According to World Soil Information (ISRIC) rate, in the space of 50 years, 12.8 million km2 of soils have thus experienced diminished fertility. With the aim of limiting such land impoverishment, which is hitting the intertropical and mediterranean zones particularly harshly, a range of reforestation programmes using rapid-growing forest species (such as eucalyptus, exotic pine or Australian acacias) was undertaken from the mid 1970s.

Establishment of bacterial and mycorrhizal symbioses provides these trees with the adaptation ability necessary for growth on virtually barren, mineral-deficient soil. Although no proof is needed as to their effectiveness for producing plant biomass in harsh environmental conditions and their utility as windbreaks to control erosion, there is little information on their potential impact on the genetic and functional biodiversity of the soil microorganisms.

A research programme run since 2005 in Senegal and Burkina Faso by an IRD team and its partners1 yielded clues for understanding the influence of exotic plants on the structure and biodiversity of these communities of fungi and bacteria. In Burkina Faso, controlled experiments showed that the development of E. camaldulensis, the eucalyptus species most often planted in the world, outside its area of origin, significantly reduced the diversity of the mycorrhizal fungi communities essential for the healthy functioning of the ecosystem.

This negative effect was also found in the soil of a Senegalese plantation of Acacia holosericea where, scarcely a few months after its introduction, the soil's microbial characteristics had completely changed. This quick-growing species had effectively selected certain species of mycorrhizal fungi and bacteria of the genus Rhizobium, ending in a reduction in the species diversity of these symbiotic communities. The soil sampled from areas surrounding the A. holosericea plantation had a balanced distribution of mycorrhizal fungi species, whereas the breakdown of the fungal spore content in soil from the plantation showed a predominance of one species and therefore a strong imbalance in the composition of the mycorrhizal fungi community.

In the knowledge that a plant ecosystem's productivity is closely dependent on a soil's mycorrhizal diversity, there is a risk that the Australian acacia might create a new ecosystem whose physical, chemical and biological characteristics will not necessarily be favourable to a recolonization of the habitat by native species. The research also demonstrated that the environments generated by this species were less resistant to water and heat stress. In a context of global climate change, such habitats could therefore experience a drastic fall in their microbial activity and thus lose their ability to be the basis of proper development of the plant cover.

The conclusions of the study conducted in Senegal in a precisely defined environment cannot, however, be generalized to tropical soils as a whole. Indeed, investigations on another A. holosericea plantation, in Burkina Faso, yielded the observation of an increase in microbial functional diversity. The contradictions between these sets of results should prompt the organizations involved in natural resources management to plan for possible introductions of exotic species case by case, taking account not only of potential impacts of the plant species under consideration for introduction, but also of the nature of the soils they are to colonize. For although this practice can yield highly satisfactory results, such as increases in the species richness of severely degraded environments, such as old mining areas, it can also upset for a long time the organization of the microbial communities which guarantee the fertility of a soil.

This research work was conducted with the support of scientists from the Département de biologie végétale of the Cheikh Anta Diop University of Dakar (Senegal) and from the Laboratoire Sol-Plante-Eau of the Institut de l'Environnement et des Recherches Agricoles (Inera) of Ouagadougou (Burkina Faso).

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