Multivariate effect gradients driving forest demographic responses in the Iberian Peninsula Marta Coll a,b , Josep Peñuelas a,b , Miquel Ninyerola c , Xavier Pons d , Jofre Carnicer a,b,e,⇑ a CSIC, Global Ecology Unit, CREAF-CEAB-CSIC-UAB, Cerdanyola del Vallès, 08193 Barcelona, Catalonia, Spain b CREAF, Cerdanyola del Vallès, 08193 Barcelona, Catalonia, Spain c GRUMETS Research Group, Department of Animal Biology, Plant Biology and Ecology, Universitat Autònoma de Barcelona, Spain d GRUMETS Research Group, Department of Geography, Universitat Autònoma de Barcelona, Spain e Community and Conservation Ecology Group, Centre for Ecological and Evolutionary Studies, University of Groningen, The Netherlands article info Article history: Received 20 November 2012 Received in revised form 25 March 2013 Accepted 3 April 2013 Keywords: Forest demography Quantile modeling Recruitment Growth Mortality Macroecology abstract A precise knowledge of forest demographic gradients in the Mediterranean area is essential to assess future impacts of climate change and extreme drought events. Here we studied the geographical patterns of forest demography variables (tree recruitment, growth and mortality) of the main species in Spain and assessed their multiple ecological drivers (climate, topography, soil, forest stand attributes and tree-spe- cific traits) as well as the geographical variability of their effects and interactions. Quantile modeling analyses allowed a synthetic description of the gradients of multiple covariates influencing forest demog- raphy in this area. These multivariate effect gradients showed significantly stronger interactions at the extremes of the rainfall gradient. Remarkably, in all demographic variables, qualitatively different levels of effects and interactions were observed across tree-size classes. In addition, significant differences in demographic responses and effect gradients were also evident between the dominant genus Quercus and Pinus. Quercus species presented significantly higher percentage of plots colonized by new recruits, whereas in Pinus recruitment limitation was significantly higher. Contrasting positive and negative growth responses to temperature were also observed in Quercus and Pinus, respectively. Overall, our results synthesize forest demographic responses across climatic gradients in Spain, and unveil the inter- actions between driving factors operating in the drier and wetter edges. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction During the last decades, climate observations prove the exis- tence of a global warming trend and an increase in the intensity and frequency of drought events in the Northern hemisphere (IPCC, 2007). In the Mediterranean Basin, unprecedented warming and extreme drought events have been reported over the last decades (Xoplaki et al., 2004; Lana et al., 2008). Moreover, future model projections predict the Mediterranean Basin to be largely affected by climate change (IPCC, 2007). By 2100 annual rainfall is expected to drop by up to 20% (up to 50% less in summer), and mean temper- atures are expected to increase by 3–4 °C(Giorgi and Lionello, 2008). Since many Mediterranean ecosystems are already water limited (Boisvenue and Running, 2006), several studies suggest that Mediterranean forests could be highly vulnerable to these fu- ture climate changes (Lindner et al., 2010). Forest responses to climate change will ultimately depend on demographic processes, like recruitment, growth and mortality, which drive plant community dynamics (Clark et al., 2011a,b). Those forest demographical processes depend on local water avail- ability and temperature conditions (Gerten et al., 2008), interacting with species-specific functional traits, phenotypically plastic re- sponses and site-specific conditions (e.g. soil, slope and forest stand attributes) (Linares and Tíscar, 2010; Martínez-Vilalta et al., 2010; Lloret et al., 2012; Carnicer et al., 2011). Several types of interactions among these driving factors may determine future demographic responses in drier areas (Clark et al., 2011b). Firstly, even for forests growing under similar climatic conditions, demo- graphic responses can greatly differ depending on site-specific for- est stand attributes, which are often closely linked to both local competition intensity and to management practices (Linares et al., 2009). For instance, in dry environments, competition for water increases with increasing tree density (Linares et al., 2010; Fensham and Fairfax, 2007; Martín-Benito et al., 2007), producing spatial gradients in drought severity that may often not correspond to local climatic gradients (Maestre et al., 2005; Greenwood and Weisberg, 2008; Linares et al., 2010). Secondly, water availability 0378-1127/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.foreco.2013.04.010 ⇑ Corresponding author at: Community and Conservation Ecology Group, Uni- versity of Groningen, Nijenborgh 7, 9747 AG Groningen, The Netherlands. Tel.: +34 935814221; fax: +34 935814151. E-mail address: jofre.carnicer@creaf.uab.cat (J. Carnicer). Forest Ecology and Management 303 (2013) 195–209 Contents lists available at SciVerse ScienceDirect Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco