Changes in soil properties and vegetation following livestock grazing exclusion in degraded arid environments of South Tunisia

Flora 205 (2010) 184–189

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Changes in soil properties and vegetation following livestock grazing exclusion in degraded arid environments of South Tunisia Kaouthar Jeddi, Mohamed Chaieb  Universite de Sfax, Faculte des Sciences, De partement de Biologie, Laboratoire de Biologie et d’Ecophysiologie des ve ge taux en milieu aride, Route de Sokra, Km 3.5, BP 1171-3000, Sfax,Tunisia

a r t i c l e in f o

a b s t r a c t

Article history: Received 11 October 2008 Accepted 17 March 2009

In this study, characteristics of vegetation and soil properties under continued grazing and exclusion of livestock for 6 and 12 years were examined in a degraded Stipa tenacissima steppe in South Tunisia. Exclosures enhance the total plant cover, the dry matter yield, the number of species per unit area and the Shannon–Wiener diversity. Some palatable species were frequently found inside the protected site. In the continually grazed site, these species are being replaced by less desirable species. Contents of soil organic matter, total nitrogen, Ca2+ and K+ in soils, as well as, water infiltration rate and basal soil respiration showed an increasing trend as time of grazing exclusion increased, from minimum values in the continually grazed area (Gr) to the maximum levels in 12 years protected area (12ex), while there was an opposite trend for Na+ concentration, EC, pH and soil hydrophobicity values. The results suggested that excluding grazing livestock on the arid degraded steppes has a great potential to restore vegetation and soil. Therefore, it must be encouraged as an alternative to stop further degradation and to combat desertification in arid and semi arid ecosystems. & 2009 Elsevier GmbH. All rights reserved.

Keywords: Livestock exclusion Vegetation characteristics Soil properties Arid ecosystem Restoration

Introduction Desertification, which includes degradation of vegetation cover, soil degradation, and nutrient depletion, is a major ecological and economical problem in the arid and semiarid areas (Le Houe rou, 2001; Whitford, 2002). Livestock grazing is one of the main causes of degradation in these areas (Keya, 1998; Le Houe rou, 1995; Li et al., 2008; Yates et al., 2000). The effects of grazing on the plant community and soils are considered destructive because of the reduction of ground cover, productivity and litter accumulation, the destruction of topsoil structure, and compaction of soil as a result of trampling (Manzano and Navar, 2000; Milchunas and Lauenroth, 1993). These processes in turn increase soil crusting, reduce infiltration, enhance soil erosion susceptibility and cause a decline in soil fertility (Hiernaux et al., 1999; Lavado et al., 1996; Yates et al., 2000). The steppes of arid Mediterranean zones are under high risk of acute degradation and they are being destroyed at a rate of 1% per year (Aidoud, 1989; Le Houe rou, 2001). Soil and vegetation degradation in these zones is caused mainly by livestock grazing (Le Houe rou, 1995, 2000). To stop further degradation, to combat desertification and to foster the recovery of the structure, composition and function of

these degraded ecosystems, some management actions have been implemented in recent years. The integral protection by excluding grazing livestock is considered an effective alternative to restore vegetation and soil in these degraded ecosystems. Indeed, the main objective of this management is recovery of the natural vegetation cover with associated benefits for soil and water conservation (Le Houe rou, 2000). Indeed, exclosures enhance vegetation cover and litter accumulation (Pei et al., 2008; YongZhong et al., 2005), increase diversity of herbaceous species (Eweg et al., 1998), improve water infiltration rate (Mekuria et al., 2007), soil fertility (Pei et al., 2008) and some biological soil properties including enzyme activities and basal respiration (Yong-Zhong et al., 2005). In the south of Tunisia, in spite of the importance assigned to the overgrazing management, the effect of exclosures on vegetation and soil is not well studied. Therefore, our experiment was conducted to investigate the effects of livestock grazing exclusion on vegetation and soil properties in an arid degraded Stipa tenacissima L. steppe.

Materials and methods Study sites

 Corresponding author. Tel.: +216 74 274 923; fax: +216 74 274 437.

