short-term effects of soil water, defoliation and rangeland condition on productivity of a semi-arid...

Journal of Arid Environments (1999) 43: 47–62Article No. jare.1999.0533Available online at http://www.idealibrary.com on

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Short-term effects of soil water, defoliationand rangeland condition on productivity of a

semi-arid rangeland in South Africa

H. A. Snyman*

Department of Grassland Science, University of the Orange Free State,P.O. Box 339, Bloemfontein, 9300, South Africa

(Received 10 September 1998, accepted 5 May 1999)

Over a 3 year period, selected sites in three categories of rangeland — good,moderate and poor — were subjected to varying levels of frequency and intensityof defoliation. Three watering regimes — normal, below-normal and above-normal — were applied. The effects of defoliation and levels of soil waterwere evaluated in terms of the production of above-ground phytomass andwater-use efficiency (WUE) in relation to evapo-transpiration (Et). For alltreatments, rangeland in good condition (RC1) produced significantly morephytomass (p40)01) and used water more efficiently (p40)01) than thosein moderate and poor condition (RC2 and 3). High intensity/high frequencydefoliation caused a significant (p40)01) increase in phytomass productionand WUE, under conditions of high soil water availability. Generally, range-land condition in interaction with the soil water content are the main determi-nants of optimal production and WUE. Statistically significant (p40)01)linear relationships between phytomass production and Et were determined forall rangeland conditions and defoliation treatments. In degraded rangeland,water is used inefficiently regardless of the quantity of water received.

( 1999 Academic Press

Keywords: above-ground phytomass production; evapo-transpiration; inten-sity of defoliation; frequency of defoliation; rangeland condition; soil waterbalance; water-use efficiency

Introduction

Faced with a rapidly growing population, which must be fed and clothed, combined witha decrease in land area available for agricultural production (Snyman, 1995) anda decline in rangeland potential (O’Connor, 1994; Snyman & FoucheH , 1991, 1993), thenecessity for the improvement and sustainable management of the rangeland ecosystemin southern Africa cannot be over emphasized. Worldwide there is an increasing interestin the development of farming systems based on sustainable principles (Snyman, 1998).It is estimated that approximately 66% of rangeland in South Africa is moderately toseriously degraded (Scheepers & Kellner, 1995). Should this deterioration be allowed tocontinue, sustainable animal production will not be possible in the long term. With the

*E-mail: [email protected]

40-1963/99/090047#16 $30.00/0 ( 1999 Academic Press

48 H. A. SNYMAN

rising cost involved in producing planted pasture, the importance of rangelands to thelivestock industry is increasing.

Rangeland covers approximately 40% of the earth’s surface with approximately 80%located in arid and semi-arid areas (Snyman, 1997a). In these areas, which contain ap-proximately 65% of South Africa’s rangelands, alternative methods by which productioncan be economically increased are limited (Snyman, 1998). Water is the primary limitingproduction factor, and must be conserved and managed as effectively as possible(Snyman, 1999). Currently, animal production within the rangelands and forestry jointlyaccount for approximately 62% of consumption of the total rainfall (Bennie et al., 1997).Rangeland vegetation should therefore be managed to use rainfall efficiently.

Knowledge of the influence of defoliation on the productivity and condition of fodderplants is extremely important for sustainable animal production and management of therangeland ecosystem. Much research has already been conducted worldwide into theimpact of varying levels and frequency of defoliation (Barnes, 1960, 1961; Opperman,1967; Rethman & Booysen, 1968; Steinke, 1975; Snyman & Opperman, 1983; Snyman& Van Rensburg, 1990). However, insufficient attention has been paid to intensityand frequency as separate factors, with the result that the influence of each is diffi-cult to quantify. Trials have been too short-term to be able to provide informationsuitable for grassland management. Changes in plant species composition can dramati-cally influence the water balance (Snyman & FoucheH , 1991, 1993), as can levels ofproduction (O’Connor & Bredenkamp, 1997; Snyman, 1998, 1999), nutrient cycling(Du Preez & Snyman, 1993) and soil loss (Snyman & Van Rensburg, 1986; Snymanet al., 1987; Snyman, 1999). Information on the interaction between defoliation, climaticvariation and rangeland condition is still very limited, hindering the development andimplementation of rangeland management strategies, particularly in areas with aridityindexes (rainfall/evaporation ratio) of lower than 20% and between 20 and 40% (LeHoueH rou et al., 1993). The purpose of this study was to investigate methods in whichwater as a limited environmental factor can be sustainably and effectively managedwithin the rangeland ecosystem in semi-arid areas.

