Using Reference Conditions to Identify Thinning Regimes Needed to Accelerate the Development of Late-Successional Characteristics in Young Douglas-Fir Dominated Forests

Quantifying the attributes of undisturbed late-successional forests is a crucial problem in the restoration and management of forest and stream ecosystems, driving both the evaluation of current forest conditions and the setting of stand management targets for specific points in the future. We identified 117 undisturbed, mature (average age  = 113 yr) Douglas-fir dominated riparian and upland stands from existing forest inventories across western Washington and Oregon for the purpose of establishing reference conditions for such stands. We did this to establish quantitative metrics for structure essential to the function of mature, late-successional forests, and to assess whether there were any important quantitative differences between upland and riparian forests. Though mature upland and riparian Douglas-fir dominated forests are structurally similar in many ways, the riparian stands have taller, larger diameter Douglas-fir trees in the overstory, and fewer small diameter Douglas-fir live trees and snags in the understory. This results in riparian forests with higher wood volumes and basal areas, indicating they are growing on more productive sites relative to the upland stands. Both riparian and upland forests have abundant large diameter (> 50 cm) live trees (175 and 70 TPH, respectively) and large diameter snags (29 and 33 TPH, respectively). The overstory is almost entirely Douglas-fir and the understory is dominated by western hemlock and western red cedar. Collectively, our data suggest that mature riparian and upland Douglas-fir dominated forests have well developed late successional structural characteristics in terms of abundant large trees in the overstory, abundant large snags, a well-developed understory of shade-tolerant trees, and canopy gaps. Riparian forests develop late successional forest characteristics more rapidly than upland forests, presumably because they are growing on more productive sites.