Seedling dynamics and functional traits of native and introduced Foeniculum vulgare: A test of invasive vigor

Tests of invasive vigor that focus on few traits can miss important differences between plants in their native and introduced range, potentially limiting their effectiveness. The inclusion of traits that are likely to affect performance at multiple life stages in the invaded range should yield more relevant information about genetic differences between ranges. Using fennel (Foeniculum vulgare), a common non-native plant in California grasslands and wetlands, a comparative survey was conducted of plants from the introduced range in California and the native range in the Mediterranean Basin. This field comparison was followed by a series of common garden, glasshouse, and growth chamber experiments to compare fennel seed and seedling traits. The field pattern matched that predicted by invasive vigor: California populations exhibited larger aboveground size and seed mass. However, after controlling for latitude of origin, comparisons of growth and morphological characteristics of introduced and native range seedlings yielded little support for invasive vigor. Growth and mortality did not differ significantly between California and Mediterranean populations through the first growing season. Additionally, Mediterranean range plants exhibited higher germination rates than their Californian counterparts, counter to the predictions of invasive vigor. As comparative studies between native and introduced range populations continue to include a variety of morphological, demographic, and physiological traits, as well as latitudinal information, we will have an increased ability to test hypotheses concerning biological invasions.