Vegetation ash and soil, resume, challenges and perspectives


Paulo Pereira
pereiraub@gmail.com
Mykolas Romeris University, Vilnius, Lithuania

After the fire the ash distributed on soil surface have a great influence on soil protection and it is an important source of nutrients for plant recuperation (Cerdà and Doerr, 2008; Pereira et al., 2010). In the study of fire effects on soil, normally, the role of ash has been neglected, because it is removed or because studies are carried out weeks after the fire, when ash was already removed by wind, water or mixed with soil. In burned areas, when the studies are carried out some weeks after the fire, there is a great probability that the fire impacts in soil are not only due temperatures, but also due the incorporation of ash into the soil profile. Thus methodologies that consider the study of fire impacts on soil several weeks after the fire should consider the ash impact (Pereira et al., 2010).

Figure 1. Wet ash covering soil surface.
Figure 1. Wet ash covering soil surface.

Ash is a very mobile and dynamic material, especially in high severity burned areas, where the intense combustion induce an important loss of mass, that it is easily transported by wind in the immediate period after the fire. This „dancing ash“can have important implications in areas further form where it was produced. The implications of this are a strong (re)distribution of ash produced at different temperatures, which will have consequences on the degree of soil protection after the fire. The initial effects of wind in ash (re)distribution are a complex spatio-temporal distribution of soil protection, that will favoured the protection of some areas and decrease in others (Úbeda et al. 2009; Pereira et al., 2013).

After the first rains, ash is eroded or incorporated into the soil profile changing the underlying soil properties (Bodí et al. 2013). However the impacts depend upon the soil properties previous to fire and fire severity which ash it is produced. Ash can decrease soil erosion, retaining water and facilitating water infiltration (Cerdà and Doerr, 2008) or seal soil, reducing the soil water storage and increase the erosion (Onda et al. 2008). When dry, ash binds strongly onto soil surface and clog soil pores (Fig 1.) (Woods and Balfour, 2010). Thus, fire effects on soil properties are very heterogeneous and difficult to predict, because 1) The same fire temperature induce different severities according to the specie (Fig. 2). In wildland environments species are mixed in forest litter (for more details about fire severity measurement according ash colour, please consult Pereira et al. 2012), 2) Fire impacts in litter and organic matter are different, producing different responses and 3) Post-fire wind and water erosion (re)distribute and rework ash and soil mixing the impacts of fire severity on both. Ash can change for a short period of time soil hydrologic properties. When produced at lower temperatures, ash is water repellent (Bodi et al. 2011) and this can increase soil hydrophobicity after low severity fires (Pereira et al. 2012b). In addition, Ash water repellency is strongly linked with the chemical composition. When produced at high temperatures, ash is wettable because fire consumes vegetation hydrophobic surfaces (Dlapa et al. 2013). Further research it is needed about the effects of the different types of ash and soil effects after the fire and how they interact.

Figure 2. Fire severity (assessed with ash colour) impacts on Quercus suber, Pinus pinea and Pinus pinaster litter (Bodí et al. 2011).
Figure 2. Fire severity (assessed with ash colour) impacts on Quercus suber, Pinus pinea and Pinus pinaster litter (Bodí et al. 2011).

The amount and type of ash nutrients and leached elements is related with the fire severity (Pereira et al. 2012), the temperature of combustion (Úbeda et al. 2009) and specie affected (Pereira et al., 2011). This will induce a great heterogeneity on the type and amount of nutrients leached from ash. Aslo ash is rich in heavy metals that could act as contaminant (Pereira and Úbeda, 2010). The nutrients leachability depends on ash pH and the amount of CaCO3 (Pereira et al. 2013a) and normally decreases with the number of leaching (Liodakis et al. 2009). With the number of leaching ash pH decreases and the amount and type of available nutrients change also (Pereira et al. 2010).
Due the complexity of the landscape (e.g type and amount of vegetation and distribution on soil surface, type of soil, topography and aspect) and meteorological conditions during the fire, the effects of fire in ash properties are very much heterogeneous. One of the main priorities of ash research is understand this dynamic that very frequently is attributed to vegetation moisture and fuel distribution. But since we know that temperature impacts are different in different species, laboratory fire simulations can consider the mix of the species existent in a certain environment, instead of analyse species individually, as previous studies have done (Úbeda et al. 2009; Bodi et al. 2011). After the fire it is important to understand ash infiltration erosion and infiltration process. Intense rainfall and steep topography might promote ash erosion, but low intensity precipitation might retain soil in soils surface. Important questions emerge, as the threshold at what ash can be infiltrated in the soil or eroded. In which type of soil, ash is infiltrated easily and what kind of ash is eroded faster, in what conditions (e.g texture, specie severity, etc). These and other questions should be addressed in order to have a better understanding about the interaction of ash and soil in burned areas.

