36 TUNISIA 



greater than its roots are capable of reaching within the first 

 few years after planting. At present Tamarix africana is used 

 for planting only on those portions of the dunes which are capable 

 of retaining some moisture during the summer. Experiments 

 are also being carried on with other drought-resistant species, 

 particularly with Acacia eburnea, Acacia decurrens, and 

 Acacia tortilis, of which the latter grows naturally in the 

 valley of Thalah. This species resists droughty years, since 

 its roots penetrate deep into the ground. During the first 

 year, however, this acacia starts with difficulty and requires 

 repeated watering so that its cultivation over large areas is 

 impracticable. The Forest Service is also conducting experi- 

 ments with Parkinsonia aculeata, a tropical shrub which origi- 

 nates from the deserts of South America, and also with Tamarix 

 articulata which, so far, have given good results. 



On the shore dunes to the east of the city of Bizerta the sow- 

 ing of Imperata cylindrica, Ricinus communis (native), and of 

 Tamarix made under cover of brush gave good results. Other 

 species, such as Acacia, Eucalyptus, Casuarina, and also Pinus 

 halipensis and Pinus maritima, have been tried but without 

 success. 



When, owing to the looseness or mobility of the sand, the 

 roots of the natural growth are bared by the wind, it may be 

 temporarily held in place with a brush cover. But frequently 

 despite all precautions, the natural regeneration does not get 

 a hold when once destroyed by overgrazing. 



Erosion. On the short steep slopes of the spring basins, 

 as well as on the reserved areas where the surface becomes so 

 caked that the water cannot be absorbed, dangerous erosion 

 has taken place. This fills the springs with sand in considerable 

 quantities and decreases their flow. When once a ravine has 

 been formed, every torrential rainfall causes serious damage. 

 Any measures designed to prevent this erosion must keep the 

 water from these downpours from collecting in large quantities 

 and from flowing in any volume. To accomplish this it must 

 filter into the soil, a process facilitated by plowing the requisite 

 number of horizontal ditches, aligned exactly on contours. 



