Mechanical thinning and prescribed fire are widely used to restore western forests after a century of fire suppression, yet we know little about how these treatments affect understory communities where plant diversity is highest. We followed understory plants and environmental factors in old-growth, Sierran mixed conifer for two pre-treatment and three post-treatment years using a full-factorial combination of burning and thinning treatments. Treatments significantly changed species composition through a highly localized combination of disturbance intensity interacting with pre-treatment vegetation patches. Pre-treatment richness was most significantly associated with soil moisture; after treatments additional variables became associated with richness and cover. Neither burning nor thinning alone significantly increased richness or cover. Species that increased significantly in cover were associated with conditions of burn/thin combinations: increased light and soil moisture caused by thinning, and burning's reduction of litter, slash, and shrub cover. Our study suggests that the means by which forests are restored affects understory diversity and cover. Prescribed fire was most effective for increasing understory diversity and reducing shrub cover, but when applied off-season, additional fuels provided by mechanical thinning increased burn area and intensity, reducing litter and slash and increasing herb richness and abundance. © 2006 Elsevier B.V. All rights reserved.