University of California

Range Plant Growth and Development

Melvin George and Kevin Rice

Introduction

The annual plants that dominate the annual grasslands and the understory of the oak-woodlands have different life cycles from perennials that dominate most of the world’s rangelands.  In this chapter we will discuss morphological and physiological changes associated with these life cycles, photosynthesis and carbohydrate storage, and how grazing can effect individual plants.

Seasonality and Life Cycles

First let’s define some common terms such as annual, perennial, grass, forb and shrub.  An annual plantis one that starts from seed each year.  A perennial plantsurvives for several years.  While perennials may produce seeds they invest a certain amount of energy to storing carbohydrates in roots and crowns for use in respiration during dormant (cold or dry) periods and initial support of regrowth at the beginning of a new growing season.

Grasses are monocotyledonous, usually herbaceous plants with narrow leaves growing from the base.   Monocotyledonous plants or monocots are one of two major groups of flowering plants (angiosperms) that are traditionally recognized, the other being dicotyledonous plants or dicots. Monocot seedlings typically have one cotyledon (seed-leaf), in contrast to the two cotyledons typical of dicots (Figure 1).  Graminoids include the grasses of the Poaceae family, as well as the sedges of the Cyperaceae family and the rushes of the Juncaceae family (Figure 2). The grasses dominate most of the world’s grasslands but also include cereals, bamboo, and turf grasses that are cultivated for food, fiber and recreation.  Sedges include many wild marsh and grassland plants, and some cultivated ones such as water chestnut (Eleocharis dulcis) and papyrus sedge (Cyperus papyrus).  Forbs are herbaceous flowering plants that are not grasses, sedges or rushes. Forbs are dicotyledonous and non-woody.  The term, forb, is frequently used in vegetation ecology, especially in relation to grasslands.  A shrub or bush is a horticultural rather than a strict botanical category of woody plant, distinguished from a tree by its multiple stems and lower height, usually less than 5–6 m (15–20 ft) tall. A large number of plants can be either shrubs or trees, depending on the growing conditions they experience (Figure 3).

Fig 1

Figure 1  Monocotyledonous (left) and dicotyledonous (right) seedlings.

 

Fig 2

Figure 2.  True grass, sedge and a rush.

 

Fig 3

Figure 3.  Perennial plants include herbaceous grasses and forbs and woody plants include shrubs and trees.

 

Life Cycles

Phenology is the study of periodic plant life cycle events and how these are influenced by seasonal and inter-annual variations in weather. Examples of life cycle events include the date of germination or seedling emergence, or the date of flowering or seed set.  A listing of phenological stages normally includes vegetative, flowering and dormancy stages for perennials.   Table 1 is a list of 12 phenological stages or stages of maturity that have been used to study seasonal changes in forage quality of annual grasses  (George and Bell 2001).

Table 1.  Numerical stage of maturity used to predict crude protein, crude fiber, phosphorus and calcium content of annual grasses, filaree and bur clover (George et al. 2001).

Table1

Each year annual plants, germinate, become established and proceed through vegetative and reproductive stages (Figure 4).  During the winter when moisture is adequate but temperatures are low, photosynthesis and plant growth are slow.  With the arrival of spring, photosynthesis and plant growth rates increase.  As soil moisture is depleted plants begin to dry and die becoming litter.  Variation in seasonal temperatures and precipitation influence the seasonality of plant growth.

Fig 4

Figure 4.    Annual and perennial plant life cycles.

 

Perennial plants live for several years and re-grow each year (Figure 4).  Therefore you will be managing the same plant year after year.  They reproduce from seed or vegetatively with tillers, stolons and rhizomes. Short lived perennials last 3-5 years whereas properly managed long-lived perennials can survive much longer. Woody perennials can live for decades and centuries.  In temperate regions many perennial plants enter dormancy to survive freezing winter temperatures. In Mediterranean regions where there is a pronounced summer dry season and mild winter temperatures, perennial grasses and forbs may enter a dry season dormancy, regrowing with the fall rains.  Much of the aboveground biomass of perennial grasses and forbs die as the cold or dry season approaches.  The remainder of the aboveground and near surface portions of these plants enter a dormant phase until the cold or dry season is over.  During cold dormancy (winter) soil moisture is stored during rainy periods and snow melt.  As spring approaches these plants use this soil moisture to begin to grow as temperatures increase.  These plants will remain vegetative for several weeks but will eventually flower and develop seeds.  As the growing season progresses seed is set and eventually dispersed.  With cooling temperatures the plants begin to enter dormancy and much of the aboveground biomass dies and dries. Most perennial grasses and forbs in the western U.S follow this winter dormancy life cycle pattern. 

Seed Banks

When thinking about seed banks in California annual rangelands, it is important to make a distinction between short-term (i.e. transient) seed banks and long-term (i.e. persistent) seed banks. Transient seed banks refer to the seeds that spend a short time in the soil, usually the time between their production and dispersal in late spring and their subsequent germination with the onset of fall rain. In contrast, persistent seed banks refer to seeds that remain in the soil for multiple years; in fact, these seeds can remain viable in the soil for many decades. The degree of difference between the species composition of the aboveground grassland community and the community hidden underground in the seed bank can be quite striking and varies among species and grassland types. 

