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- Taylor & Francis
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- © 2015 Taylor & Francis
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- 2151-3732
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- 2151-3740
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- 10.1080/21513732.2014.998284
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Abstract
International Journal of Biodiversity Science, Ecosystem Services & Management, 2015 Vol. 11, No. 2, 106–113, http://dx.doi.org/10.1080/21513732.2014.998284 a b c Mouldi Gamoun *, Bob Patton and Belgacem Hanchi a b Laboratoire d’Ecologie Pastorale, Institut des Régions Arides (IRA), 4119 Médenine, Tunisia; Central Grasslands Research Extension Center, North Dakota State University, 4824 48th Ave. SE, Streeter, ND, 58483, USA; Département de Biologie, Faculté des Sciences de Tunis, Campus Universitaire, 1060 Tunis, Tunisie (Submitted 23 April 2014; accepted 10 December 2014; edited by Neville Crossman) Livestock grazing influences arid rangelands greatly with important effects on vegetation dynamics. Two areas traditionally grazed by sheep and goats in southern Tunisia were sampled to evaluate the vegetation response to grazing management. A continuous grazing (CG) area was sampled in March 2007. A 2000 ha exclosure that had been rested for 3 years (2004– 2007), grazed for 2 months (July and August 2007), and then rested for 7 months (September 2007 to February 2008) was sampled before and after grazing, and again after the 7 months’ rest. Results show that vegetation dynamics in arid rangelands respond strongly to changes in grazing management. Our results suggest that even previously overgrazed rangelands are resilient and are able to recover if given rest periods. In the studied Tunisian rangeland that has been moderately or lightly grazed, we found that recovery improved faster compared with continuously grazed. In practice, excluding grazing livestock and the use of a rotational grazing system are available ways to restore vegetation affected by CG. Therefore, a stocking rate not exceeding the carrying capacity is vital to maintain grazing operations under changing conditions and sustain rangeland resources over the long term. Increased stocking rates generally promote rangeland degradation. Keywords: vegetation; controlled grazing; restoration; Tunisia Introduction rangelands productivity (Gamoun 2014), and it is a pre- ferred practice for conserving biological soil crusts and the As in many other arid areas worldwide, rangelands have a ecological services they provide in nitrogen fixation and key role as grazing lands for pastoral use and very important soil stabilization (Liu et al. 2009). Moreover, low-to- role in the local economy. Research on sustainable land use moderate levels of grazing can increase production com- and resources management as well as for better understand- pared with no grazing; however, the level of grazing that ing of natural processes in arid areas can contribute to maximizes production depends upon the growing condi- develop strategies to combat desertification and for improv- tions of the current year (Patton et al. 2007). Furthermore, ing people’s life (Breckle et al. 2001). Many of the world’s livestock grazing affects soil physical properties and sur- rangelands are believed to be degraded as a result of exces- face water hydrology, which might cause serious conse- sive livestock grazing (Breman & de Wit 1983; Milton quences for plant growth in a dry Mediterranean climate et al. 1994). Evidence that livestock grazing strongly affects where water is a scarce resource (Jeddi & Chaieb 2010). the structure, richness, and composition of vegetation In southern Tunisia, the concept of rangelands man- (Ibáñez et al. 2007; Louhaichi & Tastad 2010;Bo agement has become broadly accepted and implemented et al. 2013: Peters et al. 2013; Reynolds 2013; Rutherford over the last two decades. Productive management of these & Powrie 2013; Gamoun 2014). Changes in plant species rangelands has proven unlikely when the natural vegeta- composition are mostly due to the replacement of palatable tion becomes severely degraded. However, this situation by unpalatable species and annual plants when degradation can be remedied if restoration work is undertaken (Le occurs (Archer & Smeins 1991; Briske 1991; Milton Houérou 2002; Gamoun et al. 2012), and this is why et al. 1994; Tarhouni et al. 2006, 2007). Some previous rangelands protection