Revue internationale d'écologie méditerranéenne International ...
Revue internationale d'écologie méditerranéenne International ...
Revue internationale d'écologie méditerranéenne International ...
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166<br />
◆ L. ABDALLAH, M. CHAIEB & E. LE FLOC’H<br />
Abridged English version<br />
During the last few decades, arid and desert ecosystems were subjected<br />
to high and continuous human pressure. Overgrazing, clearing<br />
and deforestation were the main causes of degradation. This caused<br />
phytogenetic erosion to occur and several species to become endangered.<br />
In addition to human pressure, altering climatic conditions,<br />
marked by decreasing rainfall and increasing of potential evapotranspiration,<br />
contributed to the ecological unbalance in these arid<br />
and desert regions. Faced with these phenomena, several specialists<br />
of these regions suggested setting up restoration programs to improve<br />
ecosystem health. Many questions were put on the agenda before<br />
any action, for example which kind of phytogenetic sources could<br />
be used for restoration?<br />
Within this frame, we studied the phenology and ecophysiology<br />
of two shrub species from Saharan Tunisia. The study<br />
was carried out in natura near the town of Nefta (33°55’ N,<br />
8°08’ E ; south-west Tunisia). This site is characterized by a<br />
Mediterranean climate, upper Saharan biogeographic zone. The<br />
vegetation is a continental steppe, dominated by chamaeophytes<br />
and nanophanerophytes such as Spartidium saharae, Calligonum<br />
polygonoides subsp. comosum, Cornulaca monacantha, Retema<br />
raetam, Stipagrostis pungens. Soil is a shallow coarse, very filtering<br />
sandy top gypsum crust. Average annual rainfall is generally<br />
lower than 100 mm.<br />
Our study dealt with the phenology and water potential (Ψ H)<br />
of two Saharan shrubs, Spartidium saharae (Fabaceae) and<br />
Calligonum polygonoides subsp. comosum (Polygonaceae). The<br />
observations took place in 1999, 2000 and 2001on a monthly<br />
basis. The two species were often able to maintain a high Ψ H.<br />
For the duration of the study, midday water potential (Ψ min) was<br />
generally greater than –2.5 MPa and predawn water potential<br />
(Ψ B), were never below –1.3 MPa for Spartidium saharae and<br />
–2.3 MPa for Calligonum polygonoides subsp. comosum.<br />
In addition to a lower (Ψ B) and a larger difference water<br />
potential (ΔΨ = Ψ B – Ψ min), plant phenology suggested that<br />
Calligonum polygonoides subsp. comosum is likely to be more<br />
efficient in controlling water transfer than Spartidium saharae<br />
is. The low ΔΨ values measured during all seasons are likely<br />
to explain the low biologic activity of Calligonum comosum.<br />
As a matter of fact, this species showed relative sensitivity to<br />
drought that inhibited its photosynthetic activity and caused<br />
its leaves to fall.<br />
As Spartidium saharae developed a strong drawing and swiveling<br />
root system, it seemed more efficient in using the low rainfall<br />
generally encountered under arid climate. In the same way, the<br />
scaly foliage of this species is likely to allow adjustments in<br />
water transfer while the remaining photosynthesis is progressed<br />
by stems.<br />
In conclusion, planting a mixture of these two local species on<br />
Tunisian rangelands from the arid and Saharan zones could insure<br />
reasonable plant production. This will improve the plant cover that<br />
can better stabilize sand dunes and prevent desertification.<br />
ecologia mediterranea, tome 29, fascicule 2, 2003