E-mail addresses: [email protected] (K. Jeddi), [email protected] (M. Chaieb). 0367-2530/$ - see front matter & 2009 Elsevier GmbH. All rights reserved. doi:10.1016/j.flora.2009.03.002

The experiment was performed in an arid steppe named El Gonna (3414106600 N, 101300 2200 E), located at 20 km West of Sfax in

K. Jeddi, M. Chaieb / Flora 205 (2010) 184–189

Southern Tunisia. The climate of the region is Mediterranean lower arid with temperate winters (Emberger, 1954). The 22-year average annual precipitation and temperature was 196 mm and 18.6 1C, respectively. The rainy season is generally from midOctober to April, and the dry season from mid-May to September. Mean annual evaporation is 1200–1240 mm (measured at Sfax weather station). The landscape is dominated by Villafranchian limestone crust forming undulating hills. The soil of the area is Regosol, with friable caliches at 10–25 cm depth and gypsum outcrops (FAO, 1990). In 1995, a restoration project was initiated by Sfax Forest service. The exclosures were established gradually and grazing by domestic herbivores was gradually excluded, allowing the natural vegetation to recover. The area is occupied by an overgrazed S. tenacissima L. steppe, showing some indicator species of the presence of gypsum such as Lygeum spartum, Atractylis serratuloides, Gymnocarpos decander and Helianthemum intricatum and, some plants such as Artemisia herba-alba and Reaumuria vermiculata reflecting vegetation recovery after grazing exclusion. Vegetation sampling Records and measurements were made during spring 2007. Three sites were selected for sampling. In the first (12ex), livestock had been excluded for 12 years at the time of this research. In the second (6ex), livestock had been excluded for 6 years. In the third (Gr), the area is continually grazed. A total of eight stands, each of 20 m  20 m, were sampled within each site. Four transects (10 m long) were set up in each stand. The quadrat point method (Daget and Poissonet, 1971) was used to measure the floristic composition, the total plant cover (%) and cover (%) of each herbaceous species sampled. Observations were made every 10 cm, for a total of 100 points along each transect. The number of species per square meter was determined within 8 quadrats of 10 m2 area in each stand. Dry matter was assessed by the formula of Le Houe rou (1987) and as used by Abdallah et al. (2008): 1

DM ðkg ha

Þ ¼ r  43:173:6

ð1Þ

where r is the perennial species cover. Diversity was calculated using the Shannon–Wiener index. It combines the number of species present with the proportion of individuals belonging to each species (Begon et al., 1990). The formula used to calculate the index values is H0 ¼ 

s X

Pi ln Pi

ð2Þ

i¼1

where H0 is the diversity index value, s the number of species, Pi the proportion of individuals belonging to the ith species and ln the natural log. A high diversity means a tendency to equidistribution or regularity of various species numbers. Plant traits and names are based on Greuter et al. (1989) and Chaieb and Boukhris (1998).

185

1982) were used to analyse total nitrogen (TKN) and extractable phosphate (P), respectively. Potassium (K+), calcium (Ca2+), magnesium (Mg2+) and sodium (Na+) were determined by atomic absorption spectrometry (Atomic Absorption spectrophotometer 6800). Basal soil respiration (BSR) was measured by using air-tight 250-mL polyethylene flasks containing 5 g of soil at field capacity and vials containing 10 mL of 0.1 M NaOH (Emteryd, 1989). The flasks were kept in darkness at 25 1C for 48 h. The CO2 emitted was measured by quantifying the amount of NaOH remaining after the incubation. Soil respiration is expressed as mg CO2 g1 day1. The soil hydrophobicity was measured in the laboratory on undisturbed soil samples. We collected the soil samples by carefully inserting a Petri dish on the surface soil with the help of a knife. Once in the lab, the samples were air-dried. Then, we deposited one drop of distilled water on the soil surface by using a micropipette, and measured the time before the drop was fully incorporated into the soil. This measurement was repeated 3 times per soil sample. We evaluated infiltration capacity in each sample using the method described by Zaady (1999). Before measurements, soil samples used in the previous analysis were moistened to saturation and allowed to settle for 24 h in the dark. We drilled five 1-mm diameter holes under the Petri dishes containing the surface soil sample moistened at saturation, and fixed them to plastic funnels. Then, we added 100 mL of distilled water by using a micro rainfall simulator located 20 cm on top of the surface soil, and measured the time until the first drop of water percolated, and the total amount of water collected after 5 min. The soil infiltration rate was expressed as mL min1. Data analysis We evaluated the effect of grazing exclusion on vegetation parameters and soil properties by using one-way ANOVA, with grazing management as fixed factor. ANOVA analyses were conducted with SPSS (11.0). We used Tukey’s HSD test to perform pairwise comparisons. Covariance between soil properties was evaluated by calculating Pearson’s product moment correlation coefficient. Values of the probability lower than 0.05 were regarded as statistically significant.

