Figure 12.2. Productivity of spineless cactus (Opuntia ficus-indica var. inermis) versus rainfall in Tunisia. Adapated from Monjauze and Le Houerou (1965).
Large areas of platyopuntias are available year-round in Algeria, Mexico, and especially Brazil and Tunisia and serve as an emergency feedstock in case of drought (Table 12.1). In many arid areas (e.g., Tunisia, Mexico, South Texas, South Africa), farmers make extensive use of cacti as emergency forage, harvesting it from both wild and cultivated populations to prevent the disastrous consequences of frequent and severe droughts (Le Houerou 1992). Since 1900 in North Africa, shrubs (mainly Acacia cyanophylla and Atriplex nummularia or A. halimus) and cacti (mainly Opuntia ficus-indica var. inermis) have been planted to reduce water and wind erosion and rangeland degradation. Nearly 1 million ha have been planted in Algeria, Morocco, Libya, and Tunisia (Table 12.1). These plantations in low rainfall areas also provide feed for livestock during frequent drought periods. Because of their high water-use efficiency (Nobel 1988, 1989), the aboveground productivity of cacti tends to be much higher than that of most other plant species in arid and semiarid regions. For example, under rain-fed conditions in Tunisia and with no fertilizer application, spineless platyopuntia yields vary from 20 to 100 tons fresh weight of cladodes ha-1 year-1 as rainfalls increase from 150 to 400 mm per year (Fig. 12.2).
Marginal lands often represent fragile ecosystems. Ploughing and indiscriminate vegetation removal can result in large-scale degradation and destruction of vegetation cover. The scarcity and even disappearance of several plant species indicate the magnitude of genetic and edaphic losses (Le Houerou 1991). To reverse the desertification trend and to restore vegetation cover in such areas, appropriate integrated programs are needed for rangeland monitoring, livestock control, and conservation of natural resources.
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