Science et changements planétaires / Sécheresse


Effets des feux de savane sur le stockage et l’émission du carbone et des éléments-trace Volume 4, issue 4, Décembre 1993

Laboratoire d’Écologie, URA CNRS 258, École Normale Supérieure, 46, rue d’Ulm, 75230 Paris Cedex 05, France.
  • Page(s) : 251-63
  • Published in: 1993

Savannas roughly cover one fifth of emerged lands. They are submitted to bush fires, the influence of which on both the ecosystems and the atmosphere is recognized, but still hotly debated. Relevant data are numerous, but yet insufficient and often contradictory. Fire is a very constituent of savannas : it depends on microclimatic conditions, but is primarily bound to the nature, the structure and the physiological state of plant cover, and to such interactive phenomenons as herbivory. It both depends on and maintains savanna heterogeneity : understanding its behaviour and effects at the regional or global scale is difficult. Consubstantial to savonnas as soon as these appeared, the major effect of fire is to bring the system back to an initial stage, at least for its above-ground parts. It does not affect much the soil organo-mineral status, though the latter migh evolve in the long term in protected areas. As a whole, the effects of fire derive from vegetation changes. Late fires are the most detrimental to both vegetation and soil. On the other hand, early fires seem even more beneficial than protection. In the short term, protection results in an increased grass bio-mass ; in the mid term, production returns to its initial value ; in the long term, production decreases with tree cover development. In the very long term, total protection (including against grazing) should lead to a forested system in which storage of carbon and nutrients in the biomass (not necessarily in the soil) should prime over fluxes (exchanges with the atmosphere). At the present time, emissions or greenhouse products by fire are important, but only represent a low proportion of global emissions. In savannas, man activities increase at such a rate that present fires might soon disappear. In a near future, substantial changes in greenhouse emissions from savannas will much more depend on changes in agricultural and forestry practices than in fire occurence.