Forest fire effects on soil chemical and physicochemical properties, infiltration, runoff, and erosion in a semiarid Mediterranean region
Introduction
Semiarid Mediterranean regions are characterized by long, dry and hot summers and short, wet, mild winters (Hötzl, 2008). These conditions are favorable for wildfires and indeed, there has been an increase in the number of wildfires and total burnt area in the Mediterranean region since the 1960s (Kliot, 1996, Pausas and Vallejo, 1999, Wittenberg and Malkinson, 2009). The rise in the number of wildfires is ascribed mainly to the accumulation of combustible fuels in abandoned areas (Pausas and Fernández-Muñoz, 2012, Shakesby, 2011), afforestation of mono-specific flammable tree species (Shakesby, 2011), and climate change (Pausas, 2004, Pausas and Fernández-Muñoz, 2012). Possible harmful effects of forest fire include total or partial loss of vegetation and litter cover in the forest (e.g., Ben-Hur et al., 2011, Shakesby, 2011, Soto et al., 1997), increases in surface runoff, soil erosion, and downstream flooding (Ben-Hur et al., 2011, Wagenbrenner et al., 2006), and export of sediments, organic matter, nutrients, and pollutants that can endanger downstream aquatic and flood-zone habitats and associated human infrastructures (Ferreira et al., 2008, Shakesby and Doerr, 2006).
Reduction of infiltration rates (IRs) and an increase in surface runoff and soil erosion following forest fires have been widely reported (e.g., Benavides-Solorio and MacDonald, 2001, Benavides-Solorio and MacDonald, 2005, Inbar et al., 1997, Inbar et al., 1998, Martin and Moody, 2001, Mayor et al., 2007, Moody and Martin, 2001, Shakesby, 2011, Wittenberg and Inbar, 2009). The increase in runoff and soil erosion has been attributed mainly to: (i) increasing soil water repellency that can decrease the IR (DeBano, 2000, DeBano et al., 1998, Letey, 2001, Neary et al., 1999); (ii) a decrease in transpiration as a result of vegetation losses in the forest which, in turn, alters the soil–water relationships (Ben-Hur et al., 2011); (iii) amplification of soil proofing by transport and accumulation of ash particles (Cerdà and Doerr, 2008, Etiégni and Campbell, 1991, Larsen et al., 2009, Mallik et al., 1984, Martin and Moody, 2001, Neary et al., 1999, Pannkuk and Robichaud, 2003, Woods and Balfour, 2010).
Soils in semiarid regions are characterized by low organic matter and high expandable clay mineral contents (Singer, 2007), properties that can decrease the stability of soil structure (Ben-Hur, 2008). When these soils are exposed to the impact of raindrops, a structural seal develops at the soil surface (Ben-Hur, 2008, Morin et al., 1981). This seal is thin (a few millimeters) and is characterized by greater density, higher strength, finer pores, and lower saturated hydraulic conductivity than the underlying soil (Chen et al., 1980, Gal et al., 1984, McIntyre, 1958, Onofiok and Singer, 1984, Wakindiki and Ben-Hur, 2002, West et al., 1992), leading to a decrease in IR (Assouline, 2004, Ben-Hur et al., 1985a, Morin et al., 1981). McIntyre (1958) found that the structural seal consists of two distinct parts: an upper skin seal and a “washed-in” zone with decreased porosity attributed to the accumulation of dispersed clay particles. Its formation is a result of two complementary mechanisms: (i) physical disintegration of aggregates at the soil surface caused mainly by the impact energy of raindrops and fast wetting of the soil, and (ii) chemical dispersion of clay particles, which migrate into the soil with the infiltrating water and clog the pores immediately beneath the surface, forming the “washed-in” zone (Agassi et al., 1981, Lado et al., 2004b, Morin et al., 1981). Soil erosion involves two major processes: (i) detachment of soil material from the soil surface, and (ii) transport of the resulting sediments, mainly by surface runoff (Watson and Laflen, 1986).
Soil detachment and seal formation depend on aggregate stability, and therefore on soil components and soil-solution properties. Clay minerals, iron and aluminum oxides, CaCO3, and organic matter in the soil can act as cementing materials that hold the particles together in the aggregate against the impact energy of raindrops and fast wetting of the soil, leading to higher aggregate stability (Lado et al., 2004b, Oades and Waters, 1991, Rimmer and Greenland, 1976, Singer, 1994, Six et al., 2000). An increase in sodium adsorption ratio (SAR) and a decrease in electrical conductivity (EC) in the solution of the upper soil layer might enhance chemical dispersion of the clay and formation of the washed-in zone, resulting in decreased IR and increased runoff amount and transport of detached particles (Agassi et al., 1981, Agassi et al., 1994, Ben-Hur et al., 1998, Kazman et al., 1983, Shainberg and Letey, 1984, Wakindiki and Ben-Hur, 2002). Fire and high temperatures, however, can also affect the physical and chemical properties of the soil and the physicochemical properties of its solution (e.g., Badía and Martí, 2003, Certini, 2005, DeBano et al., 1998, Gimeno-García et al., 2000, Giovannini and Lucchesi, 1997, Giovannini et al., 1988, Giovannini et al., 1990, González-Pérez et al., 2004, Mataix-Solera et al., 2011, Neary et al., 1999). Gimeno-García et al. (2000) and González-Pérez et al. (2004) found that severe fire decreases organic matter content in the soil. Arocena and Opio (2003), Giovannini et al. (1988) and Ulery and Graham (1993) found textural changes in soils after heating them to > 200 °C. Hernández et al. (1997), Iglesias et al. (1997), Kutiel and Inbar (1993), Kutiel and Naveh (1987), Kutiel et al. (1995), Pardini et al. (2004) and Terefe et al. (2008) found that soil heating significantly alters the physicochemical properties of the topsoil solution, changing the ion composition and concentration.
