Understory functional trait adaptation to climate change is primarily mediated by soil conditions under long-term afforestation on the Loess Plateau

Soil and climate conditions drive plant trait variation in natural ecosystems. However, afforestation induces rapid and significant shifts in these environmental drivers. Yet the extent to which such abrupt shifts alter functional traits remains poorly understood. Here, we conducted a paired sub-watershed study in China's Loess Plateau, where over 70 years of afforestation and natural recovery have occurred. Functional traits of 59 herbaceous species were assessed. Combined with high-frequency microclimate monitoring and grid-based soil sampling, we used random forest and partial least squares path modeling (PLS-PM) to evaluate how afforestation-driven soil and microclimate influence trait variation. Afforestation significantly altered community-level functional traits of herbaceous plants, including morphological traits (plant height, specific leaf area (SLA)), chemical traits (leaf carbon to nitrogen ratio (C: N)), and tissue density traits (leaf and root dry matter content (LDMC and RDMC)). In contrast, individual-level responses were generally weak, with only the C: N ratio showing significant change. Compared with microclimate alterations, integrated changes in soil conditions, particularly soil water content (PLS-PM loading = 0.828), emerged as the dominant drivers of community-level trait variation. In addition, microclimate effects were minor and often opposed soil-mediated responses. Overall, afforestation gradually reshapes the functional composition of herbaceous communities through species turnover and subtle intraspecific shifts, driven by persistent changes in habitat structure. Under ongoing climate change, the influence of afforestation on understory plant traits is predominantly mediated by soil processes. These findings suggest that climate-induced impacts on herbaceous communities may likewise be buffered through soil-driven pathways.