is necessary to maintain sustainable research showed that the substitution of palatable by unpa- management and resilience (Gamoun et al. 2011; latable plants decreases rangeland productivity and plant Gamoun 2014). In practice, grazing management is simply diversity (Hobbie 1992; Grime 2001; Cingolani et al. controlling where and when animals graze over the land- 2005). Once unpalatable species have become dominant, scape (Norton et al. 2013). Additionally, it is the manip- it is difficult to undo the progressive change in vegetation ulation of the soil–plant–animal complex of the grazing by reducing or removing the impact of grazing (Noy-Meir land in pursuit of a desired result (Allen et al. 2011). & Walker 1986; Westoby et al. 1989). For this reason, the application of some management Moderate grazing (MG) can be used as a beneficial practices, such as rest before severe degradation occurs, management method to maintain species diversity and *Corresponding author. Email: gamoun.mouldi@yahoo.fr © 2015 Taylor & Francis International Journal of Biodiversity Science, Ecosystem Services & Management 107 becomes a necessity for optimizing ecosystems productivity and conserving biodiversity (Villagra et al. 2009). The rest management technique is considered a vital strategy to maintain rangeland productivity for use by human beings (Clewell & Aronson 2006); reducing graz- ing pressure can favour natural regeneration of the degraded southern Tunisian arid lands (Jauffret & Lavorel 2003). The impact of rest on rangelands dynamics can be evaluated through long-term monitoring of biotic and abiotic attributes (Aronson et al. 1993a, 1993b; Havstad & Herrick 2003). Effects of grazing management such as controlled stocking densities and grazing systems on plant species diversity and plant growth forms or func- tional groups may have important consequences for eco- system function (Hickman et al. 2004). The economic benefit offered by the rested rangeland is grazing. The benefits offered by the rested rangelands are also environ- mental including conserving biodiversity. The pastoral lands of southern Tunisia are characterized by high year-to-year variability in precipitation. This, in turn, result in: variability in plant growth; uneven provision of nutrition for sheep, goats, and camel; and limited poten- tial to carry out necessary plant management options such as rest and rotation grazing. Determining carrying capacity is a fundamental component of rangeland evaluation because it is an important management tool that connects forage supply and forage consumption (Vallentine 1990). Figure 1. Study area. Our objectives were to assess the vegetation responses to grazing management in the arid rangelands of the south- ern Tunisia and to estimate the carrying capacity of the The land has been subjected to continuous grazing rangeland based on primary productivity. (CG) throughout the year at stocking rates (1–4headha −1 −1 year ) which are greater than the long-term carry- ing capacity (Le Houérou 1969). The 1,100,000 ha of the Dahar communal rangelands are currently grazed Materials and methods by 460,000 sheep and 371,000 goats, resulting in a −1 Study site stocking density of 0.76 head ha . This level of graz- The study was carried out at the Dahar communal range- ing intensity could explain the disappearance of pasture lands, located in southern Tunisia (10°40ʹ E, 32°08ʹ N) species of Dahar (Elloumi et al. 2001). A 2000 ha (Figure 1). This area has an average altitude of 408 m portion of the area was excluded from grazing from above sea level and has an arid Mediterranean climate 2004 until 2007. The exclusion area and grazed area characterized by winter rains and summer drought (Le have similar soils, and the vegetation of the exclosure was similar to the area surrounding it at the time of Houérou 2005a, 2005b). The average annual precipitation exclusion (Le Houérou 1969). Domestic animals graze is 80 mm with a high coefficient of variance in precipita- and browse on 69 species that grow in this region. In tion (typically >30% and about 40% in our case) (Table 1). this study, two grazing areas were compared. Temperatures are generally hot in summer and cold in After 3 years of protection, improved range condition winter with a mean annual temperature of about 23°C. could lead to better forage supply and improved livestock The sole land use of the zone is rangeland. The soil is gains. Therefore, this rangeland becomes a resource that can skeletal with vast plate and moderate