Revegetating Semiarid Rangelands

Introduction

The semiarid rangelands of South Texas and Northern Mexico are much less productive than they were 100 to 150 years ago. A small percentage of this area only requires improved grazing management practices for greater productivity. Rangelands dominated by the most persistent and productive plant species only require harvest strategies that maintain healthy and productive plants. Moderately degraded rangelands may require brush management, but the most damaged rangelands require revegetation. The most degraded rangelands have microenvironments where plants require more water, but receive less. The worst of these areas simply get worse with time, even with careful management. Within the last 20 years, ecologists recognized that some damaged rangelands do not necessarily recover, just because management improves (Friedel 1991). Damaged hydrologic processes, harsh microenvironments, and few natural seed sources create barriers to natural improvement.

Although improved grazing management must be part of any long-term management plan, it is unlikely to significantly improve severely degraded ecosystems. Even complete removal of livestock does not insure secondary succession leading toward recovery. Unfortunately, the expense and risk of revegetation has limited its use. Most rangelands are less productive because they have damaged ecosystem processes and/or are dominated by species that do not contribute to management objectives. The most significant challenge of revegetating semiarid rangelands is to repair damaged processes, improve microenvironments, and establish plants that will continue recovery processes. Thus, although revegetating semiarid rangelands requires numerous important considerations, I will focus on practices that address hydrologic problems and improve microenvironments. These techniques include seedbed preparation, water harvesting, the use of mulches, and interseeding.

Hydrologic damage

Damaged hydrologic processes in degraded rangelands prevent recovery by reducing the amount of water entering the soil. Less perennial plant production adds fewer organic materials to the soil. Less soil organic matter reduces the aggregate stability of surface soils. Raindrops falling on exposed soil surfaces with low aggregate stability detach fine soil particles from the soil surface. These fine particles fill soil pores and create surface soil crusts with continuous surface sealing. After drying, surface crusts seal the soil surface, and reduce both infiltration and aeration. Plants in these soils may receive the benefit of only 10% to 50% of the precipitation that falls on the pasture. Consequently, plants in a 500-mm per year climate may produce like plants in a 50-mm to 250-mm climate. The soil surface provides insight into soil stability (ability to withstand erosive forces), hydrologic functioning (infiltration and runoff), nutrient cycling, and biological processes relating to energy capture (Figure 1). Since soil surface condition both affects and reflects these essential processes — it should be a priority when designing strategies to repair dysfunctional processes and capture additional resources. Effective strategies jump-start natural recovery mechanisms with initial treatments that: (1) increase the roughness of the soil surface with pits, contour furrows, basins, ripping or chiseling; and/or (2) add above-ground obstructions such as logs, rocks, woody debris, herbaceous litter, or man-made erosion control products. In the short-term, these soil-surface treatments and aboveground obstructions improve hydrologic and nutrient cycling processes by capturing water, soil, nutrients, and organic materials. These changes facilitate the establishment of vegetation that further increases biotic control over hydrologic and nutrient cycling processes. The additional resources captured by soil surface treatments help to establish vegetation that continues to improve resource retention by: (1) increasing soil organic matter; and (2) increasing water and nutrient holding capacities; and (3) improving soil structure.

Soil improvement strategies should be directed toward the eventual goal of retaining and using water where it falls. The only sustainable method of accomplishing these objectives is to reduce the amount of bare ground by establishing a vegetative cover. Vegetation protects the soil surface from raindrop impact, reduces surface flows, increases infiltration, and in the long-term improves soil structure. Removing vegetation and litter damages the soil surface and initiates a positive feedback system that accelerates degradation. Exposing the soil surface to raindrop impact leads to the development of sealed soil surfaces that greatly reduce the infiltration of water into the soil. Our goal should be to initiate ‘positive-feedback improvement systems' that reverse degradation and continue to improve the soil and hydraulic processes. This requires a healthy vegetative cover.

