Crown fire thresholds in exotic pine plantations of Australasia

Abstract

Fire managers in Australasia currently lack the basis that would permit them to objectively evaluate the potential for crown fire development in exotic pine plantations under any specified set of fuel, weather and topographic conditions. The existing crown fire initiation models all have inherent weaknesses or lack applicability, thereby rendering their utility questionable. A model that would enable them to predict the onset of crowning has been developed from a combination of physical insights and mathematical modelling coupled with relevant field and laboratory experiments. The six model inputs include at least two environmental parameters (ambient air temperature, in-stand wind speed) and possibly a third (slope steepness) where applicable, two surface fire behaviour characteristics (line-fire intensity, flame front residence time) and two crown fuel properties (foliar moisture content, live crown base height). The most fundamental principle incorporated in the model is that temperature rise above ambient conditions is determined by the intensity of the heat source at the ground surface, the height above ground in question, and the angle formed between the ground surface and the surface fire plume. Ignition or initial combustion of the needle foliage at the base of the live or green crown layer is in tum judged to be a function of the duration of heating experienced and the temperature achieved in the convection column at this height, assuming the presence of a pilot flame source(s). One of the unique features of the present crown fire initiation model is that the influence of within stand wind speed on the trajectory of the thermal plume has been considered in terms of its relative effectiveness in the convective heating of the lower live crown layer. This is considered to be a very significant improvement over C.E. Van Wagner's criteria for crown fire initiation and coupled with variable allowances for ambient conditions and duration of convective heating should thereby permit extrapolation to a wider range of burning conditions. Furthermore, a simplistic methodology has been formulated for deriving the needed empirical constant in the model, that essentially reflects the surface and bridge or ladder fuel characteristics of structurally dissimilar plantation stand types, based on the height of lethal crown scorching that could be obtained from low-to moderate-intensity surface fires, thereby negating the need for direct temperature measurement above the surface fire flame front. This has in turn resulted in new insights into the modelling and prediction of crown scorch height. The model has been tested against independent documentation obtained from experimental fires, operational prescribed fires and wildfires, with exceedingly encouraging results. The validity of C.E. Van Wagner's concept of a critical minimum crown fire rate of spread in relation to the crown bulk density in order to achieve continuous crowning has been substantiated for the first time in an operational setting. Also emerging from a detailed wildfire behaviour case study is evidence that under certain conditions, two distinctly different states of fire spread and intensity could exist in a given plantation fuel complex at a specific level of fuel moisture depending on the wind speed and arrangement/character of the plantation age-class mosaic. The model will allow exotic pine plantation managers the means of quantitatively and objectively assessing the various fire and fuel management practices designed to limit the incidence and impact of crown fires such as pruning, thinning, prescribed underburning, and plantation layout/design considerations (e.g., diverse age-class mosaic). The model could easily be extended to forest fuel complexes other than exotic pine plantations with a minimal of effort.