Results Total plant cover and dry matter yield When the steppe was protected from grazing for 12 years, the total plant cover increased by 2 times compared with the grazed area (Fig. 1A). Mean values ranged between 33.4% in Gr and 71.3% in 12ex and differences were statistically significant (po0.001). Calculated dry matter also increased with the increase of exclusion time (Fig. 1B). Mean values were 2 and 3.4 times higher in 6ex and 12ex, respectively, than in Gr. Significant differences were found between the three grazing regimes (po0.001).

Soil sampling and analysis Species/area and Shannon–Wiener diversity In each stand, four soil samples were taken at the depth of 0– 20 cm. A total of 32 samples in each site (4 samples  8 stands)  3 sites ¼ 96 samples were collected. Once in the laboratory, soil samples were air-dried and sieved with a sifter of 2 mm. Soil pH and electrical conductivity (EC) were determined (saturated paw method, AFNOR, 1987) by pH meter and conductivity meter, respectively. Soil organic matter (SOM) was determined by the Walkley–Black method (Nelson and Sommers, 1982). Kjeldahl’s method and Olsen’s bicarbonate extraction (Olsen and Sommers,

Results in Fig. 2 indicate that livestock exclusion influenced the number of species per unit area and the Shannon–Wiener diversity index. Species/area did not exceed 18 species m2 in the three studied sites (Fig. 2A). In 12ex, this parameter was significantly higher than in 6ex and Gr (17.6, 13.1 and 7 species m2, respectively; po0.001). In the same way, the Shannon–Wiener index (H0 ) was significantly higher in 12ex and in 6ex than in Gr (2.5, 1.98 and 1.4, respectively; po0.05; Fig. 2B).

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a

21 b

18 c

40 20 0 12ex

6ex

Gr

b

15 12 c

9 6 3

2500 a

0 12ex

2000

3

b

1500

c

1000 500 0 12ex

6ex

Gr

Fig. 1. Variations of total plant cover (%) and calculated dry matter (kg ha1) occurring where grazing was excluded for 12 years (12ex), 6 years (6ex) and in the continually grazed area (Gr). Different letters denote significant differences between study sites (Tukey’s HSD test at po0.05).

Shannon-Wiener diversity index ( H' )

Calculated dry matter (Kg ha-1)

a

60

Species / area (species m-2)

Total plant cover (%)

80

6ex

Gr

a

2.5

a

2 b 1.5 1 0.5 0 12ex

There was no significant difference between 12ex and 6ex in terms of their effect on this parameter (p40.05). Herbaceous species cover Species presenting a cover lower than of 1% are not cited in Table 1. Most of the recorded flora is composed of species indifferent to the protection (e.g. Erodium hirtum, Gymnocarpos decander, Kickxia aegyptiaca, Stipa capensis, etc.). No significant differences were found between their covers inside and outside the two protected areas (p40.05). Covers of some species such as Artemisia herba-alba, Cenchrus ciliaris, Cynodon dactylon, Echiochilon fruticosum, Helianthemum sessiliflorum and Salvia aegyptiaca were higher in 12ex than in 6ex or in Gr. Differences were statistically significant (po0.01 and po0.05). However, some other species such as Asphodelus tenuifolius, Astragalus armatus, Peganum harmala, Reichardia tingitana and Senecio gallicus were significantly more frequent outside than inside the two protected areas (po0.01 and po0.05). Soil properties Chemical and biological soil properties The results of changes in chemical and biological soil properties under different grazing regimes are presented in Table 2. SOM and TKN mean values showed a declining trend from 12ex to Gr. There was a significant difference in SOM between the three grazing regimes (po0.01). A wider average value of TKN is recorded in 12ex soils and a narrower value in the Gr soils. Significant differences were found just between these two treatments (po0.05). No significant differences were found between the three grazing regimes in terms of their effect on available P (p40.05).

