Vol. 32 – 2006 - Ecologia Mediterranea

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.
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