Long-term effects on soil forest unmanaged after a wildfire at different fire severities: historical fire at Cadiretes Massif, Catalonia, Spain

Wildfire is a common disturbance in Mediterranean forests ecosystems. However, during the last decades wildfire frequency and burned surface area have been increasing. The abandon of forest areas by the population, the change of soil uses and climate change are causing wildfires of higher magnitude and difficult to control. The affected ecosystems have several difficulties to recover due to the high temperature reached during those great fires. The effects of fire on burnt soil forests depend on many different factors, such as the intensity of fire, duration, quantity of combustible or recurrence among others. However, the understanding of wildfire effects on soil forests at long-term is still needed to improve.

The aim of this study is to monitor the long-term effects (28 years) of a wildfire (55 ha) in Cadiretes Massif on soil properties in two areas affected at different fire severities (low severity: LS; high severity: HS). An unburnt Control area (C) adjacent to the burnt area was selected to compare with LS and HS. Prior the wildfire, the area was a plantation of Pinus pinaster ssp. with some individuals of Quercus suber L. No management has been applied in the area after the wildfire. The soil properties studied were hydrophobicity, pH, salinity, extractable calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺), potassium (K⁺), Total Nitrogen (TN), Total Organic Carbon (TOC) and Soil Organic Carbon stock (SOC_stock). Moreover, non-tree biomass accumulation on soil forests was collected in three different layers: freshly fallen, dry litter and humus.

In all studied areas (C, LS, HS) a hydrophilic behavior was observed. Moreover, in both burnt areas (LS and HS) and in C area were found similar pH values and Mg²⁺, Na⁺, K⁺ concentrations (~5.3, ~591 mg kg^-1, ~142 mg kg^-1, ~244 mg kg^-1; respectively). However, significant differences (p<0.05) were found in soil salinity, Ca²⁺, TN, TOC and SOC_stock between C area (~115µS cm^-1, ~2955 mg kg^-1, ~0.26%, ~6.9%, ~113 kg C m^-2) and burnt areas (LS: ~87µS cm^-1, ~2681 mg kg^-1, ~0.22%, ~5.0%, ~72 kg C m^-2; HS: ~80µS cm^-1, ~2337 mg kg^-1, ~0.19%, ~4.5%, ~57 kg C m^-2). Related to the non-tree biomass accumulation, significant difference was found in the accumulation in humus between C (~232g m^-2) and burnt areas (LS: ~116g m^-2; HS: ~100g m^-2). No significant differences were found in the accumulation of freshly fallen (~275g m^-2) and dry litter (~111g m^-2).

These results could indicate that at long-term, soil properties of burnt areas are partially able to recover and reach similar values to those of unburnt areas. However, it also seems that some soil parameters need more time to reach similar values than unburnt areas, especially after high severity fire episodes