The effects of pre-planting treatment strategies on understory vegetation in an urban forest patch

Urban forest patches experience a wide variety of anthropogenic and environmental stressors, including recurrent canopy gap-creating events, herbivory pressure, and invasive species. Forest restoration strategies to overcome these stressors frequently rely on enhancing tree regeneration, including tree-planting. Pre-planting treatment strategies are a crucial preparatory step in establishing appropriate site conditions to ensure successful long-term tree regeneration. However, many pre-planting treatments rely on synthetic herbicide, which can be associated with adverse environmental outcomes. Due to the negative perceptions and effects of synthetic herbicides, which are a common tool in forest restoration, many cities, towns, and parks have begun to ban the use of these chemicals. Our objective was to compare mechanical removal with synthetic and organic herbicide applications to determine which pre-planting strategy most effectively reduces non-native plants and enhances native plant cover. We conducted this study within a 16-hectare Liriodendron-Quercus forest located in Philadelphia, Pennsylvania, USA, over a two-year period. Organic herbicide demonstrated the lowest efficacy at reducing non-native plants (14.2% reduction), followed by mechanical removal (-18.6%). Synthetic herbicide application had the highest efficacy (-46.1%). Although organic herbicide had the lowest efficacy, it did reduce non-native cover. Species-specific responses showed similar trends among pre-planting treatments. Additionally, we found that the timing and frequency of herbicide application may support higher efficacy in organic herbicide use. Our study supports species-specific management focused on above- and below-ground control mechanisms. This study provides best-management practices for pre-planting treatments and highlights the impacts these treatments have on diversity, composition, and restoration success. • Organic herbicide reduced non-native cover, but the effect was modest. • Synthetic herbicide was most effective, followed by mechanical removal. • Species-specific management is a valuable tool in managing non-native species.