6ex

Gr

2

Fig. 2. Variations of species/area (species m ) and the Shannon–Wiener diversity index occurring where grazing was excluded for 12 years (12ex), 6 years (6ex) and in the continually grazed area (Gr). Different letters denote significant differences between study sites (Tukey’s HSD test at po0.05).

This is completely the same for Mg2+ soil content. Ca2+ and K+ soil content showed a similar pattern, with the highest value in 12ex and the lowest value in Gr. For the nutrients, significant differences were found between the three grazing regimes (po0.05 and 0.001). Conversely to these ions, Na+ in soil showed a highly significant declining trend from Gr to 12ex (po0.01). Electrical conductivity and pH were also significantly higher in Gr than in 6ex and 12ex, respectively (po0.05). No significant differences were found between the two last grazing regimes (p40.05). Basal soil respiration was highest in 12ex, intermediate in 6ex and lowest in Gr. No significant differences were found between the two last grazing regimes (p40.05). BSR and SOM were positively correlated (r ¼ 0.752, p ¼ 0.024).

Physical soil properties We found significant effects of grazing regimes on physical soil properties (Table 3). Soil in 12ex showed higher infiltration rate and lower infiltration time than in 6ex (po0.01) and Gr (po0.001), respectively. Infiltration rate and time for the first drop to percolate were significantly correlated with soil organic matter (r ¼ 0.755, p ¼ 0.012, and r ¼ 0.702, p ¼ 0.012, respectively). Soil from the grazed site was significantly more hydrophobic than the soil from the two protected sites (po0.05). Hydrophobicity was negatively correlated with soil organic matter (r ¼ 0.705, p ¼ 0.023). The magnitude of the changes in physical soil properties inside the protected areas was generally higher than for chemical soil

K. Jeddi, M. Chaieb / Flora 205 (2010) 184–189

Table 1 Variations of herbaceous species cover (%) occurring where grazing was excluded for 12 years (12ex), 6 years (6ex) and in the continually grazed area (Gr). Species

Argyrolobium uniflorum (Dc) Jaub. et Spach. Artemisia herba-alba Asso. Asphodelus tenuifolius Cav. Astragalus armatus Willd. Bromus madritensis L. Cenchrus ciliaris L. Cynodon dactylon (L.) Pers. Diplotaxis harra (Forssk.) Boiss. Echiochilon fruticosum Desf. Erodium laciniatum (Cav.) Willd. Fagonia cretica L. Gymnocarpos decander Forssk. Hammada scoparia (Pomel) Iljin Helianthemum kahiricum Del. Helianthemum sessiliflorum (Desf.) Pers. Herniaria fontanesii J. Gay. Kickxia aegyptiaca (L.) Nabelek. Lycium shawii Roem. & Schult. Lygeum spartum Loefl. ex L. Paronychia arabica (L.) DC. Peganum harmala L. Plantago albicans L. Reichardia tingitana (L.) Roth Retama raetam (Forssk.) Webb Salvia aegyptiaca L. Scorzonera undulata Vahl Senecio gallicus L. Stipa capensis Thunb. Stipa tenacissima L. Teucrium polium L. Thymelaea hirsuta (L.) Endl.

Herbaceous species cover (%) 12ex

6ex

Gr

p

1.2 8.2 0.5 0 2.2 3.06 5.1 1.2 5.8 1.01 1.3 3.4 1.5 4.1 7.5 1.02 1.7 1.5 2.7 1.6 0 5.1 0 1.26 3.8 2.7 0 5.8 13.4 1.24 0

0 6.2 1.2 2.3 1.8 1.8 3.2 2.6 3.5 1.6 2.4 5.2 0 4.2 5.7 1.7 1.8 0 3.2 0 0 4.7 0 0 2.1 2.3 0 7.3 14.5 1.63 0

0 1.6 4.3 5.6 5.3 0 0 3 1.2 0.7 0 4.3 0 3.9 1.01 0 0 0.6 0 0 4.8 4.5 3.5 0 0 1.9 4.6 8.6 17.2 0 1.5

NS ** * ** NS * ** NS * NS NS NS NS NS ** NS NS NS NS NS ** NS * NS * NS * NS NS NS NS

Significance: *po0.05, **po0.01 and NS ¼ non-significant.

properties. Indeed, the protection increased soil organic matter, nitrogen, Ca2+ and K+ content by 31%, and decreased Na+ by 41%. However, infiltration rate showed a 2-fold increase, and infiltration time and hyrophobicity decreased by 42% and 46%, respectively.

