Reindeer grazing has contrasting effect on species traits in Vaccinium vitis-idaea L. and Bistorta vivipara (L.) Gray

That reindeer grazing can have large effects on plant communities is well known, but how reindeer grazing affects plant traits and plant carbon (C) and nitrogen (N) allocation has not been studied to the same extent. This study was conducted in a sub-arctic dry heath in northern Sweden. 17-year-old reindeer exclosures were used to test whether reindeer grazing affects the C:N ratio (a plant quality index), and the δ13C and δ15N (indicators of changes in C and N dynamics) as well as the C and N content of above- and below ground parts of the evergreen dwarf shrub Vaccinium vitis-idaea L. and the perennial forb Bistorta vivipara (L.) Gray. A lower C:N ratio was found in B. vivipara compared to V. vitis-idaea suggesting a higher grazing pressure on that species. We found that grazing reduced the total C content, by 26%, and increased the δ15N, by 1‰, in the leaves of B. vivipara, while no changes were observed in V. vitis-idaea. Fine roots of B. vivipara had higher δ13C (1‰) and δ15N (2.5‰) than the leaves, while such differences were not found in V. vitis-idaea. The results also highlight the importance of analysing both above- and belowground plant parts when interpreting natural variations in δ13C and δ15N.