Long-term Ecological Research in Forest Park: Tracking Urban Impacts to Forest Structure and Productivity

Current and historic land use practices as well as exposure to an urban environment can impact forest structure and function. Past and ongoing research in Forest Park, a large urban forest in Portland, Oregon, suggests that mature Douglas-fir (Psuedotsuga meziesii) dominated conifer stands in the more urban end of the park are not developing certain late successional features. Notably, they lack a shade-tolerant conifer understory composed of western hemlock (Tsuga heterophylla) and western red-cedar (Thuja plicata). Here, I present three lines of investigation taken from several 1-hectare permanent long-term ecological research (LTER) plots in Forest Park. First, I investigated whether plot-level differences in the intensity of past-land use, namely clearcut logging and fire, has created legacy effect on soil health, as measured by soil organic matter, soil pH, and depth of organic horizon. Second, I present 5-year growth and mortality data to characterize productivity patterns in these plots. Last, I analyzed light detection and ranging (LiDAR) data covering Forest Park to determine if canopy structural features diverged across urban and rural sections of the park. Overall, the results indicate some legacy of soil impact to urban areas most effected by past intensive logging. However, forest productivity, mortality, and structural patterns were generally consistent with stand age, and did not show strong patterns related to urban proximity. These results indicate that the lack of shade tolerant species might be most limited by ground level impacts. These results help inform park managers that regeneration of late successional tree species may require active restoration practices that focus on establishing more optimal growing sites