Discussion Effect of grazing management on vegetation parameters The results presented in this study demonstrate that protection from livestock grazing in arid S. tenacissima steppe has an effect on vegetation structure and composition. This effect has important consequences for restoration of plant species diversity and community structure in Mediterranean ecosystems (Le Houe rou, 2001). The increase of the total plant cover inside the protected areas is in agreement with studies showing positive effects of protection on this parameter (Belsky, 1992; Brown and Al Mazrooei, 2003). This increase can be explained by the improvement of soil conditions (temperature, moisture, nutrient cycling) inside the protected sites which favour the regeneration and the development of herbaceous species (Yates et al., 2000). It is the same for the dry matter yield (Pei et al., 2008) which also, according to the calculations, underwent a high, significant increase with protection period. By contrast, continuous grazing resulted in less vegetation cover and biomass production. This is in agreement with results obtained by Yong-Zhong et al. (2005) in Inner Mongolia (China). These authors noted that due to frequent

187

Table 2 Chemical and biological soil properties (0–20 cm) where grazing was excluded for 12 years (12ex), 6 years (6ex) and in the continually grazed area (Gr). Soil properties (0–20 cm)

SOM (%) TKN (mg g1) P (mg kg1) Mg2+ (mg kg1) Ca2+ (mg kg1) K+ (mg kg1) Na+ (mg kg1) EC (dS m1) pH BSR (mg C CO2 g1 day1)

Sites 12ex

6ex

Gr

p

1.3470.03a 0.7770.05a 38.271.09a 3.2170.01a 21.672.03a 18.271.14a 7.4471.12a 1.3370.02a 7.7571.07a 3.770.2a

1.0270.02b 0.6170.03ab 38.472.05a 3.370.04a 17.571.07b 12.370.87b 10.771.3b 1.470.06a 7.8970.08a 3.170.11a

0.670.03c 0.4870.06b 40.272.35a 2.870.05a 1571.4c 9.4370.86c 15.371.45c 2.0770.08b 8.2271.02b 2.470.34b

** * NS NS * *** ** NS NS *

Standard errors are shown. Significance: *po0.05, **po0.01, ***po0.001 and NS ¼ non-significant. Different letters denote significant differences between soil samples (Tukey’s HSD test at po0.05).

Table 3 Physical properties of the soil where grazing was excluded for 12 years (12ex), 6 years (6ex) and in the continually grazed area (Gr). Sites

Infiltration rate (mL min1)

Infiltration time (s)

Hydrophobicity (s)

12ex 6ex Gr p

18.6971.07a 13.3471.01b 8.770.24c **

22.271.33a 28.372.25b 43.7273.55c ***

4.770.07a 5.270.09a 9.270.11b *

Standard errors are shown. Significance: *po0.05, **po0.01 and ***po0.001. Different letters denote significant differences between soil samples (Tukey’s HSD test at po0.05).

trampling by sheep and cattle, the ground surface became bare and exposed to wind erosion. This resulted in soil coarsening and loss of soil fertility, which in turn influenced the amount of vegetation the area can support. Not only was the number of species per unit area increased in the protected sites, but also was the Shannon–Wiener diversity. Comparable findings were reported by Shaltout et al. (1996) and Eweg et al. (1998) and supported other studies showing a high degree of covariation between species/area and the Shannon–Wiener diversity (Pielou, 1975). Zhang (1998) noted that change in plant species diversity in relation to grazing or the cessation of grazing depend on resource partitioning and competitive patterns in vegetation. Many of the species such as Artemisia herba-alba, Cenchrus ciliaris, Cynodon dactylon, Salvia aegyptiaca, Echiochilon fruticosum and Helianthemum sessiliflorum were found to be significantly more abundant inside than outside the protected areas. They are known for their high acceptability and palatability (Chaieb and Boukhris, 1998). As a consequence, in grazed sites these palatable species are very threatened and become replaced by less palatable species which are often considered less desirable such as Asphodelus tenuifolius, Astragalus armatus and Peganum harmala (Abdallah et al., 2008; Chaieb and Boukhris, 1998). Other species such as Senecio gallicus and Reichardia tingitana that occur only in the free grazing area are some weeds of disturbed habitats (Shaltout et al., 1996).

