In heterogeneous environments, such as Mediterranean-type forests, species segregate spatially in response to selective abiotic and biotic filters occurring throughout plant ontogeny (Grubb 1977; Nathan & Muller-Landau 2000). In most long-lived plant populations, these environmental sieves play an extensive role in determining species distribution during early life history stages (Ricklefs 1977; Kitajima & Fenner 2000). A common way of analysing the influence of environment heterogeneity on these younger demographic stages is to compare the suitability for the recruitment of different microhabitats with contrasting abiotic conditions (e.g. Battaglia, Foré & Sharitz 2000; Rey & Alcántara 2000; Gómez-Aparicio 2008). However, habitat filtering often occurs at very fine spatial scales, which demonstrates a need for the quantification of the most influential environmental factors operating at different sequential life history stages. Detailed studies explicitly measuring continuous gradients of microhabitat conditions are, therefore, essential to fully understand the optimal conditions where natural recruitment successfully occurs (concept of regeneration niche, sensuGrubb 1977) and thus to verify the existence of species habitat partitioning.
An accurate characterization of microhabitat-specific suitability for recruitment requires a ‘linking-stages’ approach, where plant recruitment is explored across consecutive demographic stages (seeds, seedlings, saplings) that are connected by transitional processes (dispersal, germination, emergence and survival) (Herrera et al. 1994; Clark et al. 1999). All these demographic processes are microhabitat-dependent (Schupp & Fuentes 1995; Hulme 1997), but they are not always influenced similarly by the same microhabitat conditions (Gómez-Aparicio 2008; Norden et al. 2009). This is particularly common in Mediterranean ecosystems, where recruitment-driving processes are frequently discordant due to different microhabitat associations through subsequent life history stages (Jordano & Herrera 1995; Rey & Alcántara 2000; Traveset et al. 2003). Discordant patterns of microhabitat suitability over the life cycle of the plant lead to ontogenetic conflicts since the most favourable sites for one stage may not be the most favourable for others (Schupp 1995). At the species level, a well-known demographic conflict involves the differential responses between seeds and seedlings (i.e. seed–seedling conflict), in which microhabitats with a high predation risk for seeds exhibit more favourable conditions for subsequent seedling establishment and vice versa (Houle 1992; Herrera et al. 1994; Schupp 1995; Rey & Alcántara 2000). At the community level, ontogenetic conflicts might also occur, involving changes in species’ performance rankings across demographic stages (Baraloto, Goldberg & Bonal 2005a). In spite of their potential contribution as mechanisms of species coexistence, among-species rank reversals through ontogeny remain poorly understood (Clark & Clark 1992; Baraloto, Goldberg & Bonal 2005a), especially in Mediterranean-type ecosystems.
Recruitment-driving processes are not only influenced by microhabitat conditions but also by several intrinsic plant traits, such as seed size. Similarly to the above-discussed ontogenetic conflicts across microhabitats, opposing selective pressures among demographic stages have been described in relation to seed size due to the compromise between increasing seedling performance and reducing attraction for predators, as well as favouring successful seed dispersal (Moegenburg 1996; Alcántara & Rey 2003; Gómez 2004). Although there may be a considerable intraspecific variability in seed size, theoretical models have predicted the evolution of an optimal propagule size based on the existence of a trade-off between offspring number and size (Smith & Fretwell 1974). However, the occurrence of ontogenetic conflicting pressures may hinder the identification of the seed size that maximizes the overall probability of recruitment (i.e. optimal seed size). This attribute may also contribute to interspecific performance differences and thus promotes the coexistence among species differing in seed size (Kneitel & Chase 2004; Baraloto, Forget & Goldberg 2005b). To gain an accurate view of the selective pressures acting on seed size and the extent to which interspecific differences in this trait favour that potentially competing species do coexist, it is advisable to consider the entire life cycle of the organisms (Moles & Westoby 2004). However, this has seldom been explored in long-lived perennial species due to the enormous costs for obtaining long-term data for different species across multiple life history stages (see Baraloto, Forget & Goldberg 2005b).
The present study comprises an array of information on the regeneration ecology of two dominant coexisting oak species – the deciduous Quercus canariensis Willd and the evergreen Quercus suber L. – that differ in several functional traits, such as seed size. In the study area, adults of both oak species segregate along environmental gradients of topography and soil moisture, forming forests with distinct canopy cover. Quercus suber tends to dominate in habitats with a lower availability of soil water and nutrients where the overstorey canopy is relatively sparse (Pérez-Ramos 2007), whereas Q. canariensis is more abundant in moister habitats with denser canopies (Urbieta, Zavala & Marañón 2008a). At regional scale, about 60–70% of inventoried forest plots with Q. suber and/or Q. canariensis lacked natural regeneration (Urbieta et al. 2011).
In this paper, we used a multi-stage demographic approach aimed at characterizing the main stage-specific probabilities of recruitment (seed survival, seed germination, seedling emergence and survival during the first 3 years of life) in the two coexisting oak species along a wide and continuous gradient of plant cover (used here as a surrogate of microhabitat conditions). In addition, we investigated the implications of seed size variation, using a broad range of seed mass for the two studied oak species. We calibrated linear and nonlinear likelihood models for each of these consecutive life history stages and calculated overall probabilities of recruitment along plant cover and seed mass variation. These models were further validated through comparisons with the natural distribution of saplings at the study area. Specifically, we aimed to answer the following questions: (i) Which are the most critical demographic processes for oak recruitment?; (ii) are there ontogenetic conflicts, both within- and among-species (i.e. species rank reversals), along a continuous gradient of plant cover?; and (iii) are there opposing selective pressures on seed size among subsequent demographic stages?
By answering these questions, we seek to gain insights into the characterization of the regeneration niche of oak species with contrasting functional attributes, including seed size, and their relationships with distribution patterns of adults along the landscape. The identification of ontogenetic conflicts, both within- and among-species, will enable us to better understand the underlying mechanisms that promote species coexistence in Mediterranean mixed-oak forests. The information provided by this study will also contribute to develop ecologically based management and restoration strategies in Mediterranean forests.