Are Large-Scale Differences in Temperature and Reindeer Management Regime Affecting the Quality of Reindeer's Summer Forage?

The chemical balance between essential nutrients and defense compounds in plants determines the quality of the forage available to herbivores and can be modified by both environmental conditions and herbivores themselves. We investigated whether climate and herbivory affect nutrient and defense compound concentrations across plant functional groups. Concentrations of nutrients-nitrogen (N) and phosphorus (P)-and defense compounds-silicon (Si) and phenolics (Ph)-were measured in plant samples from the locally most abundant species, collected in northern Norway across a gradient in summer temperature and different reindeer grazing regimes. Nutrient and defense compound concentrations varied substantially across species and plant functional groups. In addition, nutrients (N, P) declined over the summer, while defense compounds (Si, Ph) accumulated. Sites with a warmer climate had a stronger decrease in nutrient concentrations over the season. We found no evidence that long-term contrasts in reindeer herbivory intensity affected the average nutritional quality within plant species. Overall, our results suggest that spatial variation in the nutritional quality in Arctic tundra vegetation is mainly determined by the species composition and by consistent changes over the summer season. In comparison, reindeer herbivory and local climate seem to have relatively little impact on the average nutrient and defense compound concentrations of the plant species, suggesting they mainly alter forage quality through effects on the functional and taxonomic composition of the vegetation.