The understory layer encompasses the majority of plant species diversity in forested ecosystems and may be sensitive to timber harvest disturbance. We hypothesize that (i) uneven-aged, low-intensity silvicultural systems can maintain understory plant diversity and support late-successional species following harvest disturbance; (ii) retaining and enhancing stand structural complexity can increase understory plant diversity in northern hardwood–conifer forests; and (iii) plant responses are influenced by interactions among canopy structure, soils, and climate processes. Experimental treatments include single-tree selection and group selection, both modified to increase structural retention, and a third technique designed to promote late-successional forest structure and function, structural complexity enhancement. Four replications of each treatment were applied to 2 ha units in Vermont and New York, USA. Understory vegetation was monitored 2 years pre- and 4 years post-treatment. Results show that over time, understory responses were strongly affected by overstory treatment and less influenced by soils and drought. All treatments succeeded at maintaining overall composition and diversity. However, late-successional diversity increased significantly in structural complexity enhancement units compared with group selection units. These results indicate that while conventional uneven-aged systems can maintain understory plant diversity, variations that retain or enhance structural complexity may be more effective at retaining late-successional species.