the place of c4 grasses in temperate pastures in australia 1996.pdfthe place of c4 grasses in...

New Zealand Journal of Agricultural Research, 1996, Vol. 39: 527-5400028-8233/96/3904-0527 $2.50/0 © The Royal Society of New Zealand 1996

527

The place of C4 grasses in temperate pastures in Australia

W. H. JOHNSTON

NSW Department of Land andWater Conservation

Wagga Wagga Soil Conservation Research CentreP.O. Box 146, South Wagga WaggaNSW 2650, Australia

Abstract This paper explores the potential roleof summer-active C4 grasses in temperate areas inAustralia from an ecological perspective. The mainfunctional differences between C3 and C4 plantsare briefly outlined. Taken together, the character-istics and environmental requirements of introducedC3 perennial pasture cultivars lessen their naturalcompetitive ability except in well-watered and coolhabitats. C4 plants are more competitive underconditions of high temperature and solar radiation.Within the C4 group, grasses belonging to the sub-family Panicoideae usually prefer humid, wetenvironments and generally decline in importancewith increasing grazing pressure and increased soilnitrogen (N) levels. Genera of Eragrostoideaegenerally prefer hot, dry habitats; they increase indominance with increased grazing pressure and Navailability. These responses give the C4 group asa whole a wider adaptive range and at thecommunity level provide versatility in responsesto changed conditions such as grazing. Based ontheir preferred zones of natural distribution, itappears that introduced C3 pasture species are sownwidely outside the area where they would befavoured to persist. The ecological impacts ofgrazing by domestic livestock and pastureimprovement are briefly outlined with particularemphasis on changes in botanical composition.There are similarities between changes in Australiangrasslands and those reported in South Africa. It isconcluded that species which naturally increase in

A96012Received 15 January 1996; accepted 10 July 1996

abundance in grazed pastures (such as some generaof Eragrostoideae) would offer considerableadvantages for the development of naturallysustainable pastures. Such pasture species wouldoffer a number of side-benefits including reducedrates of pH decline and deep drainage of excesswater to water tables. These two problems representmajor threats to agricultural sustainability in thetemperate zone and both result from the inabilityof C3 grass-based pastures to grow actively insummer.

Keywords C4 grasses; C3 grasses; nativegrasslands; pasture ecology; grazing

INTRODUCTION

The C3 perennial grasses sown for pastures intemperate Australia (Phalaris aquatica L., Dactylisglomerata L., Lolium L. spp., and Festucaarundinacea Schreb. (Oram 1990)) have a relativelynarrow adaptive and physiological range whichlimits their usefulness to the more mesic habitatsof the Tablelands, near slopes and coastal lowlands(Kemp & Dowling 1991; Ward & Quigley 1992).The growth of these species is depressed by lowtemperatures in mid winter when moisture is usuallyplentiful, and by the combination of unreliablerainfall, high irradiance, and high evaporation insummer (Colman et al. 1974; Smith & Stephens1976; Edwards 1979). In southern New SouthWales, the period of the year when C3 plants growat near-optimum rates rarely exceeds 12 weeksexcept in areas which regularly receive more than700 mm of rainfall annually (Edwards & Johnston1978).

There is increasing evidence that the adaptiverange of current perennial pasture grasses limitsattainment of revegetation and pasture productionobjectives over large proportions of the landscape.Problems of low persistence, increasing soilacidification, and dryland salinity are widespread(Johnston 1992). Available cultivars are poorlyadapted to shallow, stony, acid, infertile soils, more

528 New Zealand Journal of Agricultural Research, 1996, Vol. 39

arid and steeper classes of land, and to slopinglands generally near the limit of their climaticsuitability.

Because alternative, better-adapted species arenot yet commercially available, introduced C3species are widely sown in situations where theyare bound to persist poorly. Poor persistence or"pasture decline" is usually ascribed to grazingmanagement and other factors such as soil fertilityand soil pH (e.g., PDP 1992); however, lack ofadaptability may also be a potent factor in theirdemise.

This paper examines the potential role ofsummer-active C4 grasses in temperate areas inAustralia from an ecological perspective. It is notargued that C4 grasses should replace themainstream C3 species (including Medicago sativaL.) in areas and situations where they are naturallyproductive and persistent, but that taken over thelandscape as a whole, introduced and native C4grasses offer a complementary role to that of thegrasses currently sown in pastures particularly onsteeper and less productive landscapes.