Although the relative contribution of different grassland species to transient and persistent seed banks can vary widely, there are some general trends. Grasses, although they are often the dominant component of transient seed banks, are much less likely to contribute to persistent seed banks. The seed of most grass species germinates in the year of its production and rarely survives more than a year or two in the soil (Young et al. 1981). Even when focusing specifically on transient seed banks, the relative contribution of annual and perennial grasses to these short-lived seed banks can be very different. A study of a mixed annual and perennial (Stipa sp.) grassland in the Central Valley found that, even in areas dominated by perennial grass cover, there were essentially no perennial seeds in the soil seed bank. In contrast, annual grasses had a well-developed transient seed bank even in areas where they were a very minor component aboveground (Major and Pyott 1966). Compared to grasses, forbs have a tendency to produce a more persistent seed bank. A five year study in a California annual grassland indicated that seed carry over rates (a measure of seed bank persistence) were more than 10 times greater in forb species than in annual grasses (Young et al. 1981). Among forbs, legumes have the greatest rates of seed carryover and thus can often form very long-lived persistent seed banks. Seed coat impermeability to water (hard seed) is a form of persistent seed dormancy that is widespread in legumes and is further described below under the section on Germination. Seed longevity may explain the dominance of legumes in persistent seed banks in grasslands. Hull (1973) compared germination rates for a number of North American rangeland species that had been stored in unheated sheds for 14 to 41 years. Seeds of legumes retained their viability longer and had much higher rates of germination than any of the grass and forb species tested, regardless of storage time.

Spatial variation in the longevity of soil seed banks can depend on the frequency of soil disturbance and the formation of gaps in both plant canopy and litter layers. Germination cueing represents the capacity of a seed to detect a potential “opening” for establishment and may involve a number a physical cues such as light quality or soil temperature fluctuations. For example, Rice (1985) reported that diurnal temperature variation among three types of microsites in annual grassland (under grass litter, bare soil, and buried under gopher mounds) significantly affected rates of dormancy release in broad-leaved filaree (Erodium botrys). Microsite variation in soil temperature germination cues in this hard seeded forb might have a strong influence on seed bank depletion and longevity by affecting the loss rate due to germination.

Germination

Seed germination (Figure 5) initiates a series of changes that ultimately lead to a mature plant and reproduction of the species.  Favorable temperature and moisture are essential for successful development of the seedling during the first critical stages of growth.    Rapid germination and growth results in a high demand for light, moisture, nutrients and other plant growth requirements. 

 

Figure 5.  Animation of the germination process (please select "Germination Process"  after selecting the link to How Grass Grows (Ervin  et. al. 2004).

 

Fully developed seeds contain an embryo and food reserves, wrapped in a seed coat.  Under favorable conditions, the seed begins to germinate and the embryonic tissues resume growth, developing towards a seedling.  Some plants produce varying numbers of seeds that lack embryos, these are called empty seeds, and never germinate. 

While water is required, temperature, oxygen and light quality may also influence germination. Mature seeds are often extremely dry and need to take in significant amounts of water, relative to the dry weight of the seed, before cellular metabolism and growth can resume. The uptake of water by seeds is called imbibition, which leads to the swelling and the breaking of the seed coat. When seeds are formed, most plants store a food reserve with the seed, such as starch, proteins, or oils. This food reserve provides nourishment to the growing embryo. When the seed imbibes water, hydrolytic enzymes are activated which break down these stored food resources into metabolically useful chemicals.  After the seedling emerges from the seed coat and starts growing roots and leaves, the seedling's food reserves are typically exhausted; at this point photosynthesis provides the energy needed for continued growth and the seedling now requires a continuous supply of water, nutrients, and light.  When a seed germinates the new seedling leaf grows toward light.  This is called phototropism.

Oxygen is required for respiration during germination.  Oxygen is found in soil pore spaces but if a seed is buried too deeply within the soil or the soil is waterlogged, the seed can be oxygen starved. Some seeds have impermeable seed coats that prevent oxygen from entering the seed, causing a type of physical dormancy which is broken when the seed coat is worn away enough to allow gas exchange and water uptake from the environment. Hard seed produced by some legumes, such as rose clover, can allow seed to survive in soil for more than 20 years. 

Temperature also influences germination.  Seeds from different species and even seeds from the same plant germinate over a wide range of temperatures. Seeds often have a temperature range within which they will germinate, and they will not do so above or below this range. Many seeds germinate at temperatures slightly above room-temperature 60-75 F (16-24 C), while others germinate just above freezing and others germinate only in response to alternations in temperature between warm and cool. Some seeds germinate when the soil is cool 28-40 F (-2 - 4 C), and some when the soil is warm 76-90 F (24-32 C).

Some seeds require exposure to cold temperatures (vernalization) to break dormancy.  Seeds in a dormant state will not germinate even if conditions are favorable. Some seeds will only germinate following hot weather and others must be exposed to hot temperatures during a forest fire which cracks their seed coats. Germination in many seeds, especially small seeds, is regulated by light and many seeds, including species found in forest settings, will not germinate until an opening in the canopy allows sufficient light for growth of the seedling.  Some seeds need to pass through an animal's digestive tract to weaken the seed coat enough to allow the seedling to emerge.

Variability in the rate of germination exists between and within species.  Seed size has been shown to be a critical factor in promoting seedling vigor.  In legumes and other forbs, seed coat hardness or impermeability often retards germination but spreads germination over years which is a survival advantage for the species.  In general, germination is reduced with increasing age of seeds. 

