Dispersal of the non-native invasive species Crassula helmsii (Crassulaceae) may involve seeds and endozoochorous transport by birds

Abstract

Reproductive abilities are essential to assess pathways of introduction, modes of dispersal, and possibilities for effective on-site remediation of invasive species, as well as to identify areas at risk and develop adequate biosafety protocols. Crassula helmsii (Kirk) Cockayne, an amphibious plant from New Zealand and Australia, was introduced in Great Britain in the early 1900s (Swale & Belcher, 1982) and now occurs throughout most of the British Isles. It was not recorded in continental Atlantic Europe until the 1980s (Margot, 1983) and it is still spreading rapidly there, especially in the Low Countries. Due to its proliferous growth and ensuing negative consequences (Robert et al., 2013), control of this highly invasive species is drawing considerable attention; efforts have so far met with little success. Dispersal of C. helmsii in Europe is believed to depend exclusively on the distribution of vegetative propagules (stem fragments or turion-like apical parts) by water, man, or animals. Overwintering also occurs in a vegetative state. As with many aquatic plants, minute fragments with a single node allow regrowth. Vaughan (1978) mentioned that the species ‘seems to set good fruit’ in Britain, but Dawson & Warman (1987) considered it likely that seeds from British plants are unviable, reporting that some were retrieved from soil samples but that these did not germinate. Germination experiments with UK material at CEH Dorset were unsuccessful (Brunet, 2002), whilst Delbart (2011) recovered only aborted seeds from three populations in southern Belgium. Recent reviews reiterate dependence on vegetative parts for dispersal, overwintering, and regrowth after management, noting uncertainty about seed viability in Europe (e.g. EPPO, 2007; Lansdown, 2012; Minchin, 2008; Willby, 2008) and current management strategies are entirely based on this presumption (Delbart et al., 2011). The rapid regrowth after sod-cutting at sites in Flanders, Belgium, where care was taken to remove even the smallest fragments, and observations of water birds grazing on stands of Crassula, led us to (re)consider (1) reproduction by means of seeds, and (2) the possibility of endozoochorous transport of vegetative propagules. Dense tufts of flowering C. helmsii were collected from the coastal dune nature reserve at D’Heye, Bredene, Belgium (51u149540N, 2u599190E) in October 2013. Plants were put into plastic bags and transferred to dark storage at 4uC for eight weeks, after which, wilted flowers with seemingly well-developed brown fruits (follicles) were removed by cutting the pedicel just below their base. Dissection of one hundred follicles yielded 30 mature seeds with a maximum of two per follicle. According to the literature, a single flower may produce two to five seeds, each c.500 mm long (EPPO, 2007). The elliptical seeds in our case were slightly smaller, 385–425 mm, and presented a characteristic rugulate surface texture (Fig. 1). 300 flowers were sown in a shallow tray with fine sand and covered with 2–3 mm of sand. Another 1000 were distributed in a similar tray but mixed with the substrate to a depth of 3–4 cm. The trays were placed in a growing chamber with 14 hours of fluorescent illumination at 18uC and 10 hours of darkness at 12uC. The substrate was kept moist by allowing demineralised water to be soaked up from below. The first seedling appeared on the twenty-fifth day in both trays. Emergence ceased completely after 43 days. Overall, 21 plants developed, 86% emerging within 32 days after sowing. The germination percentage was approximately ten times higher (18%) when seeds were at or very close to the surface than when distributed to a depth of a few centimetres; germination was epigeal. Given the small size of seeds and abundant flowering from early summer up to late autumn, we conclude that reproduction from seeds should not be dismissed as a means for site colonisation by C. helmsii or in its reestablishment after control in continental *Corresponding author: luc.denys@inbo.be