• Global environmental change is altering the Earth's ecosystems. However, much research has focused on ecosystem‐level responses, and we know substantially less about community‐level responses to global change stressors.
• Here we conducted a 6‐yr field experiment in a high‐altitude (4600 m asl) alpine grassland on the Tibetan Plateau to explore the effects of nitrogen (N) addition and rising atmospheric CO₂ concentration on plant communities.
• Our results showed that N and CO₂ enrichment had synergistic effects on alpine grassland communities. Adding nitrogen or CO₂ alone did not alter total community biomass, species diversity or community composition, whereas adding both resources together increased community biomass, reduced species diversity and altered community composition. The observed decline in species diversity under simultaneous N and CO₂ enrichment was associated with greater community biomass and lower soil water content, and driven by the loss of species characterised simultaneously by tall stature and small specific leaf area.
• Our findings point to the co‐limitation of alpine plant community biomass and structure by nitrogen and CO₂, emphasising the need for future studies to consider multiple aspects of global environmental change together to gain a more complete understanding of their ecological consequences.

中文翻译：

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氮和二氧化碳富集对高寒草地生物量和群落结构的协同作用。

• 全球环境变化正在改变地球的生态系统。但是，许多研究都集中在生态系统层面的应对措施上，而我们对社区层面对全球变化压力源的应对措施知之甚少。
• 在这里，我们在青藏高原的高海拔（4600 m asl）高寒草地上进行了为期6年的田间试验，以探索氮（N）添加和大气CO ₂浓度升高对植物群落的影响。
• 我们的结果表明，N和CO _2的富集对高寒草地群落具有协同作用。单独添加氮或CO ₂不会改变总的群落生物量，物种多样性或群落组成，而将两种资源加在一起会增加群落生物量，减少物种多样性并改变群落组成。在同时富集N和CO _2的情况下，观察到的物种多样性下降与群落生物量增加和土壤含水量降低有关，并受到以高大的身材和较小的比叶面积为特征的物种流失的驱动。
• 我们的发现指出了氮和CO ₂对高山植物群落生物量和结构的共同局限性，强调未来研究需要综合考虑全球环境变化的多个方面，以更全面地了解其生态后果。