CC BY
52
АРИДНЫЕ ЭКОСИСТЕМЫ, 2012, том 18, № 2 (51), с. 54-61
=—=— ОТРАСЛЕВЫЕ ПРОБЛЕМЫ ОСВОЕНИЯ ЗАСУШЛИВЫХ ЗЕМЕЛЬ
ДИНАМИКА РАСТИТЕЛЬНЫХ СООБЩЕСТВ НА САХАРСКИХ
ПАСТБИЩАХ В ТУНИСЕ
© 2012 г. М. Гамун*, Б. Ханчи**, М. Неффати*
*Институт аридных регионов, лаборатория экологии пастбищ
Тунис, 4119 Меденин. E-mail: gamoun.mouldi@yahoo.fr **Факультет Наук Университета Туниса, отделение биологии
Тунис, 2092 Тунис.
Поступила 12.08.2011
Пастбища Сахары в южном Тунисе являются важной кормовой базой для выпасаемого скота. Эффективное управление ресурсами пастбищ оказывается невозможным в условиях жесточайшей деградации естественной растительности и при отсутствии мер по её восстановлению. Главной целью исследования стала оценка влияния системы оградительных заборов, организованной на одно-, двух- и трехлетние периоды, на растительность Сахары на трех ключевых участках с песчаными и известняковыми почвами. Результаты показали, что видовое богатство растительности и растительная биомасса увеличились за время оградительных от стравливания мер, а проективное покрытие оказалось выше на песчаных почвах, нежели на известняковых. В то же время на участках с высокой пастбищной нагрузкой песчаные почвы оказываются более продуктивными в сравнении с известняковыми, при том, что последние более устойчивы к вытаптыванию. Ключевые слова: растительность, деградация, отмена выпаса, почвы, проективное покрытие, видовое богатство, пастбищная продуктивность.
DYNAMIC OF PLANT COMMUNITIES IN SAHARAN RANGELANDS OF TUNISIA
© 2012. M. Gamoun*, B. Hanchi**, M. Neffati*
*Institut des Régions Arides, Laboratoire d'Ecologie Pastorale Tunisia, 4119Médenine. E-mail: gamoun.mouldi@yahoo.fr **Faculté des Sciences de Universitaire Tunis, Département de Biologie Tunisia, 2092 Tunis, Campus Universitaire Tunis El Manar II
The desert and Saharan rangelands of southern Tunisia are an important source of feed for grazing herds. Productive management of these rangelands has been found impossible if the natural vegetation becomes severely degraded unless restoration work is undertaken. The main objective of our study was to evaluate the impact of protective fencing for periods of one, two and three years duration on Saharan vegetation at three sites characterised by sandy and limestone (skeletal) soils. Results showed that vegetation covering, measured in terms of species richness and plant biomass, increased with time of protection from grazing, and that covering was more marked on the sandy substrate than on the limestone substrate, while under heavy grazed area the opposite occurred, thus I can deduce that the sandy soil are more productive than the limestone soil, whereas the latter are more resistant to animals trampling.
Keywords: vegetation, degradation, excluding grazing, soil, cover, species richness, pastoral productivity.
The North African steppic rangelands, located between the isohyets 50 and 300 mm, are mainly composed of very sparse vegetation (Le Houérou, 1995). Bordering the Saharan zones, the natural plant covering becomes scarce; and grasses appear no more than a short time period (Schiffer, 1971). These steppic rangelands have undergone various anthropogenic practices. Moreover, climatic conditions, human
population density and land use history are the major factors which affect the intensity of these practices; overgrazing, woodcutting and growing population (Aidoud et al., 2006).
