Secondary Forests Matter: Botanical Diversity and Structure Supporting Plecturocebus Oenanthe Persistence in Fragmented San Martín Forests, Peru

Plecturocebus oenanthe (Callicebinae) is Critically Endangered and restricted to fragmented premontane forests of San Martín, Peru. Severe habitat loss and altered forest structure threaten population persistence, and baseline ecological knowledge of habitat requirements is lacking. We characterized floristic composition and forest structural attributes across occupied habitats to identify botanical and structural features essential for species persistence. We conducted the first systematic characterization across nine forest plots in five fragments, combining one-hectare inventories in extensive areas with transect-based sampling in reduced patches. We recorded 237 morphospecies (161 identified to species level) across 53 families, dominated by Lauraceae (27 species), Moraceae (24), Leguminosae (21), and Melastomataceae (19). Alpha diversity indices (Shannon-Wiener: 3.89–5.78; Simpson: 0.89–0.98) indicated medium-to-high biodiversity, whereas low intersite floristic similarity (Jaccard: 0.08–0.21) reflected microhabitat heterogeneity. Extensive fragments exhibited inverted-J diameter distributions and taller, continuous canopies (mean height 15.5–17.5 m; crown height 4.9–5.8 m; basal area 16.69–27.28 m²/ha), whereas reduced fragments were dominated by small stems (88.14% in the 2.5–10 cm class), lower stature (mean 6.6–9.4 m), and reduced basal area (9.17–36.48 m²/ha). Pioneer species (Cecropia concolor, Jacaranda macrocarpa, Miconia spp., Pourouma guianensis, Vochysia ferruginea) dominated across fragments. P. oenanthe exploits secondary patches through ecological flexibility; however, persistence depends critically on retained vertical complexity, crown continuity, and botanical diversity, enabling safe arboreal movement and dietary breadth. Effective conservation requires maintaining canopy closure, height uniformity, and structural recovery through protection from extraction and forest restoration in both primary and secondary stands.