Disturbance severity and threshold responses in the boreal forest

This essay discusses three potential models relating disturbance severity to post-disturbance stand composition in the boreal forest: (1) continuous, where changes in disturbance severity cause a proportional and continuous change in stand composition; (2) discontinuous, where a threshold disturbance severity exists beyond which major changes in composition occur; and (3) the cusp, where thresholds exist and coexistence of two alternative compositional states is possible at the same disturbance severity. Any of the three models may be appropriate in different stands or different parts of the boreal forest. If the actual model does not correspond to that assumed by forest managers, then forest harvesting practices may cause unexpected sudden changes in forest composition. Disturbance severity under the natural disturbance regime changes so dramatically, from one disturbance to the next, that oscillations in composition over time are likely to be individualistic and irregular, rather than stable. Harvesting operations are mainly done during winter, resulting in a very low disturbance severity in comparison to the natural regime, which included crown fires. Major changes in species compositon over large parts of the boreal forest are likely to result.