Functional aspects of C3 and C4 photosynthesisAccording to Smith & Brown (1973), C4

photosynthesis occurs in about 50% of the speciesof grasses on the earth. Adaptive and evolutionaryaspects of the pathway have been outlined byOsmond (1987) and Downton (1971). The C4

pathway is essentially a CO2-concentratingmechanism, which by operating at low stomatalconductances, allows carbon (C) fixation at muchlower water cost than with C3 plants. The mainadvantage at low and middle latitudes is that underthe climatic conditions typical of summer (hightemperature, light intensity, and evaporativedemand), the pathway provides a water useefficiency advantage unmatched by C3 plants(Black 1971; Osmond et al. 1982). This gives C4

plants a competitive advantage (Black et al. 1969).For C3 species, photorespiration increases under

conditions of high light intensity and temperature(greater than about 25°C) (Black 1971; Osmond etal. 1982). This phenomenon which is not known tooccur in C4 plants, may significantly reduce net Cfixation (Black et al. 1969). Other differencesinclude that the temperature optima for photo-synthesis by C4 plants is generally higher than forC3 species (30-40 versus 10-25°C); light saturationcurves show that C4 plants need more sunlightexposure than C3 plants; and, unlike C4 plants, C3plants require open stomates and rapid gas exchange

with the atmosphere for efficient photosynthesis atlow light intensities and temperatures (Black et al.1969; Osmond et al. 1982). High transpiration inC3 plants regulates leaf temperature by increasinglatent heat loss in proportion to heat load; C4 plantsdepend to a much greater extent on sensible heattransfer for dissipating excess radiant energy(Downes 1969).

There is no evidence that C4 plants areintrinsically less sensitive to water stress (Osmond1987); however, there is ample phytogeographicevidence that C4 plants prefer (and perhaps C3plants avoid) light, saturated, xeric habitats butthis has not been linked to their biochemical andphysiological differences except by inference.Although there is a tendency to view the variouscharacteristics of C3 and C4 plants in isolation, andto focus on just one attribute such as water useefficiency, C3 and C4 photosynthetic pathwaysrepresent "complete packages" of characteristicswhich together confer ecological benefits enablingdifferent species to either occupy or co-exist invarying proportions over the whole range of habitatspresented by the landscape.

Botanical surveys in Australia (e.g., Costin1954; Benson 1994) reveal that C3 and C4 plantsco-exist except in extremely dry or cold habitats.In this respect the pathways should be viewed ascomplementary mechanisms which permit bio-regulation of nutrient cycling and water expenditureover time and which allow co-habitation whileminimising direct competition (Specht 1972).

Where C3 and C4 grasses grow bestA world viewIn summarising his early research detailing thecentres of origin and natural distribution of grasses,Hartley (1966) noted that grasses are one of thelargest families of flowering plants; they occur onall continents, in most habitats, and are present inalmost all types of vegetation. He also noted thatalthough grasses demonstrate an outstanding levelof adaptiveness to diverse habitats and they havebeen cultivated for a long period of time, onlyabout 40 of the total of perhaps 10 000 grass specieswhich occur in nature are used as cultivated pastureplants. Most of these species originated from thetemperate/Mediterranean zone, some from EastAfrica, and several from subtropical South America.This extremely low level of species domesticationwas also noted by Russell & Webb (1976) followinga survey of participants at the XI InternationalGrassland Congress. Given the vast genetic resource

Johnston—C4 grasses in temperate Australia 529

represented by naturally occurring grasses, it isdifficult to concede that ecotypes which would beuseful and persistent in almost all terrestrial habitatsdo not already exist in nature.

The cultivated C3 grasses from the temperate/Mediterranean zones of Europe and northern Africaowe their drought resistance to root penetrationand moisture acquisition (water spending as inLevitt (1972)) to overcome problems posed bytheir high water requirements (e.g., Johns &Lazenby 1973a,b; Feldhake & Boyer 1985). Asmoisture reserves are depleted, or in the face ofincreasing evaporative demand, agriculturallysuccessful C3 perennial grasses depend for theirsurvival on leaf senescence and dormancy coupledwith structures which minimise water loss such asdeep roots, large metaxylem vessels, heavilysuberised root endodermis, and relatively smallevaporating surfaces (MeWilliam & Kramer 1968).Species and cultivars which do not express a strongdormancy response lack persistence duringextended drought periods (Hoen 1968; Pook &Costin 1970; 1971; Hill 1985), and as rainfalldeclines or sites become more arid (Oram &Freebairn 1984; Hill 1985). Their need for year-round access to soil moisture (McWilliam &Kramer 1968) explains their poor persistence wheresoils are shallow and stony, or where root growthis impeded by physical or chemical barriers suchas low soil pH or elemental toxicities.

