ABSTRACT
The objectives of this study were to determine the impact of two exotic tree species (pine and eucalypts) on the early growth of Uapaca bojeri (an endemic tree species from Madagascar) via their influence on soil chemical, microbial characteristics, on ectomycorrhizal fungal community structures in a Madagascarian highland forest and to test the ability of an early-successional ectomycorrhizal shrub, Leptolena bojeriana, to mitigate the impacts of these exotic species. Finally, we hypothesised that L. bojeriana could act as a natural provider for ectomycorrhizal propagules. Soil bioassays were conducted with U. bojeri seedlings grown in soils collected under the native tree species (U. bojeri and L. bojeriana) and two exotic tree species (Eucalyptus camaldulensis and Pinus patula) and in the same soils but previously cultured by L. bojeriana seedlings. This study clearly shows that (i) the introduction of exotic tree species induces significant changes in soil biotic and abiotic characteristics, (ii) exotic-invaded soil significantly reduces the early growth and ectomycorrhization of U. bojeri seedlings and (iii) L. bojeriana decreased these negative effects of the exotic tree species by facilitating ectomycorrhizal establishment and, consequently improved the U. bojeri early growth. This study provides evidence that L. bojeriana can facilitate the ectomycorrhizal infection of U. bojeri and mitigates the negative effects of the introduction of exotic tree species on the early growth of the native tree species. From a practical point of view, the use of ectotrophic early-succesional shrub species should be considered to improve forest restauration after exotic invasion.

Key words: ectomycorrhizas, Uapaca bojeri, Exotic tree species, Degraded forest ecosystems, Nurse plant, Restoration ecology, revegetation strategies