Table 1. Summary of the variables.
Variable Description Type Source
Fire occurrence (response) Division into observed forest fires and pseudo-absence observations (0 = pseudo-absence, 1 = fire occurrence) binary PRONTO1
Fire size (response) Total burnt area in terrain types considered forest, with a logarithmic transformation (ha) numerical PRONTO1
Coordinates Location of the ignition point numerical PRONTO1
Year Calendar year (2014–2023) numerical PRONTO1
Trafficability Bearing capacity of the terrain (1–6; 1 = the highest,
6 = the lowest) 		categorical Soil trafficability map, Finnish Forest Centre
Slope Terrain slope at the ignition point (°) numerical Elevation model 10 m, National Land Survey of Finland
Canopy cover Proportion of ground covered by the vertical projection of tree crowns (%) numerical MS-NFI2
Dominant species Tree species, covering at least 50% of the total growing stock volume (Pine, Spruce, Deciduous, or Mixed) categorical MS-NFI2
Day of year Sequential calendar day number within the year (0–366) numerical PRONTO1
Finnish Forest Fire Index (FFI) Latest index value prior to fire in the municipality, to one decimal precision (1.0–6.0) numerical PRONTO1
Ignition cause Reclassified ignition cause, based on fire authorities' assessment (Human, Lightning, or Unknown) categorical PRONTO1
Population density Interpolated population density as of 31st December of the preceding year (persons km⁻2) numerical Population grid data 1 km × 1 km, Statistics Finland
Distance to nearest road Euclidean distance between the ignition point and the nearest road, including classes road, roadway, track, pedestrian and bicycle route, and winter road (km) numerical Topographic Database, National Land Survey of Finland
1 Finnish rescue services’ resource and accident statistics system, Emergency Services Academy Finland
2 Multi-Source National Forest Inventory, Natural Resources Institute Finland
Table 2. Summary statistics of the forest fires included in the analyses.
Year Number of fires Ignition cause (%) Fire size (ha)
Human Lightning Unknown Median Mean Sum
2014 1320 54.5 29.5 16.1 0.020 0.55 721
2015 629 73.9 3.5 22.6 0.003 0.17 107
2016 772 80.8 3.1 16.1 0.005 0.31 238
2017 746 82.0 2.1 15.8 0.010 0.73 543
2018 1892 55.1 19.7 25.3 0.010 0.48 910
2019 1071 62.9 15.1 21.9 0.005 0.28 296
2020 1046 73.6 10.2 16.2 0.008 0.65 683
2021 1019 58.3 21.6 20.1 0.005 0.56 575
2022 845 72.9 13.4 13.7 0.002 0.24 199
2023 944 64.7 16.6 18.6 0.005 0.35 330
All 10 284 65.4 15.4 19.2 0.008 0.45 460
1

Fig. 1. Maps showing a) the location of the study area within the boreal region (University of Minnesota 2017; The World Bank 2025) and b) the spatial distribution of forest fires recorded in PRONTO, visualized as a kernel density heatmap.

Table 3. Terms in the relative ignition probability model, listed in descending order of χ2-statistic. The s() denotes a spline function applied to the variable.
Term χ2-value p-value
s(population density) 1567.8 <0.001
s(day of year) 1565.6 <0.001
s(canopy cover) 679.3 <0.001
trafficability 317.9 <0.001
distance to nearest road 147.0 <0.001
s(slope) 126.3 <0.001
dominant species 0.5 0.918
2

Fig. 2. Response of relative ignition probability to a) trafficability, b) slope, c) canopy cover, d) dominant species, e) day of year, f) population density, and g) distance to nearest road. Lines represent the estimated effects and shaded areas 95% confidence intervals. In panel e), the dashed smooth shows variation in the Finnish Forest Fire Index as a function of day of year based on the observed fires. For continuous variables, tick marks indicate observation density; for categorical variables, class frequencies are shown.

Table 4. Terms in the fire size model, listed in descending order of F-statistic. The s() denotes a spline function applied to the variable.
Term F-value p-value
s(population density) 121.7 <0.001
distance to nearest road 76.7 <0.001
ignition cause 76.0 <0.001
s(day of year) 48.2 <0.001
s(Finnish Forest Fire Index) 22.7 <0.001
trafficability 8.5 <0.001
s(canopy cover) 6.8 <0.001
dominant species 6.6 <0.001
slope 1.0 0.309
3

Fig. 3. Response of fire size to a) trafficability, b) slope, c) canopy cover, d) dominant species, e) day of year, f) the Finnish Forest Fire Index, g) ignition cause, h) population density, and i) distance to nearest road. Lines represent the estimated effects and shaded areas 95% confidence intervals. For continuous variables, tick marks indicate observation density; for categorical variables, class frequencies are shown.