Seedling Establishment

With emergence of the radical during germination, seedling establishment begins and may not be considered successful until an adequate root system and leaf area has developed to sustain a high rate of growth (McKell 1972).  While there is no single attribute that influences seedling vigor and establishment, seed size and weight, rapid germination, seed age, high levels of biochemical and physiological activity, and rate of root and shoot growth are all potential indicators of seedling vigor.   Rapid root growth is fundamental to establishment and development of annual rangeland plants.  Individual plants and species may gain an advantage over competitors if they are able to maintain both rapid root and top growth.  In California, annual grasses frequently exhibit root growth rates greater than native perennial grasses (Aandrud et al. 2003).  This is one of the reasons that native perennial grass establishment is so difficult in California’s annual rangelands.

Seasonal growth rates

Growth rates in the annual rangelands following germination are rapid if temperatures are warm and slow if temperatures are cold (Table 2, http://ucanr.org/sites/sfrec/files/46258.pdf, accessed September 7, 2013).  The average germination date at the UC Sierra Foothill Research and Extension Center (UC SFREC) in Yuba County, California is October 20 and the average standing crop on December 1 is 347 lbs /acre or less than 12 percent of peak standing crop in May.  During the winter growth is slow such that the average standing crop at UC SFREC on March 1 is only about 700 lbs/acre which is less than 25 percent of standing crop of 2984 lbs/acre on May 1.  Thus in this example daily growth rates average about 5 lbs/acre/day from October 20 to March 1.  During rapid spring growth the average rates of growth are about 37 lbs/acre/day resulting in an average standing crop on May 1 of 2942 lb. /acre.  On average, little production occurs during May at UC SFREC resulting in only small differences between average standing crop on May 1 and on average peak standing crop.

Table2

On intermountain rangelands, grass growth begins with warming temperatures after a period of winter dormancy.  For example, in northeastern California there is little or no grass production from October through February or March.  Only about 10 percent of the annual production occurs in April as plants begin to grow.  In May and June growth is rapid often accounting for 50 to 75 percent of the annual production.  Growth slows for the rest of the summer with increasing temperatures in July and reductions in soil water availability(Figure 6). 

Fig 6

Figure 6.  Growing points often called buds or meristems include apical meristems, axillary buds and intercalary meristems (Ervin et al. 2004).

 

In California’s annual rangelands the grasses and forbs produced during the growing season dry and become litter as the growing season ends.  University of California has published guidelines for how much litter or residual dry matter (RDM) should be left at the end of the grazing season for soil protection and to provide mulch to protect germinating seeds (Bartolome et al. 2006).

Forage Quality

Forage quality decreases as plants progress through their yearly life cycle (Figure 7).  During the early vegetative stage of growth, before stem elongation and flowering, forage quality is greatest because cell contents and nutrient concentrations in metabolically active tissues are greater and contain less cell wall structure.  For annual plants, this stage follows germination. For perennial plants, this is the new plant growth following dormancy or regrowth following harvest.   As the season progresses cell wall increases and cell content decreases resulting in a gradual decline in protein content and digestibility.

 

Fig 7

Figure 7.  Protein, energy, vitamins and minerals decrease fiber and lignin increase as plants mature (George et al. 2001).

 

In the late vegetative stage when stems are beginning to elongate just before flowering, nutrient concentrations are usually lower than at early vegetative stages. This occurs during the adequate-green season’s spring flush of growth.  As plants begin to flower nutrients are being translocated to the flowers and nutrients in the forage continue to decline.  By the late flowering stage when the dough stage in grass seeds occurs, nutrients have accumulated in flowers and seed, resulting in a loss of nutrients in leaves and stems. This stage occurs late in the adequate-green season (Bentley and Talbot 1951).

As plants mature and begin to dry, seeds are dispersed and forage quality has declined to such an extent that it does not meet the nutritional requirements of some kinds and classes of livestock.  In the dry stage plants are cured, seed has been dispersed, and weathering is in progress. Plant nutrients are low and fiber is high.  Weathering is accelerated by rainfall that leaches nutrients from the dry residual forage.  This is the dry leached stage when forage quality is lowest.

Morphology and Development

Grass Anatomy

In this section we will discuss the morphology and development of grasses and forbs and plant reproduction.

Growing Points (buds, meristems)

There are several growing points on a grass plant (Figure 8).  Apical meristems or buds develop into the flower and are elevated during the flowering process.  Axillary buds give rise to tillers, rhizomes and stolons.  

 

Figure 8.  Growing points often called buds or meristems include apical meristems, axillary buds and intercalary meristems (to see an animation of these meristems select How Grass Grows, then select:

  • Select Shoot Development/Crown to see apical meristem and axillary buds on the crown of a grass plant.
  • SelectShoot Development/Leaf Expansion Dynamics to see the location of the intercalary meristem.
  • SelectShoot Development/Flowering to see an animation of the raising of the floral bud.
  • SelectShoot Development/Tilleringto see an animation of tiller growth from axillary buds.
  • SelectShoot Development/Rhizome and Stolon Development to see an animation of rhizome and stolon growth from axillary buds

 

Intercalary meristems allow leaves to expand following grazing or mowing.  Buds near the ground are less likely to be grazed or mowed but some buds become elevated as the season progresses increasing the risk of removal during grazing or mowing.  Delaying bud elevation reduces risk of bud removal.

