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 the late successional conifer tree species, 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.
Results indicate some legacy of soil impact to plots most impacted by past intensive logging. The urban mature plot had higher mean soil pH (5.87, se: ±0.06) compared to a rural mature, and old growth reference plots. Mean coarse woody debris (CWD) was lower at the urban mature plot (12.9 Mg/ha, se: ±1.64) compared to the rural and old growth sites. Although O layer depth was thinner at the urban mature site, soil organic matter was not significantly different across sites. Plot-level productivity and canopy structure did not show a strong relationship to urban proximity. Mortality was highest in rural and old growth plots, and was driven by density-independent causes such as large windthrow events to canopy trees.
These results suggest that past land-use and urban proximity do not have clear impacts on productivity or canopy structure in Forest Park. The lack of late-successional tree species in urban plots might be most related to reduced old-growth legacy features (remnant seed trees, coarse woody debris) following a history of intensive logging. Potential differences in nutrient cycling along an urban gradient, and the impact of English ivy (Hedera spp.) invasion on conifer regeneration will require further investigation. These results help inform park managers that regeneration of late successional tree species in some sections of the park may require active planting of a late successional species understory.