Harsh microenvironments

Plants in degraded rangelands are subjected to more wind and greater temperature extremes than similar plants in healthy rangeland environments. Under these conditions, they require more water to produce the same amount of vegetation, but they receive less water. This makes it more difficult to establish new plants and it reduces the productivity of surviving plants. Fortunately, we can design seedbeds that improve conditions for developing seedlings and we can add woody plants for lasting microenvironmental amelioration. Woody vegetation can be incorporated into rangelands as scattered shrubs or into improved pastures as part of agroforestry systems.

The physical stature of woody plants has strong ameliorating influences on the microenvironment and soil of its immediate surroundings (Figure 2). In arid and semiarid ecosystems, shrubs or trees improve microenvironmental conditions by moderating wind and temperature patterns (Farrell 1990; Vetaas 1992). Although the woody plants compete with understory plants for light, the benefits of this habitat amelioration often outweigh any negative effects. Plants passively affect their immediate environments with their physical structure by shading the soil and altering wind movements. This reduces wind speed, lowers the extremes of air and soil temperatures, and increases relative humidity. Plant structures trap wind-blown soil, nutrients, and propagules of microorganisms and other plants. Metabolic processes actively change the environment by altering temperature, humidity and the physical and chemical properties of soils. Plants gradually increase soil organic carbon and improve the water and nutrient holding capacities of the soil. The capacity of plants to modify their environment is roughly proportional to vegetation biomass, stature, and the rate of metabolic activity. Thus, sparse desert vegetation is less able to alter its environment than forest vegetation. However, the lesser plant-induced environmental alterations in arid ecosystems may still have significant biological impacts.

If we can recognize the changes that occur during degradation, we may prevent them before damage is done. This also helps us develop improved strategies for revegetating deteriorated rangelands. Revegetation is most appropriate where site degradation is too great for natural recovery or where rapid results are desired. We create suitable seedbed environments and add seeds. We have the most influence on the pace and direction of change when we control seedbed conditions and species availability. Knowledge of why seedings fail, how to overcome this problem, and how to plant seed, plant parts, or whole plants is essential to the use of artificially induced recovery strategies.

Seedling establishment is a function of the number of seeds in suitable seedbed environments (safe sites) rather than the total number of available seeds. Well-prepared seedbeds improve environmental conditions and improve resource availability by controlling established weeds. Planting strategies that maximize the abundance of safe sites and accurately place seed may be less expensive because they require less seed and reduce the risk of failure. Seedings fail during the germination, emergence, and establishment phases. Most failures are associated with seeding unsuitable sites, seeding at the wrong time, inadequate site preparation, poor-quality seed, and too few seed. Successful grass seedings are usually related to the degree of seedbed preparation, rainfall, and temperature.

Good seed-to-soil contact provides a more reliable water supply to the seed. Seed planted in excessive amounts of litter or in large air pockets are less likely to establish. Firm the soil under and around the seed. The soil should be firm above the seed and without large air pockets. Excessive amounts of organic materials cause similar problems by preventing seed/soil contact directly or by preventing closure of the drill slit. Seeding too deep, soil crusts, desiccation, wind erosion, water erosion, rodent depredation, insect damage, excessive soil salinity, and frost heaving reduces seedling emergence. Birds, rodents, and insects often eat seeds before they germinate, especially seed broadcast over the soil surface without any attempt to cover them. Soil crusting is a major factor contributing to grass seedling mortality.

Seedbed preparation is the primary activity of most revegetation activities, since it is the most labor-intensive, energy consumptive, and often determines success or failure. Ideal seedbeds are (1) firm below and above seeding depth; (2) thoroughly tilled, friable soil; (3) not cloddy or compacted; (4) devoid of established weeds; (5) without significant seedbank of weedy species; and (6) covered with moderate amounts of mulch or plant residue on surface. While these conditions benefit most seeded species, other species may require mineral-soil seedbeds, shade, or nurse plants.