There are no known variants of the C3photosynthetic pathway; however, C4 plants canbe divided into three sub-groups based ondifferences in activity of the decarboxylatingenzymes: NADP-malic enzyme; NAD-malicenzyme; and phosphopyruvate carboxykinase(Gutierrez et al. 1974). Species of the first group(referred to as NADP-ME type species) form malicacid as the first product of photosynthesis (malateformers),and those of the second and third groups(N AD-ME and PCK type species respectively) formaspartic acid (aspartate formers). Differences inleaf anatomy provide a basis for predicting pathwaysub-types (e.g., Carolin et al. 1973; Hattersley &Watson 1976; Hattersley 1984).

Gutierrez et al. (1974), Hattersley & Watson(1976), Ellis (1977), and Prendergast & Hattersley(1987) listed C4 tribes of Poaceae according to thedominant photosynthetic sub-pathways of theirgenera. Paniceae contains C3 as well as malate -and aspartate-forming C4 genera. Genera ofPanicoideae are mostly NADP-ME malate formerswhereas genera of Eragrostoideae are dominantly

NAD-ME and PCK aspartate formers. Theseassociations are shown in Table 1 for the sub-families of Poaceae as listed by Wheeler et al.(1982).

The natural distribution of C3 and C4 grasseshas been related to factors including lowtemperature, altitude, rainfall, and rainfallseasonality in North America (Terri & Stowe 1976),Korea (Lee & Chang 1985), South Africa (Vogelet al. 1978), South West Africa/Namibia (Ellis etal. 1980), Kenya (Tieszen et al. 1979), and Australia(Hattersley 1983; Prendergast et al. 1986). C3 plantsare naturally more prominent at high altitudes,high latitudes, and in high-rainfall environments.For C4 grasses, malate-forming NADP-ME speciesprefer hot, moist climates characterised by eithermonsoonal or non-seasonal high-rainfall regimes;aspartate-forming NAD-ME species prefer arid andsemi-arid, hot, dry environments with non-seasonalrainfall regimes, whereas PCK species areintermediate in their rainfall range. Several pioneerNAD-ME species within Eragrostoideae have beenidentified as possessing desiccation tolerance (Gaff& Ellis 1974).

Although many C4 species, especially NADP-ME types, are damaged by exposure to lowtemperature (Osmond et al. 1982), this is not ageneral characteristic of C4 metabolism. C4 speciesof Chenopodiaceae and Polygonaceae are commonin middle and central Asian desert habitats wherelow temperatures (< 0°C) occur for long periods ofthe year (Winter 1981).

Natural distribution of Cj and C4 grasses inAustraliaC4 species predominate over 80-85% of thecontinental area of Australia with the greatestnumber occurring in the Northern Territory andnorthern Queensland (Hattersley 1983). The largestnumber of C3 species occur on the southerntablelands of New South Wales, the uplands ofnorth-eastern Victoria, and in Tasmania. C3 speciesare most abundant (%) in the flora of south-westWestern Australia, southern-most South Australia(including Kangaroo Island), Tasmania, south-eastern Victoria and New South Wales, and LordHowe Island (Hattersley 1983).

Hattersley (1983) found that within theirpreferred temperature range, numbers of C3 and C4species increase as rainfall increases. C4 specieswere most numerous in regions with hot, wetsummers, and C3 species were most numerous inregions experiencing wet, cool conditions in spring.


The number of C4 species declined with decliningtemperature and/or decreasing summer rainfall. Thenumber of C3 species declined as temperaturesincreased and/or spring rainfall declined. Hattersleyfound that the proportion of C4 species decreasedalong the east coast south from the Tropic ofCapricorn, but increased from the coast to theinterior in the southern half of the continent.

