For more than a century, forest management in Switzerland has been based on a philosophy attuned to nature. Silviculture which imitates natural cycles and processes is believed to promote the growth of healthy, stable forests. Because of the diversity of species and ages, which are often mixed on a small scale, forest stands are able to resist many extremes of climate and, in the event of damage (mainly due to windthrow and snow breakage), the natural forest ecosystem can recover rapidly from total destruction thanks to abundant natural regeneration. Over the long term, Swiss foresters have become accustomed to harvesting 20% of the annual production as "storm timber". The most serious natural disasters are mainly due to extremes of one single climatic factor; e.g. Hurricane Vivian in 1990, which toppled over almost five million m³ of timber - the equivalent of Switzerland's average yearly timber harvest.
For a forest manager, however, it is interesting to analyse the less spectacular damage which occurs annually in natural forest stands and accounts for between 12 and 20% of the annual harvest. It appears that a combination of two or three climatic factors has a much greater effect on forest stability than an extreme of one single factor. Wind speeds of up to 100 km/hour may often be harmless to forests, whereas lower speeds may uproot mature trees over a wide area if the soil is waterlogged following a long period of rain. This kind of damage can occur throughout the year in coniferous stands, whereas broadleaved species growing in waterlogged soils may resist winds if this climatic combination comes when the vegetation period is over and they have lost their leaves. A number of such patterns of damage to forest stands is presented and their dependence on the occurrence of a combination of specific climatic factors is proposed.