Coupled Fire/Atmosphere Behavior in Highly Heterogeneous Woodlands and the Impacts of Tree Mortality on Fire Spread

In heterogeneous woodlands such as piñon-juniper in the southwestern United States, trees often are clumped with sparse patches of herbaceous and shrubby vegetation scattered between clumps. In these woodlands, the sparcity of ground fuels, short stature of the trees, and lack of continuity of the canopy create a different coupled fire/atmosphere regime than exists in more continuous forests. In order for fires to propagate in these woodlands, wind speeds within the canopy must be sufficient for the fire to traverse the gaps between trees since grasses are not generally sufficient to carry the fire across the inter-tree gaps. The balances that lead to fire spread or no spread conditions depend on many factors, including: wind speed, fuel moisture, litter density around the trees and tree density. Preliminary numerical studies with FIRETEC suggest that the heterogeneous nature of these woodlands leads to thresholds in fire behavior due to critical balances between heat production, wind speed, and the ease with which fuels are ignited. For example in piñon-juniper, there are thresholds where fires will not sustain themselves but with an increase in wind speed of approximately 50 percent in the canopy the fire will vigorously spread.

These factors can be suddenly changed in the event of wide spread mortality such as occurred in recent drought-induced bark beetle attack by Ips beetles. These infestations resulted in widespread mortality of piñon trees across vast areas of the Southwest, however they had almost no impact on the juniper trees, thus changing moisture contents in canopy fuels, litter accumulations, tree density, and canopy winds at different stages of the progression post attack. The presence of dead trees intermixed with live junipers raised concerns about increased fire hazard, especially immediately after the trees died and dead needles remained in the trees. Firefighters responding to wildfires in the region reported more severe fire behavior in impacted stands, especially during periods of strong winds which accompany extremely dry conditions during the fire season in the Southwest. FIRETEC simulations were also used to investigate the impacts of tree mortality on the coupled fire/atmosphere balances and fire spread. Various stages of stand evolution were studied, including the first year when dried needles clung to the dead trees, and when the needles dropped to the ground. The results indicate that such mortality events can alter the balances that lead to thresholds and change the rate of fire propagation.