Consistency and conditionality of Pseudotsuga menziesii biomass change dynamics through succession across ecosystems of the Cascade Mountains, USA

Background/Question/Methods

Whether through growth, mortality, or recruitment, forests are constantly changing, defining one of the key challenges in forest ecosystem ecology: understanding forest dynamics through succession. Models of forest succession provide an appealing conceptual framework for understanding forest dynamics, especially in the context of biomass change. Uncertainty in the degree to which such patterns are regionally consistent might limit the application of successional theory in forest management. In this study, we examined the responses of proportional Pseudotsuga menziesii biomass change to successional status, relative abundance, resource availability, and canopy cover change across an elevational and longitudinal gradient in the Cascade Mountains of Oregon and Washington, USA. Our objective was to assess the consistency (i.e., equivalence between climax vegetation types) and conditionality (i.e., interaction effects) of proportional biomass change responses in the dominant species, P. menziesii, across the region using repeated measurements of 9700 Current Vegetation Survey (CVS) National Forest inventory plots.

Results/Conclusions

Our results indicated that proportional biomass change for P. menzeisii responses to successional status (i.e., stand age, mean tree biomass, and canopy cover), canopy cover change, and abiotic environmental conditions varied regionally. For example, proportional biomass change decreased with mean tree biomass in all regions, with the highest rates of biomass change being observed in wetter ecosystems of the western Cascades Mountains compared to the drier ecosystems of the eastern Cascade Mountains. Biomass losses associated with reductions in canopy cover were mostly observed in drier regions. These results imply that individual mortality may be a particularly important driver of biomass loss in dry ecosystems while P. menziesii in wetter ecosystems may be more capable of taking advantage a competitor’s death, offsetting ecosystem level biomass losses. Early successional stands were most sensitive to canopy cover change. Our analysis of proportional biomass change in a regionally dominant conifer tree species (P. menziesii) emphasizes the importance of forest successional status and small-scale changes in forest structure on ecosystem productivity.