Towards modeling tree-to-tree fire spread in wildland urban-interface (WUI) fires

Wildfires in the Wildland–Urban Interface (WUI) have intensified in recent years due to climate change and continued development of communities at the WUI. Defensible space-reducing vegetation near structures-is a mitigation strategy, but its performance under wind-driven fire conditions remains unclear. This study integrates controlled experiments with Fire Dynamics Simulator (FDS) modeling to investigate ignition between trees subjected to wind-aided flame spread. A custom-designed propane burner, representing fire exposure similar to a 2 MW tree fire, exposed downwind Douglas-fir trees under flow velocities of 1.0 and 2.0 m s −1 , across varying separation distances. Total and radiative heat fluxes were measured using water-cooled gauges, and ignition behavior was evaluated through logistic regression. Flame impingement emerged as the dominant ignition mechanism, with convection significantly increasing heat transfer to downwind trees at short separation distances. Simulations reproduced key trends in heat flux and flame geometry but over-predicted radiation and under-predicted convection, especially at short separations. Critical ignition distances increased with wind speed, highlighting the role of wind-driven flame elongation and the need for flow-aware defensible space recommendations. The experiments provide insight into the use of computational fire modeling to inform defensible space guidelines tailored to specific environmental and fire conditions.