Ecological distribution and phenology of an invasive species, Cardamine hirsuta L., and its native counterpart, Cardamine flexuosa With., in central Japan

Abstract

Cardamine hirsuta is a European species that was recently introduced into Japan and its wide distribution has been confirmed in the Kanto district. To understand mechanisms of the recent spread of C. hirsuta in Japan, a comparative study of the alien species and its native congeneric species, C. flexuosa, was conducted. Habitat preferences, phenology and seed germination were examined. Cardamine hirsuta and C. flexuosa showed distinctive habitat‐preferences; the former was most common in open habitats created by recent man‐made constructions, and the latter was common in rice paddy fields and surrounding areas. The results indicate that C. flexuosa is a year‐long annual, with a mixed phenology of summer and winter germination and growth. Seed dormancy during summer was relatively weak for C. flexuosa, and some plants that germinated early in summer reproduced during the same summer–autumn period. Plants that germinated in late summer and autumn behaved as winter annuals. In rice paddy fields, C. flexuosa is a winter annual because germination is prevented by submergence during summer. Plants flower during the following spring and complete their life cycle before the fields are flooded for rice cultivation. Cardamine hirsuta showed strong seed dormancy during summer and behaved as a typical winter annual. Seeds of C. hirsuta were intolerant to submergence in water, a condition that breaks seed dormancy of C. flexuosa. The results explain the absence of C. hirsuta from rice paddy fields. It was concluded that the spread of C. hirsuta is attributable to the recent expansion of urban habitats created by human activity and has occurred without direct competition with C. flexuosa. Considering recent urbanization in many areas, it is suggested that C. hirsuta has been spreading rapidly in Japan.

Does invasion involve alternation of germination requirements? A comparative study between native and introduced strains of an annual Brassicaceae, Cardamine hirsuta

“…Cardamine hirsuta is one of the most recently introduced European plants that is widely spreading into Japan. The first record is the specimen from Tottori Prefecture collected in 1974 (Kudoh et al 1992), and the species was rapidly spreading across Honshu during the 1990s (Yatsu et al 2003). We found variation in seed germination responses among Japanese as well as European strains.…”

“…Postponing germination of spring-dispersed seeds to autumn may be adaptive in seasonal climates in Japan. Cardamine hirsuta behaves in Japan as a strict winter annual, and no summer-annual life-cycle is observed (Yatsu et al 2003;H. Kudoh, personal observation).…”

“…Recent introduction to Japan, North America, Australia, North Africa and other areas has been reported (Kudoh et al 1992. In Japan, the germination period starts in September and plants form rosettes to overwinter (Yatsu et al 2003). The flowering period extends from February to April in Japan, and the plants produce numerous seeds through auto-pollination (Yatsu et al 2003).…”

“…In Japan, the germination period starts in September and plants form rosettes to overwinter (Yatsu et al 2003). The flowering period extends from February to April in Japan, and the plants produce numerous seeds through auto-pollination (Yatsu et al 2003). Seeds are mechanically dispersed by dehiscing fruits, but dispersal distances are within a couple of meters.…”

“…Cardamine hirsuta rapidly expanded its distribution range during the 1990s in Japan. It became a common roadside weed over most of Honshu Island, the main island of Japan, and is considered to be continually spreading also to Kyushu and Hokkaido (Yatsu et al 2003;Kudoh et al 2006;. This is an exceptional case among widespread European weeds that usually had been introduced and spread in Japan much earlier (Ecological Society of Japan 2002).…”