Effects of eucalypt plantation abandonment on functional diversity of vegetation and soil microbiota in a mediterranean region

Eucalypt tree plantations have been identified as important drivers of land use change and biodiversity loss. In the Mediterranean basin, the abandonment of these forest stands is leading to a natural decay of the plantations, triggering a process of secondary succession. Despite the apparent recovery of these stands, there is not enough evidence to claim that passive restoration alone is sufficient to restore ecosystem functioning, particularly regarding the interactions between plants and soil microorganisms, which are fundamental for forest processes. This study evaluates the compositional and functional recovery of woody plant communities and the recovery of soil microbial activity and function along an abandonment gradient of eucalypt plantations including managed plantations, recently abandoned plantations, long-abandoned plantations, and native Quercus suber forests as reference ecosystems. We analyzed shrub and tree taxonomic and functional diversity based on resource acquisition traits and symbiotic associations (e.g. mycorrhizal type), alongside soil microbial activity and microbial functional diversity to evaluate biodiversity and ecosystem function recovery. We further assessed the relationship between these parameters. Results indicate that tree taxonomic diversity peaked in long-abandoned plantations, shrub taxonomic diversity remained constant along the gradient, and shrub functional diversity decreased in long-abandoned plantations. Soil microbial activity was suppressed in managed plantations, and soil microbial diversity was highest in long-abandoned stands. A negative correlation between shrub and microbial functional diversity was observed, which was mitigated when the relative abundance of ectomycorrhizal host shrubs was high. Our findings suggest that in this nutrient-limited and highly disturbed Mediterranean context, the coexistence of high shrub and soil microbial functional diversity is constrained by resource competition, unless nitrogen dynamics are mediated by ectomycorrhizal fungi. Furthermore, the results indicate that shrub community assembly shows high variability in the traits driving microbial functional diversity, and consequently microbial functional recovery is not guaranteed through passive restoration alone. Therefore, restoration actions should focus on steering shrub communities towards compositions that support high microbial functional diversity, specifically targeting ectomycorrhizal hosts and nitrogen fixers, to re-establish top-down and bottom-up plant-soil feedbacks.