Forest fire regimes are particularly sensitive to variations in the climate and to human influences. In the Alps both the manner in which the land is used and climatic changes, in particular rises in temperature and the frequency of drought periods, are probably going to bring about considerable modifications in fire regimes. The history of these fires in Valais in the 20^th century is however still little known, as is the influence of the different determining factors.
From a study of documentary archives we have therefore reconstituted the history of forest fires in Valais from 1904 to 2008. We then tried to establish whether or not the fire regime had evolved during this time by comparing descriptive statistics from the first and the second halves of the period under study. By means of correlation analyses we could then find what factors had a significant influence on the occurrence of fires.
What emerges is that forest fire activity moved towards the plain in the course of the 20^th century, probably on account of the increase in population density at lower altitudes. The seasonality of the fires also evolved: there was an outbreak of fires in the spring during the second half of the period under study, whereas in the first half fires mostly occurred in summer. On the other hand the frequency of the fires and the surface area burned annually did not differ significantly in the periods before and after 1955.
As for the balance between factors determining the frequency of fires and the surface burned annually, there has been a modification in the period under study. Although drought was a decisive factor in the first decades of the 20^th century, afterwards it seems to have declined in importance, being supplanted by other factors, notably the availability of combustible material.
The fact that at present the forest fire regime is apparently regulated by factors other than the climate means it is possible to envisage concrete measures in order to limit fire risks.