Reforestation of exotic plants can disturb fertility of tropical soils

Washington, May 30 : Scientists have determined that the reforestation of some exotic plants can surprisingly disturb the fertility of tropical soils.

With the aim of limiting land impoverishment, which is hitting the intertropical and mediterranean zones particularly harshly, a range of reforestation programmes using rapid-growing forest 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.

But, a research programme run since 2005 in Senegal and Burkina Faso by an IRD team and its partners 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.

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.

According to scientists, the organizations involved in natural resources management, should 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.

ANI