ALLAN MCGINTY, Texas Agricultural Extension Service, San Angelo, TX 76901.
Abstract: Ashe juniper (Juniperus ashei) and redberry juniper (Juniperus pinchotii) are the dominant juniper species in central and west Texas. These two species have been a component of Texas plant communities for thousands of years, but have increased in density and spread into other habitats since European settlement. Overgrazing and prevention of fire are thought to be causative factors. Dense juniper can have significant impacts on herbaceous forage production, livestock production, watershed hydrology and wildlife habitats. Long-term planning and early treatment are the keys to proper management of juniper.
There are three major species of juniper in Texas. They include Ashe juniper (Juniperus ashei), redberry juniper (Juniperus pinchotii) and eastern red cedar (Juniperus virginiana). Because eastern red cedar is found generally east of Interstate Highway 35 in Texas, this paper will focus on redberry and Ashe juniper.
Ashe juniper, also know as post cedar or blueberry cedar, occupies over 8 million acres of Texas rangelands. The greatest abundance of this plant is found on the eastern and southern portions of the Edwards Plateau, but Ashe juniper also extends into the South Texas Plains and north into the Cross Timbers and Rolling Plains areas of the state (USDA 1985).
The present range and distribution of redberry juniper overlaps with Ashe juniper in many areas, although redberry juniper is more common in the western Edwards Plateau and eastern Trans-Pecos regions. Redberry juniper also can be found in the South Texas Plains and as far north as the Rolling and High Plains, occupying over 11 million acres of rangeland in Texas (USDA 1985).
Historically, juniper is believed to have been restricted to rocky outcrops and rocky, north-facing slopes where they were protected from intense grass fires (Ellis and Schuster 1968). There is general agreement that the habitat range and overall abundance of juniper has increased since European settlement, or at least during the past 50 years (Smeins and Fuhlendorf 1997). 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). The exact cause of this increase is difficult to determine. Overgrazing by livestock is thought to be a causative factor. Purposeful or inadvertent prevention of naturally occurring fires, along with reduced fuel loads due to grazing also have an influence (Smeins and Fuhlendorf 1997).
Today, both species are found on 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 areas.
Both redberry and Ashe juniper are in the Gymnospermae class of seed-bearing or flowering plants, which include pines, firs, cypress and ephedra. Redberry juniper can be distinguished from Ashe juniper by color of the fruit (reddish to copper brown as compared to deep blue for Ashe juniper), a more multi-stemmed growth form and small flecks of a white, wax-like substance usually present on the leaves.
Redberry juniper resprouts from the base following top removal or damage. Ashe juniper is non-sprouting. Both species are dioecious (male and female flowers occur on separate plants). Male redberry junipers produce pollen during late summer through mid autumn (Ueckert 1997). The fruits on the female plants ripen a year later. Redberry juniper plants are capable of reproducing when they are about 12 years old, but reproductive maturity may be delayed until the plants are about 25 years old due to competing vegetation.
Ashe juniper flowers between December and February. Fruit development and maturation occurs from then until November, and fruit ripening and dispersal between November and April (Smeins and Fuhlendorf 1997). Ashe juniper trees begin to produce fruits when they are 3 to 4.5 feet in height (approximately 10 to 20 years old). Fruit and seed production varies greatly from tree to tree, within years, and for the same tree across years. Large trees may produce 100,000 to 250,000 berries per tree. Weather appears to be a major controlling factor in berry production, germination and establishment for both redberry and Ashe juniper (McPherson and Wright 1990, Chavez-Ramirez 1992). Above average precipitation in successive years may be the trigger factor for accelerated juniper establishment in grasslands (Ueckert 1997).
Juniper seeds are relatively heavy and do not fall far from the parent tree (Fig 1. Adapted from Smeins and Fuhlendorf 1997). One of the major dispersal mechanisms for juniper seeds is consumption by birds and mammals. Known berry foragers include racoons, ringtail cats, gray foxes, coyotes, jackrabbits, deer, and livestock (Smeins and Fuhlendorf 1997, Ueckert 1997). With respect to Ashe juniper, American robins and cedar waxwings are important bird foragers due to their very high winter populations in the Ashe juniper region. Robins are perhaps more effective dispersal agents than waxwings, due to their post-foraging behavior. Robins have a much greater range of movement away from parent trees following ingestion of juniper berries and they tend to individually visit a wider variety of perch sites within grassland areas (Chavez-Ramirez and Slack 1994). Due to the large number of juniper seeds that fall directly under the parent tree, death of that parent tree can release many seedlings that may grow rapidly due to increased light, moisture and nutrient availability. Where one large tree occurred, a cluster of small plants can develop. Thus removal of large trees by mechanical methods not followed by burning or other practices may result in rapid replacement of the removed plants.
