Sally Donovan

Sally Donovan

2020 Urban Fellow

Research Topic: Land Use Planning and Management

Faculty Advisor: Mark Bradford

Historical agriculture increases nuisance species abundance in urban forests via carbon controls on nitrogen cycling

Land-use and plant-soil feedbacks impact soil nitrogen cycling on multiple spatial and temporal scales, from individual plants to forest macrosystems. These interactions create soil legacies that alter landscape-scale variation in carbon and nitrogen cycling—key processes for forest management, such as nuisance species removal. Soil-based management strategies aim to decrease nitrogen-rich soils that offer a competitive advantage for invasive ‘nuisance’ plants that rely on rapid growth rates. However, details of how changes in plant-soil feedbacks impact nitrogen cycling in response to historical land use are not well-understood. We use observational data to test the effects of historical agriculture on nuisance vegetation, soil organic matter, and nitrogen cycling in forested plots that were not cleared for agriculture (long-established forests) and forested plots that developed 85-100 years ago on plowed fields (recent forests). We found historical disturbance is a primary control on nuisance species abundance; the underlying factors driving nuisance abundance are more strongly related to legacy disturbances than variables describing forest structure and current-day disturbances (e.g., trash, footpaths). Carbon controls on nitrogen cycling also varied by land use history. Predicted increases in labile carbon increased rates of nitrogen immobilization in long-established forests; conversely, in recent forests, increasing labile carbon transitioned mean nitrogen cycling rates from net immobilization to net nitrification. Overall, nearly 100 years after abandonment, historical agriculture remains a primary control on nuisance vegetation and nutrient cycling in temperate urban forests.