Invasion by alien plant species represents a challenge to land managers throughout the world as they attempt to restore frequent fire-adapted ecosystems following decades of fire exclusion. In ponderosa pine Pinus ponderosa forests of western North America, the response of alien species to restoration treatments has not been well documented, particularly for alien species capable of altering environmental conditions (transformers). Understanding alien species dynamics is critical for developing treatments that accomplish restoration goals while minimizing alien invasion. We used a replicated, randomized block experiment to compare the effects of untreated control and thin-only, burn-only, and thin-burn treatments on alien and transformer understorey species at multiple spatial scales (1 m2, 100 m2, and 1000 m2). Data were collected pre-treatment and for multiple post-treatment years. We compared the richness and cover of alien species and transformer species among treatments and identified environmental variables correlated with transformer species cover. Indicator species analysis was used to identify transformer species associated with specific treatments. Alien and transformer species richness and cover were significantly higher in the thin-burn than in all other treatments at all spatial scales. Thin-only and burn-only treatments showed greater alien and transformer species responses than the control at the larger 100-m2 and 1000-m2 scales. Increased transformer cover was strongly correlated with increased tree crown scorch height and removal of overstorey trees. The thin-burn treatment had four transformer species as indicators, the thin-only had one, while the burn-only and control had none. Synthesis and applications. The results show that alien species, including transformers, respond to restoration treatments, especially the combined thin-burn treatment. Therefore monitoring for alien species invasion is an essential component of a restoration program. The abundance of transformer species increased with increasing disturbance intensity, suggesting that less intense single-disturbance treatments (burn-only, thin-only) or incremental treatments may be preferred in some applications. Where more intense treatments are required to meet management objectives, specific strategies, such as seeding of native species, limiting grazing before and after treatment, and harvesting over a protective winter snowpack, may be necessary to limit alien invasion. Journal of Applied Ecology (2006) 43, 8872013897doi: 10.1111/j.1365-2664.2006.01206.x