Leaf elemental traits and stable isotopes across a Caatinga–Atlantic forest transect reveal shifts in water-use and nutrient strategies

Abstract Natural transects in fundamental resources offer rare opportunities to evaluate how environmental gradients shape plant strategies and biogeochemical cycles. We investigated leaf elemental traits (C, N, C:N) and stable isotopes (δ 13 C, δ 15 N) along a sharp precipitation gradient from the semi-arid Caatinga (500–800 mm yr⁻ 1 ) to the humid Atlantic Forest (2000–2500 mm yr⁻ 1 ) in northeastern Brazil. Using a combination of Bayesian generalized additive models and Random Forest analyses, we quantified longitudinal trends and identified the climatic, edaphic, and functional drivers of foliar composition. Foliar δ 13 C and δ 15 N declined systematically from Caatinga to Atlantic Forest, while foliar N concentration decreased and C:N increased eastward, but with much weaker explanatory power. Random Forest models highlighted precipitation, cation exchange capacity, and soil N and C stocks as key predictors of isotopic variation, whereas foliar N was most strongly linked to SLA and soil bulk density. A notable pattern was the apparent divergence between soil and foliar nitrogen: soil N content increased toward the Atlantic Forest, yet foliar N declined, underscoring the limited value of bulk soil N as an indicator of plant-available N and highlighting the role of functional traits and species turnover. We argue that the Caatinga aligns with other tropical dry ecosystems in exhibiting unusually high foliar δ 15 N, likely reflecting soil 15 N enrichment under strong water limitation and episodic pulses of gaseous N losses. These findings show how water limitation, soil processes, and leaf functional traits interact to shape resource-use strategies across one of the steepest tropical dry–wet transitions worldwide, providing new insights into the mechanisms linking the leaf economics spectrum to isotopic biogeochemistry.