slopes crossed by be exploited by improved grazing management. The present wadi dominated with a mixture of sand and gravel. The study was aimed at evaluating the possible recovery of vegetation cover is mainly dominated by Anthyllis henoni- vegetation as a result of grazing management practice. ana Batt. [A. sericea subsp. henoniana (Batt.) Maire], The 2000 ha controlled grazing area was protected Gymnocarpos decander Forssk., Stipagrostis pungens from livestock for 3 years (2004–2007), grazed for 2 (Desf.) de Winter, Haloxylon schmittianum Pomel, and months (July and August 2007) with 1700 sheep Pennisetum dichotomum (Forssk.) Delile. Pennisetum −1 (0.85 head ha ), and was then again protected from graz- dichotomum dominates the vegetation communities in ing for 7 months. Continuously grazed area allows animals wadi where the soil is mainly calcareous silt. 108 M. Gamoun et al. Table 2. Descriptions of the investigated livestock grazing treatments and sampling timetable. Date of Treatment Description measurement Continuous CG: Continuous grazing March 2007 grazing Controlled UG: Ungrazed for 3 years March 2007 grazing MG: Moderate grazing September 2007 (July and August) MGUG: Moderate grazing March 2008 followed by ungrazed (7 months of rest) unrestricted and uninterrupted access to the grazing unit throughout the year. Data collection The continuously stocked area (CG) was sampled in March 2007. The controlled area was sampled prior to grazing after 3 years of rest (ungrazed: UG) in March 2007, immediately following MG in September 2007, and again in March 2008 after a short period of rest (MG followed by ungrazed: MGUG). Table 2 describes the sampling timetable. Eight phytoecological relevés were installed, four in the controlled grazing area and four in the continuously grazed area. Each relevé consisted of three parallel, 20 m transects. These transects were assessed using the point quadrat method as defined by Daget and Poissonet (1971) and Floret (1988). A fine pin was descended to the ground every 20 cm along the transect. Each of the 100 hits per transect was recorded according to the plant species or type of ground touched. The total vegetation cover is calculated as: VC = (n/N) * 100 with n: the number of hits of all plant species and N: the total number of hits (100 hits in our case). In each area, biomass was estimated by clipping 4 sq. m quadrats. A total of 28 plots were sampled: 16 in the controlled grazing area and 12 in the continuously grazed area. Samples were dried at 100°C and then weighed. Estimations of livestock energy needs vary appreciably depending on the method. In Tunisia, based on the follow- ing assumptions (Le Houérou 1975; Le Houérou & Hoste 1977): 1 sheep = 300 feed unit per year, and 1 kg of dry matter (DM) = 0.33 feed unit, we can estimate the live- stock daily consumption for sheep as forage unit = year days = year feed unit ðFUÞ value in 1 kg of dry matter (1) Thus, from the equation above (2), we can determine the carrying capacity: Table 1. Rainfall (mm) received in Tataouine, southern Tunisia, during the period from 2000 to 2008. 2000 2001 2002 2003 2004 2005 2006 2007 2008 CV (%) January 1.27 0.25 1.77 8.64 9.91 0 9.91 0 28.18 137.53 February 10.41 0.76 41.66 9.4 0 1.27 14.73 0 0 156.03 March 1.02 0.76 3.04 15.25 19.56 1.53 0 19.05 0.76 125.56 April 2.79 6.59 1.78 3.81 8.64 0.25 9.9 2.02 0 90.45 May 10.16 9.39 5.59 1.53 1.02 0 5.08 0 5.59 90.71 June 1.52 0 0 0 0 8.38 0 0 0 252.35 July 0 0 10.41 0 0 0.51 0 0 0 284.58 August 0 0.76 1.52 0 0 0.51 0 0 3.81 172.65 September 0.76 5.59 7.87 1.02 0 9.9 12.7 0 9.39 93.94 October 47.74 0 44.95 0.25 0 4.57 9.91 6.86 4.06 145.45 November 0 0 6.35 1.27 6.6 0 9.4 0 3.55 119.68 December 0.25 13.21 0.25 38.1 1.78 21.08 33.01 37.34 6.6 94.95 Total 75.92 37.31 125.19 79.27 47.51 48 104.64 65.27 61.94 39.73 CV (%) 214.39 144.86 150.45 167.27 155.16 159.19 107.36 211.32 152.5 International Journal of Biodiversity Science, Ecosystem Services & Management 109 forage produced in excess of plants requirements for maintenance ðkg of DM per haÞ unit of land ðhaÞ (2) Daily forage consumption ðkg of DMÞ=head number of days Plant cover was reduced from 62% to 40% after graz- The Society for Range Management defines carrying ing but had recovered to 59% after 7 months’ rest and was capacity as ‘the maximum stocking rate possible which is not significantly different from the value in the UG consistent with maintaining or improving vegetation or treatment. related resources’ (SRM 1989). The carrying capacity