Vol. 32 – 2006 - Ecologia Mediterranea

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MOHAMED TARHOUNI, AZAIEZ OULED BELGACEM, BELGACEM HENCHI, MOHAMED NEFFATI 46 Figure 8 – Percent cover of plants classified with Grime types depending on the season. See Table 1 for ANOVA results. Values are means ± IC. a/b indicate differences for RS plants; 1/2 indicate differences for CRS plants and α/β for S plants. Plant cover (%) 50 45 40 35 30 25 20 15 10 5 0 a CRS C S R S S CRS C S R S S CRS C S R S S CRS C S R S S CRS C S R S S CRS C S R S S Fall Spring Well 1 phytes and chamaephytes (Waechter 1982). During the rainy season (spring), when animals can satisfy their water-needs by the consumption of the water-rich species (especially annual), the grazing activity around wells is reduced. Animal passages to water can be spaced by several months and permit to plants to regenerate. After the summer drought all b 1 a α 2 Fall Spring Well 2 Figure 9 – Percent cover of plants classified with Grime types depending on the distance from watering points. See Table 1 for ANOVA results. Values are means ± IC. a/b/c indicate differences for CRS plants, 1/2/3 indicate differences for CS plants and α/β indicate differences for S plants. Plant cover (%) 50 45 40 35 30 25 20 15 10 5 0 a 1 2 b 12 a α α CRS CS RS S CRS CS RS S CRS CS RS S CRS CS RS S 250 m β 500 m 1 km 2 km Well 1 a 3 β a α ab β bc c β αβ CRS CS RS S CRS CS RS S CRS CS RS S CRS CS RS S 250 m 500 m 1 km 2 km Well 2 b β 250 m 1 Fall Spring Well 3 annuals are died and survive only as seeds in the soil. Hence the biologic spectrum, in spring, is remarkably different from fall. The arido-active species, in particular chamaephytes, are very sensitive to degradation. The number and abundance of the aridopassive species are not correlated with the degradation degree simulated by distance a CRS CS RS S CRS CS RS S CRS CS RS S CRS CS RS S 500 m 1 km 2 km Well 3 ecologia mediterranea – Vol. 32 – 2006 2 b
from watering points. One third (1/3) of Tunisian flora is composed by arido-active species (chamaephytes and perennial grasses) versus 2/3 of arido-passive species (essentially annual plants) (Jauffret 2001). The ephemeral characters of arido-passive species and persistent characters of arido-active species represent the two major strategies to support the dry season by therophytes and chamaephytes respectively (Evenari et al. 1975). Our results indicate that the aridoactive species are more affected by the degradation degree than by climatic drought. The arido-passive species avoid dry periods by various dormancy shapes and mark the sites with very advanced degradation (Glatzle 1985). Hence, flora seems to be more adapted to abiotic conditions than the disturbance degree. We show that arido-passive plants are not affected by disturbance degree around wells. This is can be explained probably by two reasons: (i) degradation around watering points is very advanced and the floristic cortege becomes homogeneous (Tarhouni et al. 2006) and (ii) animal activity around watering points contributes in seed dissemination, provoke the extinction of more competent species and encourage annuals development. The plant species in Tunisian arid zones, in spite of some recent studies (Jauffret 2001; Jauffret & Visser 2003), are not yet classified following their adaptative strategies sensu Grime. The majority of the arid zone plant species, that occupy very drastic sites, are stress tolerant. In their study, Madon & Médail (1997) showed that annuals in Mediterranean grasslands are often S strategies but Jauffret (2001) concludes that annuals are only RS and never S strategies. These stress-tolerant and ruderal taxa (RS) are more adapted to climatic stress and human disturbances (Grime 1977). According to disturbance degree, plant species can develop different adaptative strategies. For example at intermediate disturbance the competitive plants (C) can survive, while at sufficiently high level of disturbance only fugitive species (annual) can survive (RS) (Jauffret & Visser 2003; Sagar et al. 2003). In our previous study, Tarhouni et al. (2007), we show the dominance of RS-species in the more disturbed sites (close to watering points), CRSand CS-species at medium disturbance and CS- and S-species in lower disturbance sites (further from water). ecologia mediterranea – Vol. 32 – 2006 A preliminary overview of the effects of seasonal drought and animal pressure around watering points on plant species using adaptative strategy analyses in the Tunisian arid zone The present results constitute valid hypotheses for future work which must study more wells on skeletal soils and more wells on sandy soils with more measured distances. The period of exploitation as well as the variation of geomorphologic substratum and the technical aspects of the water point (depth, daily water yield, water salt content, equipment, location, etc.) can play a determinant role in the vegetation repartition. The characteristics particular to each well can be considered as the key determinants of the animalload (number of animals) in each site (Thebaut & Batterbury 2001). Conclusion In southern Tunisia, and especially in its hyperarid part, the floristic cortege is submitted to several constraints in relation with the edapho-climatic conditions and human disturbance. These combined factors lead to increase the degradation of plant communities. Such degradation cause changes in the physiognomy and structure of vegetation cover especially in sites submitted to a strong animal pressure like watering points. The main results of this study show that the covers of plant types are lower during the fall. Some plant types are altered by the grazing gradient around watering points but some others are more affected by the seasonal drought than the animal activity. This is explained by the strong animal stamping which is amplified by climatic and edaphic drought. During the rainy season, animal activity around watering points is reduced or null. Hence vegetation surrounding wells benefits from a temporary break. The strong contribution of annual plants to vegetation cover around wells can play a beneficial role by fixing the soil particles and improving its fertility. References Aronson J. & Le Floc’h E., 1996. Vital landscape attributes: Missing tools for restoration ecology. Restoration Ecology, 4 (4): 377-387. Aronson J., Le Floc’h E., Gondard H. & Romane F., 2002. Environmental management in Mediterranean region: references and linked indicators to plant biodiversity. Revue d’écologie (Terre et Vie) Suppl. 9: 225-240. 47
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Page 5: Pr Thierry DUTOIT Éditeur en chef
Page 8 and 9: AMAR MADOUI, JEAN-MARIE GEHU, DJAME
Page 17 and 18: Écologie trophique de la Cigogne b
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Résumés de thèses Vincent OLLIVI
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Résumés de thèses Marnyye VELASQ
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