Long-term dynamics of soil chemical properties after a prescribed fire in a Mediterranean forest (Montgrí Massif, Catalonia, Spain)
Graphical abstract
Introduction
Forest fires are a natural phenomenon in many terrestrial ecosystems and many of the effects of such fires have been examined (Neary et al., 2005, Buhk et al., 2007, Úbeda and Outeiro, 2009). Studies of their impact stress that a wide range of variables need to be taken into consideration to understand their effects on the chemical properties of the soil. Their impact can vary greatly depending on the ecosystem affected (vegetation type, environmental conditions, soil type, soil moisture content), the fire type (intensity and severity) and, the post-fire meteorological conditions; but water, air, vegetation, soil and human resources are all typically significantly affected by a forest fire (Certini, 2005). Wildfires are a serious problem in areas with a Mediterranean climate owing to the hot summers and drought conditions that provide ideal conditions for the ignition and spread of fire. This problem is exacerbated by the large amounts of fuel accumulation and the continuity between forested areas. Yet, fire should be considered a key ecological factor in Mediterranean ecosystems since fire has determined their evolution and present-day landscapes along with other natural and anthropogenic impacts (Eugenio et al., 2006). Thus, it is important that fire is perceived as a natural process; however, fire events are increasing in frequency and intensity due to a range of factors, including the rural exodus and, the abandonment of traditional uses, increasing human pressure in urban-forest interface areas, changing economic interests in forest management and the greater use of forest areas for recreational uses (Vélez, 2000).
The configuration and structure of Mediterranean landscape give rise to high severity fires that spread rapidly and which are difficult to be extinguished, especially if they reach the tree canopies. Thus, it is important to identify tools that can be used at different scales to reduce fire spread and minimize their incidence and effects. At the landscape scale, forestry management is concerned with the vulnerability of forests to fire, and is based on the consideration of fire type and fire regimes in a given area (Piqué et al., 2011). It is essential to know the type of forest in any specific plot, its species composition and its characteristics in relation to its structure and state of development, since these factors can predict the spread and vigor of the fire (González-Olabarria et al., 2007, Piqué et al., 2011). Preventive forestry is a good tool for achieving forest structures with lower amounts of fuel and a greater resistance to fire spread (Vélez, 2000). This practice involves reducing surface vegetation, pruning and removal of vertical fuel, clearing so as to minimize heat transmission and the replanting of specific species that generate forest structures with low vulnerability to fire (Fernandes and Rigolot, 2007).
Prescribed fire is the planned use of fire under predetermined weather, fuel and topographic parameters to achieve clearly defined objectives (Outeiro, 2010). The practice was introduced in Europe to control fire regimes by managing fuel, counteracting the disappearance of biomass-consuming land management practices and reducing the overall fire risk. In Catalonia, in 1999, the Government set up a unit within its fire department, known as GRAF (Forest Action Support Group) (Fig. 1), to fight fires by developing innovative prevention activities. Prescribed fires, however, have also been shown to be useful in regenerating certain plant species, improving habitats for mammals, promoting recreational uses, and facilitating animal grazing of shrublands (Afif and Oliveira, 2006). Normally, prescribed fires are low intensity fires and, if managed appropriately, do not cause any damage to trees, especially in Mediterranean ecosystems where trees are resilient to fire (Certini, 2005). Additionally, prescribed fires can have different impacts on soils, water resources, biodiversity, the risk-reduction of wildfires and carbon storage; indeed, many of the impacts can be positive, including soil fertilization, the preservation and restoration of valued shrubland habitats, enhanced habitat quality for different species and the reduction of carbon emissions compared to wildfires (Fernandes et al., 2013). Yet, these actions in the environment are often controversial and by no means are this type of forest management accepted by all parties. It is therefore vital to continue undertaking research that can provide answers regarding the benefits and drawbacks of prescribed fire in order that integrated forest management planning can strike the right balance in terms of the sustainability of our forests (Outeiro, 2010).
The effects of prescribed fires on soil chemical properties were widely studied in the recent decades for different authors (Guinto et al., 2001, Arocena and Opio, 2003, Bennett et al., 2014). Some of these studies found an increase in pH values just after a prescribed fire (Úbeda et al., 2005, Granged et al., 2011) while others observed no significant changes (Marcos et al., 1999, Afif and Oliveira, 2006). Electrical conductivity increases after a prescribed fire (Granged et al., 2011, Scharenbroch et al., 2012). The nutrient availability increases after a prescribed fire because of the incorporation of ashes into the soil according to both short- and long-term studies (Úbeda et al., 2005, Outeiro et al., 2008, Brye, 2006, Scharenbroch et al., 2012). However, to our knowledge the number of studies addressing the impacts of prescribed fires in the long term is still scarce thus, it is important to keep on doing more research because prescribed fires can help us to prevent large wildfires and reduce the fire frequency. At the same time, studies about burning recurrence are also needed, as one from Jones et al. (2015), to enrich our knowledge about how much time has to pass before burning again the same area to not damage the soil. The aim of this study is to quantify the short- and long-term changes recorded in the soil chemical properties following a low-intensity prescribed forest fire in a Mediterranean forest.
Section snippets
Study area and prescribed fire
The area selected for this study is located in the north-east of the Iberian Peninsula in the coastal mountains of Catalonia within the calcareous Montgrí Massif in the province of Girona (Fig. 2). The area has a mean annual precipitation of 694 mm with spring and autumn maxima and a mean annual temperature of around 13 °C. Soils are generally shallow with depths varying between 30 and 40 cm. Calcareous rock outcrops on the surface are prevalent (25–50%) and coarse fragments are frequent, which
Results
The soil chemical parameters before the prescribed fire are shown in the Table 1.
Immediately after the prescribed fire, soil pH increased, but not significantly, from 7.00 ± 0.28 to 7.08 ± 0.34 (Tables 1; 2). One year after the prescribed fire, pH continued to rise, reaching 7.25 ± 0.45, a statistically significant increase over pre-fire values (Table 3). Nine years after the prescribed fire, soil pH had fallen to 6.68 ± 0.40, a statistically significant difference with all three previous measurements.
Discussion
Sampling conducted before and immediately after the prescribed fire revealed statistically significant differences in all elements analyzed, except soil pH which presented a non-significant increase (Tables 2; 3). However, in the following year, different trends were observed; so, while pH and total C levels continued to rise after the fire, the other parameters decreased, with some – namely, EC, available P and K+ – returning to levels that were lower than pre-fire values. In contrast, while
Summary and conclusions
GRAF Unit aimed to increase the quality and the quantity of the grassland in Montgrí Massif, but conversely nine years later Q. coccifera L. shrubland is the dominant species. Prescribed fire has reduced vegetation fuels in the short-term however; the density of shrubland is higher than prior to the fire. Regarding vegetation resprouting after the prescribed fire, some first colonizers appeared again a few months later. In addition, soil nutrients became available, which benefit the plant
Acknowledgments
This study was made possible thanks to Project CGL2013-47862-C2-1-R: “Soil quality, erosion control and plant cover recovery under different post-fire management scenarios”, sponsored by the Spanish Ministry of Economy and Competitiveness and the FPU Program (FPU13/00139) promoted by the Ministry of Economy, Culture and Sports. Thanks the Postdoctoral program I2C from “Xunta de Galicia” from its sponsorship and economical support during the development of the manuscript. We thank the GRAF team
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