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General Description of Grass Growth and Development and Defoliation Resistance Mechanisms
Llewellyn L. Manske PhD
Associate Range Scientist
North Dakota State University
Dickinson Research Extension Center
All prairies in the Northern Great Plains require management by defoliation. Defoliation management requires consideration of the biological processes of grass plants. Grass plants have developed biological processes as defoliation resistance mechanisms in response to grazing during the long period of coevolution with herbivores and from the evolutionary selective forces of fire and drought. Defoliation by livestock can be used to sustain healthy native prairie ecosystems when the biological processes of the grass plants are considered and understood.
Plant developmental morphology is the study of plant growth and development. Grassland managers need a working knowledge of grass growth and development in order to develop sound grazing management strategies, to understand when to apply specific management practices, to know the effects of various management practices on the plant communities, and to be able to anticipate the secondary effects on livestock and wildlife.
Grass Plant Parts
Grass plants consist of shoots and roots. Shoot is a collective term that refers to the stem and leaves. The shoot comprises repeated structural units called phytomers (Beard 1973, Dahl 1995). A phytomer consists of a leaf, an internode, an axillary bud, and a node (Hyder 1974, Dahl and Hyder 1977). Each shoot generally has 5 or 6 phytomers, but may have 7 or more. Collectively the nodes and internodes of the phytomers are called the stem. The vegetative stem consists of a few to several nodes and unelongated internodes, with the apical meristem at the highest node (Langer 1972). The node is the location of leaf attachment to the stem. Internodes are lengths of stem between two successive nodes. An axillary bud is a concentration of meristematic tissue capable of developing into a tiller. A leaf is divided into blade and sheath, with a collar separating the two parts. The crown of a grass plant is the lower portion of a shoot and has 2 or more nodes (Dahl 1995).
Grass Growth and Development
Plant growth is a quantitative change in plant size (Dahl 1995). Growth occurs through an increase in the number of cells by cell division in meristematic tissue (growing points) and through cell enlargement and elongation. Most new cells are produced in the apical meristem, which is located at the top of the stem. In some species the apical meristem remains near ground level (short shoots), and in other species the apical meristem is elevated before its status changes from vegetative to sexually reproductive (long shoots) (Dahl 1995).
Groups of new cells in the apical meristem form growth centers and develop into leaf primordia, which develop into phytomers. Almost all of the cells are formed while the leaf is a minute bud (Langer 1972). The oldest cells of a leaf are at the tip, and the youngest cells are at the base (Langer 1972, Dahl 1995). Elongation of cells and differentiation of cell masses into various tissue types begin at the tip of the leaf (Langer 1972).
Leaf bud primordia are formed on alternating sides of the apical meristem (Evans and Grover 1940, Langer 1972, Beard 1973, Dahl 1995). Several leaf primordia are at various stages of development at any one time. The oldest leaf is outermost, while younger leaves grow up through existing leaf sheaths (Rechenthin 1956, Beard 1973). Growth of the leaf results from an increase in cell size (Esau 1960, Dahl 1995). Cell expansion occurs in the region protected by the sheaths of older leaves. When the cells emerge and are exposed to light, expansion ceases and photosynthesis and transpiration begin (Langer 1972). The new growing leaf receives carbohydrates from roots, stems, or older leaves until the leaf's requirements for growth can be met by the leaf assimilates (Langer 1972).
A few leaf cells are produced by meristematic tissue separated from the apical meristem. This tissue, called intercalary meristem, is located at the base of the blade, the base of the sheath, and the base of the internode (Esau 1960). The leaf intercalary meristems remain in basal positions, a morphological feature that contributes to the grazing tolerance of grass plants by permitting the elevated part of the leaf blade to be removed without an accompanying cessation of growth. Intercalary meristems of leaf blades cease activity by the time the leaf collar is exposed. Once a leaf blade is fully expanded, no further growth of that blade is possible (Dahl 1995).
Individual leaves of grass plants are relatively short lived. Young middle-aged leaves are in their prime when the rate of apparent photosynthesis is maximum and the leaves begin exporting assimilates to other parts of the plant (Langer 1972). At this point the leaf has its greatest dry weight. Leaf senescence begins to occur shortly after the leaf reaches middle age. Senescence begins at the tip, the oldest part of the leaf, and spreads downward. As senescence progresses, apparent photosynthesis decreases and export of assimilates ceases (Langer 1972). The rate of senescence is influenced by environmental conditions but occurs at about the same rate as leaf appearance. During senescence, cell constituents are mobilized and redistributed to other parts of the plant (Beard 1973). This process causes weight of the leaf to decrease (Leopold and Kriedemann 1975). The percentage of dryness in a leaf blade is an indicator of the degree of senescence. Drying leaves are probably neither an asset nor a detriment to the plant.
