The role of flood regime on invasive success of exotic species growing in riparian environments

To elucidate the conditions under which exotic species become invasive, we compared the performance of Veronica anagallis-aquatica (exotic) and V. undulata (native) in Kansai, southern-central Japan, where the former species is highly invasive, with performance in a region where this species is less invasive (Kanto, eastern Japan). We compared the micro-environments, including scale and frequency of flooding, of these species in both Kansai and Kanto. We also conducted growth experiments on seedlings from both regions to compare their responses to varying water conditions. Four treatments were conducted: a dry treatment where neither roots nor shoots were flooded; a flooded treatment where both roots and shoots were flooded; a moist treatment where only roots were flooded; and a variable treatment where the water level was shifted between that in the flooded and dry treatments. We found that the flooding frequency was higher in Kansai than in Kanto, whereas the flow peaks were almost four times higher in Kanto than in Kansai. The mean stem internode length was longer in the variable treatments than in the other treatments in the V. anagallis-aquatica seedlings derived from Kansai, but did not differ among treatments in both V. anagallis-aquatica and V. undulata seedlings derived from Kanto. V. anagallis-aquatica in Kansai evolved a strategy to evade flooding by stem elongation. However, that species did not show elongation traits in Kanto, possibly because it could not respond to large-scale flooding. V. anagallis-aquatica may show adaptive evolution in specific habitats where it can respond adaptively.