Vol. 32 – 2006 - Ecologia Mediterranea

MOHAMED TARHOUNI, AZAIEZ OULED BELGACEM, BELGACEM HENCHI, MOHAMED NEFFATI
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en fonction des distances par rapport aux puits
alors que celui des thérophytes ne varie pas ; (3)
au cours du printemps, les recouvrements des
arido-passives et des arido-actives sont plus
importants ; (4) le recouvrement des aridoactives
augmente avec la distance par rapport
aux puits mais celui des arido-passive ne varie
pas ; (5) le recouvrement des CRS et celui des RS
sont plus importants au cours du printemps et
(6) les recouvrements des CS, des CRS et des S
augmentent avec la distance par rapport aux
puits à l’exception de celui des RS.
Introduction
Spontaneous plants are submitted to many
constraints such as abiotic stress and human
disturbance, particularly in arid environments.
The harsh climatic conditions are high temperatures
in summer and cool in winter. These
inter-seasonal variations constitute an important
thermal stress (Le Houerou 1995) and
affect plant germination, a crucial stage in
their establishment (Gilbert & Haferkamp
1989). Water stress, which characterizes the
arid bioclimates (including the Mediterranean
one), induces a decrease in the total dry
weight and leaf area of plants, as well as in
the dry weight and length of roots (Munns
2002; Banon et al. 2004). Human activities
and grazing pressure affect floristic diversity
(Jauffret & Visser 2003; Mahamane & Mahamane
2005; Tarhouni et al. 2006). Grazing
induces a change in the ecological balance
because animals make a consumption choice
which favors no consumed plants to the detriment
of the appreciated ones (Waechter 1982;
Boudet 1984).
In southern Tunisia, characterized by elevated
stresses and strong human disturbances, the
flora is mainly constituted by resistant and
adapted species. However, for most north-
African steppic species, comparative and specific
data describing plant adaptative strategies
are rare, and the information in the
regional flora books (Cuenod et al. 1954;
Quezel & Santa 1962, 1963; Ozenda 1977;
Pottier-Alapetite 1979, 1981) is insufficient to
study these mechanisms. The combination of
biologic types (life forms) in a biologic spectrum,
sensu Raunkiaer (1934), corresponds to
the analysis of vegetation structure and its
biologic characters (Godron 1971). The biologic
spectrum is considered as a flora adaptation
strategy to environmental conditions
especially to climatic ones (Daget 1980; Box
1987). This spectrum is an ecosystem vital
attribute and describes the structure and the
function of ecological systems; usually the
rank of life forms in the ecosystem decreases
with the degradation degree (Aronson & Le
Floc’h 1996). On the other hand, Evenari et
al. (1975) proposed a classification permitting
to assign species functional criteria in relation
with their adaptation to the harsh climatic
conditions. Indeed, these authors classified, in
accordance with Noy-Meir (1973), the arid
zone plant species in two categories: (i) the
“arido-passive” species which do not possess
a photosynthetic activity during the dry period
and (ii) the “arido-active” species which
maintain such activity, even reduced, during
this period. The C-S-R Grime model completes
the classification of plants to climatic
constraints and human disturbances (Grime
1974, 1977; Grime et al. 1988). This model
classify plants to three groups: (1) Competitive
(C) plants monopolizing resources with
their strong vegetative development; (2)
Stress-tolerant (S) plants occupying habitats
poor in nutrients and (3) Ruderal (R) plants
growing where the natural vegetational cover
has been disturbed.
In order to reduce the loss of plant diversity
and the loss of ecosystem resilience, it is necessary
to better understand and predict the
behaviour of various response groups within
plant communities (Aronson et al. 2002). It
seems that the C-S-R model, extensively used
by Grime and his collaborators to establish
management plans and make restoration recommendations,
determines vegetation
response to all the abiotic and biotic changes.
Grime’s concept is very attractive as it
describes plant species adaptation in particular
to their environment and along of disturbance
and stress gradient (Jauffret 2001).
However, the use of the CSR strategies, conceived
on the herbaceous vegetation of temperate
Europe, required an adapted interpretation
to the north-African steppe because of
the knowledge missing on the majority of its
species (Jauffret & Visser 2003).
In this study, we evaluate the influence of a
disturbance gradient and of seasonal drought
on plant species depending on their classification
in various types. The aims are to understand:
(i) What is the impact of seasonal
drought and disturbance degree on species
biologic spectrum (Raunkiaer 1934)? (ii)
What is the impact of seasonal drought and
ecologia mediterranea – Vol. 32 – 2006