Survival, Growth, and Fecundity of the Invasive Swallowworts (Vincetoxicum rossicum and V. nigrum) in New York State

Black and pale swallowwort (BSW and PSW, respectively) are perennial, herbaceous vines in the Apocynaceae that are native to Europe. The species are becoming increasingly abundant in the northeastern United States and southeastern Canada and are difficult to manage. However, we know little about the demographic parameters of these species. We determined the survival, annual rate of vegetative growth, and fecundity of mature clumps of these swallowwort species. We selected four PSW sites (three of which comprised both old-field and forest habitats) in central New York and three BSW old fields in southeastern New York. BSW is largely restricted to higher light habitats in its introduced range. In each habitat, we followed the growth of 30 to 32 randomly selected clumps of similar size (2 to 5 stems clump in the initial year) for 3 to 4 yr. Yearly survival was 99.6 6 0.3% [mean 6 standard error] for PSW and 100 6 0% for BSW. In old fields, vegetative expansion varied from 20.01 6 0.1 to 4.6 6 0.4 stems clump yr for BSW and 20.02 6 0.2 to 2.1 6 0.5 stems clump yr for PSW. In forests, PSW growth was lower with vegetative expansion ranging from 20.01 6 0.1 to 0.8 6 0.2 stems clump yr. Fecundity of PSW in 2007 and 2008 (130 6 10 viable seeds stem yr) was similar to BSW (100 6 10 viable seeds stem yr). Fecundity of PSW in forests was generally lower than PSW in old fields, but it varied greatly among sites (0 to 170 viable seeds stem yr). We found that stem growth and fecundity did not vary with clump size (stems per clump). Since vegetative expansion and fecundity rates were high in old-field habitats, but were generally low or nonexistent in forest habitats, we suggest that management of these two invasive vines be focused in higher light environments to reduce overall seed production and its subsequent spread to surrounding areas. Nomenclature: Black (or Louise’s) swallowwort, Vincetoxicum nigrum (L.) Moench., syn. Cynanchum louiseae Kartesz & Gandhi; Pale (or European) swallowwort, Vincetoxicum rossicum (Kleopow) Barbar., syn. Cynanchum rossicum (Kleopow) Borhidi.