Urban grassland restoration: which plant traits make desired species successful colonizers?

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Abstract

Questions

Which plant traits characterize successful and failed target species in urban grassland restoration? Do traits of successful target species differ from those of resident species? How do plant traits relate to environmental constraints?

Location

In-situ experimental sites on wastelands in shrinking residential areas in Berlin, Germany.

Methods

We established grassland restoration treatments and explored plant traits of successful and failed target species (plant height, specific leaf area, seed mass, seed shape, seed longevity index, CSR strategy type, plant life form). To shed light on mechanisms that shape restoration success, we also analysed the same traits of species originating from the soil seed bank and species immigrating from the surroundings. We compared both trait sets to those of resident species. With RLQ analyses we related the trait data to abundance data of species and to variables describing the environmental setting of the sites.

Results

In the third year after treatment, several plant traits differed between the successful or failed target species and the resident vegetation, e.g. successful target species tended to be as tall as resident species, whereas failed target species were smaller, suggesting insufficient competitive ability of the latter. Species that successfully recruited from the soil seed bank were taller than resident species. Small specific leaf area was important for the establishment success of target species. Trait composition of the species assemblage clearly related to the environmental setting: mean specific leaf area and the proportion of annuals increased and the proportion of C-strategists decreased with increasing human-mediated impacts on the restoration sites.

Conclusions

Our results reveal clear trait differences between successful and failed target species in grassland restoration on urban wasteland sites, demonstrating that high competitive ability is crucial for success in target species. Grassland species that are successfully integrated into urban wasteland vegetation may thus fill well-defined vacant niches, while resembling the traits of the resident species in other ways. Our results may allow generalizations and transfer to similar urban settings, as the analysed trait states were assessed as relative values compared to resident species.

Analysing trait composition of target and spontaneous species in grassland restoration on urban wastelands, we reveal clear differences between successful and failed target species (e.g., plant height). In comparison to resident and soil seed bank species, target species partially fill vacant niches, but also resemble traits of spontaneous species. High competitive ability is crucial for establishment success in target species.

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