Estimating wildfire-driven forest carbon losses using dynamic fuels under managed and afforested forests in Europe

Wildfire projections for Europe depend not only on future climatic conditions, but also on how fuels evolve as forests age, are managed, and expand through afforestation. This study focuses on the dynamic fuel representation in FLAM by linking it with the forest model G4M in a coupled framework for EU27+UK. The coupling provides scenario-consistent, annually updated fuels (from live biomass, deadwood, and litter) from G4M to FLAM, and FLAM returns burned area used to update forest carbon trajectories and fuels in G4M in the following year.

FLAM runs on a 0.5° grid with daily time steps and simulates ignition, spread, and burned area as a function of climate, fuel loads, vegetation type, and human influence. Daily temperature, precipitation, relative humidity, and wind speed are taken from ISIMIP3b bias-adjusted CMIP6 forcings (UKESM1-0-LL and GFDL-ESM4). Fuels from G4M are divided into “old” fuels (pre-2000 managed forests) and “new” fuels (post-2000 afforested forests). The combined fuel load in each grid cell is updated dynamically using the effective burned ratio, so cells with higher burned ratios increasingly draw fuel from unburned stands, while low burned-ratio cells remain dominated by managed forest fuels.

To limit repeated burning within grid cells, an annual burned ratio approach is used to reduce the effective burnable fraction where only the remaining unburned forest area can burn. To avoid unrealistic permanent fuel depletion, a recovery function reduces the effective burned ratio toward zero (parameterized with a = 0.65 over b = 25 years), implying roughly 4% of the remaining burned ratio is removed annually, consistent with multi-decadal stand recovery times and typical rotation lengths in European managed forests. Assumptions include successful regeneration after stand-replacing fires and no change in species composition.

FLAM is calibrated and validated with historical forest burned area observations showing moderate correlation (monthly correlation r ≈ 0.63; annual r ≈ 0.59). Projections for SSP1-2.6, SSP2-4.5, SSP3-7.0 show cumulative burned area of roughly ~27–35 Mha and wildfire-driven biomass carbon losses of ~290–360 Mt C. The presentation will show how this dynamic fuel coupling changes projected wildfire outcomes and what it implies for forest carbon and biomass supply.