Accessibility to seeding equipment, cost limitations, obstructions, and value of resident vegetation also dictates the choice of seedbed preparation methods. Individual seedbed preparation methods have different effects, but they all address the critical problem of weed management. The common categories of manipulating sites and seedbeds are (1) mechanical or manual; (2) chemical; (3) fire; (4) biologic; and (5) the use of mulches (covered in subsequent section).

Mechanical seedbed preparation

Although hand labor is effective, mechanical seedbed preparation often involves standard agricultural techniques such as plowing, chiseling, disking, or harrowing. Farm equipment opens and roughens the soil surface, kills existing vegetation, and facilitates the planting process. Manual site preparation techniques are labor-intensive. Farm equipment is capital-intensive and difficult to use on steep, rocky slopes. Farm equipment causes additional problems if it packs the soil. Both mechanical and manual methods loosen the soil, reduce soil surface crusts (at least temporarily), direct water into depressions, and reduce wind speed and temperature extremes for developing seedlings. Germination and survival usually increase when the soil surface is cultivated and well prepared before planting. While disturbance (cultivation) increases seedling establishment, it also increases erosion risks. Clean seedbeds are effective where wind and water erosion is not serious problems and establishment is not greatly limited by precipitation. However, clean seedbeds are more susceptible to erosion on sandy soils, slopes, or other erosive situations. Soil disturbance (cultivation) is generally not recommended where the soils are loose and slopes exceed 20%.

Although removing the existing vegetation can accelerate erosion, not removing competing vegetation almost certainly reduces seedling establishment. Blowing sand, released after vegetation removal, causes additional problems by burying seeds, exposing other seeds, and killing young seedlings. Creating furrows to roughen the soil surface reduces many problems, but complicates seed placement for most equipment. In semiarid and arid wildlands, survival is highest when the seed are placed at the bottom of furrows. Grass seed planted on the ridges between the furrows, or on the south facing sides (in Northern Hemisphere), have higher mortality. Where the climate and soils allow long periods of standing water, seedling establishment is greater on the furrow tops.

Compacted surface- and sub-soils are common problems. Ripping or subsoiling involves pulling a steel shank through soils to break up compacted subsurface layers. The shanks are over 45 cm long and spaced about the same distance apart. Deep ripping is effective at reducing the detrimental impacts of surface and subsurface compaction and increasing the precipitation-use efficiency on semiarid rangelands.

Plowing improves crusted or compacted surface soils, at least temporarily, and kills or damages competing vegetation. Moldboard plows incorporate organic materials and bury weed seed deeply enough to prevent emergence, but have high-energy requirements. Chiseling or disking temporarily break surface crusts and kill shallow-rooted weeds. Harrowing reduces soil clods before drilling.

Creating a firm seedbed is an important component of effective seedbed preparation when establishing grasses and forbs. Firm seedbeds hold water near the surface and make it easier to control seeding depth. The final mechanical operation should leave a seedbed that is loose enough for good water infiltration and firm enough to support seeding equipment and provide good seed-to-soil contact. The most common problem with mechanically prepared seedbeds is a loose, soft seedbed. Firming improves surface-soil water retention long enough for the seedlings to establish. Packing is more effective with adequate soil moisture and is less effective on dry, light-textured soils. Packing with smooth rollers can be very detrimental under wet conditions because the smooth surface is more subject to wind and water erosion. Rolling to firm loose seedbeds prior to drilling is most effective. Rolling after broadcasting to cover seed and firm the soil is effective on freshly plowed seedbeds where compaction above the seed is not excessive.

Disk-chain-dikers have disks welded to the links of a large anchor chain. The chain rotates as it is pulled behind a crawler tractor; creating about 40,000 basins ha-1 arranged in a pattern of diamond shaped basins approximately 10 cm deep (Wiedemann and Cross 1990). Attaching a chain-diker behind the disk-chain improves tillage, land smoothing, and basin formation, in a single pass. This equipment requires high horsepower crawler tractors, but is very effective on sites with relatively large amounts of brush or woody debris. These seedbeds are well suited for aerial seeding. Broadcast seeders attached to the crawler tractor or the disk-chain-diker apply seed to the area in one operation. This equipment does not cover the seed. However, the next rainfall event erodes soil to the bottom of each pit and covers the seed.

