Short communication Regional patterns of fire recurrence effects on calcareous soils of Mediterranean Pinus halepensis communities Ma `rcia Eugenio * , Francisco Lloret, Josep M. Alcan ˜iz CREAF (Centre for Ecological Research and Forestry Applications), UAB (Autonomous University of Barcelona), 08193 Bellaterra, Barcelona, Spain Received 19 April 2005; received in revised form 8 September 2005; accepted 3 October 2005 Abstract The effects of fire recurrence on soils were studied on calcareous sites of Mediterranean Pinus halepensis-dominated communities in Catalonia (NE Iberian Peninsula). Soil organic horizons and mineral soils of 15 sites consisting in two adjacent areas, one burnt only once (in 1994) and the other burnt twice (in the same 1994 fire but also once before, from 1975 to 1993) were surveyed 9 years after the last fire. Fire recurrence decreased the occurrence and dry mass of soil organic horizons. Total nitrogen concentration in L organic horizon was higher in less recurrently burnt areas. No other significant difference between once- and twice-burnt areas was found for any studied chemical parameter either in organic L and FH horizons or in mineral soils. The present study underlines the fact that fire effects on soil organic horizons are accumulated through consecutive fires. # 2005 Elsevier B.V. All rights reserved. Keywords: Calcareous soils; Fire regimes; Mediterranean-type communities; Pinus halepensis; Regional patterns 1. Introduction Fire is a dominant ecological factor in Mediterranean-type ecosystems, and changes in fire regimes – temporal and spatial patterns of fire occurrence (Gill, 1975) – can have important consequences for the conservation of Mediterranean land- scapes. In fact, shifts in historical fire regimes have been observed both in Southern California and the eastern coast of the Iberian Peninsula, which result from changes in land use, policies, and an increasing climatic fire risk (Minnich, 1983; Moreno et al., 1998; Pin ˜ol et al., 1999; Pausas, 2004). Among those, the increase of fire recurrence – number of fire events that occur at a site along a given period of time – is of special concern. Firstly, because most wildfires are not lighted by natural but by anthropogenic causes, so fire recurrence can reach extremely high values in certain areas. Along the last decade in Catalonia (a 32,000 km 2 region in the NE Iberian Peninsula), 17% of summer fires were caused by lightning, 12% by unknown causes, and the rest were all anthropogenic; the 30% of the total were intentionally lighted. For the rest of the year, percentages were 4%, 12%, and 22%, respectively (Departament de Medi Ambient i Habitatge, 2005). Conse- quently, some areas in the region have burnt six times along 1975–1998 (Dı ´az-Delgado and Pons, 2001), i.e., average fire interval (sensu Johnson, 1992) was shorter than 4 years. Secondly, it has been pointed as having the most relevant ecological consequences (Keeley et al., 1999). Recurrent fires can lead to long-term cumulative effects on some ecosystem properties, such as vegetation regeneration (Zedler et al., 1983; Dı ´az-Delgado et al., 2002). In a previous study, Eugenio and Lloret (2004) found that in Catalonia, Pinus halepensis communities affected by two fires along a 20-year-long period showed less developed structure and lower fuel amounts than those burnt only once along the same time period, despite time since last fire was the same for both types of areas. Thus, a loss of plant productivity after recurrent fires was evidenced, which could lead to shifts in community types. If cumulative effects are evidenced at plant community level, it is likely that they also occur in soils. The loss of plant productivity should be caused by a reduced availability of nutrients in mineral soils, and should result in reduced organic matter inputs to soil organic horizons. Moreover, direct effects of reburning should be evidenced on soil organic horizons, especially on those that need long periods of time to recover, www.elsevier.com/locate/foreco Forest Ecology and Management 221 (2006) 313–318 * Corresponding author. Tel.: +34 93 581 18 77; fax: +34 93 581 41 51. E-mail address: m.eugenio@creaf.uab.es (M. Eugenio). 0378-1127/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2005.10.011