>> Background information

Development

Presbribed burning was a relatively widely used treatment method of regeneration areas in the 1950s and 60s. At its peak, altogether 30 000 hectares of forest floors were burned annually. Since 1990 the annual area of presbribed burning has been small and shown a decreasing trend. During the past five years a mere 600 hectares have been burned every year.

Natural and accidental forest fires have also been rare in recent years. Prior to 1970s there were still occassionally large fires, but during the past decades airplane surveillance, dense forest road network and modern firefighting equipment, among other things, have ensured rapid extinction of accidental fires.

As unwanted events, forest firest are naturally a different issue from prescibed burning yet their effect on forest ecosystems is mostly the same. Only the volume of trees burned or killed by fire is normally much higher in unintentional fires.

The prescribed burning of regeneration is has been listed as one of the recommonded practices in the PEFC certification standard, which is widely applied in Finland (PEFC - Programme for the Endorsement of Forest Certification schemes). The present criteria states that during the present certification period which started in 2003 the annual area of prescribed burning should be larger than during the years 1998–2002. This goal has not been met.

Impact on biodiversity

Prior to modern fire prevention measures forest fires were a central distubance factor in boreal forests with a strong impact on forest structure and species assemblages. There has been great variablity in the frequency of fires among different forest types. The occurence of fires has also been affected by the size of the continuous heath forest patch along with several stochastic factors. Dry heath forests may have burned in intervals measured in tens of years while spruce dominated areas have burned only once in hundreds of years. Deciduous herb-rich forest have burned least often.

Fire affects the structure and species assemblage of a forest in many ways. Forest fires boost the creation of deciduous stands and increase the amount of dead wood sometimes in large volumes. Although the amount of dead wood created by prescribed burnings in commercial forests is quite low, many other impacts to microclimate and soil acidity, for example, are alike. Temperature variation in a burned ground layer is wider and the soil is more alkaline than in a non-burned site.

The strongest link between forest fires and species exist for those species for which fire is either a partly or entirely necessary part of their life history. Such a species is, for example, the cranesbill species Geranium bohemicum which seeds require a sudden rise of temperature to above 45 C to germinate. Besides vascular plants, fire dependent species can found among sac and imperfect fungi, possibly even tens of species.

Some 30 insect species may also be found living solely in burned forests. The majority of these are beetles. Many of the insect species feed on the sac and imperfect fungi species.

True fire specialist disappear from a burned site normally within a few years after the fire. Species feeding on dead wood may sustain themselves for longer. The open phase with high volumes of dead wood may last for 20 to 30 years during which saproxylic species are abundant.

Due to scarcity of forest fires and prescribed burnings many fire associated species have become rare. There were altogether 34 extinct and endangered and 11 vulnerable fire species in the last red-list assesment completed in year 2000.