Plants as our teachers: Long-term Responses of Dwarf Shrub and Bryophyte Communities to Nutrient Addition in a Northern Swedish Island System | Vad växterna kan lära oss: Effekter av långvarig näringstillförsel på växtsamhällen i ett ö-system i norra Sverige.

Nutrient enrichment can significantly impact plant communities and ecosystem dynamics. This study explored the long-term effects of nitrogen (N) and phosphorus (P) additions on dwarf shrub and bryophyte communities within a retrogressive island system in northern Sweden. Using 17 years of data, I assessed standing plant biomass, species richness, and diversity to understand how these plant communities respond to sustained nutrient. Nutrient additions generally reduced biomass, species richness, and Shannon diversity, with variations depending on species and treatment duration. Bilberry (Vaccinium myrtillus) and Lingonberry (Vaccinium vitis-idaea) declined significantly under N and P additions. Black crowberry (Empetrum hermaphroditum), which initially benefited from nutrient addition, showed no long-term biomass increases, suggesting adaptation constraints or shifts in limiting factors over time. Bryophyte species, including Mountain fern moss (Hylocomium splendens) and Red-stemmed feathermoss (Pleurozium schreberi), showed marked declines, highlighting their sensitivity to nutrient-driven stress. Contrary to my hypotheses, island size, representing varying degrees of retrogression, had limited influence on most variables, suggesting retrogression plays a minor role in modulating long-term nutrient enrichment effects. Additionally, Non-Metric Dimensional Scaling (NMDS) analysis showed a gradual shift in species composition across island sizes over time, but no significant increase in resource-acquisitive plants. These results indicate that nutrient addition alters community composition and reduces ecosystem diversity, likely due to nutrient imbalance or physiological stress rather than competitive shifts. Overall, these findings highlight the long- term effects of nutrient enrichment, emphasizing the vulnerability of species adapted to nutrient-poor conditions and the complexity of plant community responses in retrogressive ecosystems.