Reducing nutrient availability and enhancing biotic resistance limits settlement and growth of the invasive Australian swamp stonecrop (Crassula helmsii)

Janneke M. M. van der Loop,Hein H. van Kleef,Leon P. M. Lamers,Rob S. E. W. Leuven,Janne Tjampens,Joost J. Vogels

doi:10.1007/s10530-020-02327-0

Janneke M. M. van der Loop, Hein H. van Kleef + Show 4 more

Open Access

https://doi.org/10.1007/s10530-020-02327-0

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Abstract

The invasive Australian swamp stonecrop, Crassula helmsii, is a perennial amphibious herb originating from Australia and New Zealand. In freshwater wetlands of North-western Europe, this alien plant species is invasive due to its efficient colonization of empty niches. The establishment of dense C. helmsii growth is threatening native biodiversity and functioning of freshwater ecosystems, especially oligotrophic wetlands with high disturbance and nutrient enrichments. As the effects of these potential drivers of ecosystem degradation are generally difficult to determine in the field, we tested the competitive strength of C. helmsii in a greenhouse experiment with two native competitor species of the same habitat type, Pilularia globulifera and Littorella uniflora. Sods dominated by either of the native species, as well as bare soils, were collected from the field and manually infested with propagules of C. helmsii. Settlement and growth of C. helmsii was assessed after five weeks. In addition, the effect of nutrient enrichment by water bird feces on competition was studied by adding waterfowl droppings. C. helmsii was able to settle successfully in all treatments, but P. globulifera and L. uniflora dominance reduced settlement success and growth of C. helmsii. On vegetated sods, the addition of waterfowl droppings had a low effect on the performance of C. helmsii, however, this treatment significantly increased biomass production on bare soils with low nutrient availability. We conclude that both absence of native competitors and eutrophication, including guanotrophication by waterfowl, explain the establishment success and invasiveness of C. helmsii. Given the fact that eradication of C. helmsii is very challenging, our results imply that management should focus on a combination of increasing local species densities and abating eutrophication. This will strongly limit the window of opportunity for invasion of C. helmsii and enhance resistance by native plant communities.

Highlights

Australian swamp stonecrop Crassula helmsii (T.Kirk) Cockayne is a perennial amphibious plant
Plant tissue N:P ratios (g/g) of C. helmsii strongly differed between locations after experiment
Settlement of C. helmsii was very high on bare soils in both nutrient-rich and nutrient-poor treatments, but strongly regulated by the presence of native vegetation

Summary

Introduction

Australian swamp stonecrop Crassula helmsii (T.Kirk) Cockayne is a perennial amphibious plant. P. globulifera grows on soils with a relatively high organic content, whereas L. uniflora thrives on more oligotrophic, mineral soils Both species form dense swards limiting space and available nutrients for C. helmsii, potentially affecting its settlement and growth. Geese droppings contain many poorly digested plant fragments, often still capable of vegetative regeneration (Denys et al 2014) Can these two processes greatly enhance propagule pressure over large areas, but nutrient release from feces works as a natural fertilizer (Dawson and Warman 1987; Robert et al 2013; Dean et al 2015; Brouwer et al 2017; Hussner et al 2017). An additional treatment with waterfowl feces was included to assess the potential facilitation of invasion through guanotrophication

Experimental setup

Results

Discussion