This broad distribution pattern is evident alsoin vegetation surveys in south-western New SouthWales which show that as rainfall declineswestwards from the Great Dividing Range,summer-growing C4 species increase in relativeabundance (Table 2). C3 species predominate onlyin the high alpine areas of the Great DividingRange (Costin 1954; Benson 1994) and westward,in the shaded, moist, flood-plain habitats dominatedby tall Eucalypts camaldulensis forest communities(C. W. E. Moore 1953a). According to Downton's(1975) list, most of the Chenopods which increasein relative importance on the heavier soils of theRiverine Plain are also C4 species; most are NAD-ME aspartate-forming species (Jacobs & Chapman1984)).

Despite the apparent advantages of C4 photo-synthesis for perennial plants in drier environmentsand habitats, most of the exotic grasses which havebecome naturalised in southern New South Wales

are C3 annual species which utilise embryodormancy as a mechanism for escaping hot, drysummer conditions. In the Monaro and Tablelandsregions, 38 of the 44 naturalised grasses listed byCostin (1954) were C3; in the south-western slopes,20 of the 22 naturalised grasses listed by C. W. E.Moore (1953a) were C3; whereas 11 of the 12introduced grasses listed by Cunningham &Milthorpe (1981), and all 16 naturalised grasseslisted by Semple (1986) were C3.

In addition to Poaceae, C4 species also occur inCyperaceae (sedges and rushes) (26) and in theDicotyledonous families Aizoaceae (6),Amaranthaceae (51), Boraginaceae (2),Caryophyllaceae (2), Chenopodiaceae (97),Compositae (1), Convolvulaceae (1),Euphorbiaceae (45), Nyctaginaceae (16),Portulacaceae (8), and Zygophyllaceae (3).(Numbers in brackets refer to the number of C4

species within the families given by Downton(1975) who also listed 327 C4 species of Poaceae.)

Climatic influences on distribution patterns.

In southern Australia, the major climatic elementsof rainfall, temperature, evaporation, and relativehumidity are co-variables controlled largely byaltitude and latitude (e.g., Edwards & Johnston1978). On the western fall of the Great Dividing

Table 1 Association between photosynthetic pathway types, taxonomic groups, and centres of natural distributionin Poaceae (after Guterrez et al. 1974; Hattersley & Watson 1976; Ellis 1977).

Family1

Pathway type

Sub-families

Tribes

Dominantlymalate formers

Panicoideae

MaydeaeAndropogoneae(Paniceae)2

Arundinelleae

Poaceae

Dominantlyaspartate formers

Eragrostoideae

(Aristideae)2

PappophoreaeChlorideaeEragrosteaeSporoboleaeZoysieae(Trioidieae)2

(Gramineae)

Arundinoideae

ArundineaeEriachneae

DominantlyC3

Pooideae

AgrosteaeAveneaeTriticeaePoeaePhalarideae

Bambusoideae

BambuseaeOryzeaeEhrharteaeStipeae

Centres of naturaldistribution

High rainfall Hot drytropicala; climatesb;

Paniceae: long, wet season non-seasonal rainfall

Andropogoneae: monsoonal, highwinter temperatures

All originate from cool temperate/Mediterraneanzones0; cool summer temperatures, shadedhabitats

'Classification as outlined and discussed by Wheeler et al. (1982) for New South Wales grasses.2Parentheses indicate tribes which do not fall within pathway type nominated. Genera of Aristideae and Triodieae

are dominantly malate formers. Malate forming as well as aspartate forming species of genera of Paniceae occur;Panicum includes malate and aspartate formers as well as C3 species.

aHartley (1958a; 1958b); bHartley & Slater (1960); cHartley (1950).

Johnston—C4 grasses in temperate Australia

Range, annual rainfall increases coincidently withaltitude, and therefore inversely with temperatureand evaporation and it becomes seasonally morewinter-dominant with distance southward fromabout 33° latitude (Edwards 1979; Johnston 1993a).Rainfall frequency also increases with altitude sothat in high-rainfall environments the length ofrainless periods is less than in lower rainfall areas.

The use of rainfall to describe the limits ofoccurrence or usefulness of pasture species (e.g.,Kemp & Dowling 1991) integrates a number ofclimatic variables, and rainfall per se may not bethe operative factor determining where species growbest. There is evidence that irrespective of moistureavailability, C4 species are more competitive thanC3 species as temperature increases (Cook et al.1976; Pearcy et al. 1981). This highlights themultidimensional nature of factors determining therespective adaptive limits of C3 and C4 species andis consistent with the general observation that evenin high (> 700 mm) rainfall areas of south-easternAustralia, summer-active grasses are mostprominent on exposed, unshaded sites.