It's obvious that natural resources management can be fulfilled through a better understanding of the vegetation dynamics related-processes (Padilla, Pugnaire, 2006). Because grazing activity obstruct the driving force of these processes. It leads to the loss of aerial plant species biomass (Kooijman, Smit, 2000). Actually, overgrazing is generally the essential cause of natural ecosystems degradation. It incites land degradation; reduces biodiversity which homogenizes flora eventually (Tarhouni, 2008). In southern Tunisia, rangelands are overexploited because the animal needs highly exceed plant production. For most of the skeletal desert soil, the overall vegetation covering oscillates between 1 and 10%; the perennial aerial biomass is lower than 100 kg of dry matter (DM)/ha/year (Le Houerou, 1995). In addition, when the mean annual rainfall is about 200 mm, pastoral production varies from 10 to 50 FU1/ha/year (Aidoud et al., 2006). The fluctuation of forage availability, both in quantity and quality, cannot permit to set equilibrium between livestock needs and natural resources production (Hiernaux, Le Houerou, 2006). Consequently, the pasture activity cannot afford an economic income; the pastoralists have to adjust their activities to this constraint. These phenomena are intensified by the impacts of climatic changes going together with several plant and animal species developing diverse and spectacular adaptations. As a result, biological conservation of the actual fauna and flora in the Saharan zone becomes crucial to ensure their sustainable development (Ozenda, 2004). However, it is important to note that the arid zone vegetation is more or less adapted to some recurring changes having limited effects on the disappearance of species in general (Darkoh, 2003).
In order to restrain the rapid erosion of biodiversity, priorities are given to restore and conserve the most promising species. These activities guarantee a good functioning of ecosystem and, therefore, the sustainable development of local populations (Child, 2003). Three management techniques, according to J. Aronson et al. (1993), may be considered: restoration, rehabilitation and reallocation. The choice of these techniques relies on degradation level and resilience capacity of the concerned ecosystems.
The restoration is used when degradation has not yet reached the irreversibility threshold. Simple to implement and relatively not expensive, restoration is characterized by an important interface between conservation and sustainable development of ecosystems (Clewell, Aronson, 2006). It is an efficient operation for biological conservation seeing the constant increase of density, covering and productivity of plant species that indicates.
Under protection, the steppe on sandy soil is more productive than on limestone soil, whereas the latter is more resistant to animals trampling (Gamoun et al., 2010). In desert rangelands, cover, species richness, and diversity increases with increasing precipitation and falls when the precipitation decreases. However, this response varies with soil type. Species richness and diversity were more affected by decreased rainfall than vegetation cover, but the effect on limestone was more evident than sandy soil (Gamoun et al., 2011).
In this context, this study examines vegetation behaviour on different soil types in three steppic rangelands of Saharan Tunisia during different protection period. The basic quest of this paper is to evaluate restoration effect on the dynamic and productivity of these desert rangelands. In other words, what tendency is attributed to the global plant covering under and after protection? How floristic composition and productivity respond to the applied protection rangelands?
Materials and methods
This study is carried in the Dahar rangelands. Being representative of different desert rangelands in the southern Tunisia (fig. 1), and is characterized by an arid Mediterranean bioclimate with a moderate winter. Rainfall is low and sporadic; the mean annual is estimated to be around 79 mm (during the period 19872007). Temperatures are generally cold in winter and hot in summer with a mean annual of about 20.1°C. The water balance is greatly affected by the low dense soil cover and exposition to winds. The rainfall quantity is almost constant and varies between 78 and 80 mm during the three period of observation. The local soils are predominantly not suitable for agricultural practices and have exclusively pastoral vocation. Despite the fragility of its potential, the region is overexploited by livestock during pluvial years. As a result, the spontaneous flora is found more degraded and its productive capacity reduced.
1 FU: Fodder Unit: it is the equivalent of the energy produced by 1 kg of barley.
Mediterranean sea
area.
The land has been subjected to continuous grazing throughout the year at stocking rates (1 to 4 head ha-1.year-1) that are greater than the long term carrying capacity (Le Houerou, 1969). The 1,100,000 ha of the Dahar communal rangelands are currently grazed by 460,000 sheep and 371,000 goats, resulting in a stocking density of 0.76 head ha-1. This level of grazing intensity could explain the disappearance of pasture species of Dahar (Elloumi et al., 2001).
