Elsevier

Aquatic Botany

Volume 130, April 2016, Pages 45-49
Aquatic Botany

Elodea canadensis shows a higher dispersal capacity via fragmentation than Egeria densa and Lagarosiphon major

https://doi.org/10.1016/j.aquabot.2016.01.004Get rights and content

Highlights

  • We tested fragmentation and regeneration rates of three submerged plant species.

  • Elodea canadensis produced more fragments than Egeria densa and Lagarosiphon major.

  • E. canadensis fragments were longer than for E. densa and L. major.

  • E. canadensis fragments showed highest regeneration capacities.

  • The regeneration time of fragments decreased with increasing fragment length.

Abstract

Submerged aquatic plants most commonly spread via a variety of vegetative plant parts, with stem fragments being the most common dispersal units. Such rapid vegetative dispersal poses strong problems in the management of invasive aquatic plant species. Here we tested the fragmentation rate of the three major alien invaders in New Zealand waters, Elodea canadensis, Egeria densa and Lagarosiphon major, under four different flow velocities and two different light conditions in a large flow tank during the NZ summer. Subsequently, the plant fragments that were produced were tested for their regeneration capacity. E. canadensis showed the highest fragmentation rate (about 20%), followed by E. densa and L. major. Fragments of E. canadensis were longer than those of the other species. They also showed the highest regeneration rate (92%), and regeneration time decreased with increasing fragment length. The strong differences in fragmentation rates and regeneration capacity of fragments indicate that these Hydrocharitaceae differed much in their dispersal potential. These differences may explain the potential invasiveness of submerged plants, although more research is needed to quantify fragmentation under field conditions.

Introduction

The broad distribution ranges of aquatic plants have often been considered as a result of their high dispersal capability, with both sexual and asexual propagules being widely dispersed within and among water bodies (Santamaria, 2002). Strong differences in the number of propagules have been recorded between submerged species, along with a predominance for vegetative diaspores (Boedeltje et al., 2003). Alien invasive terrestrial plant species generally exhibit a high propagule pressure, and this has been considered the primary determinant of establishment success (Lockwood et al., 2005, Lockwood et al., 2009). Once established, the number of propagules that had been produced, will determine the potential for further spread of the species.

Even though some submerged alien invasive plants like Myriophyllum spicatum (Aiken et al., 1979) produce high numbers of viable seeds, in most submerged invasive aquatic plants, e.g., Elodea canadensis and other Hydrocharitaceae, seed production is lacking due to the presence of only one gender in their introduced range (Wolff, 1980, Cook and Urmi-König, 1985). Consequently, the spread of these species is limited to the dispersal of asexual propagules. But despite the lack of seed production, alien submerged aquatic plants often spread rapidly within and between waterbodies (Hussner, 2012, Umetsu et al., 2012a). Yet surprisingly, a recent field study in the Erft River found no evidence for higher fragmentation rates in the non-native invasive Vallisneria spiralis compared to native aquatic plants (Heidbüchel et al., in press).

A variety of vegetative plant parts is able to disperse within water bodies (Barrat-Segretain, 1996, Barrat-Segretain and Bornette, 2000). Sometimes whole entire plants (shoots and roots) or even large floating mats of connected uprooted plants (Heidbüchel et al., in press) disperse, but mostly rhizomes, tubers, turions, and stem fragments are common vegetative dispersal units (Barrat-Segretain, 1996, Boedeltje et al., 2003, Riis and Sand-Jensen, 2006).

Stem fragments are produced either by autofragmentation, the self-induced abscission of plant fragments (Xie and Yu, 2011) or by allofragmentation (Riis et al., 2009). Allofragmentation can be caused by disturbances, such as water flow (especially during flood events), feeding activity by water birds and fish, or human activities (Barrat-Segretain, 1996, Madsen and Smith, 1997). Weed management in particular can result in the production of a large number of fragments; e.g., up to ≥18000 Egeria densa fragments were produced during a four hour period of mechanical harvesting (Anderson, 1998). The large aerenchymatic system of submerged aquatic plants causes fragments to float up and drift to new areas. In rivers, fragments are usually drifting within the upper part of the water column (Riis and Sand-Jensen, 2006).

The high regeneration capacity of submerged plant fragments has been documented in several studies (Riis et al., 2009, Umetsu et al., 2012a), and usually increases with fragment length (Riis et al., 2009, Umetsu et al., 2012b). Under laboratory conditions even stem fragments of <1 cm regenerated within two weeks, although there were differences in the regeneration rates between species (Kuntz et al., 2014). However, even though the regeneration capacity of submerged plant fragments has been documented, little is known about the number of fragments that may be produced in the field (Fleming and Dibble, 2015). The higher the number of fragments produced, the more likely a species is able to spread within and into new water bodies.

New Zealand’s economy is largely based on primary production, and increasingly utilizes a network of irrigation and drainage systems for regulating water supply. Within such waterways, conditions often support the excessive growth of invasive macrophytes, which may impede waterflow and increase flooding risk (O’Brien et al., 2014, Wilcock et al., 1999). Plant stands affect the discharge and flow and encourage the deposition of sediment, but due to the flexible nature of submerged plants, the flow resistance of aquatic vegetation varies with flow velocity (Pitlo and Shaw, 1990). In addition, the potential for the further distribution of invasive plants via fragmentation throughout connected waterways is non-negligible. In particular the three Hydrocharitaceae E. densa Planch., E. canadensis Michx. and Lagarosiphon major (Ridl.) Moss are among the major alien invasive aquatic plants in New Zealand (Champion et al., 2014). They are considered to pose a serious threat to the functioning of the irrigation systems, however, the likelihood of spread by plant fragments through the irrigation systems with varying flow velocities has not yet been studied.

To gain insight into the potential spread of the three alien invasive aquatic plants within waterways, we studied the fragmentation rates under controlled conditions in flow-through channels to test whether (i) the number of fragments produced and the fragment sizes differ between these Hydrocharitaceae species, (ii) increasing water velocities and light will increase the fragmentation rate, and (iii) the fragments from different species will show differences in their regeneration capacity.

Section snippets

Cultivation of plant material

Plants were collected from the field 3–4 weeks prior to the experiment, and 100 apical shoots (25 cm in length) of E. canadensis, E. densa and L. major were separately planted into trays (60 × 40 × 15 cm in length, width and height). There were eight trays per species. Trays were filled with 12 cm soil, containing 0.36% particular nitrogen and 770 mg total recoverable phosphorous per kg soil, with a layer (3 cm) of washed sand on top. The plants were pre-cultivated in an outdoor tank with static water

Fragmentation

A total of 213 fragments were produced from the 2400 plant shoots (Table 1), with E. canadensis showing significantly higher fragmentation rates than E. densa and L. major (p < 0.001, F = 32.174, df = 2, Fig. 2, Table 1). Different light intensities did not affect fragment number in any species. For E. canadensis, but not the other species, the number of fragments at 0.1 m s−1 was significantly lower (p < 0.05, F = 4.554, df = 3) than at 0.2, 0.3 and 0.4 m s−1 (Fig. 2).

Additionally, the fragment lengths

Discussion

E. canadensis, E. densa and L. major are three major invaders growing in a variety of aquatic habitats, including static and flowing waters with varying light regimes (Wolff, 1980, Cook and Urmi-König, 1985), but high irradiances can cause serious stress for the plants (Hussner et al., 2010, Hussner et al., 2011). Their spread is like for most submerged aquatic plants largely based on the dispersal of vegetative plant fragments (Boedeltje et al., 2003, Barrat-Segretain, 1996). The higher the

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