Developmental Anatomy

Grasses and other plants progress through a vegetative phase, a transition phase and a reproduction phase (Figure 9).  In the vegetative phase grass shoots are mostly leaf.  Leaf blade collars remain nested in the base of the shoot and there is no evidence of sheath elongation or culm development.  In response to day-length, temperature or other environmental variables the apical meristem is gradually converted from a vegetative bud to a floral bud. This is called floral induction. This conversion phase is termed the transition phase.

Fig 9

Figure 9.  Vegetative, elongation and reproductive stages of annual plant life cycle.

 

During the transition or elongation phase floral induction begins, leaf sheaths begin to elongate and culm internodes also elongate raising the floral buds and leaf bases to a grazable height (Figure 8).  During the reproductive phase the conversion from vegetative to floral bud production is completed and the unseen inflorescence emerges from the leaf sheath.  Just before the inflorescence emerges it is covered by the leaf sheath and this is commonly called the “boot stage” of development.  Successful regrowth following defoliation depends on productive plant meristems or buds.  Plant species with growing points elevated are more susceptible to grazing than plants that keep their growing points close to the ground during most of their life cycle.

Forb anatomy

Growing Points (buds, meristems)

Forb anatomy varies between species.  This diagram of legumes identifies some common forb plant parts (Figure 10).  Like grasses and other plants, forbs have apical buds that may develop into flowers and axillary buds that produce stems, rhizomes and stolons.

Fig 10

Figure 10.  Location of growing points on a clover plant.

 

Woody plants have apical and axillary buds like those of forbs.  They also have buds on the root crown or roots that resprout following damage or loss of the aboveground stems as occurs during fire or following wood cutting.  Chamise and some oaks are strong resprouters following fire or top removal.

Reproduction

The reproductive phase is triggered primarily by photoperiod (Leopold and Kriedemann 1975, Dahl 1995) but can be slightly modified by temperature and precipitation. Some plants are long-day plants and others are short-day plants. The long-day plants reach the flowering 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 are long-day plants and reach flower phenophase before 21 June.

The 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 day length (Leopold and Kriedemann 1975). Generally, most warm-season plants are short-day plants and reach the 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.

Plant populations persist through both asexual (vegetative) reproduction and sexual reproduction (Briske and Richards 1995).  Annual plants are dependent on sexual production (seed) each year for survival.  Likewise, short-lived perennials depend on seed production. 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).The frequency of true seedlings is low in perennial grasslands and occurs 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. Sexual reproduction is necessary for a population to maintain the genetic diversity enabling it to withstand large-scale changes (Briske and Richards 1995). 

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 grazing than are those species in which a high proportion of the shoots remain vegetative (Branson 1953).

Photosynthesis  and Carbohydrates

Factors that influence photosysnthesis

When considering grazing and plants it is important to remember that 1) Plants are the only source of energy for grazing animals, 2) the formation of sugars, starches, proteins and other foods is dependent on photosynthesis, 3) plants do not get food from the soil but they obtain raw materials needed for photosynthesis and subsequent food production, and 4) when leaves are removed from plants, food-producing capacity is reduced.

Photosynthesis is the bonding together of CO2 (carbon dioxide) with H2O (water) to make C6H12O6 (sugar) and O2 (oxygen), using the sun's energy. The sugar contains the stored energy and serves as the raw material from which other compounds are made. Respiration is the opposite of photosynthesis -- the stored energy in the sugar is released in the presence of oxygen, and this reaction releases the CO2 and H2O originally bonded together by the sun's energy.

Stomata are the "pores" in leaves (and stems) through which CO2 is taken in and O2 is released during photosynthesis. Plants control when stomata are open or closed and the width of the opening .  The width of the opening is controlled by two guard cells that expand and contract to open and close the space between them.  Transpiration is the process of water loss (evaporation) through the stomata when they are open. This pulls more water and nutrients up to the top of the plant, but causes the plant to lose water and potentially dehydrate.   Water Use Efficiency (WUE) refers to how good the plant is at bringing in carbon dioxide for photosynthesis without losing much water out of its stomata. More specifically, it is the ratio of carbon dioxide intake to water lost through transpiration.

Numerous factors influence photosynthesis including leaf area, light intensity and quality, carbon dioxide content of the air, physiological efficiency, soil nutrients, water supply and temperature.  Light interception increases with leaf area (Figure 11).  Photosynthesis increases with increasing light intensity (Figure 12). Thus photosynthesis increases with increasing leaf area (Figure 13).  And it follows that forage yield resulting from photosynthesis increases with leaf area (Figure 14).

Fig 11

Figure 11. Relationship between light interception and leaf area (Brougham 1956).

 

Fig 12

Figure 12.  Photosynthesis increases with increasing light intensity. (Parsons et al. 1983a).

 

Fig 13

Figure 13.  Thus photosynthesis increases with increasing leaf area (Parsons et al.  1983b).

 

Fig 14

Figure 14.  Relationship between leaf area and herbage yield (Brougham 1956).

 

Production resulting from photosynthesis is expressed as gross primary production (GPP) and net primary production (NPP).  NPP is GPP minus respiration (R).  GPP and NPP initially increase as leaf area increases but as upper leaves begin to shade lower leaves respiration increases resulting in a decrease in NPP (Figure 15).

Fig 15

Figure 15.  Change in gross primary production (GPP), net primary production (NPP) and respiration (R) with increasing leaf area.

