Potential phytotoxic and shading effects of invasive Fallopia (Polygonaceae) taxa on the germination of native dominant species

Petr Pyšek,Petr Zákravský,Lenka Moravcová,Vojtěch Jarošik

doi:10.3897/neobiota.9.1266

Abstract

Two species of knotweeds (genus Fallopia, Polygonaceae), native to Asia (Fallopia sachalinensis, F. japonica) and their hybrid (F. ×bohemica) belong to the most noxious plant invaders in Europe and exert a high impact on invaded plant communities that are therefore typically extremely poor in species. The remarkable paucity of invaded communities points to the possible existence of mechanisms suppressing germinating populations of native species in invaded stands. In this pilot study we assessed, under laboratory conditions, whether there are phytotoxic effects of the three Fallopia congeners on seed germination of three target species: two native species commonly growing in habitats that are often invaded by knotweeds (Urtica dioica, Calamagrostis epigejos), and Lepidium sativum, a species commonly used in allelopathic bioassay as a control. Since knotweeds generally form stands with a high cover, we included varying light conditions as an additional factor, to simulate the effects on germination of shading by leaf canopy. The effects of aqueous extracts (2.5, 5.0%, and 0% as a control) from dry leaves and rhizomes of the Fallopia congeners on germination of the target species were thus studied under two light regimes, simulating full daylight (white light) and light filtered through canopy (green light), and in dark as a control regime. Rhizome extracts did not affect germination. Light treatments yielded inconclusive results, indicating that poor germination and establishment of species in invaded stands is unlikely to be caused by shading alone, but we found a pronounced phytotoxic effect of leaf extracts of Fallopia taxa, more so at 5.0% than 2.5% extract concentration. Fallopia sachalinensis exerted the largest negative effect on the germination of Urtica dioica, F. ×bohemica on that of C.epigejos, and F. japonica had invariably the lowest inhibitory effect. In the field in Central Europe, F. sachalinensis often invades less disturbed, moist riparian-like habitats where it often encounters U. dioica populations, while F. ×bohemica tends to occur more often in ruderal sites with a high representation of C. epigejos; this might imply that each of the knotweed species exhibits a stronger effect on native species that are dominant in habitats they typically invade. The weakest phytotoxic effect of F. japonica corresponds to the results of previous studies that found this species to be generally a weaker competitor than its two congeners. Although the results of our experiments cannot be taken as a direct evidence for allelopathic effects acting in the field, the demonstrated potential phytotoxic effect of invasive Fallopia species on the germination of native species suggests that allelopathy may play a role in the strong impact of knotweed invasion on species diversity of invaded communities observed in the field.

Highlights

Germination was highly significantly affected by the interaction between target species, light regime and leaf extract concentration from the three Fallopia taxa (Table 2), and by the interactions between light and leaf extracts within each of the three target species (Table 3 and Fig. 1)
In L. sativum, the 5% leaf extract from F. japonica markedly decreased germination in dark, but not in green and white light; the germination increased on 2.5% extract from F. japonica in dark, but decreased in both light regimes
In U. dioica, the decrease in germination between control and 2.5% extract from F. sachalinensis was much steeper in light regimes than in dark

Summary

Introduction

Recent research on biological invasions increasingly focuses on different types of im pacts of invasive species (Levine et al 2003; Vilà et al 2010; Pyšek and Richardson 2010), amongst which the impact on the biological diversity of invaded communities and ecosystems is perceived as of utmost importance (Levine 2000; Chornesky and Randall 2003; Sax and Gaines 2003; Richardson and Pyšek 2006; Hejda et al 2009; Winter et al 2009). Invasive plant species have recently been shown to markedly differ in their effects on species richness and diversity of native species in invaded communities; some invaders reduce the numbers of native species that persist after the invasion only to a little extent (Hejda and Pyšek 2006, 2008) while others have considerable impact on native species richness (Hej da et al 2009). Among the latter group, taxa of the genus Fallopia Fallopia taxa exhibit the most severe impacts on species richness and diversity among CentralEuropean alien plants, reducing the number of species present prior to invasion by 66 to 86% (Hejda et al 2009)

Methods

Results

Conclusion