Tom Thurow, Amy P. Thurow, Charles Taylor, Jr., Richard Conner and Matthew Garriga
Increased dominance of shrubs and trees in what had previously been grasslands or savannas is a widely observed trend that appears to coincide with European settlement of rangelands (Archer 1994). Before people altered range systems, these vegetation communities were maintained by periodic fire and the grazing /browsing patterns of native wildlife. Most pioneers did not have experience in semi-arid regions, consequently they did not anticipate how introduction of domesticated livestock and suppression of fire would alter the rangeland structure from mostly a grassland to a woodland dominated by oak, mesquite, and juniper (Taylor and Smeins 1994). These changes in vegetation result in environmental and economic tradeoffs with regard to the types and amounts of products that rangelands provide. These tradeoffs have significant implications for ranch enterprises and for the land-use patterns supported by rangelands and surrounding regions. The objective of this paper is to discuss the relationships between production in response to changes in range vegetation, generally speaking, and the specific implications of these economic and environmental tradeoffs as they affect alternative uses of rangeland in the Edwards Plateau of Texas.
As juniper cover increases, there tends to be a decrease in the herbaceous production (Clary 1974, Clary and Jameson 1981, Pieper 1990, McPherson and Wright 1990). For example, Dye et al. (1995) projected that annual herbage production on three sites near San Angelo, Texas, in a closed-canopy redberry juniper woodland would be about 85, 59 and 82% lower than the potential herbage production estimates for the sites. Conversely, as tree density is reduced as a result of brush control efforts there is an increase in herbaceous biomass production (Robinson and Cross 1970, Clary 1971, Clary 1987). For example, herbaceous biomass at the edge of western juniper (Juniperus occidentalis) canopies increased from near 0 to about 1,400 kg/ha within 4 years after the trees were killed with granular picloram (Evans and Young 1985). The sphere of influence and the magnitude of a juniper tree’s ability to reduce herbaceous production tends to be related to soil depth, with the extent of tree impact decreasing as soil depth increases (Dye et al. 1995). It is likely that this pattern is related to amount of water able to be stored in the soil profile (in general, deep soils have more storage space than shallow soils). The influence of water storage on competition and forage production was illustrated in Oklahoma where during wet years there was no difference in forage production three meters beyond the canopy, but during a dry year the forage production in tree interspaces was significantly lower than in adjacent grasslands (Engle et al. 1987).
Figs. 1 and 2 illustrate how distance from the tree canopy influences herbaceous production for a site on the Sonora Agricultural Experiment Station. These graphs compare the herbaceous standing crop on a site three years after the brush had been cut and removed (Fig. 1) with an adjacent site that had not been cleared (Fig. 2). It is noteworthy that near where the dripline had been, herbaceous production increased by two- to three-fold three years after the trees had been cut, compared with herbaceous production at the dripline when the trees were still present. The area under the trees had much better soil structure than the grass interspace due to the high amount of organic matter in the soil which was associated with the tree litter decomposition (Hester 1996). This improved soil structure results in a much greater infiltration rate under the tree than in the grass interspace (Hester et al. 1997). The dripline area is therefore likely to be an area of relatively greater water input because water which runs off the adjacent grassland interspace is able to infiltrate into the dripline soil. The extra water input, combined with the decomposition and release of nutrients from the tree litter, explains the peak of production associated with the dripline. Greater production in the grass interspace following tree removal is due to cessation of competition for water and nutrients with tree roots. The deep litter layer near the trunk had not decomposed after three years and appeared to impede herbaceous growth in that area.
The overall effect of brush clearing on forage production and hence livestock carrying capacity is shown in Fig. 3. Decomposition of the accumulated tree litter and the associated release of nutrients and greater infiltration rate explains why, in the decade or so following brush control, land formerly under brush cover produces slightly more forage than land that had always remained as grassland. The relationship between brush cover and carrying capacity is not a straight line because as brush density increases, there is an increasing amount of forage that is not readily accessible to livestock. This is particularly true for redberry juniper, which has a multi-stem growth form, thus it is difficult for large grazing animals to reach forage growing in the understory. The relationship between carrying capacity and lease value is not a straight line because as shrub density increases, the difficulty of managing the livestock increases. Therefore, the difference between the carrying capacity and the lease value relationships illustrated in Fig. 3 is attributable to the extra cost in labor that ranchers must invest on land with dense brush, thus making the lease worth less than actual livestock carrying capacity. Hunting
The value of a hunting lease is determined by a variety of subjective assessments made by the hunter. One of the primary considerations in a hunter’s calculation of lease value is perceptions concerning the quality of habitat. For the Edwards Plateau, an estimate of hunting lease values relative to brush cover is shown in Fig. 4. This relationship is not based on game density, rather it is estimated based on perceptions of brush density habitat value by average Texas hunters in the Edwards Plateau. Actually, the density of deer is unlikely to decrease as fast as these estimated Edwards Plateau lease value drop at the lower brush cover values (Terrell and Spillett 1975, Howard et al. 1987, Skousen et al 1989).