 

Carbohydrates and Carbohydrate Allocation

Carbohydrates stored in roots and stems are the "savings account" of many perennial forage plants. They are energy stores used for survival during dormancy and to start growth following dormancy, although photosynthesis takes over fairly quickly.   Sugars produced during photosynthesis are used as an energy source immediately or stored or converted to starch and then stored.  These stored carbohydrates are an energy source that the plant can draw on when photosynthesis is inadequate to meet current plant energy needs.   Dormancy is one period when photosynthesis is low or nonexistent.  Thus the plant draws on these reserves to start growth as dormancy ends.  The plant uses this energy for  such activities as 1)root replacement, 2) leaf and stem growth following dormancy, 3) respiration during dormancy, 4) bud formation and 5) regrowth following top removal.

Plant scientists use the term ‘source’ for tissues where carbohydrate is produced and ‘sink’ for tissues where carbohydrates are utilized. Plant organs can be either sources or sinks depending on stage of growth and environmental conditions.

In early spring, carbohydrates stored in seed, crown, stem base, root or rhizome tissues are the source for carbon and energy utilized in the formation of the first new leaves; which in this case are the sink (Figure 16). When enough leaf area has formed such that the surface area has sufficient photosynthetic capacity to produce more carbohydrate than required for growth, that leaf then becomes a source of sugar.  Once the leaf is fully extended, the sink for growth is removed.  Thereafter, sugars produced in excess of respiratory needs are available for translocation to sinks in other parts of the plant (Figure 17).  Meristematic tissues, which are undifferentiated growth points throughout a plant, have priority for allocation of carbohydrates. The sink for excess carbohydrates might be new leaves, tillers, roots, stems or seed production. During the stem elongation phase, the developing reproductive organs inside the stem are the sink. During seed filling, the stem is then the source and the seed is the sink. If adverse conditions limit seed filling, excess sugars may be leftover in the stem. High respiratory rates in cool season grasses during hot weather may become a sink, burning up sugars that might otherwise go towards growth or seed production. When growth slows or stops due to cold temperatures, lack of water or other nutrients, the sink is removed.  As long as there is green leaf tissue and adequate sunlight to allow production of photosynthates, accumulation may occur whenever carbohydrate production exceeds utilization.

Fig 16

Figure 16.  In early spring, carbohydrates stored in seed, crowns, stem bases, roots or rhizomes are the source for carbon and energy utilized in the formation of the first new leaves which are initially a sink.

Fig 17

Figure 17.  Once photosynthesis starts sugars are produced in excess of respiratory needs fairly quickly and excess sugars are translocated to sinks in other parts of the plant (new leaves, tillers, roots, stems or seed production).

The source-sink relationship between various plant organs is very dynamic, and can change hourly as environmental conditions affect photosynthetic capacity, respiration, and growth (Figure 18). Polysaccharides are too large for transport, so they are hydrolyzed to simple sugars for translocation. Much of the translocation of sugars from source to sink occurs in stem tissue. This is why stems are often higher in sugar concentration than leaves.

Fig 18

Figure 18. The source-sink relationship between various plant organs is very dynamic, and can change hourly as environmental conditions affect photosynthetic capacity, respiration, and growth.

 

Water and Nutrient Uptake

Roots take up water containing mineral nutrients (N, P, K, S, Ca, etc.) from the soil. When soil water content exceeds the permanent wilting point water is available to plants and nutrient uptake occurs.  In crop plants permanent wilting point is about 15 atm. of tension.  However, many arid land plants can extract soil water well below the permanent wilting point for crop plants.  However, most nutrient uptake occurs during periods when soil moisture exceeds 15 atm. of pressure. 

Complex interactions involving decomposition of rocks, organic matter, animals and microbes take place to form inorganic nutrient ions in soil water. Roots absorb these mineral ions if they are readily available. They can be tied up by other elements or by alkaline or acidic soils. Soil microbes also assist in ion uptake.

Mycorrhizae are a mutualism between plants and fungi.  The plant provides the fungus with carbohydrates and the fungus helps the plant get nutrients from the soil, like phosphorus and nitrogen. Plants make carbohydrates from carbon dioxide and sunlight and their growth is usually limited by nitrogen and phosphorus.  While fungi can absorb nitrogen and phosphorus from the soil, they can’t make their own carbohydrates. 

Secondary Compounds/Toxins

Secondary products or metabolites are compounds produced by plants that appear to have no function in photosynthesis, respiration, solute transport, translocation protein synthesis, nutrient assimilation, differentiation or the formation of carbohydrates, proteins and most lipids.  However, secondary compounds have important ecological functions in plants.  They protect plants against herbivores and pathogens and they serve as attractants for pollinators and seed dispersing animals.

Secondary compounds have been divided into three groups:  terpenes, phenolics and nitrogen-containing compounds.  Terpenes are the largest class and include diverse substances that are generally insoluble in water.  Terpenes defend against herbivores in many ways.  Pyrethroids are nonoterpenes that occur in the leaves and flowers of the Chrysanthemum species demonstrate insecticidal activity and are the ingredients of some commercial insecticides.  Conifers accumulated monoterpenes in resin ducts found in needles, twigs and the trunk.  These compounds are toxic to many insects.  Some terpenes are associated with odors that repel herbivores and toxins that are poisonous to herbivores.