Land imprinting uses heavy rollers to make an imprint in the soil surface that increases infiltration, runoff, and erosion. Imprinting is most effective on sites with few competing plants and sandy or loose soil. Imprinting is the most effective direct seeding technology in the hot, dry Mohave Desert environment because it concentrates rainfall in the basins formed by the imprinter (Holden and Miller 1993). Seed are often broadcast in front of the imprinter and pressed firmly into contact with the soil, or broadcast behind the imprinter so that splash erosion covers seed in the depressions. Very small seeds that are buried too deeply in loose soils by land imprinters, are most effectively broadcast behind the imprinter.

Chemical seedbed preparation

Herbicides improve the establishment of planted species by controlling competing vegetation without damaging planted species. This requires herbicides that are either physiologically selective or selectively applied. Properly used, herbicides with the desired physiological selectivity, will control competing vegetation without damaging desired species. Herbicides that damage seeded species are only useful if applied at a time or place that limits damage to the seeded species. Foliar-absorbed herbicides are useful when applied directly to target plants or applied before planting, providing they have no lasting soil activity. Soil- or foliar-active herbicides can maintain a chemical-fallow that reduces weed seed and limits soil water losses to weeds. Chemical fallow techniques provide excellent control over wind and water erosion at less cost than mechanical fallows with several tillage operations.

While there are numerous additional uses of herbicides during seedbed preparation, their use is too site- and species-specific for any comprehensive description here. No universal recommendations for herbicide use as a site preparation tool are possible because of differences in legal restrictions and species tolerance. Soil and environmental factors will significantly affect herbicide movement and persistence. Consequently, effective chemical site preparation requires a thorough knowledge of herbicide effects on the species and the environment. Local knowledge of herbicides, their fate in the environment, and their selective application for wildland repair activities should be used to guide the development of effective strategies.

Burning as a seedbed preparation method

Fire removes the woody debris and herbaceous litter that interferes with mechanical seedbed preparation. The effectiveness of fire in debris removal varies with environmental conditions and the amount and distribution of fuel. Burning conditions favorable for debris removal are more hazardous than burning conditions for maintenance burning. The ignition and consumption of woody debris requires hotter, drier environmental conditions. High intensity fires can damage some desired species. So, carefully assess the potential damage against the expected benefits.

Biological seedbed preparation

Biological seedbed preparation includes nurse crops, preparatory crops, and woody plants to ameliorate harsh soil and microenvironmental conditions. Although each of these three methods requires two separate plantings, the timing of those plantings is different. Nurse crops and woody plants are usually grown simultaneously with the desired species, but preparatory crops are grown and harvested (or plowed under) prior to planting the final species. Effective repair strategies not only address initial establishment concerns, they initiate natural processes that continue to improve seedbeds, and facilitate the long-term recruitment of additional plants.

Nurse crops, also called companion crops, often help establish improved pastures in humid regions and irrigated pastures. Under these conditions planting nurse crops at or near the time when the perennial species are planted has several advantages, including (1) reduced wind and water erosion; (2) less weed competition; (3) seedlings are sheltered from wind and severe temperature; and (4) nurse crop provides forage before the perennial species are fully developed. The competition, from nurse crops, must be controlled (partitioned in time or space) to increase perennial species establishment. Nurse crops delay perennial plant establishment, on most arid- and semiarid wildlands, except in years with unusually high precipitation. Nurse crops are less frequently used on water-limited wildlands, or where soil fertility is limited.

Oats (Avena fatua) and barley (Hordeum vulgare) are common nurse crops for establishing perennial plants. Common rye (Secale cereale) is too competitive for a good nurse crop, and wheat (Triticum aestivum) somewhat intermediate. Reduce competition from these nurse crops with strategies that: reduce the seeding rate of oats or barley to between 7 and 11 kg ha-1; drill nurse crops and perennial species at 90O angles or in alternate rows; and harvest the nurse crop early.

