TIMOTHY E. FULBRIGHT, Caesar Kleberg Wildlife Research Institute, Texas A&M University-Kingsville, Kingsville, TX 78363
Abstract: Rangelands dominated by brush can be tailored to enhance habitat for white-tailed deer (Odocoileus virginianus) by designing brush manipulation to achieve the appropriate structure, spatial arrangement, and dispersion of brush. A possible design includes clearing small (about 20 acres), irregularly-shaped patches scattered throughout the landscape. These clearings should total 40% of the landscape. Relatively wide corridors of brush between patches that total 60% of the landscape should remain. Areas of tall, dense, diverse brush with canopy cover 85% should be interspersed throughout the landscape. Brush in and along natural drainage areas and large, single-stemmed mesquites should not be disturbed. Certain methods designed to improve rangeland for livestock, particularly root plowing followed by seeding of exotic grasses, are detrimental to deer habitat.
Manipulating canopy cover and density of brush may improve white-tailed deer habitat by (1) reducing competition between woody and herbaceous plants resulting in increased yield of herbaceous vegetation, (2) providing openings within a shrubland matrix to serve as focal areas for feeding activity, (3) increasing use by deer of areas in which dense brush inhibits travel, and (4) increasing nutritional quality, accessibility, and palatability of browse by stimulating growth of immature sprouts from crowns and stem bases. Brush management should be designed to achieve the structure, spatial arrangement, and dispersion across the landscape of brush communities that provide the habitat characteristics required by deer. These habitat characteristics include optimum percent canopy cover and percent of the landscape supporting woody plants; openings in close proximity to surrounding screening cover; areas of diverse, dense brush; thermal cover; screening cover; intact drainage areas; and browse, forbs, and mast.
Brush management reduces the quality of habitat for white-tailed deer when applied without proper planning and without regard to the habitat characteristics required by deer. In management plans, long-term effects of brush management should be given equal consideration to short-term effects. The short-term result of brush management is frequently an increase in forbs and immature browse sprouts that are eaten by deer. While forage for deer may be greater on mechanically manipulated sites in the short-term, in many cases the long-term effect is reduced abundance and diversity of forage.
Patterns and methods of brush management must be carefully chosen and should be based on soil types, vegetation composition, and potential economic benefits.
Brush management on ranches that include livestock and wildlife enterprises can be viewed in terms of tradeoffs. For example, clearing portions of a pasture to make cattle easier to gather and increase grass yield may reduce deer densities, potential income from leasing for hunting, and real estate value of the land because too much brush was removed. Clearing a relatively small area of dense, diverse brush might reduce the potential of a ranch to produce trophy bucks if that area was preferred buck habitat.
Charactertistics of quality white-tailed deer habitat
In planning brush management, a landscape design that includes key habitat characteristics required by both male and female deer should be developed. These habitat attributes include adequate percent canopy cover; sufficient percentage of the landscape dominated by woody plants; openings in close proximity to surrounding screening cover; areas of diverse, dense brush; thermal cover; screening cover; drainage areas; and forage. These key habitat characteristics should be interspersed within the landscape and spatially arranged in sufficiently close proximity that deer can access these areas within their home range.
Shrub cover can be quantified at two spatial scales. The first scale is the percent woody plant canopy cover at a particular site. Percent woody plant cover at the site scale is measured by estimating the amount of ground surface beneath the canopy of shrubs. The second scale is the landscape scale or the percent of the landscape that supports a cover of woody plants. The percent of the landscape that supports a cover of woody plants is measured by determining the percentage of a given area dominated by woody plants versus the amount dominated by grasses, grass-likes, and forbs; i. e., the amount of woodland versus grassland.
The optimum percent canopy cover of woody plants for white-tailed deer habitat is not clearly defined. In west Texas, woody plant canopy cover averaged 43% in areas with low deer densities compared to 63% in areas with high deer densities (Wiggers and Beasom 1986). In south Texas, deer densities were greatest in areas with 43 to 60% canopy cover of brush (Steuter and Wright 1980). Greatest deer use during summer occurred on areas with 60 to 97% canopy cover of brush.