The experiment was conducted inside the protected area. The first measurement was taken before protection (spring 2005), the other measurements were taken after protection (i.e. spring 2006, 2007, and 2008). The different community types seem to indicate different soil types. The studied rangelands are composed of four plant communities that are protected since 2006. These four plant communities are each characterized by their respective soil types.
- Rangeland 1: at an elevation of 364 m and located on sandy soil is classified as sierozem. It is characterized by more complete water infiltration and lower water retention. The organic matter content is low, the pH is greater than 7 and the calcium carbonate content is about 10 to 20%. Sand grains vary from coarse to medium size and are rarely fine. This type is dominated by Stipagrostis pungens, Retama raetam and Hammada schmittiana.
- Rangeland 2: in elevation of 346 m located on limestone soil, called regosol with high skeleton content, high calcium carbonate content, high pH values and low organic matter content. The vegetation is dominated by Gymnocarpos decander and Anthyllis sericea.
Hence, we can handle vegetation by temporal and edaphic variation. In this study one takes account only the temporal and edaphic variation, since the precipitation data from the years of the observations are almost constant.
In order to assess the vegetation covering and to inventory the floristic composition, the point quadrats method (Daget, Poissonnet, 1971) was used. Experimentally, six lines of 20 meters each are installed (three lines in the sandy soil and three lines in the limestone soil) in the studied zones. Along the line, a fine pin
was descended to the ground every 20 cm in the order of 100 times. Data on plant species and soil type were recorded. These measures are carried out in the course of the spring 2005, 2006, 2007 and 2008. The covering is calculated as follow:
R = (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).
The pastoral production is determined using a destructive method. Eight plots, of four m2 each, are installed in each zone (four plots in the sandy soil and four plots in the limestone soil). Within each plot, fresh consumable parts of the encountered plant species are cute and weighed. Representative samples are dried and weighed.
The obtained dry matter (DM) permits the estimation of the pastoral production using the following formula (El Hamrouni, Sarson, 1974): Pastoral production (in FU) =0.3*DM, with: 0.3 is the average of fodder units produced by 1 kg of DM.
The obtained data is subjected to several statistical analyses using SPSS for windows software v. 11.5 (SPSS Inc., 2002).
Results
1. Vegetation cover. The one way ANOVA shows that the vegetation covering is influenced by soil proprieties (p<0.05) and protection period (p<0.001). The interaction between these factors is not significant (p>0.05). During the first two years of protection, there is no evidence of notable differences among soil substratum's (p>0.05). Nevertheless, sandy soil appears as the most relevant soil for vegetation regeneration since the covering rate increases from 34 in the first year to 69% in the third year of protection (fig. 2). On the contrary, the optimum covering rate reaches only 50% on the limestone soil in the second year of fencing. Cover was higher in the sandy soil in protected area while under heavy grazed the opposite occurred.
2. Floristic richness. Species richness differed between soil type and protection. The mean number of species identified in each year and soil samples is shown in figure 3. This figure shows that the number of species comprising the pasture increased during the study period.
Рис. 2. Изменение проективного покрытия растительности во времени в условиях предотвращения выпаса (весна 2006, 2008 и 2009) и в зависимости от почвенного субстрата. Fig. 2. Effects of the protection period and the soil substratum on the vegetation covering in the studied rangelands through protection (spring 2006, 2008 and 2009) and under overgrazing.
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2005
after protection
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2007
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□ annuals ■ perennials
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2007
limestone soil
10
2008
14
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9
8
6
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Рис. 3. Изменение видового богатства на пастбищах во времени в условиях предотвращения выпаса. Fig. 3. Variation of the floristic richness between rangelands and soil substratum through protection (spring 2006, 2008 and 2009) and under overgrazing.