Planting annual, residue-producing crops during the growing season prior to seeding a perennial species and then directly seeding into the residue is the preparatory crop method. Preparatory crops are effective because they (1) reduce wind and water erosion; (2) reduce evaporation; (3) reduce weed problems; (4) protect young seedlings from sand damage; (4) lessen seedbed temperature extremes; (7) trap snow during winter to increase soil water; and (8) have income potential (from sale of grain) that partially offsets expenses. The preparatory crop method is the most successful seedbed preparation method in the southern US Great Plains. It is most effective under dryland conditions where wind and water erosion is serious hazards. Preparatory crops reduce soil surface drying and crusting following rain. In northcentral Texas, seeding in the dead litter of preparatory crops was 88% successful, while seeding into clean, tilled seedbeds was 67% successful (GPAC 1966).

Strip cropping is a variation of preparatory cropping that has been used in the semiarid portions of the North American Great Plains where wind erosion is a serious hazard. During strip cropping, mechanically fallowed strips (each 10-m wide) are alternately seeded to perennial grasses. Grass strips alternate with similar sized strips planted to annual crops, such as cotton (Gossypium hirsutum), wheat or grain sorghum. Then after the grass strips are established the previously cropped strips are fallowed one year and planted to grasses the next year. This reduces wind erosion hazards during the entire establishment process compared to planting perennial grasses on the entire area during the same year.

Water harvesting

Specialized surface soil modifications that collect runoff water require additional investments, but are the most reliable establishment technique in many areas. Some aridland farming systems harvest water from areas treated with latex, asphalt, or wax to improve runoff efficiency (Ffolliott et al. 1994), but those approaches are uncommon on wildlands. The most common strategies include some method to harvest or concentrate runoff water. The direct benefits of water harvesting strategies such as pitting and contour furrowing are generally short lived. These soil modifications have a finite life that is determined by erosion rate, depth, and precipitation events. However, even with a short lifespan, they can establish long-lived plants that have a lasting, self-perpetuating impact on the site. Water-harvesting techniques that establish shrubs to change microenvironmental conditions and harvest wind-blown soil, nutrients, and propagules may have long-term benefits in arid- and semiarid ecosystems (Whisenant 1995; Whisenant et al. 1995; Whisenant and Tongway 1995).

Creating depressions in the soil surface to concentrate water increases seedling survival and dramatically increases agricultural productivity in arid ecosystems. Microcatchments harvest water from within 100 m of collection basins and are effective where there is no defined stream channel. They are most appropriate in arid regions with high runoff coefficients, with the basin to catchment ratio (ratio of water holding area to water harvest area) being determined by slope, rainfall characteristics, runoff rate, and the requirements of planted species. In the northern Negev Desert (99 mm mean annual precipitation) 95% of the Atriplex halimus seedlings planted in 32-m2 microcatchments established, while those only receiving direct precipitation suffered 100% mortality (Shanan et al. 1970). Atriplex seedlings established within the basins, but nowhere else. This microcatchment system greatly increased productivity. In southern Arizona (150- to 200-mm mean annual precipitation), microcatchments increased Cenchrus ciliaris (buffelgrass) productivity 5-fold over a 4-year period (Slayback and Cable 1970).

Water harvesting does not guarantee success. Seedings in water harvesting environments may fail during very dry years. Water harvesting is often unnecessary during wet years. However, water harvesting increases seedling establishment and plant production during the years that are neither too dry nor too wet. Like other risk-reduction strategies, water harvesting increases the probability of success, it does not eliminate failure. Deciding which water harvesting strategy (if any) is most appropriate for a particular application requires an understanding of local precipitation patterns and seedling establishment requirements.