Effect of grazing management on soil properties Livestock grazing affects soil attributes, which, in turn impacts the ecosystem function (Liebig et al., 2006). In many cases, soil

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loss on grazed rangeland is greater than on ungrazed rangeland (Perevolotsky and Seligman, 1998). Indeed, soil inside protected areas had a greater cover of herbaceous species, woody debris, litter and cryptogams, high levels of microtopography and little erosion (Yates et al., 2000). These conditions had a major impact on soil properties. Concentrations of SOM, TKN, Ca2+ and K+ in the investigated soils showed an increasing trend as time of grazing exclusion increased, from minimum values in Gr to the maximum levels in 12ex, while there was an opposite trend for Na+ concentration, EC and pH values. The increase of soil organic matter and nutrient content which accompanies grazing exclusion, can be a result of an increase in the amount of plant litter on the one hand and a decrease in soil compaction on the other hand (Xie and Wittig, 2004). This result in favourable living conditions for those organisms vital for the incorporation of the humus into the soil (Liu et al., 1997). Biological activity was improved to a certain degree after livestock exclusion. The greater BSR in the surface soil of 12ex compared with the Gr was expected due to the differences in soil properties, root density and microbial activity between the two sites (Yong-Zhong et al., 2005). Soil CO2 efflux is often positively related with variables such as moisture (kept constant in the comparative measurements), soil organic matter and microbial activity (Conant et al., 2000). The increase of soil salinity outside in relation to inside the protected sites, which was observed in the present study, conforms with the relevant report by Shaltout et al. (1996) in eastern Saudi Arabia. This increase in salinity might be related to increasing evaporation and soil erosion in the free grazing site. In this area, the loss of foliage and litter cover lead to an increase in the exposure of the soil surface to radiation and compaction, and facilitate the rapid conduction of heat through the soil (Yates et al., 2000). The decline in pH values in the exclosures as compared with Gr can possibly be related to more root biomass in the first sites and more active microorganism metabolism in the rizosphere (David et al., 2004). Hinsinger et al. (2003) noted that the secretion of organic acids from the roots and amounts of CO2 released from roots and micro-organisms could lead to the decrease in pH. Besides improving chemical and biological soil properties, protection from livestock grazing also improved physical soil features when compared with those in grazed area. Indeed, soils from the protected sites showed higher infiltration rate and lower infiltration time than those from the free grazing area. The surface soil hydrology results are in agreement with those shown by Castellano and Valone (2007) in Arizona and Yates et al. (2000) in South Western Australia. The last authors reported that the loss of porosity in the surface soil in heavily grazed area affected its hydrological properties. Furthermore, Ludwig et al. (1994) noted that grazing and trampling compact the soil, reduce water infiltration and increase water run-off. On the other hand, higher infiltration rates and lower infiltration times observed in soils from the protected sites were probably related to the important organic matter content which affects aggregate development and creates more macro-pores (Mapa, 1995) and perhaps also an abundance of fungal hyphae which maintain continuous pores (Whitford, 2002). Contrary to other studies (Harper and Gilkes, 1994), soil hydrophobicity in the present study was negatively correlated with soil organic matter. Furthermore, soil on the free grazing area was more hydrophobic than on the protected areas. This is somewhat surprising, as this variable is often linked to the presence of hydrophobic substances directly derived from plant residues (Doerr et al., 2000). In our case the lack of a positive relationship between soil organic matter and hydrophobicity can be explained by the fact that the content of fresh undecomposed

organic residues was probably low in the mineral soil (Buczko et al., 2005; Doerr et al., 2000).