On the sandy soil, the number of species in the grazing ban area increased from 6 to 29 species in 2008 with 14 perennials species and 15 annuals species (table). On the limestone soil, after 3 years of grazing ban, species increased to four 24 species (14 perennials species and 10 annuals species). Compared with free grazing in 2005, the species richness improved to 80% in sandy soil and to 55% in limestone soil.
The results show that perennial plants amount increases proportionally with the length of grazing ban. In the first year of protection, the limestone soil holds more perennial species whereas the sandy one does not. The erosion process seems to explain this observation, being more pronounced in sandy substratum where winds make easy soil particles movement at the beginning of protection period. Hence, both annual and perennial plant species cannot appropriately survive. However, when the fencing period is more extended, especially in second and third year of protection, the development of plant species fixes the soil particles in sandy substratum. Thus erosion decreases and the floristic richness rise simultaneously. After three years of protection, the decline of annual plants number on limestone soils is accompanied with perennial species amplification. The latter is seen as an outcome to their large competitive potential to absorb water and soil nutrients. Under heavy grazed the limestone soil is richer in species than sandy soil.
3. Pastoral production. Pastoral production had increased significantly in all grazing ban after the exclosure grazing. Figure 4 illustrates that the pastoral production increases from 19 to 89 FU/ha/year after three years of protection on sandy soils. Meanwhile, the so-called productivity is stabilizing at 43 FU/ha/year since the second year of protection on limestone soils. Addressing the impact of soil substratum on pastoral production, three years of fencing, highlight rapid plant regeneration on sandy soils.
Analysis of variance confirmed there was significant difference in pasture production sampled at the 5% level for protection and soil type. Generally, pastoral productivity was higher in the sandy soil in protected area while under heavy grazed area the opposite occurred. The difference seen in terms of rangeland production is manifested through the emergence of very palatable plant species having high productivity. In the protected areas, the growing species contribute to sand particles maintain and thus soil fertility. Afterwards, plant species, which disappeared under grazing pressure, are meeting again favourable conditions for their development. Hence, floristic successions overtake pastoral shortages in order to improve productivity.
Таблица. Виды, встреченные на двух типах пастбищ после трех лет охраны от использования. Table. Plant species occurrence in two rangeland types after three years of protection.
Sandy soil Limestone soil
perennials - многолетники
Allium roseum Anabasis oropediorum Maire
Argyrolobium uniflorum (Deene.) Jaub. & Spach Anthyllis sericea Lag. subsp. henoniana (Coss.) Maire
Aristida ciliata Desf. Atractylis serratuloides Sieber ex Cass
Atractylis flava Desf. Echiochilon fruticosum Desf.
Calligonum comosum L'Herit. Echium humile Desf.
Echiochilon fruticosum Desf. Farsetia aegyptiaca Turra
Echium humile Desf. Gymnocarpos decander Forssk.
Hammada schmittiana (Pomel) Iljin. Hammada schmittiana (Pomel) Iljin.
Nolletia chrysocomoides (Desf.) Cass. Hammada scoparia (Pomel) Iljin.
Plantago albicans L. Helianthemum kahiricum Delile
Retama raetam (Forssk.) Webb Helianthemum sessiliflorum (Desf.)
Rhanterium suaveolens Desf. Hernaria fontaneesii J. Gay
Salsola vermiculata L. Reaumuria vermiculata L.
Stipagrostis pungens (Desf.) de Winter Salsola vermiculata L.
annuals - однолетники
Asphodelus tenuifolius L. Astragalus corrugatus Bertol.
Bassia muricata (L.) Asch. Fagonia glutinosa Delile
Centaurea furfuracea Coss. & Dur. Filago germiniaca L.
Cutandia dichotoma (Forssk.) Trab. Ifloga spicata (Forssk.) Sch. Bip.
Daucus carota Murb. Launaea angustifolia (Desf.) Muschler
Enarthrocarpus clavatus Delile ex Godr. Launaea resedifolia (L.) O. Kuntze
Launaea resedifolia (L.) O. Kuntze Matthiola longipetala (Vent.) DC.
