Abstract
Since inference concerning the relative effects of propagule pressure, biotic interactions, site conditions and species traits on the invasibility of plant communities is limited, we carried out a field experiment to study the role of these factors for absolute and relative seedling emergence in three resident and three non-resident confamilial herb species on a nutrient-poor temperate pasture. We set up a factorial field experiment with two levels each of the factors litter cover (0 and 400 g m−2), gap size (0.01 and 0.1 m2) and propagule pressure (5 and 50 seeds) and documented soil temperature, soil water content and relative light availability. Recruitment was recorded in spring and autumn 2010 and in spring 2011 to cover initial seedling emergence, establishment after summer drought and final establishment after the first winter. Litter alleviated temperature and moisture conditions and had positive effects on proportional and absolute seedling emergence during all phases of recruitment. Large gaps presented competition-free space with high light availability but showed higher temperature amplitudes and lower soil moisture. Proportional and absolute seedling recruitment was significantly higher in large than in small gaps. In contrast, propagule pressure facilitated absolute seedling emergence but had no effects on proportional emergence or the chance for successful colonisation. Despite significantly higher initial seedling emergence of resident than non-resident species, seed mass and other species-specific traits may be better predictors for idiosyncratic variation in seedling establishment than status. Our data support the fluctuating resource hypothesis and demonstrate that the reserve effect of seeds may facilitate seedling emergence. The direct comparison of propagule pressure with other environmental factors showed that propagule pressure affects absolute seedling abundance, which may be crucial for species that depend on other individuals for sexual reproduction. However, propagule batch size did not significantly affect the chance for successful colonisation of disturbed plots.
Highlights
The susceptibility of a resident plant community to the establishment of new species depends on the suitability of the habitat, biotic interactions with residents and the amount of resources available to new species [1,2,3,4]
Differences between gap sizes were small in plots with litter, whereas in control plots temperature amplitude was higher in large than in small gaps
Large gaps persisted as low-competition patches throughout the first year after creation, whereas species from the resident vegetation surrounding the experimental plots re-colonised small gaps by means of vegetative growth. This is in line with [14] who found that small gaps were colonised more rapidly and showed a higher density of clonal ramets than large gaps
Summary
The susceptibility of a resident plant community to the establishment of new species (i.e. community invasibility) depends on the suitability of the habitat, biotic interactions with residents and the amount of resources available to new species [1,2,3,4]. The quality of gaps, i.e. the magnitude of changes in resources (release from resource competition) and conditions (light, temperature) in comparison with intact vegetation as well as gap longevity, depends on gap size [11]. Pronounced and long-lasting enrichment in resource supply will have the largest effects on invasibility (cf fluctuating resource hypothesis, [9]). This prediction was supported by field experiments showing that high levels of disturbance that were coupled with high fertility were most favourable for the establishment success of non-resident species [10,16,17,18,19]. The response of different species to gaps and disturbance is idiosyncratic and seems to be related to plant traits such as seed mass [2,18,20,21], life history stage [4] and germination requirements [2,18] at least initially [22]
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