Knowledge of photosynthetic capacity is crucial for fully understanding a species' invasive potential and for the development of appropriate control strategies. Although growth and reproductive data are available for the invasive swallowwort vines Vincetoxicum nigrum and V. rossicum, photosynthetic data are wanting. These herbaceous, perennial congeners were introduced from separate European ranges during the late 19th century and became invasive during the following century in the northeastern United States and southeastern Canada. Vincetoxicum nigrum has been observed growing mainly in high light environments, whereas V. rossicum occurs across a wide range of light environments, suggesting niche divergence and that different management strategies might be needed for the two species. In this work, we investigated whether the differing habitat associations of these species is reflected in their photosynthetic capacities and leaf morphology. Photosynthetic parameters and specific leaf mass were determined across a range of light environments represented by four field habitats (common garden, forest edge, old field, and forest understory) and two greenhouse environments (high and low light). In the high-light common garden habitat, V. nigrum achieved 37% higher maximum photosynthetic rates than V. rossicum, but photosynthetic performance of the two species was the same in the forest edge habitat. Additionally, species' performance was virtually identical in high light, low light, and transitions between high and low light regimes in the greenhouse. Specific leaf mass of V. nigrum was 17% higher in the common garden and 19% higher in the greenhouse compared with V. rossicum. Both invasive Vincetoxicum spp. appear capable of growing within a broad range of light environments and their management should be similar regardless of light environment. Other explanations are required to explain the scarcity of V. nigrum in low light natural areas.
Nomenclature: Black swallowwort, Vincetoxicum nigrum (L.) Moench. [Cynanchum louiseae Kartesz & Gandhi]; pale swallowwort, Vincetoxicum rossicum (Kleopow) Barbar. [Cynanchum rossicum (Kleopow) Borhidi].
Management Implications: Vincetoxicum nigrum and V. rossicum are problematic invasive perennial vines in the northeastern United States and southeastern Canada. Although several facets of their biology and ecology have been previously investigated, this is the first study of their photosynthetic capacities. We found that V. nigrum has a higher maximum photosynthetic rate than V. rossicum in a high-light common garden, where both species were planted. In the introduced range, V. nigrum occurs almost exclusively in high-light habitats in natural areas and, although this species might perform better in high light environments, V. rossicum is an invader of both high- and low-light natural areas. Regardless, we found that the two species showed nearly identical patterns of photosynthetic response in an intermediate light forest edge habitat, where they occurred naturally, and under high- and low-light greenhouse conditions. Generally, where light availability was limited, the species performed the same, and only where light was virtually unlimited (common garden) did V. nigrum show a photosynthetic advantage over V. rossicum. Based on these results and despite its general absence in low-light natural area habitats, V.