Lotus pusillus Viv. Savignya parviflora (Del.) Webb
Matthiola longipetala (Vent.) DC. Schismus barbatus (L.) P. Beauv.
Medicago minima Grufb. Scorzonera undulata Vahl.
Paronychia arabica (L.) DC.
Savignya parviflora (Del.) Webb
Schismus barbatus (L.) P. Beauv.
Silene arenarioides Desf.
Thesium humile Vahl.
Discussion
Monitoring vegetation dynamics under different fencing periods shows many effects on vegetation physiognomy and pastoral production. The covering rate increases during the fencing process. The floristic richness and pastoral productivity were two essential parameters for studying the natural vegetation response to restoration modes (patterns). Tracking plant richness allows measuring the structural resilience of a given ecosystem; while pastoral production helps the study of its functional resilience. Furthermore, these indicators were tightly linked to the regeneration mechanisms amongst coexistent species which depends on disturbances type, frequency and intensity. The relationship between richness and pastoral productivity can be discussed according to more than a single approach: The first supposes that plant communities, rich in species, should be more productive since they have the lowest nutrients loss (Tilman et al., 2001), based on the fact that the nutrients uses fluctuate from one plant specie to another. These variations can fully optimize the nutrient availability and reach, therefore, a better productivity of plant communities (Tilman et al., 1996). Eventually, it appears that our results confirm it: the highest productivity belongs to the richest vegetation. However, the existence of retroactive effect of productivity on diversity may make this hypothesis enough
controversial (Grime, 1988). The productive communities contain generally a plant species with a large size and more important biomass. As a consequence, plant species were increasingly competitive towards light and ground although their renewal decreases. Therefore, the ecological processes and plant production as well were provided with a small number of species which dominate the overall community.
Рис. 4. Изменение продуктивности во времени на разных типах пастбищ в условиях предотвращения выпаса. Fig. 4. Variation of pastoral production (in UF / ha / year) depending on soil type and the protection period.
At the beginning of the succession process, the floristic composition is more expanded on limestone soils than on the sandy one where it consists of just 10 species before protection. In reality, due to the grazing pressure, herbaceous species were completely and easily eradicated on sandy soils compared to limestone soil where plants were well fixed to the soil upper layer. On sandy soils the biggest plant species like Stipagrostis pungens and Retama raetam subsist owing to their well adjustment to drought and human impacts. Floristically, post-protection sandy soils were ameliorated so that they turn out to be more productive than the limestone reg, whereas the latter were more resistant to animals trampling (Gamoun et al., 2010). In the short term, healthy signs of fencing on vegetation were evident. Different restoration aspects encourage rangeland regeneration which affects directly but differently annual and perennial plants. Palatable plants possibly increased after protection, such as; Nolletia chrysocomoides, Argyrolobium uniflorum, Aristida ciliata, Echiochilon fruticosum, Plantago albicans, Anabasis oropediorum, Helianthemum sessiliflorum and Hernaria fontaneesii. Generally annuals species have been considered palatable plant, their number increases immediately after the protection.
Besides, precipitations have strong influence on annual species while the perennial, with a great inertia, respond slowly to rainfall variations.
Conclusion
The restoration of degraded rangelands provides essential ecological services to the livestock. It allows actually inter-annual and spatial variability through accelerating natural vegetation covering regeneration. In this context, our paper aims to evaluate the impacts of fencing techniques on Saharan vegetation of three protected areas in southern Tunisia using three ecological indicators: covering, floristic richness and pastoral production.
Main results are: 1) covering rate, richness, and pastoral production are much more important for the sandy soils than the limestone one, 2) difference of annuals and perennials distribution according to rangelands characteristics 3) after three years of fencing, plant regeneration is remarkable and 4) the sandy soil are more productive than the limestone soil, whereas the latter are more resistant to animals trampling.
On the other hand the limestone soil supports better overgrazing.
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