In south Texas, mature bucks preferred areas with canopy cover 85% and with dense screening cover (Pollock et al. 1994). Brush management planning should focus on having areas with 85% brush canopy cover interspersed within the landscape. Brush management is not recommended for white-tailed deer habitat improvement on areas with <60% canopy cover of woody plants.
Mature bucks in south Texas preferred areas with a large number of different brush species (18-20) (Pollock et al. 1994). This may relate in part to nutrition, because one brush species might be more nutritious during one part of the year whereas another might be more nutritious during another part of the year. Also, different brush species produce mast crops at different times of the year. A large number of different brush species with different sizes and foliage characteristics might also result in denser screening cover. Areas that have naturally high levels of shrub diversity should not be disturbed.
Natural openings with little or no brush serve as morning and evening feeding areas and are often the centers of nighttime activity for deer (Inglis et al. 1986, Bozzo et al. 1992). There are probably several interrelated reasons deer congregate in openings. Deer may be attracted to openings when feeding because predators are more visible. Forage may also be more abundant or easier to access in openings. Deer densities may be greater in habitats where openings are scattered within a landscape dominated by brush. Gee et al. (1991) suggested that the optimum percentage of wooded area for deer in the Cross Timbers of Oklahoma and Texas is 40-60% of the landscape, with patchy, irregularly shaped openings <200 yards wide composing the remainder of the landscape. White-tailed deer densities increased after 50% of the landscape was cleared by chaining of cedar (Juniperus spp.) and live-oak (Quercus virginianus) in strips in the Texas Hill Country and declined on areas that were 80% cleared (Rollins et al. 1988). Dr. Sam Beasom and I did a study during the 1980’s in which we roller-chopped 20% of a 1,000 acre pasture covered by guajillo (Acacia berlandieri) and blackbrush (Acacia ridigula) each year for 4 years in separate but adjacent roller-chopped strips alternating with non treated strips. The greatest difference in deer densities occurred during the second year of the study, after we had roller-chopped 40% of the pasture (20% the first year + 20% more the second year). Deer densities quadrupled in the roller-chopped pasture compared to the untreated pasture.
Deer prefer areas where they can break visual contact with threatening entities by moving a short distance (Inglis et al. 1986). An important function of woody plants in deer habitat is providing screening cover for concealment. Brush must be >1 yard tall to serve as screening cover. Mature bucks prefer areas with taller screening cover. Mature bucks in south Texas heavily used areas where average seasonal canopy height was 16 feet and did not use areas with brush <15 feet tall (Pollock et al. 1994). Mature bucks select taller screening cover regardless of the amount of herbaceous vegetation present.
Dense screening cover that inhibits travel, such as whitebrush (Aloysia lyciodes) thickets, may receive little use by deer (Bozzo et al. 1992). Discing narrow strips within these thickets may increase use by deer.
Deer densities are often greatest in drainage areas and brush management is strongly discouraged within and along drainage areas. In west-central Texas, bottomland habitat contained higher deer densities than all other habitat types (Darr and Klebenow 1975). Deer densities were almost 6-fold greater in bottomland habitats than in upland savannas. Chaining bottomland habitats reduced deer densities by >50%, with densities decreasing as the amount of area chained increased. The taller vegetation along drainage areas is of major importance for deer because it provides preferred loafing and bedding sites (Inglis et al. 1986). Optimum deer habitat should contain several bedding and loafing coverts/mile².
In addition to cover, woody plants provide browse and mast, and are nurse plants for shrubs and forbs eaten by white-tailed deer. Browse and cacti generally compose a large portion of deer diets during periods of low rainfall when forbs are not abundant. Valuable forbs, such as bush sunflower (Simsia calva), often grow primarily under the protection of shrubs.
Care should be exercised when attempting to classify woody plants as to their value for wildlife. Mesquite (Prosopis glandulosa) is often considered an “undesirable” shrub, but mesquite leaves often compose a significant portion of deer diets. Mesquite pods are heavily eaten by deer during the summer. Mesquite canopies are the focal point for establishment of several browse species eaten by white-tailed deer. Mesquite canopies facilitate the establishment of these species by reducing soil temperatures, increasing soil nutrients, and serving as sites of seed dissemination by birds. Groundcherry (Physalis spp.), mistflower (Eupatorium spp.), and several other forb species eaten by deer occur primarily beneath mesquite canopies.