Procedure

Experimental site

The research was conducted 5 km west of Bloemfontein (29306@S, 26357@E; altitude1350 m) in a semi-arid region with an average summer rainfall of 560 mm. In January,the average maximum daily temperature ranges from 303C to 333C, in July it isapproximately 173C. Extremes of 413C in January and 283C in July have been recorded.On average, frost occurs on 119 days each year (Schulze, 1979).

The data were collected from a typical dry sandy Highveld Grassland (Bredenkamp& Van Rooyen, 1996). The soil is a fine sandy loam of the Valsrivier form (Goedemoedfamily–1121) (Soil Classification Working Group, 1991), with a 1)8% slope. The threedistinct horizons (A: 0–200 mm, B21: 200–400 and B22: 400–900 mm) contained 22, 38and 36% clay respectively and the respective bulk densities were 1651, 1688 and1788 kg m~3. Valsrivier is one of the most important soil forms of rangeland in thecentral grassland area of South Africa.

Materials and methods

Experimental design

The experimental design is illustrated in Fig. 1. A 3]3]3 (rangeland condition, waterand defoliation treatment) factorial experiment was conducted using a split plot design

SHORT-TERM EFFECTS ON PRODUCTIVITY OF A SEMI-ARID RANGELAND 49

with sub-samples per plot (Winer, 1974). There were two replications for watertreatment and three for defoliation treatment. Analysis of variance was conducted whereall interactions for the variables were included.

Rangeland condition

Three different rangeland condition sites — good, moderate and poor (henceforthRC1, 2 and 3) — were selected on the basis of their vegetation composition and studiedover a 3 year period (1991/92 to 1993/94 season). Distances of 20 m separated the sites.RC1 is dominated by decreaser species (Foran et al., 1978) such as Themeda triandraand Digitaria eriantha, while RC2 is dominated by increaser II(a) species such asEragrostis lehmanniana and E. chloromelas. RC3 largely contains increaser II(b) species,predominantly Cynodon dactylon. The composition and basal cover were intended toreflect the compositional states which arise as a result of different histories ofgrazing on this vegetation type (Potts, 1923; Mostert, 1958; Van den Berg et al., 1975;Snyman, 1988), although the distinctions between them were not maintained rigidly, aswas the case in the study of Snyman (1998). The plots represented rangeland grazed byanimals to such conditions over time.

The basal cover and botanical composition were annually determined by pointsampling at fixed intervals along two permanent 1)4 m transect lines per defoliationsubplot (n"324 lines). Strikes on rooted, living basal plant material were recorded byspecies. In the absence of a strike, the plant species nearest to the point was recorded atintervals of 200 mm. All sites were mown to a height of 30 mm before the onset of thetreatments.

RC and classification of species were determined following Fourie & Du Toit (1983).Desirability in terms of grazing value (dry-matter production, palatability, nutritivevalue, whether perennial or annual and grazing resistance) as well as ecological status(decreaser and increaser species) followed Foran et al. (1978). Classification of dryThemeda-Cymbopogon grassveld into different ecological groups followed Fourie& Visagie (1985).

Water treatments

In this study, 18 experimental plots of 6]6 m each (six plots per rangeland condition)were used. They were hydrologically isolated from the surrounding soil by means ofplastic sheets, (250 Nm thick) extending 150 mm above the soil surface. For each RC,three water treatments were included—(1) normal, (2) below-normal and (3) above-normal—with two replications per condition, randomly allocated to the six plots in eachRC. Treatment (2) was obtained by placing tins over the sites just before a rain shower,so that between 15% and 20% of the surface was covered. The quantity of watercollected by the tins after a rain shower was determined.

For treatment (3), the sites were periodically irrigated so that soil water content wasalways maintained between veld water capacity (VWC) and permanent wilting point(PWP) (Snyman & FoucheH , 1991). The content was monitored at 100 mm intervals bymeans of a neutron probe (model CPN 503) with six access tubes inserted to a depth of1)5 m in each hydrologically isolated plot. The probe was calibrated as described bySnyman et al. (1987).

