Testing theories of disturbance in temperate and boreal forests
Conceptual frameworks of disturbance and ecosystem response will be presented, grounded with empirical field data on disturbance interactions in northern hardwood-hemlock and southern boreal forests of the western Great Lakes region. Questions considered include: Under what conditions do disturbances initiate episodes of succession? How do disturbances cause patterns of successional convergence and divergence? When do forest responses to a gradient in disturbance severity exhibit continuous versus threshold effects? And, how will climate change interact with disturbance severity?
The recent occurrence of large-scale disturbances in boreal and temperate forests has allowed limited testing of disturbance theories. Both threshold and continuous effects on forest composition occur across a gradient of disturbance severity. A large disturbance can create a mosaic of continuous and threshold effects depending on the initial states of stands across the landscape, and combined with spatial heterogeneity in disturbance severity, can lead to a mosaic of successional convergence and divergence. Threshold effects are more likely when disturbance severity and/or disturbance type are out of synch with life history characteristics of the dominant pre-disturbance tree species. The differential effects of disturbance severity and disturbance type are considerably blurred and poorly understood. Unified measures of disturbance severity that allow comparison of severity across disturbance types have been difficult to develop. In a warming climate, threshold disturbance severities that cause a change in forest state will likely become lower, leading to surprises in post-disturbance successional trajectory. Multiple disturbances such as wind throw or insect infestation followed by fire, are likely to become more common and have very high combined disturbance severities. Large high-severity disturbances are likely to be a major contributor to changes in landscape structure in a warming climate.