References

Bodí, M., Balfour, V., Pereira, P. 2011. Quan passen les flammes i el fum. El paper de les cendres segons tres joves investigadors. In: Cerdá. A. Quan es crema el Bosc. L’impacte del foc en els ecossistemes, Revista Metode, 70, 89-94.

Bodí, M., Doerr, S., Cerdà, A., Mataix-Solera, J. (2012) Hydrological effects of a layer of vevegetation ash on underlying wettable and water repelent soil. Geoderma, 191, 14-23.

Bodí, M., Mataix-Solera, J., Doerr, S., Cerdà, A. (2011) The wettability of ash from burned vegetation and its relationship to mediterranean plant species type and total organic carbon content. Geoderma, 160, 599-607.

Cerdà, A., Doerr, S.H. (2008) The effect of ash and needle cover on surface runoff and erosion in the immediate post-fire period. Catena, 74 , 256- 263. http://dx.doi.org/10.1016/S0341-8162(02)00027-9.

Dlapa, P., Bodi, M., Mataix-Solera, J., Cerdà, A.,Doerr, S. (2013) FT-IR spectroscopy reveals that ash water repellency is highly dependent on ash chemical composition. Catena, in press.

Liodakis, S., Tsoukala, M., Katsigiannis, G. 2009. Laboratory study of leaching properties of Mediterranean forest species ashes. Water, Air and Soil Pollution, 203, 99-107.

Onda, Y., Dietrich, W.E., Booker, F. (2008) Evolution of overland flow after severe forest fire, Point Reyes, California. Catena, 72, 13-20.

Pereira, P., Bodí. M., Úbeda, X., Cerdà, A., Mataix-Solera, J., Balfour, V, Woods, S. (2010) Las cenizas y el ecosistema suelo, In: Cerdà, A., Jordán, A. (eds.) Actualización en métodos y técnicas para el estudio de los suelos afectados por incendios forestales, 345-398. Càtedra de Divulgació de la Ciència. Universitat de Valencia. ISBN: 978-84-370-7887-8. Deposito Legal: V-3541-2010.

Pereira, P., Cerdà, A., Úbeda, X., Mataix-Solera, J. Arcenegui, V., Zavala, L. (2013) Modelling the impacts of wildfire on ash thickness in a short-term period, Land Degradation and Development, in press.

Pereira, P., Mataix-Solera, J., Ubeda, X., Cerda, A., Cepanko , V., Vaitkute, D. Pundyte, N., Pranskavicius, M. Zuokaite, E. (2012b) Grassland fire effects on soil organic matter, soil moisture and soil water repellency in Lithuania (North-Eastern Europe). First results. Eurosoil 2012, Bari.

Pereira, P., Úbeda, X. (2010) Spatial variation of heavy metals released from ashes after a wildfire, Journal of Environmental Engineering and Landscape Management, 18(1), 13-22.

Pereira, P., Úbeda, X., Martin, D. (2012a) Fire severity effects on ash chemical composition and water-extractable elements. Geoderma, 191, 105-114.

Pereira, P., Úbeda, X., Martin, D., Mataix-Solera, J., Guerrero, C. (2011) Effects of a low prescribed fire in ash water soluble elements in a Cork Oak (Quercus suber) forest located in Northeast of Iberian Peninsula, Environmental Research, 111(2), 237–247.

Úbeda, X., Pereira, P., Outeiro, L., Martin, D. (2009) Effects of fire temperature on the physical and chemical characteristics of the ash from two plots of cork oak (Quercus suber), Land Degradation and Development, 20(6), 589-608.

Woods, S.W., Balfour, V.N. 2010. The effects of soil texture and ash thickness on post-fire hydrological response from ash-covered soils. Journal of Hydrology, 393, 274-286.

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