Influence of competition by sown grass, disturbance and litter on recruitment of rare flood-meadow species
Introduction
Limited seed dispersal has been identified as a major constraint in the restoration of species-rich plant communities (Bakker and Berendse, 1999). Consequently, much recent research on restoration has focused on active seed introduction of target species (Jones and Hayes, 1999, Hölzel and Otte, 2003). However, even after bridging dispersal limitation by sowing, many species require specific regeneration conditions that may considerably differ from those of adult plants (Grubb, 1977). In this early life stage coexistent living and dead biomass can have quite diametrical impacts. In this context an established, closed sward was identified as one major obstacle for the successful establishment of less competitive herbaceous species whereas the impact of simultaneously sown grass is less clear (Bosshard, 1999).
On ex-arable fields, sowing grass simultaneously with target species may enhance the utilization of newly created meadows by hay-making or pasture. Thus grass sowing may be desirable practice since the integration of restoration sites into local, non-intensive farming systems (i.e. no fertilisation, first cut for hay production mid June or later) is a crucial factor for their long-term preservation (Donath et al., 2004). If the effects of sown grasses on seedling emergence of rare target-species were neutral (or even positive) the divergent interests of farmers and conservationists in flood-meadow restoration could be reconciled.
Facilitation by surrounding vegetation, dead or alive, seems to be more important under harsh site conditions, while under favourable conditions inhibition through interception of light and water often outweighs these positive effects (Xiong and Nilsson, 1999, Eckstein and Donath, 2005). These negative effects seem to be attenuated by gaps in closed swards created by disturbance, which are essential for changes in species composition through introduced seeds (Bullock et al., 1995). Seed size seems to be one important specific trait that may influence the impact of living and dead biomass on seedling recruitment: Large-seeded species are usually much more tolerant towards competition through surrounding vegetation or litter coverage than small-seeded ones (Krenová and Leps̆, 1996).
In two field experiments we analysed the main effects of sown grass on (i) germination, (ii) survival and (iii) establishment of six rare herbaceous grassland species under contrasting hydrological regimes (functional vs. fossil floodplain, i.e. direct flooding vs. indirect flooding). Additionally, we assessed the effects of litter application and topsoil disturbance on the same variables in an alluvial meadow. Beyond general effects of the treatments we were also interested in species-specific reactions of the introduced herbaceous species. To draw conclusions about the effects of the treatments beyond the phase of germination we followed the individual fate of the seedlings over a period of two years.
Section snippets
Study area
The study area is situated in the Hessian portion of the Holocene floodplain of the River Rhine about 30 km south-west of Frankfurt, Germany. This region represents one of the last and most important strongholds of many rare and endangered alluvial grassland species in Central Europe (Donath et al., 2003).
A winter dyke divides the area into two hydrological compartments: functional and fossil floodplain. Although direct flooding of the fossil floodplain by river water is prevented by winter
Effects of sown grass
In both floodplain compartments repeated measurement analyses revealed significant effects of time, species, and time × species and grass × species interactions on SP, but no effect of grass, grass × time or grass × time × species (Table 2). In case of GP, which did not increase after the fourth counting date, there was no main effect of sown grass and its interactions, while main effects were observed for species, time and their interaction (all p < 0.001; results not shown).
A combined analysis of the
Effects of sown grass
In all study species the non-significant effects of sown grass on germination during (GP) and at the end (GF) of the experiment, can be viewed as a consequence of the simultaneous development of the sown grasses and study species. By the time the coverage of grass peaked (end of July) and light interception reached levels at which germination is probably lowered (Foster and Gross, 1998, Leps̆, 1999) most of the germination of sown study species had already occurred. Additionally, below-ground
Acknowledgements
We thank Christiane Lenz-Kuhl and Josef Scholz-vom Hofe for their help with the set up of the field experiments. Lutz Eckstein and Astrid Dempfle deserve many thanks for proof-reading a former version of this manuscript. Rod Snowdon kindly corrected our English. This research was funded by the German Federal Agency for Nature Conservation (BfN).
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