Mulches

Seedbed mulches reduce soil erosion, lessen temperature extremes, conserve soil moisture, increase seed germination, and increase seedling growth. The benefits of mulches appear greatest in arid environments (Winkel et al. 1991; Roundy et al. 1997) and where weed competition is a serious obstacle. Organic materials provide nutrient supplements and improve the soil's resource retention capacity. When mulching in low moisture situations, mix leaves of rapidly decomposing species with more slowly decomposing leaves. Gravel, stones, rocks, and even oil are useful for certain applications. Gravel mulches increase germination under water-limiting conditions, unless they are too deep (Winkel et al. 1991).

Hay mulch seeding involves spreading seed-containing hay over a well-prepared seedbed. It is a favored technique for restoring native species and genotypes because it is the only way to obtain seed of some species. However, since each species produces seed at a different time, many species are absent, or under represented, from a single hay harvest. Cut the hay when the important species are at an optimum stage of maturity. After cutting, rake, dry, and stack the hay. Apply seed hay prior to the optimum seeding time for the dominant (or preferred) species within the hay. Spreading the hay by hand is labor intensive and most practical on small sites. Commercial, chopper-spreaders are available to shred and apply hay over larger areas. Typically, at least 2,000 kg hay ha-1 are required; double that rate on highly erosive sites. The hay may require anchoring where wind or water can displace the hay. Commercial hay crimpers, disking, vertically oriented coulter blades, or short-term trampling by livestock are effective at anchoring hay to the soil. Seed hay supplies seed, improves microenvironmental conditions, conserves water, and reduces soil erosion. Hay mulch seeding has long been used to heal blowouts in sandy areas by stopping sand movement and establishing a permanent vegetative cover. Sandy soils require little or no seedbed preparation before spreading the hay, if there are few competing plants. However, most situations (especially heavier-textured soils) require good seedbed preparation and should be plowed immediately prior to applying the seed hay.

Interseeding

Interseeding involves seeding herbaceous plants into an existing stand of herbaceous vegetation. Since established vegetation has a strong advantage over establishing seedlings, it is seldom advisable to drill or broadcast directly into established stands of perennial plants. Even strong competitors establish better without competition. Reduce competition from established vegetation by (1) direct seeding into severely depleted upland sites; (2) planting cool-season species into established warm-season vegetation where sufficient autumn and/or early spring precipitation occurs; and (3) using mechanical or herbicidal methods to kill strips of established vegetation. Interseeding is most appropriate where (1) erosion hazards are high; (2) complete seedbed preparation is impractical; or (3) the existing vegetation should be supplemented rather than completely replaced. Erosion hazards are reduced during interseeding, compared to complete seedbed preparation and seeding, because the site remains at least partially protected before, during, or after interseeding. Interseeding spreads seed and costs over a larger area, compared to complete stand replacement. Adding herbaceous legumes to an existing stand of grass improves forage quality. Planting dates are similar for interseeding and regular seeding approaches. Seeding rates of one-third to one-half (the full seeding rate) are typical.

Several furrow opener designs are effective when interseeding. One design used rotary blades to break up the sod in front of each double disk opener. Most interseeders use a furrow opener to remove a strip of sod from each row and then plants, covers, and packs seed into the opening. The better interseeders have good control over stripping depth, seeding depth and packing. The most effective width of the controlled strip depends on the vigor of the remaining vegetation, soil moisture, and competitive ability of the interseeded species. Furrows 20 to 25 cm wide, 5 to 8-cm deep, and 100 cm apart are common. Interseeding into highly competitive vegetation or drier environments requires wider cleared strips.

Figure 1. Cycle of soil degradation illustrating the importance of the soil surface in the continuing downward spiral of soil condition. Other pathways are possible, but this is most common. While soil surface conditions are not the causal factor in all rangeland degradation, it is the most widespread factor. These factors should be viewed as links in a chain, breaking any link can lead to the initiation of the sequence.

Figure 2. Factors influencing microenvironmental and soil conditions under and around an individual shrub or tree. After Farrell (1990).

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