Differential Elevational Diversity Between Overstory and Understory in Tropical Forests

ABSTRACT

Understanding how biodiversity patterns vary among forest vertical strata along elevational gradients is crucial for predicting ecosystem sensitivity to global change, yet such comparisons remain scarce in tropical forests. Here, we established 16 sites along a 1500‐m elevational gradient in Yunkai Mountains to compare diversity between tree and understory layers, specifically tracking two nationally protected and rare species ( Alsophila spinulosa and Angiopteris fokiensis ). Tree‐layer α‐diversity exhibited a unimodal relationship with elevation, peaking at mid elevations, whereas understory α‐diversity maintained consistently higher diversity and displayed a bimodal pattern with peaks at 400–600 m and 1000–1200 m. The β‐diversity of both tree and understory layers varied across elevational zones, primarily driven by species turnover of common dominant species rather than within‐zone heterogeneity or rare species. However, the two rare ferns exhibited distinct elevational distributions (unimodal and U‐shaped, respectively) that were decoupled from overall α‐diversity. Our study demonstrates contrasting elevational responses between tree and understory strata. Consequently, effective biodiversity conservation in tropical mountain forests must integrate complete elevational gradients with vertical stratification, explicitly prioritizing the maintenance and long‐term monitoring of understory microhabitats.