Evapo-transpiration (Et) was determined by the quantification of a soil water balanceequation (Hillel, 1971). The change in soil water supply (*W) was calculated followingMoore et al. (1988), where (#) indicates an increase and (!) a decrease in the amountof water within the rootzone. As the in"ltration capacity of the soil exceeded the amountof water applied either by irrigation or rainfall, surface runo! was ignored. Deepdrainage is di$cult to measure and was excluded. For the purpose of this study, Et was

Figure 1. Experimental design. W"water treatment: 1"normal; 2"below normal; 3"abovenormal rainfall. D"defoliation treatment: 1"low intensity]high frequency; 2"high inten-sity]high frequency; 3"high intensity]low frequency. C"rangeland condition: 1"Good;2"Moderate; 3"Poor.

50 H. A. SNYMAN

calculated as follows:Et"(I#P)!(*W)

where I"irrigation, P"precipitation and *W"change in soil water supply.


Defoliation treatments

Defoliation treatments of plants in each plot consisted of three combinations of twolevels of frequency and two levels of intensity, carried out during the 1991/92 and1992/93 growing seasons. Nine 1 m2 quadrats were defoliated per hydrological unit,with three replications per treatment as follows: (1) low intensity/high frequency(60 mm height, four times per year) (2) high intensity/high frequency (30 mm height,four times per year), and (3) high intensity/low frequency (30 mm height, once a yearduring the dormant period in winter: 30 June). Cutting at 60 mm once per year duringthe dormant period in winter was excluded, as 30 mm equals the stubble height fora veld type which is normally used to obtain the seasonal primary production (Snyman& FoucheH , 1991). The main purpose of the treatments was to identify the influence ofintensity and frequency of defoliation on seasonal regrowth and therefore primaryproduction. As RC3 is dominated by pioneer plants (perennial stoloniferous grasses),defoliation was restricted to 30 mm and 15 mm for practical reasons, instead of 60 mmand 30 mm as applied to RC1 and 2 (perennial bunchgrasses). The treatments consistedof clipping with sheep-shears at intervals of 90 days, commencing on 15 October. Thewater-use efficiency was defined as the quantity of above-ground phytomass pro-duced per unit volume of water evapo-transpirated.

Results

Rainfall and irrigation

The quantity of water applied during the different water treatments for the 1991/92to 1993/94 seasons is presented in Fig. 2. On average, 644 mm water was applied overthree seasons, which is 13% higher than the average rainfall of 560 mm for the studyarea. The plants in these irrigated plots were not subject to water stress over the threeyears. The soil profile of this water treatment revealed that it was wet to a depth of500 mm for the large part of the experimental period.

The 1992/93 and 1993/94 seasons were characterized by below average rainfall receiv-ing respectively 33% and 35% less rain than the long-term average for the study area.During 1992/93, in the months December–May, an average of only 134)2 mm of rain fell,compared to the long-term average of 403)6 mm. The 1993/94 growing period was alsocharacterized by abnormally low rainfall, with an average of 52)9 mm for the monthsDecember–February, compared to the long-term average of 240)9 mm. On average,treatment (2) received 15)2% less water than treatment (1) over the experimental period.

During the dry 1992/93 and 1993/94 seasons, treatment (3) had to be applied weeklyto keep the water content between veld water capacity and permanent wilting point. Thesoil water content of areas which were not irrigated was only occasionally higher than thepermanent wilting point. For the greater part of the post-summer of the 1992/93 and1993/94 seasons the grasses were subject to water stress. This dry period was alsocharacterized by exceptionally high temperatures. Plants in the study area which werenot irrigated were solely dependent on rainfall and never became reproductive duringthis period. By contrast, the plants in the irrigated areas were reproductive throughout,with certain species even reproducing twice during a season.

Rangeland condition and basal cover

The average RC score (expressed as a percentage of that in a benchmark site) over theexperimental period ranged from 96)33% to 30)64% and the basal cover from 8)69% to3)12% (Table 1).

Figure 2. Total amount of water (mm) received by the various treatments over the 1991/92 to1993/94 seasons. Water treatments: 1"normal, 2"below-normal, 3"above-normal rainfall.