Combining the relationships of lease value for livestock grazing and hunting leases, it is apparent that an Edwards Plateau rancher seeking to maximize hunting lease income would manage the site to approximately maintain a 30% brush cover (Fig. 4). If hunters were educated to understand that their hunting success would not be hurt (in fact, would probably be helped) if brush densities were substantially less than the 50% cover currently favored, then the maximum hunting lease income would shift to sites with approximately 20% brush cover, thereby increasing the combined livestock and hunting income to Edwards Plateau ranchers by several extra dollars per acre.
The dense heartwood of mature Ashe juniper has value for use as fence posts or for juniper oil extraction (Garriga et al. 1997). Redberry juniper or young Ashe juniper do not have sufficient heartwood to be used for either of these commercial purposes. The current market rates for Ashe juniper heartwood are about $38 per ton delivered to one of the four juniper oil mills in the Edwards Plateau, or $2-5 per acre for harvesting posts. Generally, ranchers are more concerned with the removal of juniper for range improvement than generating income from juniper harvest, therefore contracts to harvest juniper generally require cutting of all juniper. The need for old-growth heartwood and increasing labor costs make harvesting juniper for profit a unique niche and an increasingly unlikely commercial enterprise. Given current commercial markets and opportunities in Texas, the value of juniper wood does not seem to be a significant factor influencing the management of rangelands.
The influence of juniper cover and density on water fate extends beyond the ranch gate. Even though no monetary benefit is obtained by the land owner, the water that recharges Texas streams and aquifers is arguably the most valued product from rangelands. In Texas, essentially all of the surface water is already allocated to meet the demands of existing users, to meet the minimum needs of flow required to protect endangered aquatic species, and to maintain the viability of the coastal wetlands which are important for the fish/shrimp industry and for wildlife. Groundwater pumping is occurring at a rate far greater is being replenished (Van der Leeden et al. 1990). Despite this, the Texas Water Development Board (1990) projects municipal and industrial demand for water to increase 186% by 2040.
How will this additional demand for water be met? Basically, the citizens of Texas must make some difficult decisions because the development of the Texas economy cannot continue on its current expansion pace unless either more water is made available (that is, through development of new supplies or more efficient water use), or the existing supply of water is reapportioned among the users (that is, one sector gets less water so that another sector can continue to grow). Increasing the availability of water is generally viewed as preferable to the politically-divisive reallocation of water rights, therefore options that can increase the amount of water availability merit careful consideration.
A barrier to previous policy discussions of brush control as a means of increasing water yield was the lack of understanding about to how much water yield is influenced by shrub cover. Fig. 5 illustrates the relationship of shrub cover (approximately 2/3 juniper, 1/3 oak) to water yield developed at the Sonora Agriculture Experiment Station.
It is apparent that significant increases in water yield occur only after most of the brush is removed. There is not a linear relationship between brush cover and water yield because when some brush is cleared, the remaining brush and grass have the potential to use water at a faster rate. Accordingly, at high brush densities, removal of a portion of the brush is likely to result in a moderate water yield increase at first, but after several years of canopy and root growth of the remaining brush, there is unlikely to be a difference in water yield. That is why it is necessary to remove most of the brush from a range site to achieve sustainable, significant increases in water yield. The relationship depicted in Fig. 5 between water yield and brush cover helps to explain why many of the seeps and springs that were historically present throughout the Edwards Plateau no longer flow. It also explains why many ranchers have observed that clearing brush around dormant springs can cause them to flow again. On a broader scale, the graph also implies the significant extent to which an increase in brush on the Edwards Plateau over the last half century has impacted the water supply to streams and aquifers of central Texas.
There are several barriers to brush control for enhancing water yield from rangeland:
The ranching industry does not produce income sufficient to cover the full economic costs of brush control. The recent elimination of wool and mohair subsidies further constrained the ability of ranch enterprises to pay for brush control by reducing the reliability of ranch revenues from livestock enterprises. Therefore, it is unlikely that ranch enterprises in the Edwards Plateau will be able to pay for an increased effort in brush control on their own.