Seed germination is know to vary tremendously between Ashe juniper trees. Some trees never produce seeds that germinate while other trees produced seeds with 55% germination. Individual trees also have their own germination pattern. Some were highest in the fall, others in mid-winter and others in late winter. This combination of events, i.e., synchronous maturation within a tree and asynchronous maturation between trees, effectively favors seed consumption and dispersal by animals and retains dispersal agents such as robins and waxwings in the area for a long period of time (Smeins and Fuhlendorf 1997).
The soil can contain a large seed bank of juniper seeds at any point in time. Owens and Schliesing (1995) found between 1197 and 1436 seeds/m2 in the leaf litter under Ashe juniper trees and between 275 and 366 seed/m2 in the soil on a site near Uvalde. A similar study at the Sonora Experiment Station found between 11,000 and 23,000 seed/m2 in the leaf litter and 250 to 650 seed/m2 in the soil (Blomquist 1990). Although there is little information concerning redberry juniper seed longevity, Ashe juniper seeds are know to survive for only a short period of time in the soil. In a study at the Sonora Experiment Station, seed germination had dropped to less than 1% regardless of being on the soil surface or buried in the seed bank. Although this may seem to limit the ability of Ashe juniper to produce new plants, 1% of the hundreds of thousands of seeds produced by a mature stand of ash juniper would still be a significant number for potential recruitment into the population (Smeins and Fuhlendorf 1997).
Seed establishment is possible across a great variety of soil/vegetation conditions. Although junipers are more readily established in rocky, relatively, open sites, they can and do establish on relatively deep soils with a continuous cover of herbaceous plants. In the latter case, increase in size and numbers may be slowed due to herbaceous competition (Smeins and Fuhlendorf 1997).
Maximum growth rates for redberry juniper generally occur between June and September (McPherson and Wright 1989). Based on tree ring analysis, redberry junipers occupying upland sites grow about 2.3 in/yr in height during years 1- 10, 2.0 in/yr during years 11 – 20 and 1.7 in/yr during years 21 – 30. On deeper soils, growth rates are 3.1, 2.7 and 2.2 in/yr in years 1 -10, 11 – 20 and 21 – 30, respectively.
Based on historical aerial photographs, Ashe juniper at the Sonora Experiment Station showed a steadily increasing growth rate in canopy area over a 30-year period (Fig. 2. Adapted from Smeins and Fuhlendorf 1997). Average individual canopy area increased 0.58 m2/yr from 1955 to 1966, 1.40 m2/yr from 1966 to 1973 and 1.94 m2/yr from 1973 to 1995. Great variation in growth rates existed between individual trees (Smeins and Fuhlendorf 1997).
Redberry junipers resprout profusely from the cotyledonary node if the aboveground portion of the plant above this area is damaged or removed. The cotyledonary nodes of many saplings become covered by soil after about 8 to 12 years. This characteristic suggests that redberry juniper less than this age should be susceptible to grassland fires or to cutting at ground level (Smith et al. 1975).
Junipers can have a dramatic impact upon grassland plant communities they invade. In a study near San Angelo, Texas, annual herbage production (1,156 lb/ac) on a low stony hill range site 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). It was projected that once these junipers create a closed-canopy woodland annual forage production would be approximately 283 lb/ac, or an 85% decrease from the sites potential. Such impact on grassland plant communities also have a profound effect on livestock carrying capacities. In the absence of mature junipers, this site could support approximately 1 animal unit/20 acres, compared to 1 animal unit/135 acres when the site becomes a closed-canopy woodland.
A study in Nolan County, Texas quantified the relationship between redberry juniper canopy cover and forage production on a very shallow range site (Gerbolini 1996). The relationship was found to be curvilinear. Forage production decreased at an increasing rate until the juniper canopy reached about 34%, then decreased at a decreasing rate as juniper canopy cover continued to increase (Fig. 3. Adapted from Gerbolini, 1996).
The impact of mature redberry junipers on the herbaceous plant community intensifies with increasing proximity to the juniper trunks. Beneath mature redberry juniper canopies 55 to 97% of the soil surface is covered by a dense layer of dead juniper leaves. Densities of plants are 65% to 90% less than in areas beyond the canopies. Grass and forb production decreases dramatically from 20 feet beyond the edge of mature redberry juniper canopies to the juniper trunks (Ueckert 1997).