There is clearly a need to identify the climatic,edaphic, and topographic limits within which themainstream pasture cultivars are naturallyproductive and persistent. The informationpresented here indicates that C4 grasses enjoy acompetitive advantage in medium-low (less thanabout 700 mm), unreliable, or seasonally summer-dominant rainfall areas. If this is the situation,current pasture cultivars are only truly adapted to arelatively small proportion of the temperate zoneand to sites which by virtue of their position in thelandscape, naturally enjoy a long period of moistureavailability. Addressing the pasture needs of thegreater proportion of the temperate zone requires a

Table 2 Proportion of C3 and C4 species withinPoaceae and (Poaceae + Chenopodiaceae) over a broadtransect through southern NSW as reported in a numberof botanical surveys.

Zone

Monaro and tablelandsa

South-western slopes'1

Central-western plains0

Southern Riverine Plaind

Riverine Plaine

Poaceae%C 3

7134273934

%C

2966736166

Poaceae +Chenopodiaceae

4 %C3

6220121317

% C 4

3880888783

Summarised from: aCostin (1954); bMoore (1953);'Cunningham & Milthorpe (1981); dSemple (1986);eLeigh & Mulham (1977); and Mulham & Jones (1981).

531

considerable expansion in the range and physio-logical characteristics of species available for usein grazed pastures.

Factors which modify the competitiveness ofC3 and C4 grassesNon-climatic and non-geographic factors whichimpact on the survival of perennial grasses includecompetitive relationships, especially with C3 annualgrasses and legumes, and factors related to soilmoisture availability, fertility, and nutrientresponses including response to soil pH. Withinany one region, these factors may vary over shortdistances, and may interact with responses tofertiliser application, grazing, and other manage-ment variables.

Some ecological impacts of pasture improvementand grazing

There have been wholesale changes in theoccurrence and frequency of individual plantspecies and plant species groups in response toclearing, grazing, cultivation, and plant introductionthroughout the temperate zone. This has resultedin displacement of the original complex vegetationby relatively simple communities having a similarbotanical composition over a wide range ofenvironments and soil conditions (Moore 1959).In southern Australia, the general changes havebeen from summer- to winter-growing species, fromperennials to annuals, and from native to introducedspecies (Moore 1953b; Moore 1959,1965; Whalleyetal. 1978).

The effects of grazing on mixed grasslands hasmost often been ascribed simply to defoliation andtrampling (e.g., Donald 1970; Whalley 1970;Southwood 1972; Taylor & Whalley 1976), andeven though some of the species which disappearearly in response to grazing are also reputed to berelatively unpalatable (Robinson & Lazenby 1976),this simple cause/effect model has not beenseriously challenged.

The changes induced by pasture improvementand grazing in the grasslands of the temperate zoneembody elements of each of the successionalmodels discussed by Connell & Slatyer (1977).Invasion by "outside" species is facilitated bychanged light, moisture, and nutritional relation-ships triggered largely by clearing and applicationof fertiliser. C3 plants, particularly annuals, arefavoured as invaders because their life cycle iscomplementary rather than antagonistic to that of


the resident vegetation and their preference forshade (Story 1967) facilitates invasion of tall grasscommunities. For invading species, patterns ofgermination, biomass accumulation, flowering, andseed set are particularly important determinants ofsuccess (Biddiscombe et al. 1954; Cocks & Donald1973; Lodge 1981; Amor 1985). Invading species,particularly legumes, pave the way for furtherchange (these are the facilitators).

Increased soil nitrate levels during and at theend of summer (Simpson 1962; Moore 1965) andreduced soil moisture availability in spring favourshort-lived, nitrogen (N)-responsive grasses andnitrophyllous weeds (e.g., Biddiscombe 1953;Smith 1965; Hutchinson 1970; Lodge & Roberts1979; Taylor et al. 1985). At the same time, and asthe proportion of surviving summer-active plantsdeclines, excess nitrate is leached and soils slowlyacidify (Helyar 1987; Simpson 1987; Crocker &Holford 1991). Annual water use declines becauseless water is expended in summer and increasingamounts of water drain to water tables in autumnand winter (Johnston 1993a). Phosphorus fixationand immobilisation (Cornish & Myers 1977; Lewiset al. 1987; McLaughlin et al. 1988), combinedwith declining soil pH and increasing levels ofseasonally available nitrate reduces the com-petitiveness of annual legumes (Williams 1980;Carter et al. 1982; Wolfe & Lazenby 1973b). Thisleads ultimately and inevitably to pasturedegeneration and the loss of sown species (Cook etal. 1978a, b).

