BIOMASS AND CHEMICAL-COMPOSITION OF COMMON FOREST PLANTS IN RESPONSE TO FIRE IN WESTERN NORWAY

The biomass and chemical composition in six dominant field and bottom layer species was followed during four years after a fire in a Pinus s ylvestris forest in western Norway. Three fire intensity levels were d istinguished: low, medium and high. The overall biomass in the differe nt species was estimated from the biomass per shoot, the shoot densiti es in pure stands of the investigated species and the percentage cover at medium burned sites. Corresponding measurements were made at contr ol plots outside the burned area. A strong post-fire increase in the o verall biomass of Calluna vulgaris, Polytrichum spp., Deschampsia flex uosa and Pteridium aquilinum was found, against a much slower regrowth of Vaccinium myrtillus and V. vitis-idaea. The Vaccinium species and Calluna were the dominant species at the control plot. Accumulation of carbohydrates seemed to take place in green and non-green tissue of P teridium and Deschampsia. A corresponding accumulation of nitrogen was found in green and nongreen Calluna and in non-green Vaccinium myrtil lus tissue, and of phosphorus in green and non-green tissue of both Va ccinium species and Calluna as a result of fire. The regrowth of Callu na was mainly from seeds that appeared to be present in the soil befor e the fire. The regeneration of Pteridium and the two Vaccinium specie s took place almost exclusively vegetatively from below-ground rhizome s that had survived the fire. Three years after the fire the overall b iomass at the burned site was higher than at the control site, due to improved light and nutrient conditions. However, the biomass is expect ed to decrease in the future and the species composition will probably change as nutrients are leached out of the soil and Pinus and deciduo us trees (Betula pubescens and Salix caprea) regenerate from seeds and roots, leading to increased competition in the field and shrub layer.