While the response of understory plant species diversity to environmental factors has been widely studied, the simultaneous reactions of taxonomic, functional, and phylogenetic diversity to overstory and forest characteristics remain poorly understood. This study investigated how forest structure, overstory diversity, and topographical features collectively influence these three facets of understory biodiversity in the Boutaleb forest (Saharan Atlas in North-eastern of Algeria). We sampled 54 plots, quantifying understory taxonomic, functional, and phylogenetic diversity, and used linear models to assess the effects of overstory forest characteristics on these biodiversity facets. Results revealed that the three diversity facets respond differently to the considered factors. Taxonomic diversity was negatively affected by Aleppo pine (Pinus alepensis) prevalence, canopy openness, and altitude, but positively influenced by holm oak (Quesrcus ilex) dominance and south-facing slopes. Functional diversity increased with tree richness and canopy openness but decreased with holm oak dominance and basal area. Phylogenetic diversity decreased with Aleppo pine dominance but increased in dense holm oak stands. We also uncovered contrasting relationships between these biodiversity facets across forest types, reflecting contextdependent biodiversity patterns. Our findings provide comprehensive insights into understory plant community dynamics and offer important guidance for forest managers and conservationists in Mediterranean forest ecosystems.