of grasslands determines how many animals can be sup- ported by the grasslands’ annual biomass production (Neupert 1999; Wang et al. 2005). In contrast, it is the Effect of grazing treatment on primary productivity maximum stocking rate that will achieve a target level of Analysis of variance indicated significant differences in pro- animal performance in a specified grazing system that can ductivity between means (F =12.28, P <0.0001) in 3,56 be applied over a defined time without deterioration of the productivity. Productivity was significantly less on the CG grazing land (Allen et al. 2011). Thus, livestock stocking −1 area (42 kg DM ha ) than on the controlled grazing area densities are seen as a contributing factor to bush −1 after 3 years of no grazing (UG: 210 kg DM ha )in March encroachment and land degradation. of 2007. Biomass production was reduced by 60% within the −1 MG (85 kg DM ha ) to a level that was not significantly greater than CG, but increased biomass production was Data analysis observed in March 2008 when grazing is excluded again −1 Analysis of variance was used to test for differences in (MGUG: 173 kg DM ha ) to a level which was not statis- cover, species richness, and productivity between CG, UG, tically different from the UG condition (Figure 3). MG, and MGUG, and P < 0.05 was used to determine the significance in all tests. Fisher least significant difference at P = 0.05 was used for means comparison when the Effect of grazing treatment on species richness F-test was significant. Perennial and annual herbaceous species were the main contributors to the palatable biomass consumed by live- stock and were likewise affected by the grazing treatments. Results The perennial species were more affected by grazing Effect of grazing treatment on plant cover manipulations (F = 70.07, P < 0.0001) than the annual 3,12 Plant cover was significantly less on the continuously species (F = 85.10, P < 0.0001). The number of per- 3,12 grazed area in March 2007 than on the controlled grazing ennial species was significantly less on the CG area (6.8 area at all three sampling periods (F = 46.83; species) in March 2007 than on the controlled grazing area 3,44 P < 0.0001, Figure 2). at all sample periods (Figure 4). The number of perennial Figure 2. Plant cover on the continuously grazed area (CG) in Figure 3. Biomass on the CG area in March 2007 and the area March 2007 and the area protected from grazing for 3 years in protected from grazing for 3 years in March 2007 (UG), after it March 2007 (UG), after it had been grazed for 2 months (MG), had been grazed for 2 months (MG), and after 7 months of rest and after 7 months of rest (MGUG). Values are means ± standard (MGUG). Values are means ± standard errors. Different letters errors. Different letters indicate significant differences (P ≤ 0.05). indicate significant differences (P ≤ 0.05). 110 M. Gamoun et al. Figure 4. Perennial species richness on the continuously grazed Figure 5. Annual species richness on the continuously grazed area (CG) in March 2007 and the area protected from grazing for area (CG) in March 2007 and the area protected from grazing for 3 years in March 2007 (UG), after it had been grazed for 3 years in March 2007 (UG), after it had been grazed for 2 months (MG), and after 7 months of rest (MGUG). Values 2 months (MG), and after 7 months of rest (MGUG). Values are means ± standard errors. Different letters indicate significant are means ± standard errors. Different letters indicate significant differences (P ≤ 0.05). differences (P ≤ 0.05). species was reduced from 33 to 23 after MG but had maintenance of the vegetation. Moreover, it is necessary increased to 28 after 7 months’ rest although still signifi- to leave at least 40% of the vegetation so that grazing cantly less than the value of the previous year. does not damage seedlings and so that fast recovery can The number of annual species was significantly less on be achieved in 1 year of higher precipitation. the CG area (4.0 species) than on the controlled grazing area after 3 years of no grazing in March 2007 (UG: 36 species) (Figure 5). The number of annual species was Discussion reduced by 2 months of MG (6 species) to a level that was A previous study in the area has indicated that CG has a wide not significantly greater than CG, but had increased after 7 range of effects on the composition, diversity, and rangelands months of rest (MGUG: 32 species) to a level approaching production (Gamoun, Chaieb, et al. 2010;Gamoun 2014). to