Monitoring of windthrow occurrence and progression in spruce forests in Britain

Abstract

The results of monitoring windthrow occurrence in eight forest areas in Britain are presented. The extent of windthrow was calculated from aerial photographs, and combined with stand data using a geographic information system. The amount of windthrow occurring over a 6-year period and the dimensions of the windthrown gaps are summarized. The rates of windthrow are contrasted with expectations based on the current windthrow hazard classification. Windthrow occurred in each of the forest areas during the monitoring period, and the amount varied from year to year and from site to site. The gap size distribution remained highly skewed with a predominance of small gaps, despite an increase in numbers across all gap size classes and an increase in maximum gap size. Calculation of a summary shape index (the fractal dimension) indicated that there was no overall change in gap shape between the start and end of the monitoring periods. Windthrow was more frequent and of a higher rate than attritional damage recorded in windprone forests in other countries. However, the rate of windthrow progression was lower than previously recorded in similar forests in Britain due to variability in storm magnitude and silvicultural practice. The windthrow increment was not adequately represented by the existing hazard classification and work is now underway to replace it with a risk classification.