From nursery to field: context-dependent benefits of native mycorrhizal inoculation on Khaya madagascariensis seedlings in degraded highland ecosystems of Madagascar

Restoring vegetation on degraded tropical slopes remains a major challenge due to strong edaphic constraints limiting plant establishment. Although arbuscular mycorrhizal fungi (AMF) are known to enhance plant performance, their effectiveness under field conditions and across contrasting environments remains insufficiently understood. In this study, we investigated how site-specific soil properties and native AMF communities influence the performance of Khaya madagascariensis , an endemic tree species used in ecological restoration, from nursery to field conditions across two degraded sites in Madagascar (Boeny and Betsiboka). Soil analyses revealed marked edaphic contrasts between sites, associated with significant differences in AMF community structure. Betsiboka exhibited higher spore density (5 900 ± 601 vs 3 674 ± 537 spores per 100 g soil), diversity (H’ = 1.545 vs 1.107), and evenness (0.862 vs 0.688). Under nursery conditions, AMF inoculation significantly enhanced seedling performance, increasing shoot biomass (p < 0.0001), root biomass (p < 0.05–0.01), and mycorrhizal colonization rates (from ~ 20–23% to ~ 52–59%, p < 0.01). After transplantation, inoculated seedlings generally showed improved performance; however, the magnitude and direction of this effect varied between sites. A stronger positive response was observed at Boeny, whereas at Betsiboka the effect was weaker and, in some cases, associated with reduced survival. These results demonstrate that AMF-mediated benefits are not universal but strongly context-dependent. The stronger response under more limiting conditions suggests that AMF symbiosis plays a key role in alleviating environmental constraints. This study highlights the importance of site-adapted microbial communities and supports the integration of plant–soil–microbe interactions into restoration strategies for degraded tropical ecosystems.