## 2 Material and Methods  
### Study Area and Fire History  
The study area lies in the Serra de Senadelles, between the villages of Rocallaura (municipality of Vallbona de les Monges) and Nalec, Lieida (41\({}^{\circ}\)31\({}^{\prime}\)28.0\({}^{\prime}\) N 1\({}^{\circ}\)07\({}^{\prime}\)12.1\({}^{\prime}\) E, 615 m a.s.l.) in the Northeast of the Iberian Peninsula (Figure 2). The study area's forest is composed of a dominant tree stratum of _Pinus halepensis_ Mill (42 aa CORINE) [25]. The shrub stratum consists mainly of _Phillyrea latifolia_, _Quercus coccifera_, _Pistacia lentiiscus_, _Quercus llex_, _Erica multiflora_ and _Rosmarinus officinalis_. The herbaceous stratum is dominated by several species of moss, _Brachypodium retusum_ and some individuals of the _Orchidaceae_ family (Figure 1). The area has a normalized difference vegetation index (NDVI) of between 0.2 and 0.4 [26]. The _Pinus halepensis_ reaches an average height of 7 m to 11 m with a typical diameter at breast height of between 15 cm and 20 cm and a basal area of 20 m\({}^{2}\)/ha to 30 m\({}^{2}\)/ha. Leaf biomass oscillates between 3 and 5 t/ha, and total biomass between 60 t/ha and 120 t/ha [27]. The trees have a maximum age of between 70 and 80 years and have not, in fact, been cut or burned since the Spanish civil war (1936-39). A line of trenches dug in the area during the conflict, and the associated risk of explosion of ancient artefacts, explains why the forests have been exposed to little management activity since the late 1930s. This fact, combined with rural abandonment, has resulted in the growth of moss and an increase in forest floor duff in zones with little slope. The geological substrate is formed by Cenozoic sedimentary deposits. The lower part of the substrate contains marks with calcareous intercalations (Rupelian-Chattian); the middle contains shales, sandstones and calcareous forms (Rupelian-Chattian); and, the upper part contains clays, silts and fine-grained sandstones with intercalations of conglomerates (Rupelian) [30]. Soils are classified as _Haploxroll lithtic_[31] and the characteristics have been specified in Table 1. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline
**Horizon** & **Depth** **(cm)** & **pH** & **Calcium** & **Fine Soil** & **Organic** & **Inorganic** & **Total** & **C/N** & **Density** \\  & **(cm)** & **pH** & **Carbonate (\%)** & **(\%)** & **Matter (\%)** & **Matter (\%)** & **Carbon (\%)** & **Ratio** & **(Mg/m\({}^{3}\))** \\ \hline O * & 1–12 & - & 2.0–6.7 & - & 44.4–80.8 & 19-25-56.6 & 25-04-13.1 & - & 48–191 \\ A & 5–40 & 7.6–8.5 & 11.3-36.3 & 53–98 & 7.1-20.7 & 79.3–92.9 & 8.0–13.1 & 23–45 & 550–1570 \\ R & \(>\)40 & - & - & - & - & - & - & - & - \\ (Calerous) & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & & & & & & \\ \hline \end{tabular}  
* Organic stratum or duff layer, composed by Oe + Oa. Horizon parallel and immediately below to O horizon (A) and bedrock (R).  
\end{table} 
Table 1: Soil characteristics of the study area. Figure 1: Images of the study area close to re-ignition spots. Left: tree distribution and understory overview. Right: organic horizon and moss cover. Figure 2: Land use change in the area affected by the 2016 Rocallaura Fires. Sampling area located with a point. Left: orthophoto of the study area as it is in 2015. Right: historical orthophoto of 1945–1946 (American flight series 45–46). Adapted from: Institut Cartografic i Geologic de Catalunya (ICGC) [28; 29]. On 23 June 2016, the first fire broke out affecting 103 ha of forest and agricultural land near the village of Rocallaura (Vallbona de les Monges, l'Urgell) (Figure 3). On 25-29 June, various re-ignitions were recorded by the firefighting crews in different areas of the burned zone. On 4, 7 and 10 July, re-ignitions were registered in the forest floor and on tree trunks and stumps that continued to sustain smoldering combustion. No rain occurred during the period. On 18 July another re-ignition was registered burning 500 m\({}^{2}\).