Environmental Conditions Associated with Natural Shade-Tolerant Conifer Regeneration in Forest Park, Portland, Oregon

Forest Park is a 5,100-acre urban forest located in Portland Oregon. Due to its proximity to urban development the park has been impacted by various anthropogenic stressors including logging, fragmentation, invasive species, air pollution and recreation use. This legacy of land degradation coupled with natural disturbances has resulted in changes to forest structure, composition, and function—threatening the long-term sustainability of the park. Forest Park provides many ecosystem services, and the local community depends on its sustained ecological integrity. Past research in Forest Park had identified a lack of Western hemlock and Western red cedar seedlings and saplings in the most “urbanized” section of the park when compared to a reference old growth section of the park. The successful regeneration of these species is a critical development process that leads towards structurally complex old-growth stand conditions. There is concern from Forest Park stakeholders that past land use history, periodic disturbances, and urbanization have set sections of the park on an altered trajectory towards a deciduous hardwood alternate state. A targeted sampling approach was used to locate juveniles of target shade-tolerant species and to assess the abiotic and biotic factors at those locations. Both species’ presence was negatively associated with increased vegetative cover. Western hemlock juveniles were primarily found established on nurse logs. Decreased canopy cover was also associated with higher tree vigor. Further work will involve identifying significant environmental factors associated with presence and vigor of each species to help guide future management efforts