Revue internationale d'écologie méditerranéenne International ...

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