Exposing the soil to wetting–drying cycles can significantly change the concentration and composition of ions in the soil solution which, in turn, can affect seal formation, soil hydraulic properties, and soil loss under consecutive rainstorms (e.g., Ben-Hur et al., 1985b, Ben-Hur et al., 1989, Hardy et al., 1983, Levy et al., 1986, Morin and Benyamini, 1977). Rajaram and Erbach (1998) showed that exposing a clay loam soil to wetting–drying cycles result in an increase in aggregate cohesion and size, but its stability decreased with an increase in drying stress. Wagner et al. (2007) found that wetting–drying cycles initiate aggregate evolution irrespective of soil clay content, although high clay content yielded more stable aggregates. All of these findings suggest that interactions between forest fire, soil heating, and wetting–drying cycles can affect the soil structure and seal, runoff, and soil loss under consecutive rainstorms. These interactions, however, have been little studied or documented.
Wildfires differ in terms of intensity and severity, and therefore their impact on soil properties, runoff and erosion can be diverse (Keeley, 2009). Even within a specific wildfire, local variations in lithology, topography, plant composition, fuel-load distribution, and microclimatic conditions in the forest can result in a heterogeneous spatial distribution of fire intensity and severity (Kutiel et al., 1995, Lavee et al., 1995, Shakesby, 2011). Therefore, in a forest exposed to fire, this spatial distribution can lead to several effects on the underlying soil, such as: soil barely affected by the fire, soil exposed to direct fire, and soil exposed to the heat of the fire only, with no direct flame contact. The objective of the present work was to study the effects of different fire and heating conditions on the physical, chemical, and physicochemical properties of soil, and their impact on IR, surface runoff and soil loss under consecutive rainstorms. We focused on Pale rendzina, a very common soil in forests of the Eastern Mediterranean region.
Section snippets
Experimental site, soil sampling, and tested treatments
The studied area was a planted forest located near the city of Safed in northern Israel (32°58′39″N, 35°30′22″E). The forest stand is a combination of Aleppo pine (Pinus halepensis) and Turkish pine (Pinus brutia) with a mixture of old (> 60 yr) and young trees. The average altitude of the forest is 840 m above sea level with a typical Mediterranean climate: average annual temperature and precipitation are 22 °C and 600 mm, respectively. The soil in the forest is a sandy clay loam, Pale rendzina (
Results and discussion
The physical and chemical characteristics of the unburned soil, soil exposed to direct fire, heated soil, and their respective water extracts are presented in Table 1. The fire treatments were found to differentially alter the soils' characteristics. Heating the soil to 300 °C significantly decreased its organic matter content compared to the unburned soil (Table 1), as a result of its combustion by the high temperature during the heating process. Similar results were found by Giovannini et al.,
Summary and conclusions
- •
The various fire treatments changed the physical and chemical properties of the studied soils. Heating the soil to 300 °C combusted the organic matter and significantly decreased its content in the soil. Moreover, the contents of clay and sand decreased and that of silt increased. These changes in mechanical composition were attributed mainly to: (i) dehydration of 2:1 clay minerals leading to strong interactions among the clay particles, which formed silt-sized particles and less clay-sized
Acknowledgments
This project was funded by Xunta de Galicia Project 07MRU007103PR and the Smaller–Winnikow Fellowship Fund for Environmental Research.
References (99)
- et al.
Prescribed fire-induced changes in properties of sub-boreal forest soils
Geoderma
(2003) - et al.
The effect of ash and needle cover on surface runoff and erosion in the immediate post-fire period
Catena
(2008) The role of fire and soil heating on water repellency in wildland environments: a review
J. Hydrol.
(2000)- et al.
Soil hydrophobicity variations with depth and particle size fraction in burned and unburned Eucalyptus globulus and Pinus pinaster forest terrain in the Águeda Basin, Portugal
Catena
(1996) - et al.
Soil water repellency: its causes, characteristics and hydro-geomorphological significance
Earth-Sci. Rev.
(2000) - et al.
Physical and chemical characteristics of wood ash
Bioresour. Technol.
(1991) - et al.
Soil and water degredation processes in burnt areas: lessons learned from a nested approach
Catena
(2008) - et al.
Water repellency as conditioned by particle size and drying in hydrophobized sand
Geoderma
(2013) - et al.
The effect of fire on soil organic matter — a review
Environ. Int.
(2004) - et al.
Mineralogical and chemical modifications in soils affected by a forest fire in the Mediterranean area
Sci. Total Environ.
(1997)