Soil Shapes Community Structure Through Fire

Oecologia (2010) 163:729–735 Author's personal copy DOI 10.1007/s00442-009-1550-3 COMMUNITY ECOLOGY - ORIGINAL PAPER Soil shapes community structure through Wre Fernando Ojeda · Juli G. Pausas · Miguel Verdú Received: 20 November 2008 / Accepted: 10 December 2009 / Published online: 8 January 2010 © Springer-Verlag 2010 Abstract Recurrent wildWres constitute a major selecting Introduction force in shaping the structure of plant communities. At the regional scale, Wre favours phenotypic and phylogenetic The ecological advantage conferred by a trait (or set of clustering in Mediterranean woody plant communities. traits) on individual members of a species under a given set Nevertheless, the incidence of Wre within a Wre-prone of environmental conditions allows the species to occupy region may present strong variations at the local, landscape that particular habitat (i.e. environmental or habitat Wlter- scale. This study tests the prediction that woody communi- ing). This habitat Wltering is one of the key processes struc- ties on acid, nutrient-poor soils should exhibit more pro- turing community assemblages (van der Valk 1981; Keddy nounced phenotypic and phylogenetic clustering patterns 1992; Webb et al. 2002). than woody communities on fertile soils, as a consequence In many ecosystems, recurrent wildWres constitute a of their higher Xammability and, hence, presumably higher major selecting force in shaping the structure and function propensity to recurrent Wre. Results conWrm the predictions of plant communities (Bond and van Wilgen 1996; Bond and show that habitat Wltering driven by Wre may be and Keeley 2005; Pausas and Keeley 2009). All Mediterra- detected even in local communities from an already Wre- nean-type climate regions, except the Chilean one, include Wltered regional Xora. They also provide a new perspective Wre-controlled plant communities (Cowling et al. 1996), from which to consider a preponderant role of Wre as a key which are characterized by species having traits that ensure evolutionary force in acid, infertile Mediterranean heath- the persistence of populations under recurrent Wres (Keeley lands. 1986). In woody plants, two basic Wre-associated traits have been traditionally described: resprouting ability (resprouter/ Keywords Community phylogenetics · Fire traits · non-sprouter), and Wre-induced germination [Wre-induced Mediterranean heathlands · Nutrient-poor soils · (P+)/non-Wre-induced germination (P¡); Pausas et al. Phenotypic clustering 2004; Pausas and Verdú 2005]. P+ is widely accepted as an adaptive trait state to the recurrent presence of Wre (Bond and van Wilgen 1996; Keeley and Bond 1997; Pausas et al. 2006). At the regional scale, frequent Wres favour an over-repre- Communicated by Jon Keeley. sentation of P+ species in woody plant communities of the Mediterranean basin (Verdú and Pausas 2007). This over- F. Ojeda (&) representation of a particular phenotype (i.e. phenotypic Departamento de Biología, Universidad de Cádiz, W Campus Río San Pedro, 11510 Puerto Real, Spain clustering) is a consequence of habitat ltering of those e-mail: [email protected] species having the focal trait state (P+). When the trait is evolutionarily conserved, phenotypic clustering subse- J. G. Pausas · M. Verdú W quently determines phylogenetic clustering (Webb et al. Centro de Investigaciones Sobre Deserti cación W (CIDE, CSIC-UV-GV), Apartado OWcial, 2002). Indeed, the re-induced germination trait (P+/¡) is 46470 Albal, Valencia, Spain strongly conserved in the woody Xora of the Mediterranean 123 730 Author's personal copy Oecologia (2010) 163:729–735 Basin and woody plant communities under high Wre fre- habitats under the hypothesis that acid, nutrient-poor soils quency show both phenotypic and phylogenetic clustering shape the phenotypic and phylogenetic community struc- (Verdú and Pausas 2007; Pausas and Verdú 2008) ture through Wre. Cavender-Bares et al. (2004) found strong phenotypic clustering among co-occurring Quercus species sharing Wre-related traits in Floridian oak communities, thus Materials and methods evidencing habitat Wltering. Although this pattern of Wre-driven phenotypic and phy- Study area logenetic clustering of Mediterranean plant communities has been reported at the regional scale, little is known at the The northern (European) side of the Strait of Gibraltar local, landscape scale, where marked diVerences in the Wre region, at the westernmost point of the Mediterranean regime may also occur (e.g. van Wilgen et al. 1990; Clarke Basin, stands out within the Mediterranean for the singu- 2002). At this scale, soil features such as rockiness (Clarke larity of its plant biodiversity (Rodríguez et al. 2008). and Knox 2002) or fertility (e.g. Kellman 1984) may cause Most of this region