52 H. A. SNYMAN

RC1, 2 and 3 were dominated by Themeda triandra, Eragrostis lehmanniana andTragus koelerioides, which formed 57%, 40% and 41% respectively of the speciescomposition. In the RC1 irrigated sites, T. triandra increased by 15%. Digitaria erianthanoticeably decreased by an average of 49% under conditions of water stress. Sporobolusfimbriatus increased with an average of 66% in sites receiving less water. The speciescomposition of the other rangeland conditions was not influenced much by irrigation.The same trend was noticed by Snyman & Opperman (1983) in a similar climate andsoil form.

The basal cover of the RC1 irrigated sites increased by an average of 0)5% over theexperimental period, compared with a decrease of 0)4% in unirrigated plots receivingless than normal rainfall. The basal cover of RC2 and 3 was not noticeably influenced byvariation in soil water content.

Above-ground phytomass production

The average above-ground phytomass production for different water and defoli-ation treatments for every rangeland condition over the three seasons is shown in Fig. 3.Regardless of water and defoliation treatments, the production of the differentrangeland conditions differed significantly (p40)01). RC1 produced 31% and 76%on average more than RC2 and 3, respectively. A significant interaction (p40)013) wasestablished between the productions of RCs1 and 2 in both irrigated and unirrigatedplots. The former produced 48% and 54% more (p40)01) than the latter for rangeland ingood and moderate condition, regardless of defoliation treatment. The small (p'0)05)differences in production of RC3, regardless of quantity of irrigation, is noticeable(Fig. 3). Due to the particularly dry 1992/93 and 1993/94 seasons, production of thenormal (rainfed) (1) and less than normal treatments (2) did not differ significantly(p'0)05) for all of the RC or defoliation treatments. Except for RC3, the production ofirrigated sites did not differ (p'0)05) between defoliation treatments.

Table 1. Percentage species composition, ecological status totals, basal cover andrangeland condition score at each condition class for the 1991/92 to 1993/94 seasons

Ecological Species Benchmark Good Moderate Poorstatus (RC1) (RC2) (RC3)

Decreaser Digitaria eriantha 0)17 12)10 2)10Panicum stapfianumSporobolus fimbriatus 0)52 4)16Themeda triandra 81)06 57)10 13)10

Decreaser total 81)75 73)36 15)20Increaser II(a) Cymbopogon plurinodis 1)21 2)14 0)12

Digitaria argyrograpta 4)24 2)17 4)12 1)00Eragrostis chloromelas 8)13 8)16 14)16 0)05Eragrostis lehmanniana 9)12 40)15 24)12Heteropogon contortus 0)35

Increaser II(a) total 13)93 21)59 58)55 25)27Increaser II(b) Eragrostis obtusa 0)09 0)10 11)47 7)14

Triraophisandropogonoides

0)09 12)19 3)16

Increaser II(b) total 0)18 0)10 23)66 10)30Increaser II(c) Aristida congesta 0)26 0)20 3)14 21)75

Lycium tenue 1)12Tragus koelerioides 1)47 0)90 2)16 41)23Walafrida saxatilis 1)30 1)55

Increaser II(c) total 4)12 1)10 5)30 64)53Increaser II total 18)26 13)67 87)51 100)00Rangeland

condition score920)00 886)23 661)79 281)92

Rangelandcondition (%)

100 96)33 71)93 30)64

Basal cover (%) 9)06 8)69 6)32 3)12


The frequency of defoliation significantly (p40)01) influenced the production of theirrigated sites for all the RCs. Production of plants in frequently defoliated irrigated sitesin RC1 and 2 was not significantly (p'0)05) higher than those less frequently defoli-ated, but was so (p40)01) for RC3. In all the irrigated sites, the high frequency ofdefoliation produced 14%, 12% and 62% more production for RC1, 2 and 3 respective-ly. Intensity of defoliation only had a significant (p40)01) influence on production inirrigated sites—high intensity produced 15%, 14% and 35% more (p40)01) phytomassin RC1, 2 and 3 respectively, regardless of the level of irrigation. The highest orderinteraction (RC, water treatment and defoliation) in the analysis of variance was notsignificant (p'0)05) for production.

