## 4 Discussion
### Probability of Ignition and Fuel Consumption in the Duff Layer  
The most important factors for the ignition and consumption of the soil duff layer are the maximum temperatures reached and the duration of these temperatures during a forest fire [41]. The longer the time during which the duff is subject to high temperatures, the lower the heat energy needed for ignition [45]. Thus, the minimum heat flux estimated for the ignition of a peat bog, with a moisture content of 10%, is 10.5 kW/m\({}^{2}\) during 3 min of high temperatures and 5.8 kW/m\({}^{2}\) during 30 min [45]. The composition of the duff (mineral content, moisture, bulk density and depth) also appear to determine the probability of its ignition and consumption [7]. The mineral content is one of the most important properties determining the smouldering combustion of fine organic layers [46], as it was in this case study. The moisture content of duff layer in Mediterranean summer months can vary between 5% and 30% [44; 47], but this depends greatly on their state of decomposition and on their composition [23; 48]. At these relatively low rates of moisture content, oxidation at the combustion fronts can produce enough energy to dry the duff and ensure propagation spread [10; 22]. Moreover, increasing temperature trends and longer periods of drought in the Mediterranean [13; 14], together with the consequent increase in water stress [15; 16], are likely to increase this risk of ignition and consumption and propagation within the duff layer. Moisture content with a 50% ignition probability (P\({}_{50}\)) is defined as the threshold at which ignition occurs [7] (Figures 6 and 7). The ignition results show that, due to extremely low levels of moisture content in the period of fire risk in the Mediterranean, the soil duff layer can easily combust. For ignition to occur in this study, approximately 75% mineral content (25% organic content) at 5% moisture content was estimated to be sufficient. The results also show that an increase in bulk density reduces ignition and propagation probability [8; 9]. Most studies of combustion in the duff layer agree that an increase in mineral content decreases ignition probability [8; 23]. However, the addition of soluble salts, such as calcium and magnesium, can act as catalysts in the oxidation of carbon, favouring the propagation of combustion [46]. This study has not undertaken a characterization of soluble salts in the duff layer, but in light of the high concentration of Ca\({}^{2+}\) cations in the mineral soil it can be considered that the mineral composition of the duff layer is playing a role on the estimated combustion parameters. The results of the study show that to obtain a consumption of above 90% and 60% of the duff sample (Cons\({}_{90}\) and Cons\({}_{60}\) respectively) and to achieve subsequent propagation [46], the mineral content values need to be lower than 55% and 40%, for moisture contents of 5% and 25%, respectively. Here, it is worth mentioning that [PERSON] and [PERSON] [23] did not include bulk density in their model because they did not observe a significant influence in the percentage variation of the consumption of their samples. The spatial variability in consumption can be determined by the percentage (>60% and >90%) of samples consumed. In this study, we have estimated that at a moisture content of 60%, half the samples would consume more than 60% of their organic content. If the moisture content (5% and 15% MC) is fixed at the levels prevailing during the re-ignitions, between 35% and 40% of the samples would consume more than 90% and all the samples would consume more than 60% of their organic content. These results highlight the heterogeneity of the O in terms of inorganic content and how this can influence spatial propagation after a fire. We should stress the importance of other factors that may favour ignition and increase consumption over a prolonged period.