CSIRO PUBLISHING International Journal of Wildland Fire 2009, 18, 430–441 www.publish.csiro.au/journals/ijwf Prediction of fire occurrence from live fuel moisture content measurements in a Mediterranean ecosystem Emilio Chuvieco A,B , Isabel González A , Felipe Verdú A , Inmaculada Aguado A and Marta Yebra A A Department of Geography, University of Alcalá, E-28801 Alcalá de Henares, Spain. B Corresponding author. Email: emilio.chuvieco@uah.es Abstract. The present paper presents and discusses the relationships between live Fuel Moisture Content (FMC) measurements and fire occurrence (number of fires and burned area) in a Mediterranean area of central Spain. Grasslands and four shrub species (Cistus ladanifer L., Rosmarinus officinalis L., Erica australis L. and Phillyrea angustifolia L.) were sampled in the field from the spring to the summer season over a 9-year period. Higher seasonal FMC variability was found for the herbaceous species than for shrubs, as grasslands have very low values in summertime. Moisture variations of grasslands were found to be good predictors of number of fires and total burned surface, while moisture variation of two shrubs (C. ladanifer L. and R. officinalis L.) was more sensitive to both the total burned area and the occurrence of large fires. All these species showed significant differences between the FMC of high and low occurrence periods. Three different logistic regression models were built for the 202 periods of analysis: one to predict periods with more and less than seven fires, another to predict periods with and without large fires (>500 ha), and the third to predict periods with more and less than 200 ha burned. The results showed accuracy in predicting periods with a high number of fires (94%), and extensive burned area (85%), with less accuracy in estimating periods with large fires (58%). Finally, empirical functions based on logistic regression analysis were successfully related to fire ignition or potential burned area from FMC data. These models should be useful to integrate FMC measurements with other variables of fire danger (ignition causes, for instance), to provide a more comprehensive assessment of fire danger conditions. Additional keywords: fire danger, fire risk. Introduction Wildland fires play a critical role in vegetation succession (Pausas and Vallejo 1999), soil degradation (Almendros et al. 1990) and watershed runoff changes (Robichaud 2005), while making up an important proportion of global greenhouse emis- sions (van der Werf et al. 2004) and land use transformations (Stolle et al. 2003), especially in the tropical forests. Although several factors need to be considered for a com- prehensive evaluation of fire danger (Chuvieco et al. 2003b), most operational danger indices rely almost exclusively on mete- orological data because they are relatively easy to obtain and provide a good prediction of fuel moisture content (FMC) sta- tus (Deeming et al. 1974; Van Wagner 1974). The Canadian Fire Weather Index (Van Wagner 1974), for instance, is based on three moisture codes that account for water variations in three fuel layers, whereas the US National Fire Danger Rating System (Deeming et al. 1978) considers four different mois- ture codes (1 h, 10 h, 100 h and 1000 h). In both systems, the moisture codes are expected to estimate the water content of dead components, which are the most flammable and easy to estimate from meteorological observations, because they mainly depend on atmospheric conditions, material thickness and chem- ical composition (Viney 1991). The FMC of live plants has more complex interactions with atmospheric and soil water capacity, because it is largely influenced by species’ water bal- ance mechanisms (Slavik 1974). In fact, neighboring plants with the same atmospheric conditions may have very different FMC values, depending on their physiological characteristics and drought-resistance strategies: root system, leaf thickness, leaf morphology, chlorophyll content, etc. Consequently, in fire danger assessment, a general distinction is made between moisture content of dead materials lying on the forest floor (senescent grasses, dry leaves, small twigs, and organic material in the topsoil) and live species, among which a further division can be made between FMC of leaves and stems. FMC is commonly expressed as the amount of water per dry mass of the fuel (commonly expressed in percentage, but also as gH 2 O per g dry mass). FMC has proved inversely related to ignition probability owing to the fact that part of the energy necessary to start a fire is used up in the process of evaporation right before the fire starts to burn (Dimitrakopoulos and Papaioannou 2001). Additionally, FMC dilutes volatiles and excludes oxygen from the combustion zone. However, water content also affects fire propagation as the source of the flames is reduced with humid materials, therefore reducing flammability (Viegas 1998). Although laboratory experimentation has proved the rela- tions between FMC and ignition delay, ignition potential or © IAWF 2009 10.1071/WF08020 1049-8001/09/040430