Statistically significant (p40)01) relationships were established between above-ground phytomass production and Et for all RCs and defoliation treatments, except forRC3 subjected to high frequency/low intensity, where the relationship was not signi-ficant (p40)05) (Fig. 4). The index value of agreement (d) (Willmot, 1982) is veryhigh in all treatments and varies between D"0)85 and D"0)99. In all treatments therelationships were linear.

Comparison of the slopes of the regressions of RCs subjected to the same defoliationtreatment differed significantly (p40)01) from each other, while treatments didnot produce much difference (p'0)05) from each other within a single RC. The

Figure 3. Average above-ground phytomass production of plots representative of the differ-ent rangeland conditions at different defoliation and water treatments over the 1991/92 to1993/94 seasons. Least significant differences are for the interaction of rangeland condi-tion]soil water content. Coefficient of variation (production)"9)22%. Water treatments:1"normal, 2"below-normal, 3"above-normal. Defoliation treatments: 1"lowintensity/high frequency. 2"high intensity/high frequency, 3"high intensity/low frequency.( ) RC1; ( ) RC2; ( ) RC3.

54 H. A. SNYMAN

production of RC1 and 2 increased considerably with an increase in Et, while RC3maintained a relatively low production, regardless of the quantity of soil water (Fig. 4).

Water-use efficiency

The average water-use efficiency (WUE) of the RCs at differing levels of irri-gation, and intensities and frequencies of defoliation is shown in Fig. 5.

Figure 4. Relationship between above-ground phytomass production (kg ha~1) and evapo-transpiration (mm) for different rangeland conditions, different intensities and frequen-cies of defoliation. (—) RC1; (– – –) RC2; (– . – . –) RC3. 1"Low intensity/high frequency:Rangeland condition: Good: y"!312)3#2)61x, r2"0)89, D"0)97, n"27, p40)01. Mode-rate: y"!536)4#2)49x, r2"0)94, D"0)98, n"27, p40)01. Poor: y"0)38#0)59x,r2"0)59, D"0)85, n"27, p40)05. 2"high intensity/high frequency: Rangeland condition:Good: y"!563)1#3)35x, r2"0)87, D"0)96, n"27, p40)01. Moderate:y"!936)7#349x, r2"0)95, D"0)99, n"27, p40)01. Poor: y"!383)8#1)479x,r2"0)91, D"0)97, n"27, p40)01. 3"High intensity/low frequency: Rangeland condition:Good: y"!839)9#3)58x, r2"0)93, D"0)98, n"27, p40)01. Moderate: y"!

921)6#3)27x, r2"0)96, D"0)99, n"27, p40)01. Poor: y"!71)1#0)41x, r2"0)77,D"0)93, n"27, p40)01.


A significant interaction (p"0)014) was obtained between the WUE of differentRCs and the irrigated and non-irrigated sites. In all sites, WUE differed significantly(p40)01) between RCs. The WUE between treatments (1) and (2) differednon-significantly (p'0)05), but did differ significantly (p40)01) from the irrigated

Figure 5. Mean water-use efficiency (WUE) over the experimental period for differentrangeland conditions under different soil water contents and defoliated at differentintensities and frequencies. Least significant difference is for the interaction rangelandcondition]soil water content. Coefficient of variation (WUE)"10)11%. Water treatments:1"normal, 2"below-normal, 3"above-normal. Defoliation treatments: 1"lowintensity/high frequency, 2"high intensity/high frequency, 3"high intensity/low frequency.( ) RC1; ( ) RC2; ( ) RC3.

56 H. A. SNYMAN

(treatment (3)) sites. The irrigated sites for RC1, 2 and 3, regardless of defoliationtreatment, produced 2)9, 2)2 and 2)2 kg ha~1 respectively for every mm Et. Theintensity and frequency of defoliation influenced the WUE non-significantly (p'0)05)in the unirrigated sites, regardless of RC. In irrigated sites, high intensity/high frequencyof defoliation caused a significantly (p40)01) higher WUE, regardless of RC resultingin 15%, 15% and 49% more efficient water-use in RC1, 2 and 3 respectively. Thehighest WUE of 2)81 kg ha~1 mm~1 occurred in RC1 subjected to high intensity/highfrequency. RC1, 2 and 3 produce 1)99, 1)38 and 0)50 kg ha~1 respectively for every mmevapo-transpiration, regardless of water and defoliation treatments. The highest order

