Equilibrium Moisture Content of Dead Fine Fuels of Pubescent Oak (Quercus pubescens Willd.) and Holm Oak (Quercus ilex L.)

Abstract

The moisture content of dead fine forest fuels is a central component of nearly all fire behaviour and fire danger rating systems. For modelling purposes, equilibrium moisture content curves are an important input parameter. When a dead fine fuel is exposed to an environment of constant temperature and relative humidity, its moisture content increases or decreases until it reaches a steady state called the equilibrium moisture content. It is an important characteristic of a dead fine fuel, as it defines the end points toward which the moisture content tends. The equilibrium moisture content is a function of fuel temperature, relative humidity and fuel type, as well as of whether the particle has been adsorbing or desorbing moisture. The main objective of this study was to derive equilibrium moisture content curves (isotherms) for dead fine fuels of important Mediterranean oak species, pubescent oak (Quercus pubescens Willd.) and holm oak (Quercus ilex L.). Experimental measurements of twenty litter samples (10 g) per tree species were used for determination of equilibrium moisture content values. Isotermal stepped testing was conducted in the growth chamber through each sorption phase, at 21.1 °C, over a range of relative humidity values 20% to 90%, in 10% steps. The equilibrium moisture content curves were obtained using nonlinear least-squares fitting based on the measurements and Van Wagner’s model. Both adsorption and desorption measurements followed a typical sigmoid shaped curve. Equilibrium moisture content was an average of 1.5% and 1.1% higher for desorption than adsorption, demonstrating typical hysteresis. The experimental equilibrium moisture content values for pubescent oak litter were within the range of those published for other broadleaves species, while the equilibrium moisture content values for holm oak litter are among those published for conifers.