Brush management methods
Brush management methods fall into three broad classes: mechanical, chemical, and fire. Mechanical methods can be further divided into plant removal or top removal. Plant removal methods include grubbing individual plants, root plowing, discing, railing, cabling, chaining, bulldozing, and variations of these basic methods. These brush management techniques only temporarily reduce canopy cover and density of woody plants. Brush reestablishes to the degree that re treatment is needed within 10-20 years following root plowing, the most severe treatment. Reestablishment of brush occurs because seeds of woody plants remain in the soil even if all the woody plants are killed, and animals disperse seeds to treated areas. These seeds eventually germinate and woody plants dominate the vegetation once again. Less severe treatments, such as discing and herbicide treatments are more temporary than root plowing.
Top removal treatments include roller chopping, aerating, and shredding. These treatments do not kill brush, but only remove the top growth of the plants. Thus, the treatments must be repeated every 2-3 years to maintain suppression of woody plants.
Mechanically clearing large tracts of deer habitat (1,000 acres or more), without leaving portions of the landscape with adequate woody cover, substantially reduces white-tailed deer populations (Inglis et al. 1986). Root plowing large areas is the most destructive brush control method to white-tailed deer habitat. The initial effects of root plowing include loss of most of the key habitat characteristics for deer that I previously discussed. Root-plowed areas may support an abundance of forbs when rainfall is adequate, and these may attract deer. However, during drought deer do not utilize root plowed areas because browse is not available. Clearing large areas by other methods, such as chaining, also reduces deer densities until brush reestablishes.
Root plowing and discing kill herbaceous plants and reseeding is usually required to increase grass production following treatment. Seedbed preparation and seeding increase the cost of the treatment. Exotic grasses such as buffelgrass (Cenchrus ciliaris) and old world bluestems (Bothriochloa spp. and Dichanthium spp.) are commonly planted after root plowing. Planting of these grasses further degrades the value of the landscape for white-tailed deer habitat because (1) grasses form only a small portion of deer diets; (2) exotic grasses tend to form dense stands and out compete native grasses and forbs; (3) buffelgrass and old world bluestems produce chemical compounds that inhibit growth of desirable native forbs such as Illinois bundleflower (Desmanthus illinoensis) (Nurdin and Fulbright 1990); (4) soil chemical quality is lower beneath monocultures of exotic grasses than beneath diverse stands of native grasses (Dormaar et al. 1995); (5) primary productivity in more diverse plant communities is more resistant to drought and recovers more fully following drought (Tilman and Downing 1994); and (6) greater plant species diversity has been associated with greater biomass stability through time, greater resistance to grazing by single species of ungulates, and with greater resilience after grazing (McNaughton 1985). A final disadvantage of mechanical brush control methods that disturb the soil is that they often make the habitat susceptible to invasion by undesirable plants such as goldenweed (Isocoma spp.).
Grass production may be high for several years following root plowing and seeding. However, on heavier-textured soils in the western South Texas Plains little herbaceous vegetation remains 20 or more years after root plowing, in part because of soil compaction by livestock. In contrast, forbs may be abundant on root plowed areas in the eastern portions of the South Texas Plains where rainfall is higher.
Brush eventually reestablishes after large areas have been cleared and quality of treated areas as habitat for deer increases. However, the quality of the habitat on areas where brush has reestablished following brush management is often lower than the quality of the habitat before treatment. Brush species preferred by white-tailed deer for browse and mast are often absent on rangeland that has been root plowed. The plant community that reestablishes following root plowing is composed of woody legumes such as mesquite. Thus, root plowing causes a shift from a diverse woody plant community to a near monoculture of woody legumes (Fig. 1).
The woody legume community that reestablishes following root plowing may remain for a very long time. Dr. Fred Guthery and I developed a computer model that simulated the process of brush reestablishment following root plowing over long periods of time. Our model predicted that root-plowed areas must recover for at least 150 to 200 years after the treatment before they again support the diversity of browse species originally present.