Phenolic compounds from leaves, roots and decaying litter are sometimes the source of allelopathic compounds that reduce germination and growth of nearby plants.  Lignin is a complex phenolic compound found in cell walls of xylem and other conducting plant tissues.  While lignin strengthens and protects plants it deters feeding by animals because it is relatively indigestible.  Flavenoids are a large class of phenolic compounds that serve to attract animals for pollination and seed dispersal.  Some isoflavenoids in legumes have anti-estrogenic effects resulting in infertility of sheep with clover rich diets.  Tannins are also phenolic compounds that are common in woody plants.   Tannins are general toxins that may significantly reduce growth and survivorship of many herbivores when added to their diets.  Cattle and deer commonly avoid plants and plant parts with high tannin contents.  Tannins and other phenolics can bind dietary protein in cattle and other animals.

Nitrogen-containing compounds include alkaloids and cynogenic glycosides which are toxic to humans and other animals.  Most are biosynthesized from common amino acids.  Large numbers of livestock deaths are caused by ingestion of alkaloid-containing plants such as lupines, larkspur and groundsel.  Likewise cyanogenic glycosides are widely distributed in the legume family and in sorghum (Sorghum spp.) and corn (Zea mays) as well as some species in the rose family.

Some secondary compounds inhibit growth of other plants.  Allelopathy is the inhibition of the growth and development of one plant by another. Plants communicate with each other in a variety of ways. Plants use allelopathy as a means to guard their own space and protect their resources. Allelopathy is a strategy to reduce competition. For example, one way for a tree to protect its root space is to make other trees' roots die off using allelopathy. The tree can then pull more water from the soil for itself.

Grazing and Plant Growth

Detrimental Grazing Effects

Grazing removes leaves where photosynthesis occurs.  Photosynthesis is required for growth of new buds, leaves, stems, and roots, Photosynthesis is required for production of seeds, and for storage of carbohydrates.  Consequently, the removal of photosynthetic tissue by grazing can have detrimental effects including:  1. Reduced carbohydrate production and storage, 2. Reduced leaf and stem growth, 3. Reduced seed production, and 4. Reduced root growth.   Reduced growth can result in reduced competitive ability.

Grazing effects on the productivity of a plant are influenced by the season, intensity, frequency and duration of grazing.  Competition from neighboring plants can also influence productivity of individual plants.  A grazed plant whose neighbors are not grazed may be at a competitive disadvantage.  Thus grazing of surrounding plants can influence the productivity of an individual plant. 

Annuals

Grazing of annual grasses often results in reduced flowering and seed production (Gutman et al. 2001) with little detrimental effect on vegetative components of the plants.  Greater tillering following defoliation often compensates for the loss of photosynthetic tissue following defoliation.  The large investment of annual plants in seeds ensures its annual regeneration under moderate grazing and, possibly, even under intense grazing in the vegetative phase. However, intense grazing into the reproductive phase can severely reduce the survival of annual grasses to maturity and seed production in the same year (Noy-Meir and Briske 1996).

Season of defoliation can influence the productivity of annual grasses.  Leaf elongation in soft chess brome (Bromus hordeaceus) is complete by the time the leaf blade emerges from the sheath.  Removal of emerged leaves is not followed by regrowth of the leaf.  Instead only young leaves that have not emerged from the sheath regrow following clipping.  If clipping is delayed until flowering, removal of the terminal bud will result in cessation of growth of that shoot.  If a portion of the terminal bud remains the shoot continues to grow (Laude 1957).  While other grasses and forbs may react differently to clipping during the vegetative and flowering period, this illustrates the importance of season of defoliation. 

The growing season on annual rangelands is short.  During the growing season annual plants move from the vegetative state to flowering and maturity rapidly.  The effects of rest following grazing depends on the phenological stage and the amount of time remaining in the growing season.  Cold weather may slow regrowth following winter grazing.  During rapid spring growth regrowth following grazing is rapid but by the time a normal rest period is over the plants may have flowered and began to mature as they near the end of the growing season. 

Perennials

Holochek et al. (2004) reviewed several North American grazing studies and concluded that perennial grasses can be grazed without damage if 50 to 70 % of the leaf and stem material by weight is left intact as a metabolic reserve to support regrowth.  Without this reserve, plant productivity and growth can be reduced.  Grazing that is too close and too frequent can slow recovery from grazing and in the long run is detrimental to plant productivity, competitive ability and survival.  Grazing that is close but followed by an adequate rest period that allows  for regrowth and recovery is not detrimental but each plant species differs in the intensity and frequency of grazing it will tolerate.  Rotational grazing provides rest following grazing.  Continuous grazing can be supported by many species if grazing intensity is low enough to maintain adequate leaf area.  Long grazing periods that result in grazing of regrowth shortly after it is produced can suppress regrowth and recovery rates, especially when grazing is close. Long or continuous grazing periods that maintain adequate leaf area can be supported by many perennial grasses and forbs.  The three ryegrass plants in Figure 19 illustrate the effect of frequency and intensity of defoliation on individual plants.

  • Plant A was allowed to grow for three months without grazing, note the healthy root system. 
  • Plant B was clipped to 3 inches every three weeks for three months, it also has a relatively healthy root system. 
  • Plant C was clipped to 1 inch every week for 3 months, note the weak root system. 
  • Plant B was properly managed.  It was not clipped too intensely (closely) and it had a three week regrowth or recovery period between each clipping. 
  • Plant C was clipped to closely and too frequently to maintain adequate rooting depth and may not survive a drought.

Fig 19

Figure 19.  Effect on productivity of  different frequencies and intensities of clipping.

