Black And Pale Swallow-Wort (Vincetoxicum Nigrum And V. Rossicum) Sites In North America And The Impact Of Abiotic Soil Factors On Their Occurrence And Growth

Abstract

The alien invasive vines, black and pale swallow-wort are currently spreading across eastern North America, invading parklands, old fields, restored forest sites, and other natural areas. These plants spread by wind-borne seed and can form dense stands where they become established. Their current ranges remain distinct with little overlap; however, it is unclear how much further their ranges are likely to expand and whether their ranges will eventually overlap. Availability of this information will be valuable for the management of these two species, especially in the context of biological control. Light availability has been shown to impact swallow-wort growth and fecundity, but the effects of other abiotic factors such as soil characteristics have not been examined. Preliminary observations and anecdotal information have associated black swallow-wort with low-pH, low-fertility soils and pale swallow-wort with high-pH, high-fertility soils. We conducted a soil sampling and site mapping study, a field experiment, and a growth chamber experiment to determine the impact of abiotic soil factors on the occurrence and growth of both swallow-wort species. The soil sampling study provided an overview of habitat and soil characteristics found in areas colonized by the swallow-worts. Location data, site descriptions, and soil samples were collected from areas occupied by the swallowworts from their introduced range. A total of 27 soil samples were analyzed for black swallow-wort and 68 samples for pale swallow-wort. Contrary to the expectation that either species would be constrained to a narrow soil pH range, both species were found on soils that ranged widely in pH (4.7-7.9). In general, pale swallow-wort tended to be found on higher fertility alfisols and black swallow-wort on lower fertility inceptisols, though both species occurred on a variety of soil types. The two species were associated with minimally disturbed sites, especially the edges of trails and roadways, abandoned fields, and secondary growth forests. Our findings suggest that soil types across a range of pH are susceptible to invasion by these swallow-wort species; however, it was unclear whether growth and fecundity of these two invasive vines is similar across these various soil types and pH. To determine if swallow-wort emergence and performance is affected by soil pH or soil type, we conducted a common garden field experiment during two years and a soil incubation growth chamber germination. In the common garden experiment, differences in growth varied primarily by species, with black swallow-wort plants having greater biomass and fecundity than pale swallow-wort plants. In the growth chamber experiment, species had no effect on the initial seed germination and emergence of seedlings. Soil pH (three levels in the field study, 12 levels in the growth chamber study) had minimal effects on swallow-wort growth. Soil type had some effect on stem height, with black swallow-wort growing taller than pale swallow-wort on a high-fertility soil, contrary to our original hypothesis. Pale swallow-wort had greater biomass allocation to roots than black swallow-wort, which is consistent with other studies. Overall, plant growth of the two swallow-wort species was vigorous for all soil type and soil pH treatment combinations suggesting that these two species can colonize and grow well in a relatively wide range of soil pH conditions. Thus, although their current ranges in North America do not appear to overlap substantially, this may not be due to these two invasive plants having narrow species-specific soil preferences as may have been postulated. From a management perspective, our results suggest that the current range of these two species is likely to continue to increase and that early detection rapid response (EDRR) programs should be established in susceptible regions not yet colonized by these two invasive vines.