Wildfires are important disturbances in boreal forests across Canada, affecting both managed and unmanaged forests. For decades, managers and regulators in Canada have assumed that high severity (i.e. post-fire tree mortality) fires occurring at relatively low frequencies dominate boreal forests. However, mounting evidence goes against this assumption, showing that historical fire regimes were probably of mixed severity, whereby smaller and frequent low-to-moderate severity fires were mixed with infrequent high severity ones. Understanding these patterns of fire severity and their consequences for forest resilience is essential for forest and fire managers. At large scales, vegetation, climate and topography are important drivers of mixed severity fire (MSF) regimes. Although complex fire behaviour models exist, most simulate individual fires by reproducing hourly/daily fire behaviour, and lack temporal integration with vegetation dynamics (fire-vegetation feedbacks). Conversely, MSF models often impose spatial severity patterns defined a priori, which impairs predictions of fire patterns under changing environmental conditions. To address these gaps, we are developing a landscape dynamic model that can simulate MSF regimes at large spatio-temporal scales, using the SpaDES (SPAtial Discrete Event Simulator) framework. In our model, spatio-temporally heterogeneous severity arises from feedbacks between fire, vegetation, topography and climate, rather than being defined a priori from observed fire severity data, which is used instead to calibrate and validate model parametrisation. Natural fire dynamics (i.e. no fire suppression) are simulated using a percolation model governed by two main parameters, the probability of a cell catching fire from a burning neighbour, and the probability of persistence (the cell burns for more than one time step). Vegetation dynamics are simulated using the LANDIS Biomass model and, together with topography and climate, affect the two fire parameters. Fire severity is calculated a posteriori as a function of fire duration and forest structure.
Results/Conclusions
Preliminary results show that incorporating fire-vegetation feedbacks largely determines spatio-temporal MSF patterns. Conversely, topography drives spatial heterogeneity in fire regimes, modulating fire behaviour and severity for a given type of vegetation, while climate has a threshold effect. Under extreme burning conditions, high fire severity becomes dominant regardless of topography and forest composition and age. Yet, high severity fire cannot be sustained once vegetation dynamics are accounted for, since forest composition shifts towards more fire resistant species, which in turn drive lower severity fires. Our next step will be to validate/calibrate the simulated temporal fire dynamics using observed fire patterns from dendrochronological data.