The long-term loss of preferred browse plants after root plowing may impact deer less severely in higher rainfall areas than in drier areas. White-tailed deer densities and nutritional status on areas root plowed 17-18 years previously and on areas of virgin brush in the eastern South Texas Plains were compared by Ruthven et al. (1994). The root plowed areas supported primarily huisache (Acacia smallii), whereas the virgin areas supported an abundance of desirable browse species. There was virtually no difference in white-tailed deer densities or nutritional status between the root plowed and virgin areas. However, in vitro dry matter digestibility of browse was lower in root plowed than in untreated areas (Ruthven and Hellgren 1995).
In contrast to findings on deer use of Ruthven et al. (1994), Pollock et al. (1994) found that mature bucks were less likely to use areas that had been root plowed and reinvaded by mesquite than areas of nontreated brushland in the more arid western South Texas Plains. In another south Texas study, sites with <50% woody canopy cover were used more heavily by deer if brush composition was more diverse (Steuter and Wright 1980).
Reseeding is not required following chaining, roller chopping, and shredding. The downside of these treatments is that brush density and canopy cover usually increase beyond pretreatment levels unless the treatments are repeated periodically. For example, on a clay loam site in south Texas, canopy cover of woody plants was 97% in 1984 on a site roller-chopped between 1953 and 1955 compared to 54% on an adjacent non treated site (Fulbright and Beasom 1987). Roller chopping or shredding are particularly undesirable treatments if mesquite is abundant. A single-stemmed mesquite will grow back as a multi-stemmed mesquite after roller chopping or shredding, forming a dense thicket. Another disadvantage of top-removal treatments is that brush species diversity declines with repetition of the treatments. For example, I found that repeated shredding at 3-year intervals for 9 years resulted in loss of Texas kidneywood (Eysenhardtia texana). Texas kidneywood is highly preferred by deer and its loss from the habitat is undesirable.
Herbicide application is an alternative to mechanical brush management. Several herbicides are available that effectively kill mesquite but most commonly applied herbicides will not kill all brush species. We applied the herbicides triclopyr and picloram to mixed brush in the Texas Coastal Bend during May. The herbicide killed most of the mesquites in our study area, but granjeno (Celtis pallida), brasil (Condalia hookeri), and several other brush species browsed by white-tailed deer were not killed. The herbicide killed branches of several of these browses, but the plants grew back.
Herbicides can be applied from fixed-wing aircraft, from a helicopter, or by hand-held sprayers. Herbicides can be sprayed very accurately from a helicopter and such application is conducive to “patch” patterns of clearing (e.g., spraying 20-acre clearings interspersed in brushland) (Fig. 2). Strip patterns are usually applied with fixed-wing aircraft. “Backpack” sprayers can be used to treat individual plants.
One advantage of herbicides over mechanical brush control methods is that herbicides do not disturb the soil. Reseeding to restore grasses is not needed after herbicide application. Finally, standing dead brush remains following herbicide application, which may provide screening cover for deer.
Disadvantages of herbicides are that forbs preferred by deer are usually decimated by the herbicide. Two to 4 years may be required for forbs to grow back in abundance similar to what existed before herbicide application. Deer may make very little use of treated areas until forbs return to their original abundance. Another disadvantage is that herbicide treatments may be more expensive than certain mechanical treatments.
Prescribed burning is a useful habitat management tool for thinning brush and increasing forbs. Non sprouting woody plants such as juniper are readily killed by fire, whereas sprouting species such as mesquite are only top-killed. In general, winter burns increase forb abundance more effectively than late winter or early spring burns.
Brush sculpting patterns
The objective of brush management to improve habitat for deer should be to create a habitat mosaic that includes the following features: woody plant canopy cover of 60% composing 60% of the landscape with areas of heavier cover with tall, diverse brush and dense screening cover and undisturbed drainage areas dominated by brush interspersed throughout; and openings surrounded by brush and around 20 acres in size scattered about and composing 40% of the landscape (Fig. 2). With the pattern of cleared openings scattered in a landscape of brush, deer could travel from one clearing to the next while remaining in relatively wide corridors of standing brush that provides screening cover. Many ranches may not contain all of these habitat attributes and these ideas will have to be adapted to what is available. Brush management should generally not be applied when canopy cover of woody plants is <60%.