Roots grow from the nodes that are in the crown and are on or below the ground. The internodes located in the crown and associated with roots and rhizomes do not elongate (Dahl 1995). Adventitious roots develop in parenchyma tissue at the nodes, just below the internodal intercalary meristem (Langer 1972). It appears that all roots have a limited life span, probably of little more than a year at most. Within the root system, turnover of root material is continuous, involving senescence, death, decay, and new formation.
Grass plants reproduce by two processes, asexual reproduction and sexual reproduction, which correspond to vegetative and reproductive phases, respectively. The dates for the initiation of vegetative growth for perennial graminoids are variable with species and local environmental factors, primarily temperature and photoperiod (Langer 1972, Dahl 1995), and also precipitation (McMillan 1957, Trlica 1977). The early growth is dependent on carbohydrates stored in the roots, rhizomes, or stem bases (Trlica 1977). Vegetative shoots develop from a main shoot by the process of tillering. A tiller is a shoot derived from vertical growth of an axillary bud (Dahl 1995) and is a complete unit with roots, stem, and leaves. There are two types of tillering: intravaginal and extravaginal. Intravaginal tillers grow vertically, close to the main shoot and within the enveloping leaf sheath, and tend to have a tufted or bunch-type growth habit (Dahl and Hyder 1977, Dahl 1995). Extravaginal tillers penetrate the enveloping leaf sheath and grow horizontally away from the main shoot for a distance before beginning vertical growth. This type of tillering results in the spreading or creeping growth habit of sod-forming plants (Dahl and Hyder 1977, Dahl 1995). If this horizontal growth is below the soil surface, the structure is called a rhizome (Beard 1973); if the growth is aboveground it is called a stolon (Dahl 1995). Rhizomes may be either continuous, producing tillers at progressive intervals, or terminal, producing 1 tiller when the apex turns up and emerges from the soil (Dahl 1995). Stolons have continuous growth and form tillers at progressive nodes (Dahl 1995). All young tillers are dependent on the main shoot for carbohydrates until they have developed their own root systems and mature leaves (Dahl 1995). After the tiller becomes independent, it remains in vascular connection with other tillers (Moser 1977, Dahl and Hyder 1977, Dahl 1995).
Reproductive growth can begin after the plant has attained a certain minimum amount of vegetative development (Dahl 1995). The status of the apical meristem changes from vegetative to reproductive between the 3.0 and 3.5 leaf stage (Frank 1996, Frank et al. 1997); flower bud primordia develop on the apical meristem, formation of new leaf primordia is inhibited, and no more leaf primordia can be laid down (Esau 1960, Langer 1972). The previously formed leaf bud primordia continue to grow and develop. The flower bud primordia develop into the inflorescence, with the apical dome becoming the terminal spikelet. Inflorescence initiation cannot be detected without destruction of the plant, but shortly after initiation the developing inflorescence enlarges, and swelling of the enclosing sheath, the first external evidence of flower stalk development, is noticeable. This stage of flower stalk development is occasionally referred to as the "boot" stage. At this point, 4 or 5 upper internodes, along with the attached leaf sheaths, elongate very rapidly. This short phenophase is referred to as head emergence phenophase. The inflorescence reaches near-maximum height shortly after emergence, and flowering and fertilization soon follow. According to Langer (1972), the flowering phenophase (anthesis) occurs when the lodicules enlarge and separate the lemma and palea, which are a pair of bracts that protect each floret. The feathery stigma (female part) spreads out. The anther filaments elongate and expose the anthers (male parts), which dehisce and liberate pollen. Wind- promoted cross pollination is the most common process of sexual reproduction in grasses. Moved by the wind, pollen may land on the stigmas. About 30-40 hours after pollination, fertilization occurs. Some needlegrasses (Stipa) reproduce by self-pollination prior to opening of florets (cleistogamy) (Dahl 1995). Some bluegrass species (Poa) can produce seed without fertilization (apomixis) (Beard 1973). Fertilization (union of male and female gametes) starts the seed development phenophase; the embryo is formed and starch is deposited to form a grain (caryopsis). When the grain is fully formed, it can be shed. Some seeds are shed immediately, and some remain with the inflorescence all winter unless loosened by wind or physical contact from animals.