Loss of annual legumes is generally accom-panied by an increase in species which utilise soilnitrate (annual grasses and a range of herbs as wellas N-responsive perennial grasses) so that there isan inverse relationship between populations oflegumes and N utilisers over time (Carter & Day1970; Wolfe & Lazenby 1973a, b; Mclvor& Smith1973; Arnold & Anderson 1987). In the short term,the ratio of legume to grass is highest where highrates of superphosphate are applied (Wolfe &Lazenby 1973a, b; Simpson et al. 1974; Langlandset al. 1979), where pastures are grazed heavily(McLachlan 1968; Carter & Day 1970; Taylor etal. 1985), and where they have been sown withcompanion C3 perennial grasses (Cook et al. 1978a;1978b; Campbell & McDonald 1979). The loss ofclover content over time (3—5 years) (Carter et al.1982) has not been satisfactorily explained but nodoubt it is a dynamic and complex process relatedto a switch in the competitive advantage of non-legumes as fertility increases (Anderson &

McLachlan 1951; Arnold & Anderson 1987). Thiscould be because of stimulation of non-legumes ordepression of legumes as fertility rises, or acombination of both.

All the introduced C3 pasture species, bothannual and perennial, have about the same growthrhythm and consequently they all compete for thesame space, light, water, and nutrient resources atabout the same time and they are all largely dormantand senescent at the same time. Their need totranspire water freely during their seasons of activegrowth means they depend to a greater extent oncurrent rainfall to meet their water requirementsbecause stored soil water is rapidly expended(Snaydon 1972; Johns & Lazenby 1973a; Guobin& Kemp 1992). If their obligate need for adequatemoisture is not met their persistence declines,especially during extended dry periods (Pook &Costin 1970; Brownlee 1973; Peake et al. 1975;Wolfe & Southwood 1980).

Land managers in the south-eastern temperatezone are encouraged to achieve and maintain apasture dry matter balance of about 40% grass,40% legume, and 20% other (useful) species. Thisis difficult and temperate pastures are renownedfor their instability (Smith 1965; Carter et al. 1982;Scott 1993). In practice, botanical composition maychange widely within and between years (e.g.,Wolfe & Lazenby 1973a). This is a barrier to theattainment of theoretical pasture composition goalsand there is a need to seek greater understanding ofthe ecology of grazed temperate pastures, especiallythe dynamics of pastures where annual plantscontribute a high proportion of total biomassproduction.

The trade-off between the growth of somespecies and the suppression of others underconditions of relatively fixed resource availabilitycan be expected to limit the annual yield of perennialand annual C3 species growing together (Hill 1985)unless the perennial has some capacity to producedry matter out of phase with the annual components(Colman et al. 1974; Wolfe & Southwood 1980;Hill 1991). This is the major advantage offered byperennial C4 grasses (Johnston & Cregan 1979;Johnston 1993b; Posler et al. 1993).

Annual C3 grasses and legumes will always beimportant components of Australian temperatepastures. These species are well placed to influencethe persistence of perennial grasses which dependon saving water in winter and spring to meet theirneeds later in summer (Begg 1959; Sharma 1976;Dunin & Reyenga 1978; Talsma & Gardner 1986).

Johnston—C4 grasses in temperate Australia

Although some authors believed that indigenousgrasses were out of step with present-day climaticpatterns in southern Australia (Specht & Rayson1957; Donald 1970), out-of-phase growth andphenological responses (Holland 1968) areimportant strategies for reducing competition andmaintaining diversity in complex vegetationcommunities (Groves 1965; Holland 1968;Williams 1971; Tremont 1994). The loss of speciesdiversity which accompanies pasture improvementand grazing of native grasslands, and the shifttowards species typical of drier habitats (Moore1959) suggests that competition for water is animportant issue for the persistence of C4 species. Itmay also be an issue for C3 perennial grasses inpastures dominated by vigorous Trifoliumsubterranean L.

