Historical pre-settlement conditions in ponderosa pine ecosystems ranged from savannas (< 30% canopy cover) with contiguous grasslands and scattered tree groups, to forests with isolated mosaic-meadows surrounded by trees. We use the term mosaic-meadows for non-treed areas that weave around individual trees and tree groups, supporting diverse understory plant communities in ponderosa pine ecosystems. The long-term sustainability of ponderosa pine ecosystems may depend on mosaic-meadows that provide fine fuels and support frequent, lowseverity fires. Increasing tree densities over the past century have eliminated mosaic-meadows and contiguous grasslands from many ponderosa pine landscapes. The link between restored ecosystem structure and function is often assumed but not empirically demonstrated, so we assessed the impact of mosaic-meadows (areas > 6m from overstory trees) on understory vegetation in five recently thinned ponderosa pine stands and one longundisturbed stand in Colorado. We also compared historical spatial patterns in mosaic-meadows to current conditions in ponderosa pine stands and determined whether thinning treatments are recreating historical conditions. Mosaic-meadows in ponderosa pine ecosystems declined substantially from a mean of 55% of stand area in 1860–1875 to 7% in 2010–2013 prior to tree thinning. The loss of mosaic-meadows has ecological consequences for the function of ponderosa pine ecosystems because spatial patterns of mosaic-meadows strongly influenced understory vegetation. At our sites, understory cover increased by 3%/m with distance from overstory trees, and understory richness increased from 6 species/m2 within 1m of overstory trees to 9 species/ m2 at 10m away from overstory trees. Cover and richness of understory vegetation (especially native forbs) responded rapidly to creation of mosaic-meadows within 1 to 4 years after treatment. Thinning treatments at two stands brought the coverage of mosaic-meadows within the historical range of variation, but there was a noticeable lack of mosaic-meadows>12m from overstory trees at all treated stands. Restoration of the fundamental ecological characteristics of ponderosa pine ecosystems should intentionally include variably sized mosaic-meadows.