Understorey vegetation plays a significant role in the structure and function of forest ecosystems. Controlling understorey vegetation has proven to be an effective tool in increasing tree growth and overstorey development. However, a long-term consequence of the practice on plant diversity is not fully understood. Here, we analyzed early development of overstorey and understorey vegetation of four ponderosa pine plantations established decades ago in northern California, USA. These experiments were intended to examine the effect of understorey control on tree growth, including additional effects of tree density or fertilization. Over the years, trees and understorey vegetation were measured repeatedly for height and crown width. We found that responses of understorey vegetation to overstorey varied at different developmental stages. First, understorey cover increased with overstorey cover until it peaked at a certain age or overstorey height and cover, which varied with site quality. Second, herbaceous species reached their peak earlier than shrubs; shrubs subsequently suppressed herbaceous species. Fertilizer effects on plant species diversity were site specific and density effects were non-significant except at the highest density. These results demonstrate that both fertilization and density manipulation influenced understorey cover and diversity. Such differences are relevant in vegetation management planning efforts.