Experimental development of a fire management model for Jarrah (Eucalyptus Marginata Donn ex Sm.) forest.

Abstract

Accumulations of flammable fuel and seasonal hot, dry weather has ensured that fire is an important environmental factor which has shaped jarrah forest ecosystems of south-west Western Australia. Today, fire impacts on all aspects of jarrah forest management, including timber and water production, recreation and wildlife conservation. Fire management involves controlling destructive wildfires and applying prescribed fires over a wide range of burning conditions to achieve a variety of protection, production and conservation objectives. A sound scientific understanding of the behaviour, physical impacts and long term ecological and commercial effects of fire is essential to planning and implementing fire regimes and suppression activities pertinent to current and foreseeable management. Existing forest fire behaviour guides developed in the 1960s from small low intensity experimental fires set under mild conditions perform adequately over the low fire intensity range, but are deficient at predicting the behaviour of moderate and high intensity fires burning under warm, dry conditions. Another shortcoming is that they do not attempt to predict physical impacts of fire which give rise to ecological responses or commercial losses. This thesis describes laboratory and field experiments designed to model the behaviour and some important physical impacts of fire in jarrah forest over a wide range of potential burning conditions. Fire behaviour and fire impact models were developed for a standardjarrah. forest fuel type; the structure, composition, dynamics and combustion properties of which were studied in detail. Most variation in equilibrium headfire rate of spread on level terrain was best explained by the product of a power function in wind speed and a power function in fuel moisture content. Headfire rate of spread was independent of the quantity of fuel per unit area. Forced convection and flame contact appeared to be the primary mechanisms for flame spread in wind driven fires which burnt across then down into the eucalypt litter fuel bed. Conversely, the rate of spread of zero wind fires and backfires was directly related to the quantity of fuel burnt, suggesting that radiation was the primary mechanism for flame spread in this situation. The transition from a fire spreading primarily by radiation to one spreading primarily by convection occurred at a wind speed of 3 - 4 km h-1. For zero wind conditions, rate of spread and slope were best related by an exponential equation form and fire shape was described by a power function in wind speed. Flame size was a function of rate of spread, fuel quantity consumed and fuel moisture content. Immediate physical impacts of fire on vegetation and soil were examined in three zones and coupled with fire behaviour variables and factors affecting heat transfer by fitted regression models. Impact above the flames (crown scorch height), was dependent on flame height, fire intensity and the season in which the fire occurred. Impacts in the flames (stem damage and mortality), were dependent on the quantity of fuel consumed, fire intensity and bark thickness. Soil heating was a function of the quantity of fuel consumed, soil moisture and fuel moisture. A soil heating index was developed which allows numerical characterisation of fire-induced soil heating. The fire behaviour and fire impact models developed by this thesis provide a scientifically based system for using fire as a tool for multiple use forest management