The reproductive phase is triggered primarily by photoperiod (Roberts 1939, Leopold and Kriedemann 1975, Dahl 1995) but can be slightly modified by temperature and precipitation (McMillan 1957, Leopold and Kriedemann 1975, Dahl and Hyder 1977, Dahl 1995). Some plants are long-day plants, and others are short-day plants. Long-day plants reach the flower phenological stage after exposure to a critical photoperiod and during the period of increasing daylight between mid April and mid June. Generally, most cool-season plants with the C3 photosynthetic pathway are long-day plants and reach flower phenophase before 21 June. Short-day plants are induced into flowering by day lengths that are shorter than a critical length and that occur during the period of decreasing day length after mid June. Short-day plants are technically responding to the increase in the length of the night period rather than to the decrease in the day length (Weier et al. 1974, Leopold and Kriedemann 1975). Generally, most warm-season plants with the C4 photosynthetic pathway are short-day plants and reach flower phenophase after 21 June.
The annual pattern in the change in daylight duration follows the calendar and is the same every year for each region. Grassland management strategies based on phenological growth stages of the major grasses can be planned by calendar date after the relationships between phenological stage of growth of the major grasses and time of season have been determined for a region, with consideration of a possible variation of about ± 7 days to accommodate annual potential modification from temperature and precipitation (Manske 1980).
Plant populations persist through asexual (vegetative) reproduction as well as sexual reproduction (Briske and Richards 1995). Vegetative growth is the dominant form of reproduction in semiarid and mesic grasslands (Belsky 1992), including the tallgrass, midgrass, and shortgrass prairies of North America (Briske and Richards 1995). True seedlings develop only infrequently in established grasslands and only during years with favorable moisture and temperature conditions (Wilson and Briske 1979, Briske and Richards 1995), in areas of reduced competition from older tillers, and when resources are easily available to the growing seedling. Reproductive shoots are adapted for seed production rather than for tolerance to defoliation (Hyder 1972). Grass species that produce a high proportion of reproductive shoots are less resistant to continuous heavy grazing than are those species in which a high proportion of the shoots remains vegetative (Branson 1953). Sexual reproduction is necessary for a population to maintain the genetic diversity enabling it to withstand large-scale changes (Briske and Richards 1995). However, production of viable seed each year is not necessary to the perpetuation of a healthy grassland ecosystem.
Defoliation Resistance Mechanisms
Grass plants have developed resistance mechanisms to grazing. Plants that have grazing resistance characteristics have the ability to persist in a grazed plant community. Grazing is more than removing herbage from grass plants (Langer 1972). Grazing changes physiological processes in all parts of the plants. Grazing alters the microclimate of the plant community by changing light transmission, moisture relations, and temperature. Grazing changes the soil environment and affects soil organism activity. Grazing resistance characteristics are described in two categories: internal mechanisms and external mechanisms. Internal mechanisms are associated with herbivore-induced physiological processes (McNaughton 1979, 1983). External mechanisms involve herbivore-mediated environmental modifications (Briske and Richards 1995). The internal mechanisms are divided into two subcategories: tolerance mechanisms and avoidance mechanisms (Briske 1991). Grazing tolerance mechanisms facilitate growth following defoliation and include increased meristematic activity and compensatory physiological processes (Briske 1991). Grazing avoidance mechanisms reduce the probability and severity of grazing; avoidance mechanisms include anatomical and growth form characteristics as well as chemical defenses that deter herbivory through the production of secondary compounds reducing tissue accessibility and palatability (Briske 1991). Grazing resistance in grass is maximized when the cost of resistance approximates the benefits of resistance. Plants do not become completely resistant to herbivores because the cost of resistance at some point exceeds the benefits conveyed by the resistance mechanisms (Pimentel 1988).
Defoliation removes leaf area, immediately disrupting plant growth and photosynthesis. When defoliated by large herbivores, plants adjust through internal tolerance mechanisms during a transition period when physiological functions are modified. The resulting increased leaf photosynthetic capacity and increased carbon and nitrogen allocation enable defoliated plants to compensate for foliage losses. These processes become engaged immediately following defoliation and occur over a period of several days. Unfavorable environmental conditions at the time of defoliation can limit growth, delaying or slowing plant recovery (Briske and Richards 1995).