lies within Los Alcornocales Natural heterogeneous burn patterns and thus determine divergent Park (ca. 1,700 km2) and has a rugged topography, albeit Wre regimes in terms of frequency and severity. Indeed, soil no high elevations (500–1,100 m asl). These mountain fertility decreases the Xammability of plant fuels and thus chains are mainly formed by folded siliceous Oligo- lessens Wre propensity (e.g. frequency) of plant communi- Miocene sandstone (González-Donoso et al. 1987). In ties (Kellman 1984; Orians and Milewski 2007). In con- mountain tops and ridges, this sandstone gives rise to very trast, plants on acid, nutrient-poor soils accumulate large acid, weathered soils, characterized by a high content of amounts of polyphenolic compounds (Northup et al. 1998; soluble aluminium (Ojeda et al. 1995, 1996), an indicator Kraus et al. 2003), most of which are Xammable (Orians of severe nutrient deWciency (Woolhouse 1981; Prasad and Milewski 2007). They also hinder litter decomposition and Power 1999; Schroth et al. 2003). These acid, infertile rates and may constitute a chemical defence against herbiv- soil patches are abundant in the region, surrounded by ory (Hättenschwiler and Vitousek 2000; Kraus et al. 2003), non-acid and more fertile limestone and/or marl-derived thus favouring the retention of dead branches in the canopy soils (Ojeda et al. 1996). Acid and infertile sandstone and hence the probability of ignition and spread of Wre soils harbour open heathlands, dominated by Wne-leaved, (Schwilk 2003). low shrubs, whereas marl and limestone soils are mostly Here, we test the prediction that, at the landscape level covered by broad-leaved, sclerophyllous shrublands and within a region characterized by the recurrent occurrence of thickets (Ojeda et al. 1995, 2000). Wre, woody plant communities on acid, infertile soils should exhibit more pronounced phenotypic and phyloge- Floristic, edaphic and functional data netic clustering patterns than neighbouring communities on non-acid, fertile soils. This may be so because infertile We selected 16 plots, eight on acid, nutrient-poor, sand- plant communities should burn comparatively more often stone soils (LowFer) and eight on non-acid, fertile, lime- owing to their higher Xammability. We focused on Mediter- stone and marl soils (HiFer) from two previous studies ranean shrubland communities from two markedly diVerent (Ojeda et al. 1995; Garrido and Hidalgo 1998). Data on soil types frequent in Los Alcornocales Natural Park, at the woody species composition in 100-m line transects were northern side of the Strait of Gibraltar (southern Spain). obtained for each plot from these two sources, as well as These shrub communities occur in coastal and subcoastal soil pH and soluble aluminium (Table 1), as surrogates for mountains under mild Mediterranean conditions and the soil fertility (see above; see also Ojeda et al. 1995). LowFer region is characterized by a high Wre incidence (Ojeda et al. plots were open heathlands while the HiFer plots were 1995, 2000). They are thus assembled from a Wre-Wltered sclerophyllous shrublands and thickets. Each species was Xora at the regional scale (Verdú and Pausas 2007). This classiWed as P+/¡ depending on the ability of its seeds to scenario provides an excellent opportunity to explore resist the action of Wre and present Wre-cued recruitment on whether: (1) a presumably spatial variability in Wre inci- the basis of published information (Paula et al. 2009) and dence at the landscape scale is reXected in diVerences in the Weld observations (see Fig. 1). Based on Xammability tests phenotypic and phylogenetic structure of local communi- of Mediterranean woody species by Elvira-Martín and ties; and, if so, whether (2) Wre may still act as a Wlter in an Hernando-Lara (1989) we were able to ascertain that already Wre-Wltered regional Xora. By using detailed inven- Xammability levels were higher in LowFer community tories and functional information of the woody Xora and samples (average 79% of high-Xammable species) than soil data in local community samples we compare the struc- in HiFer ones (average 53% of high-Xammable species; ture of shrubland communities in two edaphically contrasting t test = ¡5.24, P-value = 0.0002). 123 Oecologia (2010) 163:729–735 Author's personal copy 731 Table 1 Soil pH values and concentration of soluble aluminium (in PLowFer HiFer p.p.m.) and percentage of Wre-induced-germination (%P+) species in Drosophyllum lusitanicum the eight acid, nutrient-poor, sandstone soils (LowFer) and eight non- Viscum cruciatum Myrtus communis acid, fertile, limestone and marl soils (HiFer) plots Pistacia lentiscus Fumana thymifolia Plot Soil pHa Aluminiuma %P+ Helianthemum origanifolium Cistus albidus Cistus crispus LowFer

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