Pinus yunnanensis fire spread and structural ignition risk in mountainous wildland–urban interfaces of southwestern China

Wildland–urban interface (WUI) fires pose increasing risks in mountainous regions. However, fire-spread processes in southwestern China remain poorly understood. This study quantifies the effects of wind speed, slope, and forest–building separation distance on fire propagation and structural ignition in Pinus yunnanensi s-dominated WUI areas. Laboratory burning experiments combined with fire dynamics simulator (FDS) modeling were conducted across wind speeds of 0–3.5 m/s, slopes of 0–35°, and building separation distances of 4–10 m, measuring peak temperature and radiative heat flux. Crown fuels reached 505–713 °C and 7.85–10.00 kW/m 2 , respectively. Higher wind speeds reduced ignition time (~300 to ~194 s), while slope amplified fire development, peaking at 25°. Increasing separation distance significantly lowered heat exposure, preventing ignition at 10 m under the simulated wind speed of 2.5 m/s, 0° slope, no-ember-transport condition within the 900 s simulation period and shifted heating from flame-dominated to radiation-dominated. Forest–building separation emerged as the most effective factor controlling structural ignition, indicating that defensible-space design can substantially weaken vegetation–structure thermal coupling. These findings provide process-based evidence for near-structure fuel management, settlement planning, and community wildfire adaptation in mountainous WUI regions facing increasing climate-driven fire risk.