<p>Stable carbon isotopes (δ¹³C) are extensively utilized to study intrinsic water use efficiency (iWUE) at the leaf–scale in terrestrial ecosystems, serving as a crucial metric for assessing plant adaptation to climate change. However, there is currently a lack of consensus regarding the leaf-scale iWUE variation characteristics among different functional types. In this study, we measured the δ¹³C_leaf and iWUE values of different functional plants (i.e., life forms, leaf types, and mycorrhizal types) from 120 species across distinct habitat types (i.e., hillside, near-peak, and peak) in a subtropical forest on the western slope of Wuyi Mountains, southern China. The results showed that the δ¹³C_leaf values of plants on the western slope of Wuyi Mountains ranged from −34.63‰) to −30.04‰, and iWUE ranged from 5.93 µmol mol⁻¹ to 57.34 µmol mol⁻¹. The δ¹³C_leaf and iWUE values differed significantly among plant life forms, following the order of herbs &gt; vine plants &gt; shrubs &gt; trees. The δ¹³C_leaf and iWUE values of ectomycorrhizal (ECM) species were greater than those of arbuscular mycorrhizal (AM) species despite there being no significant difference between plants with different leaf types (Simple leaves (SL) <i>vs.</i> Compound leaves (CL)). From the hillside to the peak, both at the community level and at the species level, the δ¹³C values of leaves and iWUE values of plants exhibited an upward trend. The regression analysis revealed that leaf-scale iWUE was significantly negatively correlated with soil water content and significantly positively correlated with leaf phosphorus content. The findings indicated that leaf carbon isotope fractionation and corresponding iWUE can be influenced by life form, mycorrhizal type, and soil water availability. These insights provide a deeper understanding of the coupling mechanisms of carbon, water, and nutrients among different functional plant types in subtropical forests, and offer insights into predicting plant adaptability under climate change.</p>