Carbon and nitrogen are necessary to many physiological processes within the plant. When a plant is defoliated, carbon and nitrogen levels decrease because the processes through which the plant normally acquires these elements are affected (Coyne et al. 1995). Very little if any of the root carbon is remobilized to support shoot growth (Briske and Richards 1995). The root system continues to function as a carbon sink following defoliation (Ryle and Powell 1975, Richards and Caldwell 1985). Soluble carbohydrates within the roots decline as a result of continuous utilization of carbohydrates by root respiration, nutrient absorption, and root growth (Chapin and Slack 1979, Briske and Richards 1995). Following defoliation of 50% or more of the shoot system, rapidly growing grasses in high-fertility environments reduce root growth and elongation, root respiration, and root nutrient absorption (Crider 1955). Root mortality and decomposition may begin within 36-48 hours (Oswalt et al. 1959). Some grass species adapted to growing in low-fertility environments have increased capacities for root respiration and nutrient absorption rates. These species can maintain root growth, respiration, and nutrient absorption for 48 hours following 1 severe defoliation, but 2 or more successive defoliations reduce root growth (Chapin and Slack 1979, Briske and Richards 1995).
Most of the carbon allocation for compensatory growth processes comes not from the roots but from alternative sources (Briske and Richards 1995). The carbon that may be utilized by plants for shoot growth comes from the remaining shoot tissue, stems, and rhizomes, and from alternative substrates, which include hemicellulose, proteins, and organic acids (Richards and Caldwell 1985, Briske and Richards 1995). Current photosynthetic carbon from the remaining shoot is preferentially allocated to areas of active shoot meristematic tissue and is more important for plant growth following defoliation than are carbohydrate reserves (Ryle and Powell 1975, Richards and Caldwell 1985, Briske and Richards 1995). Severely defoliated plants depend upon carbohydrate pools to initiate plant growth (Briske and Richards 1995). Carbon allocation from undefoliated tillers to defoliated tillers increases following defoliation until the defoliated tillers reestablish their own photosynthetic capacity (Welker et al. 1985, Briske and Richards 1995). The increased carbon export to defoliated tillers does not occur at the expense of carbon allocations to the root systems of undefoliated tillers (Briske and Richards 1995).
Nitrogen pools in the roots and remaining shoot tissue can be mobilized to support shoot growth following defoliation (Briske and Richards 1995). Most of the remobilized nitrogen is allocated from remaining shoot tissue; only a small portion is allocated from the root system. The amount of remobilized nitrogen from the remaining shoot is greater when the growth medium is low in available nitrogen than when the growth medium is high in available nitrogen (Millard et al. 1990, Ourry et al. 1990). Nitrate absorption within 8 hours after defoliation increases at a greater rate in grass plants grown in low-fertility environments than in grass plants grown in high-fertility environments (Macduff et al. 1989).
Defoliated plants increase photosynthetic rates of remaining foliage (Briske and Richards 1995). This compensatory photosynthesis can be induced by changes in light intensity and quality that result from grazing modifications in the microhabitat and by modifications of physiological functions caused by the indirect effects resulting from increased root-shoot ratio and mediated by cytokinins and other signals produced in the root (Briske and Richards 1995). These changes appear to affect leaf development and aging. The photosynthetic apparatus is rejuvenated, the leaf senescence rate is inhibited or reduced, and the lifespan of leaves is increased (Briske and Richards 1995). Remaining mature leaves on defoliated plants frequently develop increased leaf mass per unit area within 1-14 days after defoliation (Briske and Richards 1995). Leaves exhibiting compensatory photosynthesis after defoliation may have higher dark respiration rates, a characteristic of leaves with higher protein content (Atkinson 1986). This characteristic indicates that the foliage at the same phenological growth stage is higher in nutritional quality on defoliated plants than on undefoliated plants.
The growth rate of replacement leaves and shoots increases following defoliation. The rate of leaf area expansion following defoliation is determined by interactions among meristem type, environmental variables, and resource availability (Briske and Richards 1995). Growth is most rapid from intercalary meristems, intermediate from apical meristems, and slowest from axillary buds (Briske and Richards 1995). Expanding leaves tend to grow longer on defoliated plants than on undefoliated plants (Langer 1972). The photosynthetic rate of the regrowth leaves is higher than that of same-age foliage on undefoliated plants (Briske and Richards 1995). Enhanced leaf and tiller growth rates usually persist for only a few weeks following defoliation and are not consistently expressed in all environmental conditions or at all phenological stages within the growing season.
