Soil seed banks are important components of terrestrial ecosystems, being crucial vegetation drivers after perturbation events and in annual regeneration. Their composition and dynamics are highly evident in therophytic communities, found in dry areas or subject to frequent disturbance (Recasens et al., 1991; Lyaruu and Backéus, 1999; Caballero et al., 2003; Mayor et al., 2003; Reiné et al. 2004). On the contrary, the relationship between soil seed bank and standing vegetation is more obscure where resprouting perennials are dominant (Thompson and Grime, 1979; Leck et al., 1989). Thus, the crucial role played by soil seed banks in maintaining the biodiversity of vegetation over space and time is related to the particular constrictions and perturbation dynamics of each habitat (Fenner and Thompson, 2005). Besides the theoretical interest in this relationship, there are also implications for ecological restoration (Bekker et al., 1997; Willems and Bik, 1998; Lyaruu and Backéus, 1999).
Grime and co-workers have developed a promising approach based on seed features, concerning vegetation dynamics (Grime et al., 1981; Thompson, 1993). According to their methods and focus, the analysis of a few 'soft' morpho-functional seed traits allows one to predict for each taxa the chances of dispersal, persistence and germination. Various studies have shown that these predictions work well enough at least in temperate and Mediterranean floras (Thompson et al., 1993; Cerabolini et al., 2003; Peco et al., 2003). Thus, this approach may assist in understanding the role the soil seed bank plays in shaping standing vegetation, according to the capacities and limitations of the different morphotypes.
A good deal of information on species composition, distribution and the main habitat descriptors in Mediterranean and submediterranean vegetation has been obtained (Rivas-Martínez et al., 2002, and references therein). However, only a few studies have targeted functional aspects, while those devoted to regeneration functions are scarce. Moreover, most of these studies deal with sclerophyllous forests, whereas scrubs and pastures remain much less investigated (but see Russi et al., 1992; Peco et al., 1998a; Guardia et al., 2000; etc.), despite the diversification and species richness of these communities. In the Plana de Vic area, a wide range of species-rich pasture communities stand out against a farmed landscape, forming heterogeneous mosaics on the slopes of a number of small hills (Casas and Ninot, 1994, 1995). These hilly surfaces represent noticeable biodiversity headquarters, with very low connectiveness among them (Figure 1). Traditionally, sheep flocks maintained these pastures exploited, also acting as diaspore vectors among the pastures and between pasture areas, fallows and other marginal lands (Poschold and Bonn, 1998).
Figure 1. General view of Plana de Vic, with scattered hills (strongly eroded on South facing slopes) over the farmed plateau.
However, the abandonment of these activities, in tandem with farming intensification during the past half century has increased the actual fragmentation of the pasture units, furthering their encroachment and impoverishment (Casas, 2001).
The aim of this paper is to provide an analysis of the soil seed bank of these pastures, which on the whole are a good example of a seminatural, species-rich landscape. We have selected the two most extensive communities, which moreover show contrasting site conditions, species composition, and functional structure. The analysis included comparisons between standing vegetation and soil seed bank, between communities, and between seasons, on the basis of species composition, functional and ecological plant groups, and seed traits. Specifically, we wanted to evaluate (i) the size and the dynamics of the soil seed bank in these transitional pastures, (ii) the appropriateness of this ecological compartment in community regeneration, and (iii) the functional differences between the two communities studied in terms of soil seed bank. Scientific interest in these aspects has increased along with awareness of the ongoing biodiversity impoverishment in most seminatural landscapes, since knowledge of the structure and dynamics of soil seed banks is a key foundation for restoration management (Bekker et al., 1997; Willems and Bik, 1998).
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