Modeling wind fields and fire propagation following bark beetle outbreaks in spatially-heterogeneous pinyon-juniper woodland fuel complexes

Abstract

We used a physics-based model, HIGRAD/FIRETEC, to explore changes in within-stand wind behavior and fire propagation associated with three time periods in pinyon-juniper woodlands following a drought-induced bark beetle outbreak and subsequent tree mortality. Pinyon-juniper woodland fuel complexes are highly heterogeneous. Trees often are clumped, with sparse patches of herbaceous vegetation scattered between clumps. Extensive stands of dead pinyon trees intermixed with live junipers raised concerns about increased fire hazard, especially immediately after the trees died and dead needles remained in the trees, and later when the needles had dropped to the ground. Studying fire behavior in such conditions requires accounting for the impacts of the evolving heterogeneous nature of the woodlands and its influence on winds that drive fires. For this reason we used a coupled atmosphere/fire model, HIGRAD/FIRETEC, to examine the evolving stand structure effects on wind penetration through the stand and subsequent fire propagation in these highly heterogeneous woodlands. Specifically, we studied how these interactions changed in woodlands without tree mortality, in the first year when dried needles clung to the dead trees, and when the needles dropped to the ground under two ambient wind speeds. Our simulations suggest that low wind speeds of 2.5m/s at 7.5-m height were not sufficient to carry the fire through the discontinuous woodland stands without mortality, but 4.5m/s winds at 7.5-m height were sufficient to carry the fire. Fire propagation speed increased two-fold at these low wind speeds when dead needles were on the trees compared to live woodlands. When dead needles fell to the ground, fine fuel loadings were increased and ambient wind penetration was increased enough to sustain burning even at low wind speeds. At the higher ambient wind speeds, fire propagation in woodlands with dead needles on the trees also increased by a factor of ∼2 over propagation in live woodlands. These simulations indicate that sparse fuels in these heterogeneous woodlands can be overcome in three ways: by decreasing fuel moisture content of the needles with the death of the trees, by moving canopy dead needles to the ground and thus allowing greater wind penetration and turbulent flow into the woodland canopy, and increasing above-canopy wind speeds.