Figure 6. Relationship between water-use efficiency (kg ha~1 mm~1) and evapo-transpira-tion (mm) for different rangeland conditions and defoliation intensities and frequencies. (—)RC1; (– – –) RC2; (– . – . –) RC3. 1"Low intensity/high frequency: Rangeland condition: Good:y"1)21#0)0014x, r2"0)59, D"0)89, n"27, p40)01. Moderate: y"0)169#0)0023x,r2"0)78, D"0)93, n"27, p40)01. Poor: y"!312)3#2)61x, r2"0)89, D"0)97, n"27,p40)01. 2"High intensity/high frequency: Rangeland condition: Good: y"0)70#0)0028x,r2"0)55, D"0)83, n"27, p40)05. Moderate: y"!61#0)004, r2"0)86, D"0)96,n"27, p40)01. Poor: y"!0)59#0)003x, r2"0)36, D"0)68, n"27. 3"High inten-sity/low frequency: Rangeland condition: Good: y"!312)3#2)61x, r2"0)89, D"0)97,n"27, p40)01. Moderate: y"!0)73#0)0039x, r2"0)90, D"0)97, n"27, p40)01. Poor:y"!0)138#0)000259x, r2"0)27, D"0)64, n"27.


interaction (RC water and defoliation treatment) in the analysis of variance was notsignificant (p'0)05) for WUE.

Statistically significant (p40)01) linear relationships were found between WUE andEt in RC regardless of level of defoliation, where r2 values ranged from 0)78 to 0)90(Fig. 6). The linear relationships showed the best fit in all treatments.

58 H. A. SNYMAN

In RC3, WUE was not closely related to Et and the index value of agreement (D)varied between 0)37 and 0)64. When subjected to high intensity/low frequency defoli-ation, it was noticed that RC3 used water particularly inefficiently regardless ofquantity received. In all other defoliation treatments, WUE showed a sharp incline withan increase in evapo-transpiration, with RC1 the most efficient consumer.

Discussion

As available water is relatively limited in the arid and semi-arid areas, it is important thatthe water needs and influence of defoliation on grazing plants in this ecologicallysensitive area be investigated for sustainable animal production. Changes in the primaryproductivity of semi-arid rangeland have been shown to accompany changes in the speciescomposition of the vegetation. This study clearly showed that rangeland degradation is notonly accompanied by a decrease in productivity, but also reduced water-use efficien-cy. When rangeland is in poor condition, water-use efficiency is low, regardless of thesoil water content. Milchunas & Lauenroth (1993) also found that changes in primaryproduction and species composition were related. Where the effect of grazing onspecies composition was more extreme, primary production was reduced by up to 40%.Le HoueH rou (1984) reviewed rain-use efficiency (RUE: plant production per mm ofrainfall) in 179 sets of data, and argued that it primarily reflects perennial aerial biomassand ground cover, with RUE being substantially lower in degraded ecosystems orconsiderably higher in pristine conditions. Clear evidence of this comes from the 12-yearand 20-year study of Snyman & FoucheH (1991) and Snyman (1998) respectively, whocompared plant production, runoff and RUE in relation to vegetation conditions. Ineach year, plant production was lowest, runoff highest and RUE lowest in RC3.The average WUE over the trial period obtained in this study, regardless of defoliationand water treatments (1)99, 1)38 and 0)50 kg ha~1 mm~1 for veld in RC1, 2 and3 respectively), compares well with that obtained by Snyman (1998) over a 20 yearperiod also under semi-arid climate (2)5; 1)58 and 0)78 kg ha~1 mm~1). Actual RUEfigures throughout the arid zones of the world may vary from less than 0)5 in depletedsubdesert ecosystems to over 10)0 kg DM ha~1 mm~1 in highly productive and well-managed steppes, prairies or savannas (Le HoueH rou & Hoste, 1977; Le HoueH rou, 1984,1994; Le HoueH rou et al., 1988).