The picture presented here is one of pastures intransition rather than pastures in a steady state oflong-term sustainable production. Althoughchanges in composition and structure are triggeredby clearing, plant introduction, and grazing, theprocesses involved are more likely to be caused bychanges in light, water, and nutrient regimes, andcompetition from alien plants than trampling andgrazing per se. Simple cause/effect models fail torecognise the inevitability and complexity of theseprocesses and that they are potentially on-going.They also do not provide any insights as to how theprocesses may be disrupted, re-directed, or utilisedto advantage in grazed pastures.

C4 grasses in temperate-zone pasturesAt high elevations in the southern temperate zone,patterns of moisture availability coupled withrestricted opportunities for germination andseedling survival between November and March,would generally only favour recruitment of autumn-germinating C3 plants. Thus Danthonia spp.,Microlaena stipoides (Labill.) R. Br., and othercool-season perennial grasses (including unde-sirable species such as Agrostis capillaris L. andHolcus lanatus L.) have come to replace the tall,warm-season grasses once common in areas whichhave been pasture-improved and grazed (Munnichetal. 1991).

Except for botanical surveys such as summar-ised in Table 1, there have been few detailed reportsof the occurrence and behaviour of C4 grasses onagricultural lands in south-western New SouthWales and north-eastern Victoria subsequent tothe work of Moore (1953a, b). Nevertheless thechanges reported by Moore (1953b) and later

533

elaborated on (Moore 1959) have similarities withthe gross changes which have occurred in thewarmer, northern temperate zone despite itssummer-dominant rainfall and the more frequentopportunities for seedling recruitment duringsummer (Smith & Johns 1975). In the north,temperature and rainfall conditions are morefavourable for the growth of C4 grasses generally(Colman et al. 1974) and they have remainedconspicuous components of grazed pasturesespecially on light-textured acid soils (Williams1979; Lodge & Whalley 1989).

In southern areas, C4 grasses are mostconspicuous on shallow soil upland sites whichhave not been cultivated, and which are typicallyexposed to the north-west. They have been subjectto low grazing pressure for a long period of timeand they do not express a botanical compositiontypical of improved pastures such as high clovercontent and invasion by nitrophyllous weeds exceptclose to sheep camps. Prominent C4 species includeBothriochloa macra (Steud.) S.T. Blake, Cynodondactylon (L.) Pers, Chloris truncata R.Br., andspecies of Aristida L., Eragrostis Wolf, Sporo-bolus R. Br., and Paspalidium Stapf.

Throughout the temperate zone, the summer-active grasses which have increased in abundanceunder grazing and in response to increased soilfertility are almost exclusively NAD-ME or PCKaspartate-forming genera of Eragrostoideae (e.g.,Moore 1959; Robinson & Lazenby 1976; Lodge &Roberts 1979; Lodge & Whalley 1985; Taylor etal. 1985). Responses to nutrients applied in pots(Cook et al. 1976; Robinson 1976; Harradine &Whalley 1978) coupled with nutrient transectstudies near sheep camps (Robinson & Dowling1976; Robinson et al. 1983; Rogers & Whalley1989; Rogers 1993) and responses to heavy grazing(Lodge & Whalley 1985), indicate that the changesresult from changed nutrient status (Charley &Cowling 1968) rather than defoliation. Increasedabundance of malate-forming species such asBothriochloa macra, Themeda australis (R. Br.)Stapf, and species of Aristida L. may indicatedeclining fertility and grazing pressure possiblyfollowing a period of low-impact pastureimprovement.

Responses to grazing reported in temperateAustralia parallel the successional changes notedin pastures in Southern Africa in response to bothgrazing (e.g., Davidson 1962; Gillard 1969; Bosch& Janse Van Rensburg 1987) and N fertiliserapplication (Davidson 1964; Louw 1966; Grunow


et al. 1970) which encourages the development ofsub-climax or serai grasslands dominated by NAD-ME species of Eragrostoideae. Some climax specieshave been shown or are believed to be N-sensitive(Roux 1954; Jong & Roux 1955; Ellis 1977) andthis would be a factor in their rapid disappearancefrom pastures grazed at high stocking rates.

Selection of ecotypes having desirableagronomic attributes from within species known toincrease in abundance under grazing would be auseful approach to developing cultivars for grazedtemperate-zone pastures (Johnston 1986). Specieswithin Eragrostoideae may in addition, be welladapted to environments where the availability ofsoil moisture is variable and where moisturereserves in early summer have been depleted bycompanion annual C3 plants.

