G10 INSTAL.LACIONS DE DRENATGE, D'EVACUACIÓ I CANALITZACIONS G10.G02 ELEMENTS SOSTERRATS ( CLAVEGUERONS, POUS, DRENATGES )

of a salt marsh

Elytrigia atherica is a tall clonal grass species typical of higher salt marshes, but is gradually invading to the lower marshes. At young suc- cessional stages of a salt marsh, E. atherica is found sparsely dispersed in small groups of ramets. These patches increase in size and ramet density over time, eventually forming extensive swards as succession proceeds. This study investigates the change in the clonal diversity of E. atherica stands during colonization as a result of its reproductive strategy. Clonal diversities of differently sized patches of E. atherica were investigated on two lower salt-marsh sites of different age, 25 and 35 yrs respectively. Microsatellite fingerprint patterns were used to determine genet identi- ties and to estimate relatedness and genetic differentiation between the sites, between patches within sites and within patches. The majority of the patches on both sites contained more than one genet. On the older site the clonal diversity was higher than on the younger site. However, the clonal diversity tended to decrease with increasing patch size. Low genetic differentiation was found between the two sites, indicating habi- tat differentiation, whereas differentiation between patches within sites was high. It is reasoned that different environmental conditions could have resulted in different clonal structures: On an older marsh, the increase of successful seedling recruitment, due to more suitable envi- ronmental conditions, leads to an increase in clonal diversity. Over time, with increasing ramet density, intra-specific competition is likely to increase, resulting in a decrease of clonal diversity.

Introduction

Many clonal plant species are able to propagate both sexually (creating new genets from sexually produced seed) and vegetatively (producing genetically identical ramets, which are potentially independent plant units) (Cook 1983). It has been proposed that differ- ent modes of propagation are favourable under different environmental conditions encountered during the life of an individual or a population (Olivieri et al. 1995, Eriks- son 1997). This advantage may be expressed at the genetic as well as the ecological level (Ellstrand & Roose 1987). For example, vegetative reproduction has the advantages of physiological aid from the mother plant to the daughter plant through vegetative connectors (D'Hertefeldt & Jonsdottir 1999) and absence of the costs of sexual repro- duction (e.g. flowering, seed production) (Doust & Laporte 1991, Ronsheim & Bever 2000, Rydgren & Okland 2003). On the other hand, seed dispersal is a key component of a species capacity to establish populations in newly created patches of habitat (Piquot et al. 1998). Hence, long distance seed dispersal is a means of escaping the competitive effect of clonal crowding and kin competition (Eriksson 1997, Howe & Miriti 2004). This suggests that the recent history of a population of plants capable of both clonal and sexual reproduction is written in its genetic structure as the consequence of past envi- ronmetal conditions..

The balance between sexual and vegetative reproduction, i.e. the relative production of sexually and vegetatively produced offspring by individual plants, will determine the clonal diversity of the population (Watkinson & Powell 1993, Eriksson 1997). First, life- history traits of the species determine the allocation to both types of propagation. Second, external factors such as seedling and stochastic mortality, competition and her- bivory may have effects on clonal diversity. Biotic and abiotic factors such as inter- specific competition or facilitation and nutrient availability may affect this balance as well. Thus, the balance between sexual and vegetative reproduction is dynamic and may show spatio-temporal dependence on the prevailing environmental conditions (Mandu- jano et al. 1998, Nabe-Nielsen & Hall 2002).

The wheatgrass species, Elytrigia atherica, is capable of both vegetative and sexual reproduction. In the past, the occurrence of E. atherica was limited to the higher eleva- tions of salt marshes (Bakker 1989, Van Wijnen et al. 1997). However, during the past three decades, the species has been expanding exponentially at higher elevations in many European salt marshes (Bakker et al. 1993). This increase in E. atherica has led to a concurrent decrease in species diversity (Leendertse et al. 1997, Bockelmann et al. 2003). In addition, the species has recently been expanding its range rapidly by invading lower elevations of the marshes (Bockelmann 2002, R.M. Veeneklaas unpublished data),.

At young successional stages with low elevations, E. atherica can be found sparsely dispersed in groups of ramets that we will refer to as patches. Patches may establish from a single, or a few seedlings, followed by vegetative spread. These patches increase in size and ramet density over time (R.M. Veeneklaas unpublished data). Such densely packed patches may trap dispersed seeds during tidal inundations. Eventually the

CLONAL STRUCTURE OFELYTRIGIA ATHERICA

patches merge and form extensive swards in late successional stages, dominating most other salt-marsh species (Olff et al. 1997, Van Wijnen et al. 1997). Based on the observa- tion that both biotic (e.g. herbivory, Kuijper et al. 2004) and abiotic (e.g. nutrient avail- ability, Van Wijnen & Bakker 1999) conditions acting on E. atherica are very dynamic over time, it is likely that the clonal diversity of E. atherica stands during colonisation will change as a result of a changing reproductive strategy (Kik et al. 1990). In addition, autonomous patch processes, such as the increase of intra-specific competition in denser patches (Tilman 1988), may influence the clonal diversity. It is, however, difficult to predict the direction of change in clonal diversity during succession, as some environ- mental factors could have opposing effects or have different influences on vegetative or sexual reproduction (Kik et al. 1990).

Previously, studies on the genetic diversity of E. atherica in European salt marshes were conducted on macrogeographical scales (Bockelmann et al. 2003, Refoufi & Esnault 2006). However, a specific study aimed at clonal diversity on a microgeographi- cal scale is necessary in order to understand the clonal structure of invasive, clonal plants such as E. atherica. The aim of this study is to assess 1) what mode of propaga- tion, sexual or vegetative, governs the successful spread of the species in new habitats, and 2) whether there is a change in the balance of sexual versus vegetative reproduction as succession proceeds? To this end the clonal diversity of E. atherica on two adjacent sites, differing in successional stage and consequently in environmental conditions but within one salt-marsh system will be measured. Genets will be distinguished by using microsatellite markers as DNA fingerprints (Bockelmann et al. 2003). Relatedness and genetic differentiation between genets and groups of genets will be estimated to assess if differences in environmental conditions between the two sites pose different selection pressures on the individuals (Stanton et al. 1997).

Materials and Methods