Understanding how the relative importance of local adaptation versus phenotypic plasticity varies across spatiotemporal scales is critical for explaining and predicting plant population dynamics. We conducted a three-year reciprocal transplant experiment with the dominant grass Stipa krylovii between contrasting sites (wetter Xilinhot and drier Beligutai) in the Inner Mongolia Steppe to determine the dominant adaptation mechanism driving variation in five phenotypic traits. Trait variation explained by site increased over time, indicating the growing importance of phenotypic plasticity. The Beligutai population demonstrated significant local advantage (∆LA) in vegetative and reproductive shoot production, whereas the Xilinhot population exhibited greater adaptive plasticity. These findings reveal temporally evolving phenotypic plasticity in S. krylovii, leading to divergent adaptive strategies between populations. This study provides empirical support for predicting species distribution under climate change and advances theoretical understanding of adaptation mechanisms in perennial clonal herbs across semi-arid regions.