In a 31-year-old ponderosa pine plantation in northern California, researchers studied how different shrub densities affected plant growth. They found that greenleaf manzanita thrived, snowbrush initially did well but then declined, Sierra plum survived in the understory, and needlegrass grew rapidly but disappeared by the end of the study. Ponderosa pine grew best in areas with no shrubs, followed by light shrub density, and poorly in medium- and heavy-shrub areas. Shrub foliar cover and crown volume per acre had a bigger impact on pine growth than shrub height or density.