Silicon versus carbon mediates defensive strategies of two dominant congeneric trees across subtropical forest elevations

Abstract Intraspecific variation of plant functional traits benefits plants to adapt diverse habitats. How subtropical trees adjust their defensive strategies (leaf Si and C-based defense compounds) across elevations remains poorly understood. In this study, sixteen plant functional traits of two dominant congeneric trees (Castanopsis eyrei and C. lamontii) were detected at 360, 700 and 1045 m above sea level (E360, E700 and E1045) in a subtropical forest. From E1045 to E360, C. eyrei prioritized C-based defense by increasing leaf tannin (+368.3%) at the cost of decreasing leaf non-structural carbohydrates (NSCs; −46.0% starch). From E700 to E360, C. lamontii employed Si accumulation as defense by increasing leaf Si (+113.5%) but with no significant changes in leaf NSCs. Soil Si availability across elevations contributes to this leaf Si accumulation. Our results implied species-specific acclimation mechanisms of forest trees to warmer conditions (from high to low elevations) in subtropical forests, highlighted the significance of Si in mitigating tree’s carbon cost by integrating Si into cost-benefit analysis and trait-based ecology, shaping future shifts in tree’s dominance and productivity in subtropical forests under climate warming.