the UG condition. Livestock grazing has caused a severe degradation of vegeta- tion both directly (by eating it) and indirectly (trampling) (Gamoun, Tarhouni, et al. 2010). However, excluding graz- Estimating the carrying capacity ing improved plant cover, flora richness, and productivity Since we know the annual net primary production of the (Deng et al. 2013;Gamoun 2014). studied rangeland, it is possible to compare it with the Under grazing, the natural vegetation cover in our present consumption/head/grazing period (or with the DM study area is mainly dominated by small chamaephytes requirements) to obtain the present stocking rate, and and some hemicryptophytes. Annual plants are absent combining this with the recommended rate of utilization, because the climatic conditions of the period of grazing determine the carrying capacity allowed to maintain the are not suitable for their development. According to optimal rate of forage use. Waechter (1982), sheep mainly prefer annual plants. In The analysis is based on pre- and post-grazing biomass. the absence of these, sheep are attracted to herbaceous Before the grazing period, biomass production is about plants like Plantago albicans L. and Cynodon dactylon −1 −1 210 kg DM ha , whereas it is reduced to 85 kg DM ha (L.) Pers., and some chamaephytes as Argyrolobium uni- after 2 months of grazing period. Afterward, forage con- florum (Decne.) Jaub. & Spach, Echiochilon fruticosum −1 sumption is about 125 kg DM ha during 2 months with (Desf.), Herniaria fontanesi J. Gay and Helianthemum utilization rates about 60%. sessiliflorum (Desf.). Grazing significantly benefited cha- Using Equation (1), the average daily consumption maephytes (Kahmen & Poschlod 2008) and can make up rate of vegetation by an animal has been estimated at the greater part of the fodder production (from 60% to 2.49 of DM per day. 80% of production) (Le Houérou et al. 1974; Floret & Equation (2) may be used to calculate the carrying Pontanier 1978). Dominance by unpalatable species has capacity. The calculated carrying capacity for this range- been proposed as a stable state for rangeland vegetation in −1 −1 land is 0.14 head ha year . This is the carrying capa- arid steppes under CG. However, our results show that city of equilibrium, which a rangeland can support controlled grazing with periods of rest leads to an impor- without being damaged. This should ensure the tant improvement of natural vegetation, and perennial International Journal of Biodiversity Science, Ecosystem Services & Management 111 species richness and productivity can be improved by The amount of biomass determines forage availability >50%. Similar results were reported by Ayyad and El- and thus constrains livestock carrying capacity. Floristic Kadi (1982), where vegetation cover, species richness, changes also lead to economic and social problems and productivity increased as a result of rest. In fact, because of their negative impact on livestock carrying grazing management can provide adequate forage for live- capacity. To calculate carrying capacity, we need to deter- stock while maintaining environmental quality. Likewise, mine the total available forage in the pasture and also Sasaki et al. (2012) suggest that nutritive value and yield determine the animal demand for forage. Livestock con- of herbage can be modified greatly in responses to live- sumes 60% of the available forage, and they were able to stock grazing. consume 2.49 kg of DM for head per day which is This management practice seems to help maintain regarded as a normal value. The rate of residual forage floristic heritage sustainability and conservation. left after grazing is about 40% which represents an impor- According to certain authors (Ayyad and El-Kadi 1982; tant step towards fast improvement and resilience. When Deng et al. 2013), controlled grazing might be of better animal numbers are too high, the result is continued over- consequences than full protection. grazing of areas and degradation, desertification, and loss In our study, two grazing treatments were compared: CG of resilience. and controlled grazing. CG results in the re-grazing of plants, This study has found that by excluding grazing from leading to overgrazing. There are also many plants that are sites after exploitation with proper stocking rate will allow completely UG and of low quality but with high quantity the fast improvement of rangelands and will provide (Gamoun 2012, 2014). In contrary, the controlled grazing another fodder resource