## 4 Discussion
### Determination of Smoldering Combustion Spread Rate  
The smoldering combustion spread rate is a multidimensional phenomenon made up of two components: in-depth vertical and surface lateral spread. Here, only the lateral spread rate could be estimated. In-depth spread could not be estimated as we did not have the resources required to quantify the calorific energy of the organic matter [50]. The spread rate falls as the moisture content rises and increases at greater wind speeds [50]. For a given moisture content, if the bulk density increases, more energy input is needed to continue smoldering combustion [9]. In contrast, high bulk density peat produces more energy due to the oxidation of more mass [2], but it tends to retain more moisture content. This water has to be evaporated by the drying subfront before the arrival of the pyrolysis and oxidation subfronts [3]. An estimated range for the smoldering combustion spread rate in peat has been described by [PERSON] et al. [50]: it varies between 25 cm/h (with wind speeds of 4.3 cm/h and a moisture content of 5%) and 2 cm/h (without any wind and 100% moisture). This study shows that, on the days that the re-ignitions took place, the mean smoldering combustion spread rate was approx. \(3.84\pm 0.34\) cm/h. This rate was estimated for an organic horizon (Oe + Oa) with a 10% moisture content, without wind and with a mean bulk density of 160 kg/m\({}^{3}\). If the surface in the study area were homogeneous, smoldering combustion could begin at \(92.16\pm 8.16\) cm/day. Starting from the fire on 23 June 2016, the spread distance could be estimated, on the condition that all the re-ignitions were a consequence of smoldering combustion and the spread was linear, homogeneous and without wind. For example, on the 25-30 of June 2016, the estimated radial distance affected by smoldering would be 1.84 m, 2.77 m, 3.69 m, 4.61 m, 5.53 m and 6.45 m, respectively. On the 4, 7 and 10 of July 2016, the burnt radial distance would reach 10.76, 12.90 and 15.67 m, respectively. And on 19 July, the radial distance would be 23.96 m. With a mean daily wind speed of 7 km/h, this rate would increase [50]. We should stress that, unlike in the model described in [PERSON] et al. [9], the bulk density of the samples in the present study did not vary in terms of their moisture content. This indicates that the spread rates obtained may be underestimated and that the actual rate could be higher, especially at high moisture contents. In contrast, the high mineral content of the study samples (35% IM in this study compared to 2.6% in [PERSON] et al. [9]) would probably reduce the spread rate [7], although the high presence of Ca\({}^{2+}\) and Mg\({}^{2+}\) cations could well increase heat transfer and, therefore, the spread rate [46]. Thus, we only estimated rates that might differ from those in reality, but, nevertheless, they should serve as a reference for further studies in Mediterranean areas. It can be assumed that the main factors responsible for the heterogeneity in the spread of smoldering combustion, during the drought months in Mediterranean ecosystems, are the mineral content and the bulk density (due to the low levels of soil moisture registered in the area). However, moisture has been shown to be the main limiting factor in the spread of fires in other ecosystems with a high concentration of organic matter [50; 51], which is why it should be considered an important factor, above all, in fires that take place at times of the year when soil water availability increases. It could also be considered an important factor in periods of drought, since small changes in the moisture of the organic horizon can result in a significant change in the consumption of organic matter during smoldering combustion [52]. The presence of roots, branches and stones can also play an important role in fuel heterogeneity and, as a result, affect smoldering combustion [47].