Elevational Shifts in Tropical Tree Leaf Traits: Interactions between Soil, Climate, Light, and Phylogeny

Understanding how tropical trees respond to complex environmental gradients is essential for predicting forest resilience under climate change. We examined variation in key leaf traits, including specific leaf area (SLA), foliar nitrogen (N) and phosphorus (P), C:N and N:P ratios, and stable isotope composition (δ13C, δ15N), in 160 tree species along a 3200-m elevational transect on Mount Cameroon. This gradient spans hyper-humid coastal rainforests to arid Afroalpine savannas, capturing sharp transitions in climate, soils, and forest structure. Leaf traits shifted nonlinearly with elevation, from acquisitive strategies at mid-elevations to conservative syndromes in lowlands and highlands. Mid-elevation forests (~1000–1500 m), characterized by moderate climate and canopy disturbance by elephants, supported nutrient-rich, high-SLA foliage. In contrast, high-elevation forests (>2000 m) exhibited low SLA, high C:N, and enriched δ13C, consistent with stress tolerance under cold, dry, and fire-prone conditions. Leaf N:P ratios indicated the strongest potential phosphorus limitation in hyper-humid lowlands, where extreme rainfall (>12,000 mm yr−1) likely promotes nutrient leaching and low soil P availability. Foliar δ15N declined markedly with elevation (from +5‰ to −5‰), consistent with shifts in nitrogen acquisition strategies and ecosystem N cycling, including increased reliance on mycorrhizal-mediated uptake associated with changes in dominant functional groups (e.g., Fabaceae vs. Ericaceae). A bimodal δ15N pattern, with enrichment in both lowland and upper montane forests, likely reflects contrasting nitrogen loss regimes associated with high rainfall and fire disturbance, respectively. Phylogenetically informed analyses revealed that climate, soils, forest structure, and evolutionary history jointly shaped trait–environment relationships, with strong phylogenetic signal in δ13C, C:N, and δ15N. Together, these results highlight the importance of integrating functional traits, isotopes, and phylogeny to predict tropical forest responses to global change.