Defoliation management can manipulate vegetative growth from axillary buds at some phenological growth stages by reducing the influence of apical dominance. Apical dominance is the physiological process by which the apical meristem from a lead tiller exerts hormonal regulation over axillary bud growth (Briske and Richards 1995). Auxin, a growth-inhibiting hormone produced in the apical meristem and young developing leaves, interferes with the metabolic function of cytokinin, a growth hormone, in the axillary buds. Auxin does not directly enter the axillary buds, and its indirect effects are not thoroughly understood (Briske and Richards 1995). Defoliation can influence tillering from axillary buds by temporarily reducing the production of the blockage hormone, auxin, within the meristem and young developing leaves (Briske and Richards 1994). This reduction of plant auxin in the lead tiller allows for cytokinin synthesis either in the roots or in the crown or for cytokinin utilization in axillary buds. The decreased level of auxin and the resulting synthesis and/or utilization of cytokinin stimulate the development of vegetative tillers (Murphy and Briske 1992, Briske and Richards 1994). Partial defoliation of young leaf material reduces the hormonal effects of apical dominance by the lead tiller, allowing some secondary tillers (Langer 1972) to develop from the previous year's axillary buds. Secondary tillers can develop without defoliation manipulation after the lead tiller has reached anthesis phenophase, but usually only 1 secondary tiller develops from the potential of 5 to 8 buds because this secondary tiller hormonally suppresses additional axillary bud development by apical dominance. When the lead tiller is partially defoliated at an early phenological growth stage, several axillary buds can develop subsequently into secondary tillers. Apparently, no single secondary tiller is capable of developing complete hormonal apical dominance following defoliation of the lead tiller at this time. Some level of hormonal control from the older axillary buds still suppresses development of some of the younger axillary buds (Manske 1996). Under some conditions the axillary buds that have most recently matured grow out to form tillers, even though older buds may exist on the crown (Busso et al. 1989). Axillary buds survive as long as the parental tiller remains alive. The longer axillary buds remain inhibited the less likely they are to form tillers (Mueller and Richards 1986). With our present level of knowledge of this mechanism, we are unable to achieve the full potential of all axillary buds developing into secondary tillers.
Stimulation of tillering by defoliation is not consistent throughout the growing season and is influenced by stage of phenological development, environmental conditions, and frequency and intensity of defoliation. Defoliation alters the timing or seasonality of tiller recruitment and may not increase the total number of tillers in many native range grasses over the long term (Briske and Richards 1995). Interaction between the physiological stage of plant development and plant defoliation is not completely understood. Defoliation during early spring, before plants have reached the third-leaf stage, exerts a negligible stimulatory effect on tillering (Olson and Richards 1988, Vogel and Bjugstad 1968). Early season defoliation negatively affects potential peak herbage biomass production (Campbell 1952, Rogler et al. 1962, Manske 1994). In some grasses defoliation during later vegetative growth promotes tiller recruitment to a greater extent than does defoliation during any other phenological stage (Briske and Richards 1995). Defoliation at the time of stem elongation but prior to inflorescence emergence stimulates tillering in several grass species (Olson and Richards 1988). Defoliation at the boot stage suppresses tillering in some warm-season grasses that are stimulated to tillering during the inflorescence emergence stage (Vogel and Bjugstad 1968). Cool-season grass species initiate lead tillers the previous fall. Vegetative reproduction through increase in tiller development from axillary buds can be beneficially stimulated by partial defoliation of lead tillers between the third-leaf stage and flowering.
Grass plants exhibit two strategies of stem elongation, described as short or long shoots. Short shoots do not produce significant internode elongation during vegetative growth, and the apical meristem remains below cutting or grazing height, continuing to produce new leaves until it changes to the reproductive phase and the flowering stalk elongates (Dahl 1995). Short shoots that remain vegetative may have the apical meristem vegetatively active for more than one growing season (Dahl 1995). Long shoots elevate the apical meristem by internode elongation while still in the vegetative phase (Dahl 1995). Many grass species with long shoots are stimulated to increase tillers by moderate defoliation prior to flowering (Richards et al. 1988). Apical meristem removal by defoliation has been shown to increase tillering in several warm-season grasses and some cool-season grasses (Richards et al. 1988, Murphy and Briske 1992) and not to stimulate tillering in some other cool-season grasses (Branson 1956, Richards et al. 1988). Long-shoot plants are nearly always decreased in pastures that are heavily grazed continuously (Branson 1953).
