Effects of wildfire and laboratory heating on soil aggregate stability of pine forests in Galicia: The role of lithology, soil organic matter content and water repellency
Research Highlights
► Wildfire-induced changes in aggregate stability are highly variable. ► Changes in aggregate stability are associated with changes in organic carbon content. ► Changes in aggregate stability can also be explained well by changes in water repellency. ► Fire severity is a key explanatory variable and should be estimated in future field studies.
Introduction
The Spanish region of Galicia, located in the NW corner of the Iberian Peninsula, is one of the most densely forested regions in Europe, reflecting the region's favourable, humid temperate climate conditions. Afforestation is widely regarded in Galicia as an important measure of soil protection, since the risk of soil erosion is high in many rural areas due to the pronounced terrain relief in combination with the elevated rainfall erosivity (Díaz-Fierros et al., 1987). During the past few decades, fast-growing tree species (predominantly pines and eucalypt) have been planted as a massive scale. This has, however, contributed to a strong increase in the occurrence of wildfires, especially in the last few years. According to the wildfire statistics for Spain for the period 1991–2006, approximately half of the wildfires occurred in Galicia, whilst Galicia only corresponds to in an area 16 % of the national forest area. These wildfires consumed a total area of 501,184 ha in Galicia. August 2006 was especially dramatic, with many wildfires closely approaching urban centres and with roughly 75,000 ha burnt by almost 2000 wildfires in no more than 12 days. One of the most serious consequences of this ecological disaster was the subsequent soil erosion in these burned areas and the resulting off-site damages caused by runoff and transported sediments in downstream terrestrial and aquatic ecosystems (Carballas, 2007). This was caused by the torrential rainfall events that occurred at the end of the summer of 2006, in combination with the lack of immediate soil conservation measures.
In unburned soils, aggregate stability is one of the factors influencing infiltration capacity and susceptibility to erosion. The occurrence of a wildfire, however, through the removal of the protective vegetation and litter cover in particular turns aggregate stability into a key consideration in post-fire measures of water, soil and nutrient conservation (Mataix-Solera and Guerrero, 2007). The effect of fire on soil aggregate stability is nonetheless far from clear. Authors like Ibáñez et al., 1983, Boix Fayos, 1997, Arcenegui et al., 2008 claim that burning enhances structural stability, whereas for example Giovannini and Lucchesi, 1997, García-Oliva et al., 1999, Mataix-Solera et al., 2002 reported the opposite.
Fire-induced combustion of soil organic matter can be expected to induce the partial or complete destruction of soil aggregates. Namely, whilst soil organic matter plays a marked role in the formation of aggregates in many soils (Oades, 1993, Roldán et al., 1994), it is the prevalent aggregation factor in the forest soils of Galicia (Díaz-Fierros et al., 1994, Varela, 2007). However, fire does not necessarily reduce topsoil organic matter content in a significant manner and low-intensity fires have even been reported to increase organic matter content (Mataix-Solera et al., 1996, Giovannini and Lucchesi, 1997, Guerrero et al., 2001). Such a fire-induced increase in organic matter content could explain why some fires are exceptional in enhancing aggregate stability rather than decreasing it (Mataix-Solera and Guerrero, 2007). An alternative explanation for fire increasing aggregate stability is through its impact on soil water repellency (Shakesby and Doerr, 2006, Varela et al., 2005, Keizer et al., 2008). Hydrophobic substances can form a thin film that partially or entirely covers aggregates and, thereby, increases their resistance to disaggregating (slaking) when wetted (Chenu et al., 2000, Mataix-Solera and Doerr, 2004, García-Corona et al., 2004, Goebel et al., 2005, Arcenegui et al., 2008).
Differences in fire intensity are likely to be of crucial importance in the contrasting findings by earlier studies on the impacts of wildfire on aggregate stability, notwithstanding the well-established fact that aggregation also depends on soil type and soil physico-chemical properties like, for example, clay and calcium carbonate content (Mataix-Solera and Cerdá, 2009) . The intensity of a wildfire, however, is not easily assessed. There exist a variety of fire intensity indicators, for example related to the colour of the ashes or the consumption of woody plan parts (e.g. Shakesby and Doerr, 2006), but their relationship with the heating regime of the soil itself is tentative at best. Therefore, various prior studies have resorted to controlled heating experiments in the laboratory to study the influence of temperature on soil properties (Giovannini et al., 1988, Soto et al., 1991, García-Corona et al., 2004).
The main aim of the present work is to determine the direct impacts of wildfires on the aggregate stability of forest soils and to clarify the role therein of fire-induced changes in soil organic matter content and water repellency. To this end, a two-fold approach is employed. Wildfire effects are studied by means of a paired sampling strategy comparing recently burned and long unburned soils at similar neighbouring sites. Heating effects are addressed by means of controlled laboratory experiments, and compared with the wildfire effects to shed further light on the importance therein of differences in fire severity. This study concerns pine forest plantations for being one of the most prevalent and, at the same time, most fire-prone forest types in Galicia. Special attention is given to comparing the region's two predominant lithologies.
Section snippets
Study area and site characteristics
The study area is located in the southern part of Galicia, NW Spain (Fig. 1). The climate of this temperate–humid Atlantic zone has a pronounced oceanic character. Mean annual rainfall is high (rainfall is about 1400 mm), and has a marked seasonal pattern, of rainy autumns and winters. Summer droughts can be pronounced with water balance deficits up to 400 mm. The temperature regime is that of temperate areas, with mean annual temperatures ranging from 8 to 15 °C. At present, forests constitute
Unburned soils
The main topsoil characteristics at the unburned sites are given in Table 3. As expected, the soils overlying granite at sites G1 to G12 are coarser than those overlying metamorphic rocks at sites S1 to S6. The former have a sandy-loam texture, the latter a loam or silt loam texture. This contrast is accompanied by marked and statistically significant differences in the sand and silt fractions but not the clay fractions. The sand fraction is significantly higher at sites G1 to G12 than sites S1
Conclusions
The main conclusions of this combined field and laboratory study on the effects of wildfire and controlled heating on aggregate stability of pine forest soils Galicia, NW Spain, are as follows:
- (i)
aggregate stability of long unburned soils is highly variable and, unlike expected, lacks a clear relationship with the underlying lithology;
- (ii)
wildfire-induced changes in aggregate stability are highly variable, as well as extent, and this can partly be attributed to the above-mentioned variability in the
Acknowledgements
This work has been by financed by the Xunta de Galicia, projects PGIDT99PXI30101A and PGIDIT02RFO30101PR and Ministerio de Ciencia e Innovación, project AGL2008-01399/FOR.
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