Vegetative overwintering and viable seed production explain the establishment of invasive Pistia stratiotes in the thermally abnormal Erft River (North Rhine-Westphalia, Germany)

Highlights

• We studied the invasive alien aquatic plant Pistia stratiotes within the Erft River.

• We documented both vegetative over wintering and viable seed production.

• Heat images showed the effect of high water temperatures on plants during winter.

• We documented the massive drift of >6000 Pistia plants per day into the Rhine River.

Abstract

Pistia stratiotes, a native of South America, has been established in the thermally abnormal Erft River (a tributary of the Rhine River) in Western Germany since 2008. The species formed mass developments since then. Plants mainly overwinter as small flat plants, which are protected against air frost temperatures due to high water temperatures of >11 °C. Heat images document that during winter the temperature of flat leaves was mostly less than 2 °C below water temperature, while large leaves above the water surface have been killed by air frost. During all years, numerous flowering plants and viable seed production were observed. We have no direct evidence of seed germination in the field, but under laboratory conditions seeds showed a high germination potential even after exposure to −4 °C. The high number of >6000 drifting plants per day into the Rhine River demonstrated the high potential for this species for spread into the Rhine River and connected water bodies, and this threat potential stresses the paramount need for effective management strategies.

Introduction

Pistia stratiotes L. is a floating aquatic macrophyte of the family of Araceae. The species is native to South America (Hill, 2003) and has become widespread in tropical and subtropical regions of the world. The species is considered a troublesome aquatic weed in its introduced range (Neuenschwander et al., 2009, Diop and Hill, 2009, Moore and Hill, 2012). P. stratiotes performs best under high nutrient conditions (Sharma and Sridhar, 1981, Henry-Silva et al., 2008), reaching plant densities of up to 1000 plants m⁻² (Dewald and Lounibous, 1990) with dry mass reaching up to 1 kg m⁻² (Reddy and De Busk, 1984). The species spreads predominantly by vegetative propagation (Sculthorpe, 1967, Dray and Center, 1989). However, seed production is documented from Africa, India, South America, Australia and Asia (Dray and Center, 1989, Harley, 1990). In Europe, seed production had only been reported from a thermally heated stream in Slovenia (Sajna et al., 2007), and from plants in The Netherlands, which have been kept under greenhouse conditions (Pieterse et al., 1981).

Due to the potential impact of dense P. stratiotes stands on both aquatic flora and fauna and human use of water bodies, eradication via management has become necessary in several countries (Diop and Hill, 2009, Neuenschwander et al., 2009, Coetzee et al., 2011). In Europe, the species has been found in several countries (Hussner, 2012), with largest populations occurring in southern Europe (Brundu et al., 2012). Besides the established population in Slovenia (Sajna et al., 2007), some ephemeral populations have been recorded in Russia, the Netherlands and the Czech Republic (Mennema, 1977, Pilipenko, 1993, Pysek et al., 2002). In Germany, the species had been documented as ephemeral in different localities (Hussner et al., 2010). Diekjobst (1984) found P. stratiotes in the thermally abnormal Erft River, but the species did not become established even though the species had been re-introduced several times into the river as ornamental waste between 1980 and 2005 (Hussner and Lösch, 2005). Since 2008, a persistent population has been documented in the Erft River downstream of the two main discharge locations of thermally heated water from the nearby opencast minings and became the second most abundant aquatic plant species in the Erft River since then (Hussner, 2014). The plant occurs in approx. 35 km length of the river, from the most upstream population in a small (∼30 m²) side arm near Bedburg (50°59′7.95″N, 6°34′41.01″E) downstream to the inflow into the Rhine River (51°11′3.25″N, 6°43′54.27″E). In this study, we investigated the winter survival and establishment strategy of P. stratiotes in the Erft River. We were specifically interested in morphological adaptations during the cold season and analyzed plants using heat imaging. We further investigated plant propagation by following dispersal into the river Rhine and experimentally testing seed germination success after different cold exposure.