Tillers recruited early in the growing season frequently become florally induced and terminate their life cycle during the same growing season, while tillers recruited later in the season frequently over-winter and resume growth the subsequent growing season (Briske and Richards 1995). The longevity of these late tillers generally does not exceed two complete growing seasons (Langer 1956, Butler and Briske 1988). Severe fall and winter defoliation has the potential to reduce grass density and production greatly the following year by reducing these late-stimulated tillers. Tiller longevity for grasses and sedges is greater at northern latitudes than at southern latitudes (Briske and Richards 1995). Plant longevity of some major northern grass species may range from 27 to 43 years (Briske and Richards 1995).
Tiller development decreases with increasing frequency and intensity of defoliation. Low levels of grazing also reduce tiller densities by decreasing tiller development and increasing tiller mortality through shading (Grant et al. 1983). The optimal defoliation intensity varies with species, stage of phenological development, and associated environmental conditions (Langer 1963). Grazing some native bunchgrass populations decreases individual plant basal area and increases total plant density (Butler and Briske 1988). However, severe grazing may reduce total basal area and tiller numbers (Olson and Richards 1988).
Internal avoidance mechanisms enhance some grass species' ability to deter herbivory by the production of secondary compounds for chemical defense and by the deposition of mineral silica in epidermal cells. Other internal avoidance mechanisms reduce plant tissue accessibility by changing growth morphology. Both heavy grazing and frequent mowing can function as selection pressure on grass plant growth morphology, causing forms to change and grow low and close to the ground. This genetically based change in growth form can occur in less than 25 years (Briske and Anderson 1992). The grazing-induced growth forms are characterized by a large number of small tillers with reduced leaf numbers and blade areas (Briske and Richards 1995). This growth form is better able to avoid grazing because less biomass is removed and a greater number of meristems remain to facilitate growth.
Long-term ungrazed grass plants shift to erect growth forms with a small number of larger tillers because of the increase in mulch accumulation and shading (Briske and Richards 1995). Shading from other plants reduces the light intensity that reaches the lower leaves of an individual plant. Grass leaves grown under shaded conditions become longer but narrower, thinner (Langer 1972, Weier et al. 1974), and lower in weight than leaves grown in sunlight (Langer 1972). Root growth is reduced because roots are very sensitive to reduction in light intensity reaching the leaves. Reduced light levels or shading has more serious effects on roots than on shoots (Langer 1972).
External mechanisms contribute to compensatory grass growth following defoliation. Grazing removes some of the aboveground herbage and increases the amount of solar radiation reaching the remaining leaf tissue. Defoliation improves plant water status as the result of an increase in root-shoot ratio and reduction of the transpiration surface. Increasing the root-shoot ratio also increases nutrient supply to remaining tissue.
An important external mechanism stimulated by defoliation of grassland plants is the symbiotic activity of soil organisms within the rhizosphere (Manske 1996). The rhizosphere is that narrow zone of soil surrounding living roots of perennial grassland plants where the exudation of sugars, amino acids, glycosides, and other compounds affects microorganism activity (Curl and Truelove 1986, Whipps 1990, Campbell and Greaves 1990). Bacterial growth in the rhizosphere is stimulated by the presence of carbon from the exudates (Elliott 1978, Anderson et al. 1981, Curl and Truelove 1986, Whipps 1990). Protozoa and nematodes graze increasingly on the proliferating bacteria (Curl and Truelove 1986) and accelerate the overall nutrient cycling process through the "fast" pathway of substrate decomposition, as postulated by Coleman et al. (1983). The activity of the microbes in the rhizosphere increases the amount of nitrogen available for plant growth (Ingham et al. 1985, Clarholm 1985, Allen and Allen 1990). The presence of vasicular-arbuscular mycorrhizal (VAM) fungi enhances the absorption of ammonia, phosphorus, other mineral nutrients, and water (Moorman and Reeves 1979, Harley and Smith 1983, Allen and Allen 1990, Box and Hammond 1990, Marschner 1992). Rhizosphere activity can be manipulated by defoliation at early phenological growth stages, when a higher percentage of the total nitrogen of the plant is in aboveground structures and a higher percentage of the total carbon of the plant is in belowground structures. At that time, partial defoliation disrupts the plant's carbon-to-nitrogen ratio, leaving a relatively high level of carbon in the remaining tissue. The plant exudes some of this carbon through the roots into the rhizosphere in order to readjust the carbon-nitrogen ratio. Under conditions with no defoliation, bacteria in the rhizosphere are limited by access to simple carbon chains (Curl and Truelove 1986). Under conditions with defoliation, the rhizosphere bacteria increase in activity in response to the increase in carbon exudates (Lynch 1982, Ingham et al. 1985). The increase in activity by bacteria triggers increases in activity in other trophic levels of the rhizosphere organisms (Curl and Truelove 1986). This elevated rate of activity increases available nutrients for the defoliated grass plant (Ingham et al. 1985, Clarholm 1985). During middle and late phenological stages of growth, carbon and nitrogen are distributed more evenly throughout the plant. Defoliation at that time does not remove a disproportionate amount of nitrogen, and very little or no exudation of carbon into the rhizosphere occurs. The decreased soil water levels that generally occur during middle and late portions of the grazing season also limit rhizosphere organism activity levels (Curl and Truelove 1986, Bazin et al. 1990).
