Effects of UV-B radiation on germination characteristics in invasive plants in New Zealand

UV-B radiation represents a potentially selective, yet little studied environmental factor for plant invasions, especially with respect to germination characteristics and seedling establishment in areas of high UV-B exposure such as New Zealand. To explain invasive potential of plant species pre-adaptation and local adaptation to selection factors in the invaded range are two frequently consulted concepts. To test for the relevance of these mechanisms, it is necessary to compare both invasive and non-invasive species, as well as native and exotic origins of invasive species. In the present study, germination success of two congeneric species pairs of the genera Verbascum (Scrophulariaceae) and Echium (Boraginaceae) were investigated under high UV-B intensities. Each genus comprised one species that has successfully invaded New Zealand grasslands and one species that was introduced but has not been invasive in New Zealand. In an among-species approach, pre-adaptation was tested by comparing germination success of native (European) origins of all four species in relation to their different invasive success in New Zealand. In a within-species comparison, native (European) and exotic (New Zealand) origins of the two invasive species were compared to test for local adaptation to UV-B in the invaded range. In both approaches, UV-B radiation inhibited the germination success of all study species. However, the comparison of invasive and non-invasive species of the two genera showed no UV-B-specific pre-adaptation of invasive species to high UV-B intensities. Higher germination success of invasive species probably led to an establishment advantage during colonization of the invaded range. Although local adaptation of exotic populations to UV-B could not be demonstrated in the within-species approach, a genetic shift in germination velocity between native and exotic origins was found. These differences may be ascribed to other relevant environmental factors, e.g. overall irradiation and drought, inducing similar plant responses as under UV-B radiation.