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ABSTRACT Questions Suburban forests help maintain biodiversity in an urbanizing world. Prescribed burns in these forests aim to reduce invasive species and promote native species but may also change the plant traits that are most common and affect beta diversity. Here, we evaluated the frequency of traits related to species' ability to persist through fire or colonize and mature between fires. We asked whether burns have changed the composition of species or traits in the understory of suburban forests over 30 years and whether these changes have led to biotic homogenization. We tested three hypotheses: (1) species composition changed over time in burned sites, but not unburned sites; (2) plant traits changed over time and with prescribed burns; and (3) changes in species and traits led to biotic homogenization of burned but not unburned sites. Location Public forests in DuPage County, Illinois, USA. Methods We used a 30‐year vegetation dataset and assembled trait data to examine changes in species composition and seven plant traits. We used PERMANOVA to determine whether species and traits changed over time or with fire and PERMDISP to measure beta diversity. We used mixed‐effects models to examine changes in the relative frequency of individual plant traits. Results Both species and traits changed over time. Prescribed burns appeared to act as a filter on species traits, but not species composition. The understories of burned plots became increasingly dominated by herbaceous species, whereas the understories of unburned plots were increasingly dominated by woody species. Across all plots, traits associated with long‐distance dispersal increased, whereas those associated with short‐distance dispersal decreased. Conclusions Burns changed the understory of suburban forests, leading to trait but not species homogenization. Natural resource managers should be cognizant of these changes, as they may have unintended consequences on ecosystem functions and services.