A publicly-funded cost-sharing program targeted to achieve increased water yields from rangelands could provide ranchers the necessary financial means to control brush in a manner that would increase water yield. Fig. 5 illustrates that public funds provided for brush control to improve water yields would get the biggest return on investment if the existing vegetation cover was converted to grassland. Fig. 4. illustrates that ranchers would maximize the lease value of the ranch when brush cover is about 30%. However, at 30% brush cover the potential for water yield is only about 1/10th that of an open grassland. Also, the long-term cost of maintaining 30% brush cover would be as great or greater than maintaining grassland. Therefore, ranchers are unlikely to participate in a cost-sharing program designed to maximize water yields from rangelands unless the cost-sharing incentives covered lost revenue-earning opportunities in addition to assisting with the costs of clearing brush.
2) Endangered Species Act Restrictions
Junipers provide nesting habitat for a variety of songbirds. One of these, the golden-cheeked warbler (Dendroica chrysoparia), is an endangered species which nests only in Texas and requires a habitat characterized as a closed canopy composed of mature Ashe juniper and oak. Vast, dense juniper monocultures, or young juniper stands that are less than 12 ft tall, are not preferred habitats for this species (Rollins and Armstrong 1994). Golden-cheeked warblers are very susceptible to failed attempts at raising young because of nest parasitism by the brown-headed cowbird (Molothrus ater), a species usually associated with grasslands. Clearing portions of closed canopy Ashe juniper and oak can therefore expose the warbler to greater vulnerability to parasitism. This is a primary concern in the debate over how large of a continuous area of closed canopy Ashe juniper-oak woodland is needed to support healthy golden-cheeked warbler populations. Resolution of this issue will determine where and how brush control on the Edwards Plateau will be compatible with the rules for critical habitat protection provided by the U.S. Endangered Species Act.
Many ranches on the Edwards Plateau are no longer the primary source of income for the owners. Therefore, the aesthetic appeal of a woodland may be of paramount concern, making it unlikely that the owner would be interested in any plan to control brush. In a similar vein, the prices of land throughout most of the Edwards Plateau exceeds its value for wildlife habitat and livestock production. Therefore, to the extent that trees are considered to enhance real estate value on the Edwards Plateau, it is unlikely that landowners will voluntarily covert the brush-covered rangelands to grasslands.
There are a variety of on-site and off-site environmental and economic ramifications regarding vegetation management on rangelands. Livestock carrying capacity would be maximized if the range was maintained as grassland. Hunting revenue, an increasingly important component of ranching income, is maximized with a brush cover of about 50%. Since most ranches rely on both livestock and hunting lease revenues, a compromise brush cover of about 30% would currently maximize the livestock and hunting lease value of the land.
Downstream citizens also have a stake in how the range is managed because much of the water recharging the region’s streams and aquifers originates on rangeland watersheds. A 30% brush cover would theoretically yield only 1/10th as much water than if the site was maintained as grassland. Since current water use patterns result in a chronic overdraft of the regions existing water supply, and since projected demands for water are expected to continue to increase, it is in the interest of downstream water users to advocate brush control. For this to happen, it would be necessary for the downstream users to develop a funding mechanism to share in the cost of brush control with the rancher.
The desire to increase water for downstream use must be balanced with the desire of many citizens to maintain a woodland cover for protection of endangered species and aesthetic values. Many recent landowners in the Edwards Plateau have sources of income other than ranching. For them, the aesthetic value of woodlands may be more important than revenue generated from the ranch. This implies that as land ownership patterns continue to change to individuals who do not depend on the ranch for income, it will become less likely (or more costly) for them to participate in brush control programs. Educating the public about the tradeoffs and consequences of brush management on the Edwards Plateau can foster informed dialog and decisions regarding these choices.
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Fig. 1. Standing herbaceous biomass three years after the brush had been cut and removed from the site at the Sonora Agricultural Experiment Station, Texas.
Fig. 2. Standing herbaceous biomass in association with tree species at the Sonora Agricultural Experiment Station, Texas.
Fig. 3. Estimated lease value and livestock carrying capacity of Edwards Plateau rangeland with different amounts of brush cover.
Fig. 4. Estimated lease value of Edwards Plateau rangeland with different amounts of brush cover.
Fig. 5. Estimated water yield associated with brush cover at the Sonora Agricultural Experiment Station, Texas.
Comments: Allan McGinty, Professor and Extension Wildlife Specialist