Increasing fire recurrence in Europe and impact on soil quality

Since the mid of the last century fire recurrence (number of fire events that occur at a site in a given period of time) has increased in the Iberian peninsula and the overall Mediterranean basin (Pausas and Fernández-Muñoz, 2012). This occurs due to fuel accumulation from land abandonment and extensive reforestation (Koutsias et al. 2012) and to extreme weather events (Camia and Amatulli 2009, Carvalho et al. 2011, Hoinka et al. 2009, Koutsias et al. 2012). The future warmer and drier climate projected for this region will further increase the risk of wildfire occurrence and of increasing fire recurrence (Giorgi and Lionello 2008). Future wildfire risk is projected to increase in Southern Europe (Lindner et al. 2010, Carvalho et al. 2011, Dury et al. 2011). The annual burned area is projected to increase by a factor of 3 to 5 in Southern Europe compared to the present under the A2 scenario by 2100 (Dury et al. 2011).

There is ample literature on the effects of fire recurrence on vegetation. Recurrent fires can lead to long-term cumulative effects at plant community level such as changes in plant composition and structure (Lloret et al 2003, Eugenio et al 2006, Santana et al 2010), losses in plant productivity (Díaz-Delgado et al 2002, Eugenio and Lloret 2004 Delitti et al 2005), and delays in post-fire plant regeneration. Such changes in vegetation are likely to be associated to changes in soil quality. For instance, it has been suggested that loss of plant productivity with subsequent fires is associated with a cumulative reduced availability of nutrients in mineral soils (Ferran et al 2005). However, although the impact of wildfires on soil nutrient content in Southern Europe has been extensively studied, only a few studies have assessed this impact on the basis of fire recurrence (Caon et al 2014).

One of the most common changes in plant communities driven by high fire recurrences in Southern Europe is the replacement of pine woodlands by shrublands. Despite the high post-fire resilience of the most common pines in the Mediterranean (Pinus halepensis, P. brutia and P. pinaster), these trees fail to regenerate when time interval between fires is shorter than the time needed to accumulate a sufficient seed bank, i.e. around 15 years (Eugenio et al. 2006, Santana et al 2010). Surprisingly, it has been poorly studied whether a shift from pine woodlands to shrublands is associated with a shift in soil fertility. Most of the available research assessing the impact of fire recurrence on soil fertility is performed in ecosystems dominated by species with resprouting ability after fire (i.e., Quercus suber woodlands or Q. coccifera shrublands), and thus, with no major shifts in plant community (e.g., Trabaud 1991, Carreira et al 1994, Guenon et al 2001, Ferran et al 2005). One of the few works studying effects of repeated burning on soils in Mediterranean pine woodlands found that, nine years after the last fire, sites burned twice in an interval of 18 years had less developed organic horizons but similar mineral soils than sites only burned once in that period (Eugenio and Lloret, 2005). The authors attributed this response to the lower vegetation development in twice- than in one-burnt areas. The lack of cumulative effects of recurrent fires in mineral soils could however not be concluded as pooled soils up to 20 cm depth were sampled, whereas maximum depth commonly affected by fires is around 5 cm (Giovannini, 1994).

Early warning indicators of soil functioning

A significant decline in soil quality has occurred throughout the entire world as a result of adverse changes in its physical, chemical, and biological properties, caused by human activity and climate change (Van Camp et al. 2004). Soil degradation processes in drylands are particularly acute due to the fragility imposed in these areas by water scarcity in combination with large human and climatic pressures. Thus, soil degradation in drylands is one of the main environmental problems worldwide, including Europe (32% of the land mass are drylands, home to 25% of the population). Moreover, this problem is expected to get worse in the face of current global change (Millennium Ecosystem Assessment, 2005; Reynolds et al., 2007), where Europe is forecasted to be one of the world’s regions most impacted. Further, both theoretical developments and empirical data provide evidence that healthy drylands can shift to a degraded state in response to small increases in human and climatic pressure once a threshold has been surpassed (Scheffer and Carpenter 2003, Rietkerk et al 2004, Schroder et al 2005, Gao et al 2011). This implies the possibility for sudden major and difficult-to-recover ecological and economic losses, what explains the research emphasis on identifying early warning indicators (Dakos et al. 2012, Kéfi et al. 2014). In this context, the identification of early warnings of changes in ecosystem functioning, including changes in soils, is prioritary for identifying areas with higher risk of degradation in response to specific pressures, being recurrent wildfires one of the most common pressures in European forest and shrublands.

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