Reproductive biology of Vincetoxicum rossicum (Kleo.) Barb. (Asclepiadaceae), an invasive alien in Ontario1

Journal of the Torrey Botanical Society, Jan-Mar 2004 by St Denis, Melissa, Cappuccino, Naomi

ST. DENIS, M. AND N. CAPPUCCINO (Department of Biology, Carleton University, Ottawa, Ontario, KIS 5B6 Canada). Reproductive biology of Vincetoxicum rossicum (Kleo.) Barb. (Asclepiadaceae), an invasive alien in Ontario. J. TORREY BOT. SOC. 131:8-15. 2004.-We investigated the reproductive biology of the alien invasive vine Vincetoxicum rossicum (Asclepiadaceae) and evaluated pollinator visitation rate in the field, evidenced by missing pollinaria, to flowers of isolated plants and those of plants in dense monocultures. Flowers in dense monocultures were visited less frequently than those on isolated plants surrounded by old-field grasses. Visited flowers were more likely to produce fruit and those fruits contained more polyembryonic seeds. However, a greenhouse experiment in which we cross- or self-pollinated flowers revealed no difference in fruit set, seed production, seed weight or embryony. This suggests that the higher fruit set and greater polyembryony of visited flowers in the field was not the consequence of visitation, but may have been the result of floral visitors having chosen the most vigorous flowers. Day-old flowers emasculated in the greenhouse experiment occasionally produced a fruit, indicating that the germination of self-pollen begins the day a flower opens. The ability to produce large numbers of selfed seeds is no doubt advantageous to this highly invasive asclepiad.

Key words: Vincetoxicum, Asclepiadaceae, pollination rates, self-compatibility.

Pale swallow-wort Vincetoxicum rossicum (Kleo.) Barb. (Asclepiadaceae; syn. Cynanchum rossicum) is a perennial herbaceous vine that was introduced to North America from Ukraine and Russia in the late 180Os (Sheeley and Raynal, 1996). It has become a pest of natural areas, forming dense monocultures in both old-field and understory habitats. Its spread in recent years throughout the Toronto and Ottawa metropolitan regions has caused it to be considered one of the important invasive plants of natural areas in Ontario (Ontario Invasive Plants Working Group, 2000). Vincetoxicum rossicum is also spreading throughout upstate New York and other northeastern states (Sheeley 1992), where it is threatening unique native habitats such as the alvar communities east of Lake Ontario (pers. comm. R M. Lawlor, The Nature Conservancy).

Vincetoxicum rossicum spreads locally and colonizes new areas by wind-dispersed seeds. Most seeds do not travel far from the parent plant (Cappuccino et al. 2002); however, occasionally seeds are caught on rising air currents and could potentially travel long distances. Since V. rossicum does not produce rhizomes (Sheeley 1992, Christensen 1998), if individual long-distance dispersers are to establish new focal populations far from the main infestation, they must be able to self-fertilize. Self-fertility is often observed in isolated or marginal plant populations (Baker 1955, Wyatt 1986, Barrett 1988, Daehler 1998a).

Many members of the Asclepiadaceae are self-incompatible and many have very low fruit-set (Wyatt and Broyles 1994), traits that would not be expected to contribute to invasiveness. Indeed, asclepiads are under-represented in global data sets of serious or widespread weeds (Daehler 1998b). A few weedy milkweeds, however, have been shown to be self-compatible (Wyatt and Broyles 1994). The black swallowwort Vincetoxicum nigrum (L.) Moench. produces viable autogamous seeds (Lumer and Yost 1995), making this species a successful invader in southern New York state. In the present study, we examine the reproductive biology of V. rossicum in eastern Ontario. We assess rates of pollen removal in the field, compare fruit-set and germination of seeds from visited and nonvisited flowers, and compare the success of self- and cross-pollination in a greenhouse experiment.

Materials and Methods. SPECIES DESCRIPTION. Vincetoxicum rossicum is a perennial herbaceous vine that climbs to a height of 1-2 m each growing season. The flat, star-shaped flowers are 1-cm across, range in color from maroon in the sun to pale beige in the shade, and are displayed in loose cymes in the leaf axils. The black corpuscula of five pollinaria are easy to see against the pale green background of the central gynostegium (Figure Ia). Each corpusculum is attached by translator arms to two pollinia (Figure Ib). Nectar is often visible at the base of the petals between the lobes of the corona (Figure Ia). Flowers are present on the plants from late May through August in Ontario, and damaged plants that resprout later in the season can produce flowers until the first frost. The flowers have a sweet, fruity odor that is not unpleasant.

The fruits of V. rossicum are slender follicles 4-7 cm long. A fruit may consist of a single follicle or two joined follicles. The fruits dehisce to release comose seeds throughout the late summer and autumn. The seeds of V. rossicum range from 3-5 mm long and 1-12 mg in dry weight. They are often polyembryonic.

Vincetoxicum rossicum is native to Ukraine and southern European Russia around the Black Sea (Pobedimova 1952). In North America it occurs in a fragmented distribution ranging from the Atlantic coast to Michigan and from Ontario south to southern Pennsylvania (Sheeley and Raynal 1996). It invades old fields as well as woodlots, and has been reported along roads, utility corridors and fencerows (Sheeley and Raynal 1996).