Foliar stoichiometry of Bornean forest seedlings across a logging–restoration gradient: species turnover and a robust calcium-balance shift

Abstract Selective logging is the most widespread disturbance in tropical forests, yet how it alters the nutrient balance of regenerating seedlings is poorly resolved, and most stoichiometric studies cannot separate community species turnover from within-species nutrient change. We asked whether multi-element foliar stoichiometry differs among unlogged primary, naturally regenerating, and actively restored forests in Malaysian Borneo, and whether any difference reflects species turnover or within-species adjustment. Six molar ratios (N:P, N:K, N:Ca, N:Mg, Ca:Mg, K:P) were computed for 15 species; because foliar nutrients were measured on samples bulked per species–plot, analyses used 128 deduplicated species–plot profiles. Multivariate stoichiometric composition differed among management types ( $$p = 0.009$$ ), but this was largely a by-product of which species were present: species identity explained most ratio variance, and the difference vanished when only species shared between unlogged and logged forests were compared. The wider stoichiometric “niche volume” (the spread of nutrient profiles) of logged forests was likewise turnover-driven. Calcium balance was the exception: Ca:Mg remained management-dependent after controlling for species identity (partial $$\eta ^{2} = 0.105$$ ), foliar calcium was consistently lower in logged forests, and within shared species N:Ca rose and Ca:Mg fell after restoration; these calcium patterns were robust to excluding the lianas and the nitrogen-fixing legumes. We conclude that among-management differences in seedling foliar stoichiometry mainly reflect post-logging species turnover, with a composition-robust calcium-balance shift the principal candidate for a genuine biogeochemical change, an important caveat for using foliar stoichiometry in restoration assessment.