 

Plants that are grazed or clipped too closely (intensity), leaving little or no residual leaf material and removing growing points from tillers, are slow to regrow  as illustrated by the purple needlegrass plant in Figure 20.  This results in delayed regrowth and may result in use of stored carbohydrates. Root growth may stop in response to heavy grazing and flowering may be suppressed. Reduced root growth and root death can result in reduced drought tolerance.  The purple needlegrass plant in Figure 21 illustrates that leaving adequate residual leaf area following grazing or clipping supports regrowth.   This plant was clipped to a stubble height of 10 cm (4 inches) leaving adequate leaf area for regrowth (George et al. 2013).

 

Fig 20

Figure 20.  Regrowth of purple needlegrass following weekly clipping to  ground level for one growing season.

 

Fig 21

Figure 21.  Regrowth of purple needlegrass following weekly clipping to 10 cm (4 in).

 

Season of grazing can influence productivity following grazing.  Regrowth rates change as plants progress from vegetative states, to flowering, seed production and maturity.  Regrowth rates are faster during the vegetative state but are often slowed during flowering and seed production.  Regrowth rates slow with the approach of dormancy.  Dormant season grazing is usually not detrimental.  For perennial plants with long growing seasons, grazing followed by rest may be beneficial to plant productivity, competitive ability and survival.  Long growing seasons occur in regions with dependable moisture throughout the year and short cold or dry seasons such as New Zealand and in irrigated pastures in California.

Most rangelands have short growing seasons.  For perennial plants with short growing seasons the effects of rest following grazing may not be realized because regrowth is slowed as the plant matures and approaches the end of the growing season.  In other words the growing season is too short for rest to be as effective as in a longer growing season.

Grazing at intensities and frequencies that reduce plant growth may also reduce the competitive ability of the plant, especially if the competitor is not being grazed. Grazed plants may be shaded by adjacent competitors.  Grazed plants with reduced root systems may be less capable of extracting water and nutrients than an adjacent ungrazed plant.  Grazed plants may produce fewer seeds.  Vegetative reproduction from tillers, stolons and rhizomes may be suppressed by grazing.

Summary

In summary, the negative effects of prolonged, heavy grazing can include:

  • decreased photosynthesis
  • reduced carbohydrate storage
  • reduced root growth
  • reduced seed production
  • reduced ability to compete with ungrazed plants
  • reduced accumulation of litter or mulch which increases water infiltration and retention, plus it protects soil from erosion.

Conversely under light or moderate grazing:

  • plant productivity may increase
  • tillering may increase
  • shading of lower leaves may be reduced
  • transpiration losses may be reduced
  • ability to compete with ungrazed plants may be improved
  • Soil protecting litter will accumulate, however, excessive accumulation can also be detrimental to seedling establishment.

Literature Cited

Aanderud, Z.T., C.S. Bledsoe and J.H. Richards.  2003. Contribution of relative growth rate to root foragingby annual and perennial grasses from California oak woodlands.  Oecologia 136:424–430.

Bartolome J.W., W.E. Frost, N.K. McDougald and J.M. Connor. 2006. California Guidelines for Residual Dry Matter (RDM) Management on Coastal and Foothill Annual Rangelands. Oakland, CA:  University of California Division of Agriculture and Natural Resources Rangeland Monitoring Series, Publication 8092. 8 p.

Belsky. A.J.  1992.  Effects of grazing, competition, disturbance and fire on species composition and diversity in grassland communities. Journal of Vegetation Science 3:187-200.

Bentley, J. R. and M. W. Talbot.  1951.  Efficient use of annual plants on cattle ranges in the California foothills. Washington D.C.: USDA Circular No. 870.  52 p.

Branson, F.A.  1953. Two new factors affecting resistance of grasses to grazing. Journal of Range Management 6:165-171.

Briske, D.D. and J.H. Richards. 1995. Plant responses to defoliation:  A physiological, morphological and demographic evaluation.  In:  Wildland Plants:  Physiological ecology and developmental morphology.  D.J. Bedunah and R.E. Sosebee (eds.) Denver, CO: Society for Range Management.  p. 635-710. 

Brougham, R.W. 1956.  Effect of intensity of defoliation on regrowth of pasture.  Australian Journal of Agricultural Research 7:377-387.

Dahl, B.E. 1995. Development morphology of plants. p. 22-58. In: D.J. Bedunah and R.E. Sosebee (eds.), Wildland plants: physiological ecology and developmental morphology. Denver, CO:  Society for Range Management.

Ervin, E.H., A.O. Abaye, C. Estes, and B. Carroll.  2004.  How Grass Grows.  Blacksburg, VA:  Virginia Polytechnic Institute and State University, College of Agriculture and Life Sciences.  http://www.plantsciences.ucdavis.edu/gmcourse/module_resources/module1/resources/howgrassgrows.swf

George, Melvin and Marya Bell.  2001.  Using Stage of Maturity to Predict the Quality of Annual Range Forage.  Oakland, CA:  University of California Division of Agriculture and Natural Resources Publication 8019.  7 pgs.

George, Melvin, Glenn Nader, and John Dunbar.  2001.  Balancing Beef Cow Nutrient Requirements and Seasonal Forage Quality on Annual Rangeland. Oakland, CA:  University of California Division of Agriculture and Natural Resources Publication 8021. 9 pgs. 

