Drivers of Phytochemical Diversity Along Elevation Gradients

Mountains compress steep environmental gradients into short distances, making elevation an ideal natural laboratory for investigating the ecological drivers of phytochemical diversity. While elevational patterns in species richness have been studied extensively, the factors governing phytochemical diversity along mountain slopes remains poorly understood. Through a systematic literature search, we synthesise evidence to evaluate the patterns, drivers and hypotheses of phytochemical diversity across elevation gradients. Reported patterns are mixed, nonlinear, and strongly context-dependent, varying among plant lineages, compound classes, and analytical approaches. Biotic-interaction hypotheses predict declining defensive chemical diversity with elevation, driven by reduced herbivore specialisation and pollinator specificity, while abiotic-stress hypotheses predict increases in stress-related chemicals toward the summit. Community-level patterns further reflect plant species richness, phylogenetic turnover, and scale-dependent sampling effects. We conclude that no universal relationship between phytochemical diversity and elevation can yet be established. Future progress such as community-level studies, manipulative experiments, and causal approaches will advance our understanding on this increasingly urgent dimension of biodiversity under global change.