A conceptual model to characterize disturbance severity in forest harvests

I present a three-dimensional disturbance severity model and illustrate the potential usefulness of the model for characterizing disturbance conditions caused by forest harvesting. Disturbance is most often described in terms of disturbance type, size and frequency, with less emphasis on severity. In addition, studies of disturbance impacts have emphasized the dominant tree and woody understory layers, overlooking the species-rich herbaceous layer. For a wide range of harvesting treatments (clearcutting to selection systems with and without site preparation), I characterized disturbance severity based on hypothesized patterns of canopy removal, understory vegetation removal and forest floor and soil modification. To illustrate the application of the model, I plotted the data from three studies in North American coniferous and mixed coniferous-deciduous forests in the three-dimensional diagram of disturbance severity, using actual measures of the degree of disturbance on each axis or estimates inferred from other related variables. The plotted points showed general agreement with the hypothesized ranges of disturbance severity for the treatments represented in these studies. The three axes of disturbance severity can be affected differently depending on disturbance type, emphasizing the importance of characterizing the degree of disturbance on each of these three axes. Short-term responses of the herbaceous layer are strongly influenced by disturbance severity on each of these three axes. Disturbance conditions must be quantified before vegetation indicators of these conditions can be identified. The conceptual model provides a framework for comparing forest harvesting treatments to natural disturbance events. These comparisons are critical to designing ecologically sustainable forest management strategies. (c) 2007 Elsevier B.V. All rights reserved.