for livestock for the next season. may have resulted in severe grazing, but plants are not grazed This, in turn, determines the resilience of the arid while they were recovering, and there is no overgrazing. rangeland. The growth rate also depends on the severity of graz- Carrying capacity of each area is not considered as a ing. When plants are severely grazed, their regeneration is fixed parameter but rather as a variable dependent on slow. When grazing is less severe, the regeneration is rainfall. Allred et al. (2014) found that the influence of relatively rapid. In arid zones, where the evapotranspira- precipitation on livestock productivity was contingent tion is higher, controlled grazing can reduce the aerial upon spatial heterogeneity. If the pastoral use in the long biomass and allow the root system to meet the plant’s term remains higher than the current level, then the gen- water needs (Le Floc’h 2001). eralized degradation risk to the ecosystem is unavoidable. Moreover, many of the plants play an important role in By picking the most palatable species, animals exercise preventing soil erosion, increasing soil deposition and different defoliation pressure on existing species, which improving drainage of the lowlands (e.g., Stipagrostis can threaten their perpetuity. pungens (Desf.) de Winter and Calligonum comosum Sustainable use of the natural vegetation and the L’Herit). reduction of land degradation processes are important to Proper stocking rates are essential for the sustained ensure the rural population’s subsistence, provide eco- management and durability of these ecosystems for main- nomic and social incomes, and improve the economic taining vegetation structure and productivity. The grazing productivity (Jauffret & Lavorel 2003; Dembélé of our rangeland during 2 months by a grazing pressure et al. 2006). not exceeding 1700 head did not reduce the plant cover below the critical point (20–25%) at which erosion appears (Le Houérou 1995), so that regeneration was Conclusion faster and grazing would be possible again in the next Stocking rate is considered the most important of all season. Khumalo et al. (2007) suggest that light stocking grazing management decisions. Arid rangelands are typi- in the desert rangelands does not increase perennial grass cally resilient and capable of regeneration even though the production compared with conservative grazing, but it process of regeneration can be delayed by natural forces could have a small benefit in maintaining perennial grass (droughts) or by the interference of overgrazing, time of cover during drought. This proper stocking rate grazing introduction, and heavier stocking rates. −1 −1 (0.14 head ha year ) is important for sustainable graz- However, we confirm that rest and controlled grazing ing management and will ensure optimal animal and for- on arid rangelands are effective tools for sustainable man- age production over long term. In contrast, improper agement of these steppe ecosystems. By controlling stock- −1 −1) stocking rates (0.76 head ha year result in overgraz- ing rates, managers conserve biodiversity while ensuring ing, decrease in plant cover and biomass production, and the continued productivity of forage. poor species richness. The difference is due to the stocking rate, which is properly maintained in our case. This stock- ing rate, which can be used during 2 months of summer Acknowledgements without detrimental effects on the rangelands resource, can We extend our special thanks to Janet Patton for valuable dis- be considered as proper carrying capacity attempts to cussions and comments on the manuscript. We also thank Mr balance between long-term forage supply and forage con- Debbabi Said, Dadi Kamel and all members of the CRDA- sumption by grazing. Tataouine for technical assistance rendered. 112 M. Gamoun et al. Gamoun M. 2014. Grazing intensity effects on the vegetation References in desert rangelands of southern Tunisia. J Arid Land. Allen VG, Batello C, Berretta EJ, Hodgson J, Kothmann M, Li 6:324–333. X, McIvor J, Milne J, Morris C, Peeters A, Sanderson M. Gamoun M, Chaieb M, Ouled Belgacem A. 2010. Evolution des 2011. An international terminology for grazing lands and caractéristiques écologiques le long d’un gradient de grazing animals. 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Journal
International Journal of Biodiversity Science, Ecosystem Services & Management – Taylor & Francis
Published: Apr 3, 2015
Keywords: vegetation; controlled grazing; restoration; Tunisia