Altitudinal Gradients of Grassland Carbon and Nitrogen Isotope Composition Are Recorded in the Hair of Grazers

Aim: The hair of grazers provides an isotopic record of environmental and nutritional signals. Here, we assess the effect of altitude on the carbon and nitrogen isotope composition of the hair of ruminant grazers and its relation to grassland vegetation, to evaluate the use of hair isotope data for ecosystem reconstruction, animal nutritional ecology and biogeochemical studies in montane environments. Location: European Alps. Methods: We sampled grassland vegetation (pure C₃) and the hair of ruminants along an altitudinal gradient (400-2500 m), and analysed their isotope composition ( $\delta^{13}C$ and $\delta^{15}N$ ). Results: were compared with published effects of altitude on $^{13}C$ in C₃ plants at the species level and on $^{15}N$ at the community level. The study was complemented with a comparison of diet and hair isotope composition in ruminants held in confinement. Results $\delta^{13}C$ of hair increased (c. 1.1‰ km⁻¹) and $\delta^{15}N$ decreased (c. 1.1‰ km⁻¹) with altitude. The same changes occurred in local grassland vegetation, and in regional to global grassland data sets. Offsets between hair and vegetation $^{13}C$ or $^{15}N$ ('diet-hair shift') were independent of altitude. Sheep (Ovis aries) and cattle (Bos taurus) exhibited a $^{13}C$ shift near +3‰, but that of goats (Capra hircus) was larger (+4.2‰) in alpine environments and in confinement. The diet-hair shift for $^{15}N$ was more variable (+2.1 to +3.6‰). Main conclusions: Grazer hair provides a faithful spatially and temporally integrated record of grassland isotope composition, useful for ecosystem and environment reconstruction. The effect of altitude on hair $^{15}N$ is important for studies of trophic relationships: an altitude shift of 2000 m produced the same effect in hair $^{15}N$ as would a shift from an animal tissue-based to a plant-based diet. The similarity of altitude effects on $\delta^{13}C$ of individual plant species, vegetation and hair indicates that the effect of altitude on species-level 'intrinsic water use efficiency scales up linearly to the community and landscape level.