Reindeer reduce biomass of preferred plant species

Abstract

Abstract. Reduced weights in reindeer that graze in pastures with high reindeer densities have raised the question if coastal summer pastures are modified by grazing. To evaluate this, the impact of reindeer grazing on standing crop was measured by the plant intercept method inside and outside grazing exclosures in the understorey of a coastal mountain birch forest in northern Norway. The understories of coastal birch forests are dominated by vascular plants and are important summer pastures to reindeer. Based on the literature, we made a priori categorization of the vascular plant species into functional groups of preferred forage, less preferred forage and forage of unknown value to reindeer.Intercept frequency was measured within the same plots on three occasions in the summer of 1996. At the end of the grazing season, total standing crop was 33% lower in open plots compared to plots protected by exclosures. However, the reduction varied between the functional groups, with only preferred forage plants being significantly reduced in standing crop (by 49%).Results suggest that reindeer have a strong annual impact on most of the preferred forage species. However, some of the preferred graminoids are tolerant of grazing and dominate the understorey despite decades of high grazing pressure. We suggest that current grazing pressure is favouring the establishment of a few grazing tolerant graminoids, and that this reduces the forage plant variability. The results are discussed in relation to the grazing optimization hypothesis and the potential importance of plant variability for pasture quality.