To a lesser extent the intensity and frequency of defoliation influenced productionand WUE during this study under normal rainfall conditions, and may play a role in theimprovement and maintenance of RC. The cumulative effects of defoliation inten-sities and frequencies from one season to another may have a greater influence on plantproduction than treatments within a specific season. Under optimal soil water content,production and WUE during this study significantly increased with higher intensitiesand frequencies of defoliation. This finding must however, be applied with caution inarid and semi-arid areas, as soil water is the limiting environmental factor for plantproduction.

Various studies have demonstrated that for grassland in semi-arid areas, production isaffected more by sward composition than by defoliation (Danckwerts & Barnard,1981; Snyman & Opperman, 1983). Although defoliation had a greater effect onthe production of RC1 than RC3 in this study, any beneficial effect occurred whenthe soil was moist. However, compensatory above-ground growth occurs at inter-mediate levels of defoliation for many species and environments, albeit at the expenseof root biomass (Weinmann, 1994; Opperman et al., 1969, 1970; Burger et al.,1975; Danckwerts & Nel, 1989; Snyman, 1993). This study agrees with O’Connor &Bredenkamp (1997) and Snyman (1998) in finding that plant production in highlyvariable climates is largely determined by rainfall and may be unaffected by animalpopulation density. Scoones (1994) argued that grazing has a limited effect on


long-term grass productivity. For grasslands with high year-to-year variability in rainfall,there is now increasing evidence that grazing-induced plant community changes arecontingent upon rainfall pattern (O’Connor, 1991; O’Connor & Bredenkamp, 1997).

Though significant linear relationships were obtained for all RCs between productionand Et, Snyman & FoucheH (1993) and Snyman (1998) found that this relationshipdeteriorates in semi-arid areas with RC3. The poor relationship observed in this studybetween WUE and Et in degraded rangeland can be ascribed to pioneer species reactingindifferently to soil water levels. The significant decrease in production as rangelandcondition deteriorates is thus a function of WUE (Le HoueH rou, 1984; Snyman, 1998,1999) and stability of the different species within a plant community (Snyman& FoucheH , 1991; Snyman, 1997b, 1999).

Over the 3 year experimental period, botanical composition and basal cover were lessinfluenced by the levels of water and frequency/intensity of defoliation in RC3 than inRC1 and 2. Had the study been extended over a longer period, differences in coverand composition might have been observed. Reviews of the effects of rainfall andgrazing on the species composition of semi-arid environments (Roux, 1996; O’Connor,1985; Snyman & Van Rensburg, 1990; Abel, 1993; O’Connor & Roux, 1995) haveunequivocally demonstrated that year-to-year variation in rainfall is a cause of markedvariation in the abundance of species. In general, the impact of grazing on speciescomposition appears to be greater in low rainfall sites, which tend to experience highrainfall variability, than in high-rainfall ones, moderated by the influence of soil types onavailable soil moisture (sandy soils are less prone to drought deficits than clay soils)(O’Connor, 1985). The critical question concerns the relative importance of grazingpressure and climatic variability on species composition. Prevailing opinion suggeststhat rainfall variability rather than grazing is the over-riding determinant of specieschange in semi-arid areas (Roux, 1996; O’Connor, 1985; Abel, 1993; Behnke& Scoones, 1993). However, it should be measured with caution, as it is based on studieswhich do not discriminate sufficiently between the timescales over which eacheffect occurs. Rainfall variability has a profound effect on annual variation inspecies abundance, but unless there is a directional trend, and despite the potential forlarge annual changes, there is no net change in species composition over the long-term.In contrast, changes as a result of grazing are small on an annual scale but are cumulativeand substantial over the long-term, because the direction of the impact of a certainpattern of grazing is usually consistent for a given species (O’Connor & Roux, 1995).Long-term stability in plant community composition may be a consequence of short-term instabilities induced by environmental variability according to models discussed byChesson & Huntley (1989).

Conclusions

The low production and poor relationship between WUE and Et of rangelands in poorcondition necessitates thorough fodder flow planning, especially during droughtperiods. This study clearly shows that the risk of drought (including anthropogenicdrought as defined by Snyman & FoucheH (1991) and Snyman (1998)), increaseswith rangeland degradation. The efficiency and cost-effectiveness with whichsoil water is converted by grazing plants into dry matter production, without deteriora-tion of resources, forms the basis of sustainability of the rangeland ecosystem.

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