Defoliation management by livestock can be successfully used to sustain healthy native prairie ecosystems when grazing is timed to coincide with phenological growth stages during which resistance mechanisms that beneficially manipulate grass growth and development can be stimulated. Successful management strategies are based on phenological growth stages of the major grasses and can be planned by calendar date for a geographical region. The phenological development of rangeland plants is triggered by changes in the length of daylight, which follow the calendar and are the same every year for each region. Phenological growth stages can be predicted by calendar date following regional determination surveys.
Management by defoliation with herbivores has the greatest beneficial effect if planned to stimulate two mechanisms: vegetative tillering from axillary buds and activity of symbiotic soil organisms in the rhizosphere. The phenological growth stages during which these two mechanisms can be manipulated are the same, between the third-leaf stage and the flowering phenophase. Little evidence has been found to suggest that defoliation at other phenological stages has beneficial stimulatory effects on grass growth.
Along with properly timed defoliation, periods with no defoliation should be provided to allow defoliated plants to complete the entire resistance mechanism process before successive defoliation events are permitted. Because the carbon and most of the nitrogen for recovery from defoliation are allocated not from the roots but from remaining shoot tissue, each defoliation event should be regulated to ensure that plants retain sufficient leaf surface to provide adequate assimilates for growth and recovery. Defoliation should never be severe. Heavy continuous grazing exceeds the abilities of the resistance mechanisms to tolerate defoliation. Grass plants subjected to continuous severe defoliation do not completely recover and can not produce at their potential levels.
Early spring defoliation, before the third- leaf stage, reduces the potential herbage production. Severe defoliation in the fall or winter has the potential to reduce grass density and production greatly the following year because late-stimulated tillers remain viable over winter, cool-season species initiate tillers the previous fall, and vegetative tillers that did not change to the reproductive phase may remain active for more than one growing season. Severe defoliation of these tillers reduces their contribution to the ecosystem the following summer.
Defoliation management designed to enhance sexual reproduction through seed production does not improve the prairie ecosystem. Seedlings contribute very little to plant production, and the energy and resources used in seed production could be manipulated into vegetative tiller production, which could improve the prairie ecosystem.
When the biological processes developed by grass plants are considered and understood, defoliation by livestock can be used to sustain healthy native prairie ecosystems. Sustainable prairie management requires that grass plant needs and biological processes be given the highest priority in the planned management strategy. Management strategies that give primary consideration to other goals may have short-term benefits but can not be sustained over the long term if they fail to incorporate consideration of grass plant growth and biological processes.
I am grateful to Amy M. Kraus and Naomi J. Thorson for their assistance with the literature search. Support for this study was provided by the North Dakota Grazing Association.
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Literature Citation for this Reviewed Range Management Report should be as follows:
Manske, L.L. 1998. General description of grass growth and development and defoliation resistance mechanisms. NDSU Dickinson Research Extension Center. Range Management Report DREC 98-1022. Dickinson, North Dakota. 12p.
This Range Management Report has been refereed and published as the following:
Manske, L.L. 1999. Can native prairie be sustained under livestock grazing? in Z. Abouguendia (ed.), 5th Prairie Conservation and Endangered Species Conference. Saskatoon, Saskatchewan. (Accepted)