investigated the influence of redberry junipers on soil properties. The upper 4 in. of soil beneath redberry junipers on the High Plains of Texas contained 20 to 40% greater concentrations of organic matter than soil in the interspaces, but the junipers did not affect soil nitrogen, potassium, phosphorus, or pH (McPherson et al. 1991). In a north Texas study, nitrate nitrogen in the surface ft of soil was lower throughout the year within a redberry juniper stand than within an adjacent area on the same soil where redberry juniper was absent (R.J. Ansley, unpublished data). During May, when the soil was moist, soil nitrate concentration in the juniper-free soil was 40% greater than in gaps between juniper trees and 50% greater than beneath redberry juniper canopies. Soil nitrate concentrations were generally greater in gaps between junipers than beneath juniper canopies throughout the year. These data suggest that redberry junipers are heavy users of soil nitrates and that the soil within their sphere of influence may be rendered less favorable for growth of other plant species (R.J. Ansley, pers. comm.). These data also suggest that the increased abundance of other shrub species and pricklypear beneath redberry junipers is due to reduced competition from grasses and forbs, rather than to more favorable soil properties beneath the junipers.
Redberry juniper has a competitive advantage over most other plants on our rangelands because it is almost entirely free of natural enemies. Juniper foliage contains volatile oils that render it relatively unpalatable to herbivores, including livestock, wildlife, and most insects (Launchbaugh et al. 1997; Rollins and Armstrong 1997; Taylor et al. 1997). Redberry juniper foliage is utilized by livestock and deer when the quality and/or availability of desirable forage and browse plants are low, but the degree of defoliation of redberry juniper is usually very low in relation to that of the more desirable plants. Redberry juniper is less palatable than ashe juniper, and this differential palatability may contribute to domination by redberry juniper where the two species’ ranges overlap on rangeland subjected to heavy goating pressure (C.A. Taylor, pers. comm.). Only one incidence of disease, a twig and needle fungus (Phomopsis blight) near Del Rio, Texas in 1977, and one outbreak of a defoliating insect, a webworm near Sweetwater in 1992, have been observed on redberry junipers by the author. Perhaps no other noxious woody plants are as free of damage from insects as are the junipers (Watts et al. 1989).
Effect on Water Resources
As juniper densities increase, water resources may deteriorate through an increase in sediment load in the overland flow, a decrease in subsurface flow, an increase in interception of precipitation, and a reduction in soil water reserves through transpiration (Thurow and Carlson 1994). It has been estimated that a large western juniper (J. occidentalis) can use about 32 gal of water/day during mid summer if soil water is readily available (Miller et al. 1987). The actual water use of redberry junipers has not been documented. However, recent research has shown that redberry juniper canopies intercept about 26% of the precipitation that is received (Thurow 1997). This water is returned to the atmosphere by evaporation. An additional 40% of the precipitation is intercepted or absorbed by the layer of juniper litter on the soil surface beneath the canopy, thus only 34% reaches mineral soil. Consequently, ranchers with dense stands of redberry juniper in a 20-in. average annual rainfall zone actually only get about 6.8 in./yr of the precipitation into their mineral soil. The desirable forage plants must then compete with the redberry junipers’ shallow, lateral roots for this already scant supply of soil water.
Influence of Redberry Juniper on Succession
Our original grasslands, as well as the redberry juniper woodlands that currently occupy millions of acres of these former grasslands, represent “stable states” or “seral stages”. The original grasslands were maintained by graminoid (grass) – driven successional processes, characterized by low grazing pressure, high fire frequency and intensity, and consequently a low probability and rate of woody plant establishment. Heavy, continuous grazing of these grasslands by cattle, sheep, and goats during the late 1800’s and early 1900’s weakened the climax grasses, caused major changes in herbaceous species composition, reduced the frequency and intensity of fires, and thus facilitated the establishment of redberry juniper. These plant communities crossed the threshold from grasslands to juniper – dominated woodlands when sufficient numbers of junipers became established and reached reproductive maturity. Juniper – driven successional processes then began predominating, characterized by debilitation of understory herbaceous plants, a general reduction in understory diversity, density, basal area, and productivity, an influx of subsidiary woody and succulent species, further reduction in fire frequency and intensity, and a high incidence and rate of juniper seedling establishment. These juniper woodlands will not revert to grasslands now, even if grazing is stopped. Furthermore, little or no improvement in range condition would occur if grazing were discontinued (Dye et al. 1995).
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Comments: Allan McGinty, Professor and Extension Wildlife Specialist