George, M.R., S. Larson-Praplan, M. Doran, and K. W. Tate. 2013.  Grazing Nassella: Maintaining Purple Needlegrass in a Sea of Aggressive Annuals.  Rangelands 35:17-21.

Gutman, M., I. Noy-Meir, D. Pluda, N. A. Seligman, S. Rothman, and M. Sternberg. 2001. Biomass partitioning following defoliation of annual and perennial Mediterranean grasses. Conservation Ecology 5(2): 1. [online] URL: http://www.consecol.org/vol5/iss2/art1

Holechek, J. L., R. D. Pieper and C. H. Herbel.  2004.  Range Management. Principles and Practices (5th ed.). Pearson-Prentice Hall, Upper Saddle River, New Jersey. 606 pgs.

Hull, A.C. 1973. Germination of range plant seeds after long periods of uncontrolled storage. Journal of Range Management 26: 198-200.

Hyder 1972.  Defoliation in relation to vegetative growth.  In:  Younger, V. (ed.).  The Biology and Utilization of grasses. New York, NY:  Academic Press, Inc.  Pgs. 74-89

Leopold, A. C. and P. E. Kriedermann. 1975. Plant Growth and Development. 2nd ed. New York, NY: McGraw-Hill. 137 p.

Laude, H.M.  1957.  Growth of the annual grass plant in response to herbage removal.  Journal of Range Management 10:37-39.

Major, J. and W.T. Pyott. 1966. Buried, viable seeds in two California bunchgrass sites and their bearing on the definition of a flora. Vegetatio 13: 253-282.

McKell, C.M.  19.  Seedling vigor and seedling establishment.  In:  Younger, V. (ed.).  The Biology and Utilization of grasses. New York, NY:  Academic Press, Inc.  Pgs. 74-89

Noy Meir, I., and D. D. Briske. 1996. Fitness components of grazing induced population reduction in a dominant annual, Triticum dicoccoides (wild wheat). Journal of Ecology 84: 439-448.

Parsons, A.J.,  E. L. Leafe, B. Collett, and W. Stiles.  1983a.  Characteristics of Leaf and Canopy Photosynthesis of Continuously-Grazed Swards.  Journal of Applied Ecology 20:117-126.

Parsons, A.J.,  E. L. Leafe, B. Collett, P. D. Penning and J. Lewis.  1983b.  The physiology of grass production under grazing. II. Photosynthesis, crop growth and animal intake of continuously-grazed swards.  Journal of Applied Ecology 20: 127-139.

Rice, K.J. 1985. Responses of Erodium to varying microsites: the role of germination cueing. Ecology 66: 1651-1657.

Wilson, A. M.  and D. D. Briske.  1979. Seminal and Adventitious Root Growth of Blue Grama Seedlings on the Central Plains.  Journal of Range Management 32:209-213.

Young, J.A., R.A. Evans, C.A. Raguse and J.R. Larson. 1981. Germinable seeds and periodicity of germination in annual grasslands. Hilgardia 49: 1-37.

List of Tables

Table 1.  Numerical stage of maturity used to predict crude protein, crude fiber, phosphorus and calcium content of annual grasses, filaree and bur clover (George et al. 2001).

Table 2.  Twenty-nine years of monthly standing crop estimates for annual rangeland at the U.C. Sierra Foothill Research and Information Center.

List of Figures

Figure 1  Monocotyledonous (left) and dicotyledonous (right) seedlings.

Figure 2.  True grass, sedge and a rush.

Figure 3.  Perennial plants include herbaceous grasses and forbs and woody plants include shrubs and trees. 

Figure 4.    Annual and perennial plant life cycles.

Figure 5.  Germination animation (Ervin et al. 2004). 

Figure 6.  Growing points often called buds or meristems include apical meristems, axillary buds and intercalary meristems (Ervin et al. 2004).

Figure 7.  Protein, energy, vitamins and minerals decrease fiber and lignin increase as plants mature (George et al. 2001). 

Figure 8.  Growing points often called buds or meristems include apical meristems, axillary buds and intercalary meristems.

Figure 9.  Vegetative, elongation and reproductive stages of annual plant life cycle.

Figure 10.  Location of growing points on a clover plant.

Figure 11. Relationship between light interception and leaf area (Brougham 1956).

Figure 12.  Photosynthesis increases with increasing light intensity. (Parsons et al. 1983a).

Figure 13.  Thus photosynthesis increases with increasing leaf area (Parsons et al.  1983b).

Figure 14.  Relationship between leaf area and herbage yield (Brougham 1956).

Figure 15.  Change in gross primary production (GPP), net primary production (NPP) and respiration (R) with increasing leaf area.

Figure 16.  In early spring, carbohydrates stored in seed, crowns, stem bases, roots or rhizomes are the source for carbon and energy utilized in the formation of the first new leaves which are initially a sink.

Figure 17.  Once photosynthesis starts sugars are produced in excess of respiratory needs fairly quickly and excess sugars are translocated to sinks in other parts of the plant (new leaves, tillers, roots, stems or seed production).

Figure 18. The source-sink relationship between various plant organs is very dynamic, and can change hourly as environmental conditions affect photosynthetic capacity, respiration, and growth.

Figure 19.  Effect on productivity of  different frequencies and intensities of clipping.

Figure 20.  Regrowth of purple needlegrass following weekly clipping to  ground level for one growing season. 

Figure 21.  Regrowth of purple needlegrass following weekly clipping to 10 cm (4 in).

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