Since 2002, mixed-severity wildfires have burned more than 65,000 hectares in the Eastern Cascades of Oregon. The objective of this study is to quantify diverse ecosystem responses and carbon transformations across four large fires that burned approximately 35% of the Metolius Watershed (115,000 ha) in 2002 and 2003. We stratified the postfire landscape by three gradients: forest type (ponderosa pine [PP] and mixed-conifer [MC]), burn severity (unburned, low, moderate, and high overstory mortality), and prefire biomass. We surveyed 64 inventory plots during 2007 and 2008 and used analysis of covariance and multiple regression to determine significant differences between response variables.
Results/Conclusions
The fires generated a complex mosaic of burn severity and ecosystem responses, including carbon transfers to the atmosphere and from live to dead pools. Tree basal area (BA) mortality ranged from 14% in low-severity PP stands to 100% in high-severity PP stands, with parallel patterns in MC stands. Additionally, fire-sensitive Abies grandis accounted for the majority of mortality in low and moderate-severity MC stands (74% and 54% of BA mortality, respectively), whereas fire-adapted Pinus ponderosa and Pseudotsuga menziesii tended to survive, representing a potential shift toward historic conditions. Postfire conifer seedling density was negatively correlated with burn severity (median range: 10,223 seedlings ha-1 in low-severity MC to zero seedlings ha-1 in high-severity PP), whereas shrub cover and biomass showed the opposite trend. Despite widespread tree mortality and associated declines in net primary productivity (NPP), non-tree NPP more than doubled in moderate- and high-severity stands, resulting in a substantial offset of decreased carbon uptake. This study highlights the heterogeneity of fire effects and diverse responses across landscape gradients (pyrodiversity) and underscores the importance of quantifying both immediate impacts and long-term recovery trajectories in ecosystems defined by complex pre- and postfire interactions (pyrocomplexity).
