POLLEN AND CHARCOAL IN LAKE-SEDIMENTS COMPARED WITH HISTORICALLY DOCUMENTED FOREST-FIRES IN SOUTHERN SWITZERLAND SINCE AD 1920

Charcoal in unlaminated sediments dated by Pb-210 was analysed by the pollen-slide and thin-section methods. The results were compared with the number and area of forest fires on different spatial scales in the area around Lago di Origlio as listed in the wildfire database of sou thern Switzerland since AD 1920. The influx of the number of charcoal particles > 75 mu m(2) in pollen slides correlates well with the numbe r of annual forest fires recorded within a distance of 20-50 km from t he coring site. Hence a size-class distinction or an area measurement by image analysis may not be absolutely necessary for the reconstructi on of regional fire history. A regression equation was computed and te sted against an independent data set. Its use makes it possible to est imate the charcoal area influx (or concentration) from the particle nu mber influx (or concentration). Local fires within a radius of 2 km ar ound the coring site correlate well with the area influx of charcoal p articles estimated by the thin-section method measuring the area of ch arcoal particles larger than 20 000 mu m(2) or longer than 50 mu m. Po llen percentages and influx values suggest that intensive agriculture and Castanea sativa cultivation were reduced 30-40 years ago, followed by an increase of forest area and a development to more natural woodl ands. The traditional Castanea sativa cultivation was characterized by a complete use of the biomass produced, so abandonment of chestnut le d to an increasing accumulation of dead biomass, thereby raising the f ire risk. On the other hand, the pollen record of the regional vegetat ion does not show any clear response to the increase of fire frequency during the last three decades in this area.