Windthrow Disturbance and Mosaic

Many studies have emphasized the role of abiotic factors in forest dynamics. Fires, catastrophic windthrow, drought etc. are all important elements in steady-state and gap-phase-dominated models. Little information is available on the consequences of these stressors in terms of rate, scale, and severity across the landscape.

Among the disturbances that modify old growth forests of the Pacific region from Canada to Alaska, the role of the gap-phase model has been overestimated. Recently Kramer et al. (2001) collected evidence about the role of windthrow in shaping temperate rain forest covers in Southeast Alaska. This disturbance is particularly evident on windthrow-prone slopes in which large-scale perturbation is dominant. In storm-protected areas, gap-phase processes are dominant, and it is in these areas that most of the timber harvest is concentrated. Slope, elevation, soil stability, and exposure to prevailing storm winds seem to determine the patch dynamics of these forests. The windthrow return interval has been estimated at 300 years, and late seral stages are maintained especially in wind-protected areas in which the gapphase dynamic is dominant. Knowledge of the dynamics of these forests is of great importance for long-term management. The removal of standing biomass in wind-protected areas modifies the scale of disturbance from the micro to large scale with unpredictable effects.

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