| Summary: | 1. Ongoing Arctic greening can increase productivity and reindeer pasture quality in the tundra. However, greening may also entail proliferation of unpalatable species, with distinct consequences for pastoral socialo-ecological systems (SES). 2. We show extensive greening across 20 reindeer districts in northern Norway between 2003 and 2020. The allelopathic, evergreen dwarf-shrub crowberry biomass increased by 60%, contrasted by smaller increases of deciduous dwarf-shrubs and stagnating forb and graminoid biomass. 3. We found evidence, although uncertain, of a negative relationship between biomass and reindeer densities, but only among forbs, the least abundant plant group. 4. Our results challenge the management decision-making, which aims at sustainable pasture management, but which assumes stationary density-dependent relationships. Adaptive management criteria sensitive to changes in focal resource-consumer relationships could help to avoid mismanaging a SES in transition. 5. Large-scale shift towards increased allelopathy may undermine the resource base of a key Arctic herbivore and pastoral SES.
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