The method or methods of brush management used to create this mosaic should be carefully chosen. There are numerous approaches that could be taken to reduce canopy cover. Methods used should be based on factors such as brush species composition and soils. A few important points to remember are: root plowing and planting exotic grasses is not recommended to create openings; top removal treatments such as roller chopping and shredding should not be used in mesquite-dominated habitats; selected brush areas should remain untreated so that they retain maximum diversity; and openings will eventually have to be retreated. Follow-up treatments may differ from initial treatments. For example, roller chopping could be followed by periodically repeated prescribed burns.
In addition to creating openings, top removal treatments such as rolling chopping and shredding are sometimes applied to stimulate growth of sprouts from stems and crown bases. This type of treatment is used primarily in south Texas where most of the shrubs are sprouting species. The new sprouts that grow following top removal are generally higher in crude protein, more palatable, and more accessible to deer than mature growth on non treated plants. Dr. Sam Beasom and I conducted research during the 1980’s to determine if availability of these sprouts would improve the nutritional plane of deer. Roller chopping was timed to provide these sprouts during late summer, when nutritional quality and availability of deer forages are often low. Unexpectedly, nutritional status of deer over a 3-year period was no different on a 1,000-acre area roller chopped in strips alternating with non treated strips area and in an untreated area. Further, the composition of deer diets was similar on the two areas.
Our laboratory analyses showed that guajillo and blackbrush that resprouted after roller chopping was high in protein. However, blackbrush regrowth was less digestible than growth on untreated plants. In other words, deer digested less of the regrowth leaves and twigs compared to leaves and twigs of plants unaffected by roller chopping.
Leaves and twigs of many of the bush species that occur in south Texas, particularly those in the genus Acacia (which include guajillo and blackbrush), produce chemicals that interfere with digestion as an adaptation against being eaten. These chemicals, often referred to as “secondary compounds” prevent deer from being able to absorb all the nutrients contained in the browse material they have consumed. When these brush species are injured by browsing or roller chopping, they can produce increased amounts of secondary compounds in the leaves and twigs that grow back after injury.
David Forbes and his colleagues at the Texas A&M University Research and Extension Center at Uvalde found that concentrations of the phenolic amines tyramine and N-methyl–phenethylamine (NMP) were greater in regrowth leaves of guajillo than in mature leaves. They also found that impaired reproductive functioning may occur in domestic sheep consuming plants containing these phenolic amines. Additional research is needed to determine if induction of tyramine and NMP in regrowth leaves affect nutrition and reproduction of white-tailed deer. David Hewitt and Tyler Campbell at Texas A&M University-Kingsville are currently conducting research to determine the effects of tannins on antler growth of white-tailed deer. Tannins are a class of secondary compounds common in native Texas woody plants.
Kelley Stewart, Lynn Drawe, and I recently completed a study on the Welder Wildlife Refuge near Sinton in which we found that deer used roller chopped areas with the greatest amount of brush regrowth more heavily than roller chopped areas with lower amounts of regrowth during summer. Thus, deer are attracted to roller chopped strips by the availability of browse regrowth even though our earlier study showed that diet composition did not differ on roller chopped and untreated areas.
The objective of brush management to improve habitat for deer should be to create a habitat mosaic that maximizes the value of the landscape as habitat. Soils, plant community composition, past management history, and objectives of the landowner vary greatly from one ranch to the next. Consequently, the best approach to tailoring brushy rangelands to enhance white-tailed deer habitat varies from ranch to ranch. Management plans for improving habitat for white-tailed deer should therefore be based on general concepts that can be modified and adapted to fit the specific circumstances. These general concepts include:
1. Clear small (about 20 acres) irregularly shaped patches scattered throughout the landscape. These clearings should total 40% of the landscape. Leave relatively wide corridors of brush separating the patches that total 60% of the landscape.
2. Areas of tall, dense, diverse brush with canopy cover 85% should be interspersed throughout the landscape to provide cover for mature bucks.
3. Avoid disturbing brush in and along natural drainage areas.
4. When manipulating brush, leave large, single-stemmed mesquites intact.
5. Use the brush control method best suited to the habitat. Root plowing is not recommended.
6. Do not plant exotic grasses such as old world bluestems and buffelgrass.
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Updated: Mar. 18, 1997