As juniper increases on rangeland, the amount of precipitation that reaches the soil surface can be reduced. Hester (1996) reported a canopy interception loss of 25.9% and 36.7% of gross precipitation for redberry and Ashe juniper, respectively. The higher interception loss for Ashe juniper is due to the denser canopy of Ashe juniper, which results in more surface area to intercept rainfall. For comparison, live oak canopies intercept approximately 25.4% of gross rainfall.
Once rainfall passes through the canopy it must also pass through the litter layer under the plant before it reaches the soil surface. It is estimated that the litter under Ashe and redberry juniper intercepts 40 to 43% of gross rainfall (Thurow and Hester 1997). As a result of interception loss via the canopy and litter, only 20.3 and 34% of the annual rainfall reaches the mineral soil surface under the canopies of Ashe and redberry juniper, respectively. This compares to 82% and 89% of annual precipitation that reaches the soil under bunchgrass and shortgrass cover, respectively. This has important impacts on stream and aquifer recharge. As vegetation cover changes from grasses to dense juniper woodland a greater proportion of precipitation will leave rangeland via evaporation and therefore less water is available for producing herbaceous forage or for deep drainage and runoff (Thurow and Hester 1997).
Although juniper invasion of rangeland generally has a negative impact on livestock production, it can be beneficial for wildlife, at appropriate densities. Juniper is a common item in deer diets, especially in the Edwards Plateau. Studies in Kerr County (Bryant et al. 1981, Warren and Krysl 1983, Waid et al. 1984) reported Ashe juniper comprising 0 to 34% of white-tailed deer diets. Nelle (unpublished data) found that redberry juniper comprised over 50% of the diets of deer in Terrell County in January, February, October and December. In general, juniper is considered only “fair” in terms of it’s value as deer forage (Armstrong 1991). Nutritionally, juniper averages only 7% crude protein for the foliage and 14% crude protein for seeds (Rollins and Armstrong 1997).
Juniper berries are common in gray fox, ringtail and raccoon scats. Juniper berries are occasionally found in quail or wild turkey crops. Chavez-Ramirez (1992) identified 19 species of birds and 9 species of mammals that consumed juniper berries at the Sonora Experiment Station.
One of the most important attributes of juniper for wildlife is in providing shelter. Thermal cover allows animals to compensate for temperature extremes. Junipers are probably much more valuable for thermal cover in the winter than in summer and in colder climates than in warmer ones (Leckenby 1977).
Large dense stands of juniper are not beneficial for either wildlife or livestock. Rollins et al.(1985) produced a series of 20 acre clearings with progressively smaller strips of brush between the clearings to identify how much escape cover was necessary for deer. Their findings suggested that as much as 70 percent of the range could be cleared mechanically without adversely affecting deer use of habitats or deer populations.
Two endangered birds, the golden-cheeked warbler and the black-capped vireo are associated with Ashe juniper. Golden-checked warblers nest only in Texas and the primary nesting habitat is characterized as a closed canopy of Ashe juniper and oak (Wahl et al. 1990). There is no evidence of the warbler being found within pure stands of redberry juniper.
The black-capped vireo does not require Ashe juniper for any particular habitat requirement, it is merely associated with other plants that often occur in conjunction with Ashe juniper. Studies at Ft. Hood have shown that vireos actually select against areas with dense cedar (Rollins and Armstrong 1997).
Juniper has been a component of Texas plant communities for thousands of years (Bryant and Shafer 1977). The basic geographic range has probably not changed greatly since European settlement, although increases in density have occurred, and in some instances juniper has spread into habitats where it previously was absent or of limited abundance (Smeins and Fuhlendorf 1997). As juniper density increases, production of herbaceous vegetation decreases, which in turn reduces livestock production from rangeland. Dense stands of juniper can also have a profound effect on watersheds, by intercepting rainfall and reducing the amount of water that reaches the soil surface and thus is available for plant growth, deep percolation, and runoff.
Juniper can be of value for wildlife as a food source, nest site or by providing escape cover and thermal protection. Large, dense stand of juniper are not beneficial to either livestock or wildlife. The optimum amount of juniper depends of individual landowner management goals. Juniper densities can be managed using fire, chemical, mechanical or biological methods. Proper management depends on detailed long-term planning and early treatment.
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Updated: Mar. 18, 1997