Abstract: Aims Plant functional traits reflect the trade-off mechanism for resource acquisition, and plants can be classified as resource-acquiring and resource-conserving based on the combination of traits. Nitrogen (N) is essential for plant growth, and increased N deposition will affect plant traits and resource acquisition strategies by altering the ecosystem nitrogen cycle. However, most studies about the effects of N deposition on plant traits and strategies have focused on the tree layer, and there is a lack of research on understory plant communities, especially in boreal forests. Methods This study relied on thirteen years of N addition experiments in Xing'an larch forests to explore the effects of N addition on understory plant traits at the species and community levels, and to analyse changes in plant resource use strategies. Important findings The results showed that N addition promoted plant height, specific leaf area and relative cover of acquisition plants (e.g. Betula fruticosa and Deyeuxia angustifolia), while suppressed plant height, relative cover and photosynthetic rate of conservative plants (e.g. Vaccinium vitis-idaea), suggesting that N addition favoured the growth of plants with acquisition strategies. At the community level, N addition significantly promoted leaf N content, specific leaf area, and plant height; and significantly reduced leaf phosphorus content of community-weighted mean traits, change in which were mainly caused by intraspecific variation. In contrast, N addition did not significantly change the functional dispersion of most traits, and the changes were mainly related to interspecific variation. More importantly, N addition induced a shift in the resource use strategy of boreal forest understory plant communities from conservative to acquisitive, thereby contributing to community net primary productivity. In summary, this study reveals the changes in resource use strategies of boreal forest understory plant communities and their relationship with plant community composition and growth response in the context of long-term N addition.

Key words: boreal forest, nitrogen deposition, plant traits, resource use strategy, intraspecific variation, interspecific variation