Soil amendments alter understory vegetation composition and functional diversity in poorly regenerated logging sites in Quebec, Canada

Soil amendments are increasingly used in boreal forest plantations to enhance seedling growth, but their effects on other compartments such as understory vegetation remain poorly understood. This study evaluates the impact of amendments [biochar (2.6 Mg ha⁻¹), wood ash (7 Mg ha⁻¹), and manure (105 Mg ha⁻¹)], applied alone or in combination, on understory vegetation after two growing seasons. We measured diversity indices and evaluated understory vegetation community composition. The effects of amendments on plant functional traits were assessed at the species level. We also examined functional diversity and calculated the community-weighted mean to assess the impact of amendments on the functional composition of the plant community. Our results highlight that manure significantly increased Shannon index of diversity from 1.87 to 2.13, with more grasses and non-native legumes with an acquisitive strategy and competitive ability. Functional diversity was the highest for manure treatments (=19.90) and the lowest for treatments without manure (=16.80). In contrast, biochar and wood ash did not significantly alter plant diversity. Community composition was similar between the biochar and control treatments, while wood ash amendment, despite overlapping in plant composition with biochar, resulted in additional species. Biochar and wood ash treatments contained more ruderal and forest herbs and woody species typical of forest disturbances, with wood ash significantly increasing leaf nitrogen concentration by 9 % compared to treatments without wood ash. Together, these findings suggest that soil amendments alter diversity of understory vegetation and act as functional filters on plant communities.