Ponderosa pine (Pinus ponderosa) is one of the most widespread conifers in western North America and has recently experienced high levels of bark beetle-induced tree mortality. This study explored the impact of beetle-induced mortality and wind speed on fire behavior during the pre-outbreak (“green stage”), immediately post-mortality when dead needles remain on trees (“red stage”), and when needles drop to the ground (“gray stage”) in southwestern ponderosa pine forests. Researchers used data from 60 field plots across five national forests in northern Arizona and developed fire simulations with the physics-based model HIGRAD/FIRETEC, allowing them to account for effects of spatial heterogeneity and dynamic winds on fire behavior.
Results suggest that bark beetle-caused tree mortality can dampen, amplify, or have little effect on subsequent fire disturbances and their ecological effects depending on level of tree mortality, time since outbreak, and wind speeds.
Fire rate-of-spread (ROS) increased as forest canopy fuels transitioned from the green to the red stage, with the greatest difference in fire ROS between red stage and green stands occurring at low wind speeds. Even just 20% tree mortality resulted in 15% greater ROS in red-stage stands over green stands, and high levels of mortality resulted in 90% greater ROS.
Total canopy fuel consumption increased with higher levels of mortality during the red stage but was unaffected at low levels of mortality and high wind speeds.
Fire simulations in the gray stage revealed a general decrease in ROS relative to the green stage and total canopy fuel consumption either decreased or did not change. The one exception was in the low wind, low mortality simulations, where both fire ROS and canopy fuel consumption were predicted to be higher in the gray stage than in green stands.
More details available on the project page Simulated fire hazard following bark beetle outbreaks in ponderosa pine: Amplification, attenuation, and a few ecological surprises.