Effects of fuel and weather conditions on forest fire behaviour in Southern Sweden in oak dominated forests

Sammanfattning: Fire activity is influenced by weather and climate, fuels, ignition agents, and human activities. Fire suppression policy in Sweden resulted in a gradual decrease in the annually burnt areas, changes in fuel loads and a decline in species diversity. Prescribed burning is recommended as a management tool for fire-prone ecosystems to support conservation policies and, eventually, mitigate large high-intensity wildfires. This study analysed fire behaviour as a function of fuel characteristics and weather conditions in oak dominated forests in Southern Sweden. I was specifically interested in thresholds indicative of accelerated fire spread and conditions leading to increased consumption of ground fuels. I also studied whether regional weather indices provide predictive power with respect to local fire behaviour. Data from 105 ignition experiments at seven study sites laid the ground for the database used in analyses. Data collection involved the collection of meteorological on-site data, pre- and post-fire fuel inventories, and measurements of fire behaviour. The minimum wind above 2.5 m/s and the temperature above 15°C positively affected fire spread. High relative humidity and the low amount of dry fuel had a negative effect on fire spread. The fire spread slowed down when the value of relative humidity exceeded 35 % and the fuel amount dropped below 1000 g/m2. Fire spread slowed down when the relative humidity exceeded 40 %, and litter depth fell below 0.05 m. Total fuel consumption grew when the fuel amount exceeded 3000 g/m2 and the fire spread rate fell below 0.05 m/s. Fuel amount and relative humidity affected total consumption. Total consumption grew when the fuel amount exceeded 1800 g/m2, while the relative humidity fell below 30 %. Regional weather indices were poor predictors of fire behaviour. The study identifies thresholds of fire behaviour in oak dominated forests that can be used by forest managers to plan prescribed burnings. The study highlights the need for further research covering a wider gradient of weather conditions.