Conclusions The arid Tunisian steppe is ecologically very fragile. Continuous grazing causes a considerable decrease in total plant cover, dry matter, species/area and the Shannon–Wiener diversity, an increase in soil salinity and further loss of soil OM, total N, Ca2+ and K+ content. Moreover, livestock grazing alters soil physical properties and surface water hydrology which might render serious consequences for plant growth in a dry Mediterranean climate where water is a scarce resource. The observed variations in all these parameters indicate that the steppe is in the stage of very strong degradation. Integral protection by excluding livestock grazing enhances vegetation recovery, productivity, and development of herbaceous species. Concentrations of SOM, TKN, Ca2+ and K+ in soils, as well as, water infiltration rate and basal soil respiration improved progressively following 6- and 12-year exclusion of livestock. This indicates that the steppe can recover and is recovering, indeed. From a perspective of ecological restoration as well as nutrient recycling and soil biological recovery which affect ecosystem function, a viable option for arid steppes management should be to adopt protecting practices in the initial stage of steppe degradation. Grazing management plans and subsequent operational decisions should include both economic and biological considerations. Therefore, they should focus upon possibilities that allow local population to use the land for pasture by organizing a rotational grazing system, and prevent grazing pressure by distributing the livestock more uniformly and ensuring that carrying capacity is not exceeded in order to maintain soil and vegetation conditions. More studies to better assess the time scale of exclusion are also needed to understand better the ecology of this fragile ecosystem. References Abdallah, F., Noumi, Z., Touzard, B., Ouled Belgacem, A., Neffati, M., Chaieb, M., 2008. The influence of Acacia tortilis (Forssk.) subsp. raddiana (Savi) and livestock grazing on grass species composition, yield and soil nutrients in arid environments of South Tunisia. Flora 203, 116–125. AFNOR, 1987. Recueil de normes francaises, qualite des sols, methodes d’analyses. 1. edit. Association francaise de normalisation (Afnor), pp. 19–30. Aidoud, A., 1989. Contribution a l’e tude des e cosyste mes steppique pˆature s des hautes plaines Alge ro-Oranaises (Alge rie). Ph.D. Thesis, Universite des sciences et technologies, H. Boumediene, Alegria, 248 pp. Begon, M., Harper, J.L., Townsend, C.R., 1990. Ecology, Individuals, Populations and Communities, second ed Blackwell, Oxford 945 pp. Belsky, J., 1992. Effects of grazing, competition, disturbance and fire on species composition and diversity in grassland communities. J. Veg. Sci. 3, 187–200. Brown, G., Al Mazrooei, S., 2003. Rapid vegetation regeneration in a seriously degraded Rhanterium epapposum community in northern Kuwait after 4 years of protection. J. Environ. Manage. 68, 387–395. ¨ Buczko, U., Bens, O., Huttl, R.F., 2005. Variability of soil water repellency in sandy forest soils with different stand structure under Scots pine (Pinus sylvestris) and beech (Fagus sylvatica). Geoderma 126, 317–336. Castellano, M.J., Valone, T.J., 2007. Livestock, soil compaction and water infiltration rate: evaluating a potential desertification recovery mechanism. J. Arid Environ. 71, 97–108. Chaieb, M., Boukhris, M., 1998. Flore succinte et illustre e des zones arides et sahariennes de Tunisie. Association pour la Protection de la Nature et de l’Environnement. L’Or du temps, Sfax, Tunisie, 290 pp. Conant, R.T., Klopatek, J.M., Klopatek, C.C., 2000. Environmental factors controlling soil respiration in three semiarid ecosystems. Soil Sci. Soc. Am. J. 64, 383–390. Daget, P., Poissonet, J., 1971. An ecological analysis method of prairies. Criteria’s of application. Ann. Agron. 22, 5–41. David, L.J., Hodge, A., Kuzyakov, Y., 2004. Plant and mycorrhizal regulation of rhizodeposition. New Phytol. 163, 459–480. Doerr, S.H., Shakesby, R.A., Walsh, R.P.D., 2000. Soil water repellency: its causes, characteristics and hydro-geomorphological significance. Earth Sci. Rev. 51, 33–65. Emberger, L., 1954. Une classification biologique des climats. Recueil des travaux du Laboratoire de Botanique, Se rie botanique 7, 3–43.

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