MONITORING OF WINDTHROW OCCURRENCE AND PROGRESSION IN SPRUCE FORESTS IN BRITAIN

The results of monitoring windthrow occurrence in eight forest areas i n Britain are presented. The extent of windthrow was calculated from a erial photographs, and combined with stand data using a geographic inf ormation system. The amount of windthrow occurring over a 6-year perio d and the dimensions of the windthrown gaps are summarized. The rates of windthrow are contrasted with expectations based on the current win dthrow hazard classification. Windthrow occurred in each of the forest areas during the monitoring period, and the amount varied from year t o year and from site to site. The gap size distribution remained highl y skewed with a predominance of small gaps, despite an increase in num bers 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 e nd of the monitoring periods. Windthrow was more frequent and of a hig her rate than attritional damage recorded in wind-prone forests in oth er countries. However, the rate of windthrow progression was lower tha n previously recorded in similar forests in Britain due to variability in storm magnitude and silvicultural practice. The windthrow incremen t was not adequately represented by the existing hazard classification and work is now underway to replace it with a risk classification.