Stratigraphy of stable isotope ratios and leaf structure within an African rainforest canopy with implications for primate isotope ecology

The canopy effect describes vertical variation in the isotope ratios of carbon (δ(13)C), oxygen (δ(18)O) and partially nitrogen (δ(15)N) within plants throughout a closed canopy forest, and may facilitate the study of canopy feeding niches in arboreal primates. However, the nuanced relationship between leaf height, sunlight exposure and the resulting variation in isotope ratios and leaf mass per area (LMA) has not been documented for an African rainforest. Here, we present δ(13)C, δ(18)O and δ(15)N values of leaves (n = 321) systematically collected from 58 primate food plants throughout the canopy (0.3 to 42 m) in Côte d’Ivoire, West Africa. Besides leaf sample height and light availability, we measured leaf nitrogen and carbon content (%N, %C), as well as LMA (n = 214) to address the plants’ vertical resource allocations. We found significant variation in δ(13)C, δ(18)O and δ(15)N, as well as LMA in response to height in combination with light availability and tree species, with low canopy leaves depleted in (13)C, (18)O and (15)N and slightly higher in %N compared to higher canopy strata. While this vertical isotopic variation was not well reflected in the δ(13)C and δ(15)N of arboreal primates from this forest, it did correspond well to primate δ(18)O values.