Unlike C4 species which are naturally betteradapted to dry habitats, the scope for increasingthe adaptability of C3 perennial grasses to dryenvironments is limited by their need to be high-yielding in order to be competitive during theirgrowing period, and their physiological need tobecome dormant and thus uncompetitive at theonset of stress. High short-term growth rates wouldbe a prerequisite to achieving high yields over alimited growing season. However, high rates ofgrowth invariably lead to rapid water use, requiringin turn, a stronger dormancy response to ensurepersistence. Thus the more persistent grassescharacteristically have shorter growing seasons andstronger dormancy responses (Hoen 1968;Biddiscombe et al. 1977; Oram & Freebairn 1984).

Improved water use efficiency has beenproposed as a means of selecting higher-yieldingC3 genotypes for dry climates (Farquhar & Richards1984). However, there is evidence that increasedwater use efficiency may be gained at the expenseof yield (Johnson 1990; Virgona 1993 ). This isconsistent with natural selection for water-efficientplants operating most strongly in water-limited,and thus yield-limited, environments. Although theecological importance of relationships betweenyield, water use efficiency, and environment iscomplex and not fully understood (Osmond 1987),it has been recognised that plants with intrinsicallylow growth rates may be better adapted to adversesites irrespective of how efficiently they grow(Parsons 1968; Downes 1970). In this respect, wateruse efficiency may be less important than the abilityof species to synchronise their phenology withperiods of adequate water supply and to possess awell-developed capacity to maintain and re-

establish from seed following catastrophic events(e.g., Williams 1971).

Weaknesses and opportunitiesThroughout southern Australia, a major weaknessof present-day pastures is that both the quality andquantity of feed on offer declines from a peak inspring to a trough in summer and autumn (Ulyatt1981). In drawing • attention to the managementdifficulties posed by the disparity between thesepasture production patterns and the grazing needsof animals, Willoughby (1970) was of the viewthat future research should aim to increase theavailability of forage during the usual periods oflow forage supply because these set the limit to thesize of animal populations, their- production perhead, and their reproductive rates on a year-roundbasis.

Because production from pastures consistingof species with either C3 or C4 growth rhythmswill always be cyclic, it is unlikely that a single"ideal" species or cultivar will occur in nature.Management to increase the proportion of "morevaluable year-long green species" (Wilson &Simpson 1992) is therefore a somewhat idealisticaim. It would be a more practical and attainablegoal to increase the year-long availability of greenforage by utilising the specific adaptive andphysiological attributes of C3 and C4 species whichallow them to grow out of phase with each other.

The nutritive value of both C3 and C4 grasses isrelated to herbage maturity and as the proportionof stem increases during stem elongation and thedigestibility of stem declines after flowering,nutritive value declines markedly (Ulyatt 1981).Although it is generally considered that C4 grassesare inferior in herbage quality compared to C3grasses (Ulyatt 1981), and this has been shown tobe the situation for a range of native grasses innorthern New South Wales (Lodge & Whalley1983; Archer & Robinson 1988), taken season byseason, quality, like dry matter production, showsan out-of-phase rhythm. The nutritive value of C3grasses is highest in winter and declines throughsummer, whereas the quality of C4 grasses is highestin summer and declines in autumn and winter.Taken together with their growth pattern, C4 grassesoffer a high degree of complementarity with theannual and perennial species already available foruse in pastures. The use of moderately palatable C4grasses in mixed pastures may reduce fluctuationsin forage supply as well as increase water use andthe uptake of mineral N during summer and autumn.


This may reduce rates of soil acidification andwater table recharge which are major threats toagricultural sustainability.

CONCLUSIONS

Some C4 perennial grasses enjoy a number ofadaptive traits not evident in the mainstreamintroduced C3 perennial grasses. These C4 grassescan grow during the warmer months of the yearand persist on poorer classes of land in seasonallywater-deficient environments. Some genera ofEragrostoideae seem particularly adapted toconditions associated with grazing such as N build-up, aridity during early summer, and high lightintensity. Species within these genera may offerconsiderable potential in the search for naturallysustainable pastures for lower-rainfall areas andrelatively arid situations in the temperate zone.

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the place of c4 grasses in temperate pastures in australia 1996.pdfthe place of c4 grasses in...

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